Technical ReportPDF Available
HYDROMODIFICATION ASSESSMENT AND
MANAGEMENT IN CALIFORNIA
Eric D. Stein
Felicia Federico
Derek B. Booth
Brian P. Bledsoe
Chris Bowles
Zan Rubin
G. Mathias Kondolf
Ashmita Sengupta
Technical Report 667 - April 2012
Hydromodification Assessment and
Management in California
Commissioned and Sponsored by California State Water Resources Control
Board Stormwater Program
Eric D. Stein Southern California Coastal Water Research Project
Felicia Federico University of California, Los Angeles - La Kretz
Center for California Conservation Science
Derek B. Booth University of California, Santa Barbara
Brian P. Bledsoe Colorado State University, Fort Collins
Chris Bowles CBEC, Inc., Eco-engineering
Zan Rubin University of California, Berkeley
G. Mathias Kondolf University of California, Berkeley
Ashmita Sengupta Southern California Coastal Water Research Project
April 2012
Technical Report 667
Acknowledgements
We would like to thank the California State Water Resources Control Boards for their financial support
to develop this document and for their invaluable input in terms of the priority technical and
management needs associated with hydromodification. In particular, we thank Greg Gearheart and Eric
Berntsen of the State Water Board's Storm Water Program, and Dominic Roques of the Central Coast
Regional Water Board, for their input, review and overall guidance throughout the process. Their
contributions were essential to helping to focus the document on areas of highest importance for the
future of hydromodification management.
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TABLE OF CONTENTS
Executive Summary .................................................................................................................................. ES-1
1. Overview and Intended Uses of the Document ....................................................................................... 1
1.1 Overall Objectives and Intended Audience ........................................................................................ 1
1.2 Rationale and Justification ................................................................................................................. 1
1.3 Need for an Expanded Approach ....................................................................................................... 2
1.4 Scope and Organization ..................................................................................................................... 4
2. Hydromodification Science ...................................................................................................................... 5
2.1 Introduction ....................................................................................................................................... 5
2.2 Hydrology Overview ........................................................................................................................... 5
2.3 Impact of Urbanization ...................................................................................................................... 6
2.3.1 Decreased Interception ............................................................................................................... 6
2.3.2 Infiltration ................................................................................................................................... 7
2.3.3 Increased Connectivity and Efficiency of the Drainage System .................................................. 8
2.3.4 Decreased Infiltration into Stream Beds ..................................................................................... 8
2.4 Changes in Instream Flow ................................................................................................................ 10
2.4.1 Moderate Stormflow ................................................................................................................. 11
2.4.2 Large, Infrequent Storms .......................................................................................................... 11
2.4.3 Baseflow .................................................................................................................................... 11
2.5 Changes in Sediment Yield ............................................................................................................... 12
2.6 Impacts on Channel Form and Stability ........................................................................................... 13
2.6.1 Physical Principles Underlying Channel Impacts ....................................................................... 14
2.6.2 Natural Variability in Stream Systems ....................................................................................... 15
2.6.3 The Role of Sediment Transport and Flow Frequency in Channel Morphology ....................... 15
2.6.4 Applicability to California Streams ............................................................................................ 16
2.6.5 Factors Determining Extent of Impacts .................................................................................... 17
2.6.6 Impacts on Other Types of Receiving Waters ........................................................................... 18
2.6.7 Influence of Scale ...................................................................................................................... 18
2.7 Impacts on Fluvial Riparian Vegetation ........................................................................................... 19
2.8 Impacts on In-Stream Biota.............................................................................................................. 20
2.9 Conclusions ...................................................................................................................................... 22
3. Framework for Hydromodification Management .................................................................................. 23
3.1 Introduction and Overview .............................................................................................................. 23
3.2 Background on Existing Strategies and Why They are Insufficient .................................................. 25
3.3 Development of Comprehensive Hydromodification Management Approaches ........................... 27
3.4 Watershed Mapping and Analysis Identification of Opportunities and Constraints .................... 28
3.5 Defining Management Objectives.................................................................................................... 30
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3.5.1 Protect ....................................................................................................................................... 30
3.5.2 Restore ...................................................................................................................................... 31
3.5.3 Manage as New Channel Form ................................................................................................. 32
3.6 Selecting Appropriate Management Objectives .............................................................................. 33
3.7 Framework for Determining Site-Specific Control Requirements ................................................... 35
3.8 Off-site Compensatory Mitigation Measures .................................................................................. 36
4. Overview of Assessment and Prediction Tools ...................................................................................... 39
4.1 Introduction ..................................................................................................................................... 39
4.2 Background ...................................................................................................................................... 40
4.3 Organizing Framework ..................................................................................................................... 41
4.3.1 Descriptive Tools ....................................................................................................................... 41
4.3.2 Mechanistic and Empirical/Statistical Models with Deterministic Outputs ............................. 44
4.3.3 Strengths, Limitations and Uncertainties .................................................................................. 47
5. Monitoring ............................................................................................................................................. 50
5.1 The Purpose of Monitoring .............................................................................................................. 51
5.2 Programmatic Monitoring at the Regional Scale ............................................................................. 53
5.2.1 Defining Watershed Context ..................................................................................................... 53
5.2.2 Determining the Effectiveness of Permit Requirements .......................................................... 53
5.3 Monitoring at the Local Scale .......................................................................................................... 54
5.4 Developing a Monitoring Plan.......................................................................................................... 55
5.4.1 Design of a Monitoring Plan ...................................................................................................... 55
5.4.2 Constraints (Step 2 of the Monitoring Plan) ............................................................................. 56
5.4.3 What to Monitor (Step 3 of the Monitoring Plan) .................................................................... 60
5.5 Recommendations ........................................................................................................................... 72
5.5.1 Programmatic Monitoring......................................................................................................... 72
5.5.2 Local Monitoring ....................................................................................................................... 72
6. References .............................................................................................................................................. 74
APPENDIX A GUIDANCE FOR APPLICATION OF HYDROLOGIC AND HYDRAULIC ANALYSES ................. 108
APPENDIX B APPLICATION OF SUITES OF MODELING AND ASSESSMENT TOOLS ................................. 109
APPENDIX C ADAPTIVE MANAGEMENT ................................................................................................. 129
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LIST OF FIGURES
Figure 2-1. Vegetation reduces runoff by intercepting a portion of the total rainfall and preventing
water from entering the drainage system. (Illustration by Jennifer Natali). ........................... 7
Figure 2-2. Stormwater flowpaths are shortened and quickened through paving, building, soil
compaction, and sewer infrastructure. The rapid concentration of streamflow increases
storm peaks. Rapid runoff and reduced infiltration prevent groundwater recharge.
(Illustration by Jennifer Natali). ................................................................................................. 9
Figure 2-3. Increased surface runoff causes an extension of the channel network. This occurs through
increased channel erosion or through constructed networks (to manage increased surface
flow). The expanded channel network delivers runoff to downstream reaches much more
efficiently. (Illustration by Jennifer Natali). ............................................................................ 10
Figure 2-4. Increased runoff efficiency causes higher magnitude peak flows, shorter duration runoff
events, decreased baseflow, and dramatic increases in small storms that may have
generated little or no runoff under pre-development conditions. (Illustration by Jennifer
Natali). ..................................................................................................................................... 10
Figure 2-5. Increased sediment yields occur during the land-clearing and construction phases of
development. Post-construction sediment yields decrease, though the rate of decrease
varies considerably depending on the degree of channel instability caused by the
construction phase and by increased runoff. (Illustration by Jennifer Natali). ...................... 13
Figure 2-6. Lane’s Balance, showing the interrelationship between sediment discharge (Q
s
), median bed
sediment size (D
50
), water discharge (Q
w
), and channel slope (S). ......................................... 14
Figure 2-7. Land use changes, hydrology, geomorphology and ecology are closely and complexly
interrelated. (Adapted from Palmer et al. 2004). .................................................................. 20
Figure 3-1. Framework for Integrated Hydromodification Management.................................................. 23
Figure 3-2. Undermining of grade control and erosion of banks downstream of structures intended to
stabilize a particular stream reach. Left photo is looking upstream at drop structure; right
photo is looking downstream from the drop structure. ......................................................... 27
Figure 3-3: Example of a hydromodification management decision-making process. .............................. 35
Figure 4-1. Organizing Framework for understanding hydromodification assessment and management
tools. ........................................................................................................................................ 40
Figure 5-1. Sample requirements for confidence of 95% (α = 0.05) and power of 80% (β = 0.20). Figure
from Pitt and Parmer 1995. ..................................................................................................... 60
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LIST OF TABLES
Table 2-1. Examples of Relationships between Flow Regime Attributes and Physical Habitat
Characteristics (adapted from Roesner and Bledsoe 2002). .................................................... 21
Table 3-1. Recommendations for implementation of watershed-based hydromodification management,
organized by the scale of implementation and the time frame in which useful results should
be anticipated. .......................................................................................................................... 25
Table 4-1. Recommendations for the application and improvement of tools in support of the proposed
management framework. ......................................................................................................... 39
Table 5-1. The recommended purpose(s) of monitoring associated with hydromodification control
plans, organized by the scale of implementation and the time frame in which useful results
should be anticipated. .............................................................................................................. 51
Table 5-2. Thresholds for rejecting potential "reference" sites................................................................. 68
Table 5-3. Compilation of metrics used in the five regional B-IBI’s described in the text. ........................ 71
ES-1
EXECUTIVE SUMMARY
Most jurisdictions in California are now required to address the effects of hydromodification through
either a municipal stormwater permit or the statewide construction general permit. Hydromodification
is generally defined as changes in channel form associated with alterations in flow and sediment due to
past or proposed future land-use alteration. Hydromodification management has emerged as a
prominent issue because degradation of the physical structure of a channel is often indicative of and
associated with broader impacts to many beneficial uses, including water supply, water quality, habitat,
and public safety. Conversely, reducing hydromodification and its effects has the potential to protect
and restore those same beneficial uses. Although hydromodification has the potential to affect all water
body types, this document focuses on assessing and managing effects to streams because they are the
most prevalent, widely studied, and arguably most responsive type of receiving water.
Hydromodification by definition results from alteration of watershed processes; therefore, correcting
the root causes of hydromodification ought to be most effective if based on integrated watershed-scale
solutions. To date, such a watershed approach has not been adopted in California; most
hydromodification management plans simply consist of site-based runoff control with narrow, local
objectives and little coordination between projects within a watershed. Furthermore, each municipality
is required to develop its own approach to meeting hydromodification management requirements
rather than drawing from standard or recommended approaches that facilitate regional or watershed-
scale integration. Long-term reversal of hydromodification effects, however, will require movement
away from reliance on such site-based approaches to more integrated watershed-based strategies.
This document has two goals, and hence two audiences. The first goal is to describe the elements of
effective hydromodification assessment, management and monitoring. The audience for this goal is
primarily the State and Regional Water Boards, since meeting this goal will require integration of
watershed and site-scale activities that are likely beyond the responsibility or control of any individual
municipality. Success will require fundamental changes in the regulatory and management approach to
hydromodification that will likely advance only iteratively and potentially require one or more NPDES
permit cycles to fully implement. The second goal of this document is to provide near-term technical
assistance for implementing current and pending hydromodification management requirements. This
goal can be achieved by municipalities within the construct of existing programs and therefore the
primary audience for this aspect of the document is local jurisdictions. Achieving this goal will facilitate
greater consistency and effectiveness between hydromodification management strategies, giving them
a stronger basis in current scientific understanding.
Watershed analysis should be the foundation of all hydromodification management plans (Figure ES-1).
This analysis should begin with a documentation of watershed characteristics and processes, and past,
current, and expected future land uses. The analysis should lead to identification of existing
opportunities and constraints that can be used to help prioritize areas of greater concern, areas of
restoration potential, infrastructure constraints, and pathways for potential cumulative effects. The
combination of watershed and site-based analyses should be used to establish clear objectives to guide
management actions. These objectives should articulate desired and reasonable physical and biological
ES-2
conditions for various reaches or portions of the watershed and should prioritize areas for protection,
restoration, or management. Strategies to achieve these objectives should be customized based on
consideration of current and expected future channel and watershed conditions. A one-size-fits-all
approach should be avoided. Even where site-based control measures, such as flow-control basins, are
judged appropriate, their location and design standards should be determined in the context of the
watershed analysis.
Figure ES-1. Framework for Integrated Hydromodification Management.
Monitoring
Watershed Management Actions
Stream Restoration
Floodplain Management
Flow and Sediment Management
New Development Site Controls and
Mitigation Requirements
On-site Actions
Off-site Actions
Other Entities or Programs
New Development Site Analysis
Watershed Hydromodification Management
Opportunities/Constraints
Management Objectives
Framework for Determining Site Control Requirements
Valuation Method for Mitigation
Watershed Analysis/Mapping
Watershed Characteristics and Processes
Current Land Use and Stream Conditions
Past Actions/Legacy Effects
Proposed Future Actions/Changes in Land Use
ES-3
An effective management program will likely include combinations of on-site measures (e.g., low-impact
development techniques, flow-control basins), in-stream measures (e.g., stream habitat restoration),
floodplain and riparian zone actions, and off-site measures. Off-site measures may include
compensatory mitigation measures at upstream locations that are designed to help restore and manage
flow and sediment yield in the watershed.
Project-specific analysis and design requirements should vary depending on location, discharge point,
and size. The range of efforts may include:
o Application of scalable, standardized designs for flow control based on site-specific soil type and
drainage design. The assumptions used to develop these scalable designs should be
conservative, to account for loss of sediment and uncertainties in the analysis and our
understanding of stream impacts.
o Use of an erosion potential metric, based on long-term flow duration analysis and in-stream
hydraulic calculations. Guidelines should specify stream reaches where in-stream controls
would and would not be allowed to augment on-site flow control.
o Implementation of more detailed hydraulic modeling for projects of significant size or that
discharge to reaches of special concern to understand the interaction of sediment supply and
flow changes.
o Analysis of the water-balance for projects discharging into streams with sensitive habitat. This
may include establishment of requirements for matching metrics such as number of days with
flow based on the needs of species present.
Achieving these goals will require that hydromodification management strategies operate across
programs beyond those typically regulated by NPDES/MS4 requirements. Successful strategies will need
to be developed, coordinated, and implemented through land-use planning, habitat management and
restoration, and regulatory programs. Regulatory coordination should include programs administered
by the Water Boards, such as non-point source runoff control, Section 401 Water Quality Certifications
and Waste Discharge Requirement programs, and traditional stormwater management programs. It
should also include other agency programs, such as the Department of Fish and Game Streambed
Alteration Program and the Corps of Engineers Section 404 Wetland Regulatory Program. Thus, all
levels of the regulatory frameworkfederal, state, and localwill need to participate in developing and
implementing such a program. The integrated watershed-based approach will likely take one or more
permit cycles (i.e., at least ten years) to fully implement.
Short- and long-term recommendations for management are summarized in Table ES-1 below.
ES-4
Table ES-1. Recommendations for implementing watershed-based hydromodification
management.
Programmatic: State and
Regional Water Boards
Local: City and
County Jurisdictions
Establish consistent standards for HMPs
Promote use of watershed approaches in
HMPs to move away from reliance on
project-based management actions
Develop a valuation method to determine
appropriate off-site mitigation
Transition to a broader set of monitoring
endpoints including flow, geomorphology,
and biology
Implement watershed analysis of
opportunities and constraints related to
hydromodification
Implement a broader set of tools to improve
on-site management actions
Develop institutional capacity to oversee
and review modeling and assessment tools
Develop capacity for information/data
management and dissemination
Develop watershed-based regulatory
programs and policies for hydromodification
management
Integrate hydromodification management
needs into other regulatory programs (e.g.
TMDL, 401/WDR)
Develop institution capacity to implement
watershed-based hydromodification
programs
Incorporate hydromodification and other
water quality management into the land use
planning process
To successfully accomplish these various recommendations for implementation, both agencies and
private-sector practitioners will need to make use of a range of analytical tools. Such tools generally fall
into three categories: descriptive tools, mechanistic models, and empirical/statistical models. Models
may be used deterministically and/or in a probabilistic manner. These different types of tools can be
selected or combined, depending on the specific objective, such as characterizing stream condition,
predicting response, establishing criteria / requirements, or evaluating the effectiveness of management
actions. Selection of tools should also consider the type of output, intensity of resource requirements
(i.e., data, time, cost), and the extent to which uncertainty is explicitly addressed. It is important to note
that deterministic modeling without accompanying probabilistic analysis may mask the uncertainties
inherent in predicting hydromodification effects. Short-term and long-term recommendations for the
application and improvement of tools to support the management framework are shown in Table ES-2.
Although there is sufficient scientific and engineering understanding of hydromodification causes and
effects to begin implementing more effective management approaches now, improvements should be
informed and adapted based on subsequent monitoring data. To be useful, monitoring programs should
be designed to answer questions and test hypotheses that are implicit in the choice of management
actions, such that practices that prove effective can be emphasized in the future (and those that prove
ineffective can be abandoned). The focus of monitoring efforts, however, needs to be tailored to the
time frame of the questions being addressed and the implementing agency (Table ES-3), reflecting the
dual goals and audiences of this document.
ES-5
Table ES-2. Recommendations for the application and improvement of tools in support of the
proposed management framework.
Time Frame
Programmatic: State and
Regional Water Boards
Local: City and
County Jurisdictions
Short-term
(<10 years)
Develop quality control and standardization
for continuous simulation modeling
Perform additional testing and demonstration
of probabilistic modeling for geomorphic
response
Pursue development of biologically- and
physically-based compliance endpoints
Work cooperatively with adjacent
jurisdictions to implement hydromodification
risk mapping at the watershed scale
Implement continuous simulation modeling
for project impact analysis
Long-term
(1+ decades)
Improve tools for sediment analysis and
develop tools for sediment mitigation design
Develop tools for biological response
prediction
Improve tools for geomorphic response
prediction
Expand use of probabilistic and statistical
modeling for geomorphic response
Apply biological tools for predicting and
evaluating waterbody condition
Table ES3. Recommendations for hydromodification monitoring.
Time Frame
Programmatic: State and
Regional Water Boards
Local: City and
County Jurisdictions
Short-term
(<10 years)
Define the watershed context for local
monitoring (at coarse scale)
Evaluate whether permit requirements are
making positive improvements
Evaluate whether specific projects/
regulations are meeting objectives
Identify the highest priority action(s) to take
Long-term
(1+ decades)
Define watershed context and setting
benchmarks for local-scale monitoring (i.e.,
greater precision, if/as needed)
Demonstrate how permit requirements can
improve receiving-water “health,” state-wide
(and change those requirements, as needed)
Evaluate and demonstrate whether actions
(on-site, instream, and watershed scale)
are improving receiving-water conditions
Assess program cost-effectiveness
Identify any critical areas for resource
protection
Identifying and, ultimately, achieving the desired conditions in receiving waters requires multiple lines of
evidence to characterize condition in an integrative fashion. At their most comprehensive, the chosen
metrics should include measures of flow, geomorphic condition, chemistry, and biotic integrity.
Biological criteria are key to integrative assessment: in general, biological criteria are more closely
related to the designated uses of waterbodies than are physical or chemical measurements. This
understanding is reflected in the State’s proposed bio-objectives policy, which includes explicit links to
hydromodification management.
ES-6
In summary, transitioning from the current site-based to a more effective watershed-based approach to
hydromodification management that addresses both legacy and future impacts will require cooperation
between the State and Regional Water Boards and local jurisdictions. Both technical and
regulatory/program approaches will need to be updated or revised altogether over the next several
permit cycles to realize this long-term goal. Substantial resources will be necessary to realize these
goals; therefore, opportunities for joint funding and leveraging of resource should be vigorously pursued
from the onset. This cooperative approach should replace the current fragmented efforts among
regions and jurisdictions.
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1. OVERVIEW AND INTENDED USES OF THE DOCUMENT
1.1 Overall Objectives and Intended Audience
Regulation and management of hydromodification is in its infancy in California. As with any new
endeavor, initial attempts to meet this need is unproven, inconsistent, and relatively narrow in focus.
To improve on existing efforts, the State Water Resources Control Board (SWRCB) has engaged a team
of experts to provide technical support to both regulators and permittees for development of
Hydromodification Management Plans (HMPs) and their associated permit requirements. This resulting
document has two goals and hence two audiences.
The first goal of this document is to provide broad perspectives on what would constitute effective
hydromodification assessment, management and monitoring, based on our current best scientific
understanding of the topic. The audience for this goal is primarily the State and Regional Water Boards,
since meeting this goal will require integration of watershed and site-scale activities that are likely
beyond the control or responsibility of any individual municipality. Success will require fundamental
changes in the regulatory and management approach to
hydromodification that will likely be possible only iteratively and
potentially requiring one or more NPDES permit cycles to fully
implement. The State and Regional Water Boards will need to
provide leadership in implementing these changes, but they will
also need to work cooperatively with permittees so that planning,
management and monitoring programs can be adapted to operate
in a more integrated manner over the broader spatial scales and
longer time frames that are necessary to achieve genuine success.
Furthermore, hydromodification management plans will need to
address preexisting conditions from previous (i.e., legacy) land
uses. Clearly, addressing such past effects will require approaches
beyond regulation of new development.
The second goal of this document is to provide near-term technical assistance for implementing current
and pending hydromodification management requirements. This goal can be achieved by municipalities
within the construct of existing programs, and therefore the primary audience for this aspect of the
document is MS4 permittees. Achieving this goal will facilitate greater consistency and effectiveness
between HMPs, giving them a stronger basis in current scientific understanding, and will also serve as
initial steps toward realizing the broader goal stated above.
1.2 Rationale and Justification
The process of urbanization has the potential to affect stream courses by altering watershed hydrology
and geomorphic processes. Development and redevelopment can increase impervious surfaces on
formerly undeveloped landscapes and reduce the capacity of remaining pervious surfaces to capture
and infiltrate rainfall. The most immediate result is that as a watershed develops, a larger percentage of
This document provides broad
perspectives on what would
constitute effective
hydromodification assessment,
management and monitoring,
based on our current best scientific
understanding of the topic. The
document also provides near-term
technical assistance for
implementing current and pending
hydromodification management
requirements.
Page-2
rainfall becomes surface runoff during any given storm. In addition, runoff reaches the stream channel
much more efficiently, so that the peak discharge rates for floods are higher for an equivalent rainfall
than they were prior to development. This process has been termed hydromodification. In some
instances, direct channel alteration such as construction of dams and channel armoring has also been
termed “hydromodification.” Such direct alterations are not the focus of this document. Rather, this
document focuses on the geomorphic and biological changes associated with changes in land use in the
contributing watershed, which in turn alter patterns and rates of runoff and sediment yield. These
changes can result in adverse impacts to channel form, stream habitat, surface water quality, and water
supply that can alter habitat and threaten infrastructure, homes, and businesses.
The State and Regional Water Boards have recognized the need to manage and control the effects of
hydromodification in order to protect beneficial uses in streams and other receiving water bodies. This
recognition has led to the inclusion of requirements for development of “hydromodification
management plans” (HMPs) in many Phase 1 and some Phase 2 Municipal Stormwater (MS4) permits.
Most HMPs require the permitted municipalities to develop programs and policies to assess the
potential effects of hydromodification associated with new development and redevelopment, to require
the inclusion of management measures to control the impacts of hydromodification, and to develop
monitoring programs to assess the effectiveness of HMP implementation at controlling and/or
mitigating the impacts of hydromodification.
Development of HMPs is challenging for several reasons. First, there are few accepted approaches for
assessing the impacts of hydromodification. Traditional modeling tools are generally untested and may
be difficult to apply or inappropriate for use in some California watersheds and streams. Responses of
streams to hydromodification are difficult to assess, given inherent climatic variability and the highly
stochastic nature of rainfall and the resulting response of streams to runoff events. There are few local
examples or case studies from which to draw experiences or conclusions.
As a result of these challenges, individual HMPs to date have utilized a variety of approaches with little
coordination or consistency between them. Little information is available on the relative efficacy of any
of these approaches. Furthermore, where approaches and tools developed for HMPs in one region of
the State (or even from a different region of the country altogether) have been used in subsequent
HMPs elsewhere, there has been little or no consideration of the effect of regional climatological or
physiographical differences on the transferability of analytical techniques and tools.
1.3 Need for an Expanded Approach
Current site-based hydromodification management approaches are limited in their ability to address the
underlying processes that are responsible for most deleterious impacts of hydromodification.
Hydromodification effects, by definition, are watershed-dependent processes that are influenced by
water and sediment discharge, movement, and storage patterns that may be occurring up- or
downstream of a specific project site. Ideally, then, the first step of any hydromodification management
plan (HMP) should be a watershed analysis; management of processes at the site or project scale should
be done only in the context of such a watershed analysis. Understanding larger-scale processes
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facilitates prioritization of activities in areas of greatest need and allows for management measures to
be located where they have the largest potential benefit, even if that is not on or adjacent to the project
site where the current impact is occurring. It also allows for expansion of site based management
beyond simple flow control and/or channel stabilization toward strategies that consider flow, sediment,
and biological conditions as an integrated set of desired endpoints.
Because watershed boundaries are often not the same as geopolitical boundaries of cities or counties,
incorporation of watershed analysis will require leadership from the State and Regional Water Boards.
Changes to the current regulatory structure may be necessary to accommodate inter-jurisdictional
cooperation and regional information sharing. Similarly, program implementation by both large and
small municipalities must include mechanisms that allow site-specific decisions to be informed by
watershed-scale analysis.
This document is intended to help address some of these
challenges and needs by providing technical recommendations,
both to state and regional program developers and to local
implementing agencies, for assessment, modeling,
development of management strategies, and monitoring. This
document can support current HMP development and, at the
same time, serve as a first step toward achieving the longer
term goals of more integrated, watershed-based
hydromodification management.
Adopting this broader approach means that managing the
effects of hydromodification cannot be the purview of the
stormwater (MS4) program alone. Effective management of
hydromodification will require coordinated approaches across programs at the watershed scale that
address all aspects of runoff, sediment generation and storage, instream habitat, and floodplain
management. Various SWRCB programs have the opportunity and ability to contribute to the goals of
comprehensive hydromodification management, including the non-point source control program, water
quality certifications, waste discharge requirements, basin planning, SWAMP, and the emerging State
Wetland Policy and Freshwater Bio-objectives program. Each of these programs can take advantage of
the tools and approaches outlined in this paper to contribute to coordinated management of
hydromodification in order to protect beneficial uses and meet basin plan objectives. Furthermore,
successful control and mitigation of hydromodification effects will support other programs by improving
water quality, enhancing groundwater recharge, and protecting habitat. Therefore, hydromodification
management can be a unifying element of many programs and support integrated regional watershed
planning.
It is important to note that hydromodification has the potential to affect all water body types; therefore,
HMPs should address potential effects to all streams and receiving waters. Because streams are most
directly affected by hydromodification, they have been the focus of current regulatory requirements
and, therefore, most HMPs. Consequently, this document emphasizes tools and approaches applicable
Current site-based approaches are
limited in their ability to address the
underlying processes that are
responsible for hydromodification
impacts.
Effective management of
hydromodification will require
coordinated approaches across pro-
grams at the watershed scale that
address all aspects of runoff, sediment
generation and storage, instream
habitat, and floodplain management.
Page-4
to fluvial systems, which are broadly defined to include wadeable streams, large rivers, headwater
streams, intermittent and ephemeral drainages, and alluvial fans (although new specific tools may be
necessary for assessment and management of alluvial fans). We recognize, however, that
hydromodification can also affect nearshore and coastal environments, including bays, harbors, and
estuaries, by altering estuary channel structure, water quality, sand delivery, siltation, and salinity.
These effects have been less extensively studied or documented and have received substantially less
attention in current hydromodification requirements. Future efforts should more directly address
hydromodification effects to all receiving waters, but the information is not presently available to
provide equally comprehensive guidance here.
1.4 Scope and Organization
This document is not intended to be prescriptive or to serve as a “cookbook” for development of
hydromodification management strategies. Rather, it is a resource to evaluate the utility of existing
tools and approaches, and it proposes a framework for integrating multiple approaches for more
comprehensive assessment and management. This framework should be used to aid in the
development of HMPs that are appropriate for specific regions and settings and take advantage of the
best available science. It can also be used to improve consistency in assessment and monitoring
approaches so that information collected across regions and programs can be compiled and leveraged
to provide more comprehensive assessments of the effectiveness of management actions. Ultimately,
such consistency should improve the effectiveness of all programs.
The authors, a team of technical experts, developed the content for this document in consultation with
agency staff and regulated entities. The document begins with a brief general discussion of the effects
of hydromodification and stream response mechanisms, providing the best available science to support
subsequent recommendations. The main body of the document focuses on presenting a proposed new
management paradigm where site-based management is nested within an overall watershed
assessment that accounts for past, current, and proposed future land use. The body of the document
also includes a discussion of existing tools and how they can be used more effectively and appropriately
to evaluate potential impacts and guide decisions on selection and design of management practices.
The third major section of the document focuses on monitoring that includes evaluation of hydrologic,
geomorphic, and biologic conditions with an overriding goal of adaptive management. The document
concludes with several technical appendices that offer specific guidance on the appropriate application
of tools and models within the existing HMP approaches, and a bibliography of resources.
Page-5
2. HYDROMODIFICATION SCIENCE
2.1 Introduction
Land-use changes can alter a wide variety of watershed processes, including site water balance, surface
and near-surface runoff, groundwater recharge, and sediment delivery and transport. Although
alteration to these watershed processes (referred to collectively as hydromodification) can affect many
elements of a landscape, the focus of this document is on impacts to stream systems. Furthermore,
while this paper will often refer to urbanization, it is recognized that other types of land-use changes
(grazing, agricultural, forestry, etc.) can have similar impacts. This section reviews relevant hydrologic
processes and summarizes the impact of urbanization on hydrologic, biologic, and geomorphic systems,
and it describes our current understanding of the physical mechanisms underlying these impacts. This
provides a foundation for establishing assessment tools and predictive models, as well as for developing
management and monitoring programs.
Although not addressed by this report, urbanization also has a range of effects on water quality (Heaney
and Huber 1984, Brabec et al. 2002) by increasing pollutant loads (Owe et al. 1982), increasing nutrient
loads (Wanielista and Yousef 1993, Hubertz and Cahoon 1999), and
diluting dissolved minerals through increased runoff and decreased
infiltration and soil contact (Loucaides et al. 2007). As a result of
both its physical and chemical effects, urbanization also affects the
integrity of biota (Heaney and Huber 1984) including fishes (Klein
1979, Weaver and Garman 1994, Wang et al. 2000) and
invertebrates (Sonneman et al. 2001, Wang and Kanehl 2003).
These impacts are acknowledged and evaluated in the discussion of
monitoring Section 4, but the details of their interactions and effects
are not otherwise addressed here.
2.2 Hydrology Overview
To understand the effects of urbanization, the basic processes of the hydrologic system must be
highlighted. A watershed’s drainage system consists of all the features of the landscape that water
flows over or through (Booth 1991). These features include vegetation, soil, underlying bedrock, and
stream channels. Urban elements such as roofs, gutters, storm sewers, culverts, pipes, impervious
surfaces such as parking lots and roads, and cleared and compacted surfaces fundamentally change the
rate and character of hydrologic processes. Generally, the hydrologic changes associated with
development and urbanization increases the speed and efficiency with which water enters and moves
through the drainage system. In undeveloped watersheds, only a portion of the precipitation that falls
ever enters the stream channel. Instead, precipitation may be: 1) evaporated off the ground surface or
intercepted by vegetation and evaporated; 2) transpired from the soil; or 3) infiltrated deeply into
regional aquifers. For the portion of precipitation that ultimately enters the stream, the rate and
processes of delivery vary between watersheds, with important implications for how urbanization will
affect runoff.
Land-use changes can alter a wide
variety of watershed processes,
including site water balance,
surface and near-surface runoff,
groundwater recharge, and
sediment delivery and transport.
Alteration to these watershed
processes are referred to
collectively as hydromodification.
Page-6
Flow can be classified as stormflow (or “quickflow”) if it enters the stream channel within a day or two
of rainfall (Dunne and Leopold 1978). Quickflow occurs through 1) infiltration excess (also called
Horton) overland flow, wherever rainfall intensity exceeds the infiltration capacity of the soil and
water flows over the ground surface; 2) saturation excess overland flow, where overland flow occurs
following filling of all pore space in surface soils; 3) shallow subsurface flow, where water flows
relatively quickly through permeable shallow soils (but still more slowly than either Horton or saturation
overland flow); and 4) precipitation directly into stream channels. Conversely, water that infiltrates
more deeply is classified as delayed flow, because it travels slowly as deep groundwater and emerges
into a stream slowly over time.
As a storm progresses, runoff patterns and rates can change, even within the same catchment. For
example, surficial soils may become saturated during the course of a storm (or a storm season) as the
water table rises, and this can induce a shift in runoff from shallow (or even deep) subsurface flow to
the quickflow process of saturation excess overland flow (Booth 1991). Even under scenarios in which
rainfall intensity exceeds infiltration capacity, Horton overland flow will not be connected to stream
channels until surface depressions are filled.
2.3 Impacts of Urbanization
The archetypal model of development involves clearing vegetation; grading, removing, and compacting
soils; building roads and stormwater sewers; constructing buildings; and re-landscaping. The specific
ways in which these activities alter runoff processes are discussed below. Development may also
directly alter stream, such as through channel straightening, levee construction, and flood control
reservoirs; however, discussion of the impacts of these alterations is beyond the scope of this
document.
2.3.1 Decreased Interception
When rainfall occurs in a watershed, some of the precipitation will be intercepted by vegetation and leaf
litter and prevented from entering the stream channel network (Figure 2-1). The percentage of
precipitation that can be intercepted varies according to cover type and the character of rainfall (rainfall
intensity, storm duration, storm frequency, evaporation conditions) (Dunne and Leopold 1978). The
effectiveness of interception decreases as a storm progresses because once the surface area of a tree is
completely wetted, water will drip off leaves and run down the vegetation as stem flow. Typically, 10-
35% of precipitation is intercepted by trees and 5-20% by crops, though these amounts vary widely
(Dunne and Leopold 1978, Xiao and McPherson 2002, Reid and Lewis 2009, Miralles et al. 2010). In
urban environments where vegetative cover is greatly reduced, landscape-scale interception may be
lower by an order of magnitude (Xiao and McPherson 2002). Precipitation that is not intercepted enters
the drainage system. Thus, the mere reduction in interception in urban areas may produce the
hydrologic equivalent of a storm that is 10-30% larger.
Page-7
Figure 2-1. Vegetation reduces runoff by intercepting a portion of the total rainfall and preventing
water from entering the drainage system. (Illustration by Jennifer Natali).
The influence of urbanization on climate is complex and varied. For example, urbanization has been
shown to increase temperature (Kalnay and Cai 2003), increase or decrease wind speeds (Oke 1978,
Balling and Brazel 1987, Grimmond 2007), increase pan-evaporation rates (Balling and Brazel 1987), and
increase shading of the ground surface (Kalnay and Cai 2003). In most studies of urban hydrology, the
dynamics of evapotranspiration (ET) are typically, explicitly or implicitly, ignored (Grimmond and Oke
1999). This exclusion exists because of the widespread assumption that urban ET is negligible compared
to rural areas with higher proportions of vegetation-covered soils (Chandler 1976, Oke 1979). In cases
such as urban deforestation in the temperate Eastern United States, it is appropriate to assume a net
loss of ET due to urbanization (Bosch and Hewlett 1982, Sun et al. 2005, Roy et al. 2009). However,
spatial variability and the site-specific dynamics of climate, vegetation, and land-use should be
considered carefully in arid and semi-arid regions where vegetation is limited prior to development. In
drier climates (including much of southern California), primary productivity (and ET) may be
substantially increased through the irrigation of urban landscaping (Buyantuyev and Wu 2008).
2.3.2 Decreased Infiltration
Infiltration in urban areas is decreased due to several factors: impermeable surfaces such as roads,
parking lots, and roofs prevent infiltration by blocking water from reaching soils; heavy-equipment
construction operations cause soil compaction and degrade soil structures; construction projects may
remove surface soils and expose subsurface soils with poorer infiltration capacity; vegetation-clearing
and bare-earth construction increase erosion and loss of topsoil (Pitt et al. 2008). The effect of
impervious surfaces is intuitive, visible, and dramatic (Booth and Jackson 1997), but not all impervious
areas affect runoff processes equally. For example, if an impervious surface is built over clayey soils
with poor infiltration, the overall runoff rates will be less affected than if built over sandy soils with high
natural infiltration rates. While the loss of pervious area has received substantial attention within
scientific and policy communities, until recent years considerably less attention has been paid to the
effects of compaction and the reductions in infiltration capacity of soils (Pitt et al. 2008). Commonly, an
area of green is assumed to be permeable, but playing fields and even ornamental lawns may have very
Page-8
low infiltration capacities (Pitt et al. 2008). A study of urban runoff in Washington found that
impervious areas generated only 20% more runoff than what appeared to be green, pervious areas of
lawns (Wigmosta et al. 1994). Factors such as excavation and lawn-establishment methods appear to
be more significant for infiltration than any other factor including grain size of the original sediments
(Hamilton and Waddington 1999). Tillage may increase infiltration slightly, while compost or peat soil
amendments can increase infiltration by 29 to 50 percent (Kolsti et al. 1995).
2.3.3 Increased Connectivity and Efficiency of the Drainage System
Rainfall in urban areas moves quickly as overland flow into storm sewers and the stream channel
network (Figure 2-2). The delivery of precipitation into urban stream channels is extremely efficient,
transforming essentially all precipitation into stormflow and creating nearly instantaneous runoff.
Under natural conditions, in contrast, most runoff to streams is via groundwater paths that typically flow
at least one or two orders of magnitude slower than surface water. Thus converting subsurface flow
into surface stormflow has dramatic consequences. Furthermore, artificial surfaces such as roofs,
pavement, and storm sewers are 1) straight, which shortens the travel distance required for delivery
into the channel network; and 2) smooth, which decreases friction
and allows flow to travel more quickly than in natural channels
(Hollis 1975). Storm sewer systems increase the density of
“channels,” which further shortens runoff travel distances (Figure 2-
3). In particular, upland regions that may not have had any surface
channels prior to urbanization are frequently fitted with storm
sewers, which dramatically increase delivery efficiency into the
channel network (Roy et al. 2009). In sum, urbanization transforms
watershed processes and flow paths that were once slow, circuitous,
and disconnected into engineered and non-engineered systems that
are highly efficient, direct, and connected.
2.3.4 Decreased Infiltration into Stream Beds
Concreting of bed and banks, channel narrowing, and channel straightening limit infiltration from a
stream into the ground. Concrete channel margins create infiltration barriers, while channel narrowing
and straightening limit the surface area accessible for infiltration and also create a less complex channel.
Channel complexity such as pools, riffles, steps, and debris dams create hydraulics that slow flow
velocities and also divert water into the subsurface (Lautz et al. 2005). In arid and semi-arid watersheds
where streams may flow only occasionally, infiltration through bed, banks, and floodplain areas may
significantly lower peak flows and may sustain aquifers vital to regional water supplies and natural
habitats (Kresan 1988, Dahan et al. 2008). Increasing recognition is being paid in the scientific literature
to the infiltration services provided by natural channels and floodplains (Macheleidt et al. 2006,
Schubert 2006).
In contrast to the slow measured
runoff to natural streams by
surface and subsurface pathways,
the delivery of precipitation into
urban stream channels is
extremely efficient, transforming
essentially all precipitation into
stormflow and creating nearly
instantaneous runoff.
Page-9
Figure 2-2. Stormwater flowpaths are shortened and quickened through paving, building, soil
compaction, and sewer infrastructure. The rapid concentration of streamflow increases storm
peaks. Rapid runoff and reduced infiltration prevent groundwater recharge. (Illustration by
Jennifer Natali).
Page-10
Figure 2-3. Increased surface runoff causes an extension of the channel network. This occurs
through increased channel erosion or through constructed networks (to manage increased
surface flow). The expanded channel network delivers runoff to downstream reaches much more
efficiently. (Illustration by Jennifer Natali).
2.4 Changes in Instream Flow
The instream flow changes resulting from urbanization depend upon site-specific watershed and
development characteristics, but typically they include modification of the timing, frequency,
magnitude, and duration of both stormflows and baseflow. Urbanization has been shown to increase
the magnitude of stormflows, increase the frequency of flood events, decrease the lag time to peak
flow, and quicken the flow recession (Figure 2-4; Hollis 1975, Konrad and Booth 2005, Walsh et al. 2005).
Because the effects of urbanization manifest differently for different components of the hydrograph, the
hydrologic alterations of moderate storms, large storms, and baseflow are discussed individually below.
Figure 2-4. Increased runoff efficiency causes higher magnitude peak flows, shorter duration
runoff events, decreased baseflow, and dramatic increases in small storms that may have
generated little or no runoff under pre-development conditions. (Illustration by Jennifer Natali).
Page-11
2.4.1 Moderate Stormflow
Urbanization of a watershed can drastically increase the frequency and magnitude of small and
moderate flow events (Hawley and Bledsoe 2011). The magnitude of flow amplification increases
generally in proportion to the amount of impervious area (Leopold 1968, Hollis 1975). For example,
flows with a return period of one year or longer were shown to be unaffected by paving 5% of the
watershed, yet the magnitude of a one-year flow could be more than ten times higher when 20% of a
watershed is paved (Hollis 1975). In undeveloped watersheds, small storms may not generate any
overland flow or streamflow increase at all, because interception, infiltration, soil absorption, and
evapotranspiration contain all the precipitation.
The change to a flashier regime with larger magnitude streamflow
generated from small and moderate storms has two primary
consequences. First, the stream power and sediment-transport
capacity of the stream increase significantly, potentially creating
channel erosion and/or stressing instream biota. Second, the
season of stormflow is likely to be extended. In undeveloped
watersheds, early or late-season storms typically do not generate
significant runoff because soils are dry, can effectively absorb most precipitation, and therefore do not
generate overland flow or streamflow. Antecedent moisture conditions are less important in urban
watersheds where overland flow is generated regardless, and streamflow is generated by even a small
storm in a dry watershed. Through magnifying small and moderate storms, urbanization may increase
the duration of sediment-transporting and habitat-disturbing flows by factors of 10 or more (Booth
1991, Booth and Jackson 1997).
2.4.2 Large, Infrequent Storms
In large storms with return intervals of 10 or more years, the influence of urbanization is less
pronounced though still present. Whereas a 1-year stormflow may be increased by ten times by paving
20% of the watershed, historical data from humid-region watersheds suggest that the peak magnitude
of a 100-year flood would not even be doubled (Hollis 1975). The diminishing influence of urbanization
on floods of higher recurrence intervals is understood by recognizing that the hydrologic processes of
large storms resemble the processes of urban runoff. Essentially, a 100-yr flood is an event that is long
in duration, severe in intensity, and likely occurs when soils are already wet. Even in an undeveloped
watershed, a storm of this magnitude can typically generate (saturation) overland flow and transport
water efficiently into the channel network in a manner more generally comparable to an urban setting.
2.4.3 Baseflow
Urbanization does not affect instream baseflows consistently. Many studies have documented baseflow
reductions and/or lowered groundwater levels that have been attributed to decreased infiltration
(Simmons and Reynolds 1982, Ferguson and Suckling 1990) and groundwater extraction (Postel 2000).
In extreme cases, baseflow in urban watersheds can disappear completely during drought years, dry
Urbanization of a watershed can
drastically increase the
frequency, duration, and
magnitude of small and moderate
flow events by factors of 10 or
more.
Page-12
seasons, or even between storm events during the wet season. The effect of reducing infiltration may
be counteracted in urban and suburban landscapes, however, through irrigation of lawns, parks, golf
courses, and other water inputs such as septic systems, leaky pipes, and sewage treatment outflow
which typically import water from outside the watershed and contribute to both streamflow and
groundwater recharge (Konrad and Booth 2005, Walsh et al. 2005, Roy et al. 2009). Indeed, imported
water volumes in very dense cities may be an order of magnitude greater than precipitation. Lerner
(2002) judged that leakage in water importation and delivery infrastructure typically ranges from 20-
50%, and in general this leakage will increase groundwater recharge in urban areas. Similarly, other
studies have found municipal irrigation capable of raising groundwater levels and causing surface
flooding (Rushton and Al-Othman 1994) and changing ephemeral streams into perennial streams (Rubin
and Hecht 2006, Roy et al. 2009). In summary, the magnitude and direction baseflow and groundwater
recharge alteration depends on climate, land use, water use, and the infrastructure system of the
watershed. There are no simple “rules.
2.5 Changes in Sediment Yield
The role of watershed sediment yield in the behavior of watersheds was first characterized
systematically by Wolman (1967) in a three-part conceptual framework of how rivers respond to urban
development, in which 1) pre-development quasi-equilibrium conditions are followed by 2) a period of
active construction involving grading, vegetation removal, and bare earth exposed to erosion; and 3) the
establishment of an urban landscape consisting of pavement, houses, gutters and sewers etc. The
construction period is marked by an increase in sediment (typically 2-10 times pre-development rates)
produced from bare surfaces and the disturbances associated with construction (Chin 2006). The
sediment produced during construction is often deposited within
stream channels, initiating aggradation and/or channel widening.
Following the construction period, sediment production decreases
(Figure 2-5) and runoff increases, resulting in increased transport
capacity and the potential for severe channel erosion that can result
in channel enlargement of commonly 2-3 (and as much as 15) times
the original channel cross-section (Chin 2006). Changes in post-
construction sediment production rates are not well studied, though
case studies have found sediment yields in post-construction watersheds to be somewhat higher than
rural, undeveloped basins.
Post-construction sediment loads are typically derived from channel enlargement as a result of
increased peak flows and the legacy of construction-phase disturbance (Trimble 1997, Nelson and Booth
2002). The rate of decline in post-construction sediment yields is therefore predominantly controlled by
the degree of channel instability caused by the construction phase and the effect of increased peak
flows. If the channel margins are armored, densely vegetated, or otherwise erosion resistant, sediment
yields may decline quickly following urbanization. If channel instability ensues, elevated sediment yields
may persist for decades or more.
The combination of increased
runoff and decreased sediment
production can result in channel
enlargement of commonly 2-3
(and as much as 15) times the
original channel cross-section.
Page-13
Figure 2-5. Increased sediment yields occur during the land-clearing and construction phases of
development. Post-construction sediment yields decrease, though the rate of decrease varies
considerably depending on the degree of channel instability caused by the construction phase
and by increased runoff. (Illustration by Jennifer Natali).
2.6 Impacts on Channel Form and Stability
Channel form and stability reflect both hydrologic and geomorphic processes. Changes to runoff
characteristics and sediment supply can affect all aspects of stream morphology, including planform,
cross-sectional geometry, longitudinal profile, bed topography (e.g., pools, riffles), and bed sediment
size and mobility. While many factors influence the type and degree of impacts (discussed below), a
suite of commonly observed morphological changes due to hydromodification include channel
enlargement (incision and widening), decreased bank stability, increased local sediment yield from
eroding reaches, overall simplification of stream habitat features such as pools and riffles, changes in
bed substrate conditions, loss of connectivity between channel and floodplain (Segura and Booth 2010),
and changes in sediment delivery to coastal waters (Jacobson et al. 2001). Impacts may also propagate
upstream as headcuts resulting from reductions in base level due to excess erosion. Likewise,
tributaries entering downstream of a developed area may also experience the upstream propagation of
headcuts due to base level reductions of the mainstem.
In addition to Jacobson et al. (2001), two well-researched literature reviews of morphological impacts
(as well impacts to riparian habitat and biota) can be found in: “Impacts of Impervious Cover on Aquatic
Systems” by The Center for Watershed Protection (2003) and “Physical Effects of Wet Weather Flows on
Aquatic Habitats: Present Knowledge and Research Needs” published by Water Environment Research
Foundation (Roesner and Bledsoe 2003). Note that these two studies differ significantly in how they
Page-14
synthesize and interpret the reviewed literature, and the CWP publication acknowledges that it does not
necessarily apply to streams in the arid west.
2.6.1 Physical Principles Underlying Channel Impacts
A convenient conceptual framework for the physical impacts of hydromodification on stream
morphology is “Lane’s Balance” (Lane 1955; Figure 2-6). This framework encapsulates a fundamental
(albeit qualitative) relationship between the hydrologic and geomorphic processes that balance water
flow and sediment in a channel. It expresses the condition of sediment transport capacity, as controlled
by water discharge and slope, in broad balance with the supplied load and size of bed sediment for a
channel in equilibrium. An increase in streamflow or a decrease in sediment supply (for example) will
typically initiate a corresponding decrease in slope and/or increase in grain size in order to reestablish
equilibrium. That decrease in slope is expressed by channel incision or degradation. In contrast, an
increase in sediment supply or decrease in streamflow will typically result in aggradation and a
corresponding increase in slope.
Figure 2-6. Lane’s Balance, showing the interrelationship between sediment discharge (Q
s
),
median bed sediment size (D
50
), water discharge (Q
w
), and channel slope (S).
Slope and grain size are not the only modes of adjustment, as stream channels have many more degrees
of freedom in responding to changes in streamflow and sediment supply. For example, Schumm (1969)
extended Lane’s Balance to include width, depth, sinuosity, and meander wavelength. More
quantitatively (and more complexly), adjustments to channel form resulting from hydromodification are
controlled by interactions among flow-generated shear stresses (described by hydraulic equations for
open channel flow, as a function of channel geometry, roughness, and longitudinal slope), inflowing
sediment load, and the shear strength of the bed and bank sediments (a function of their size
distribution and cohesiveness).
Page-15
2.6.2 Natural Variability in Stream Systems
Understanding natural variability in streams is critical to predicting and assessing anthropogenic
impacts. A stream may be considered stable” or “at equilibrium” when its overall planform, cross-
section and profile are maintained with no net degradation or aggradation within a range of variance,
over extended timeframes (Mackin 1948, Schumm 1977, Leopold and Bull 1979, Biedenharn et al. 1997).
Such systems can often withstand short-term disturbances without significant change. Even without
discrete disturbances, natural streams may be in a state of dynamic equilibrium (Schumm 1977), where
the channel exhibits stability over the long term even while actively migrating laterally such that erosion
of outer banks is accompanied by sediment deposition and bar building on inner banks. Streams may
also be fluctuating between aggradation/ degradation/ stability, all within a limited range of conditions.
A large-scale event, like a flood or landslide, can cause dramatic changes in channel form, but the
channel will often re-established its pre-event planform, geometry and slope over time.
In contrast, a persistent alteration like hydromodification can cause the rate of change to increase. As a
result, the channel may begin an evolutionary (or catastrophic) change in morphology, leading to
enlargement and instability. A geomorphic threshold is the condition at which there is an abrupt and
significant channel adjustment or failure because the channel has evolved to a critical situation. It is the
condition at which the proverbial straw breaks the camel’s back. Channels that are near a geomorphic
threshold can exhibit significant adjustments in response to a relatively small degree of
hydromodification. For example, a channel with banks that are near the height and angle for
geotechnical failure may widen abruptly due to slight incision.
2.6.3 The Role of Sediment Transport and Flow Frequency in Channel Morphology
Extensive research has been devoted to establishing specific relationships between flow frequency and
characteristics of channel morphology. The concept of “effective discharge” was introduced by Wolman
and Miller (1960), using a magnitude-frequency analysis to assess the effectiveness of flow events to
transport sediment. They concluded that, for the rivers in their analysis, relatively frequent events
(occurring on average about 1 times/year) are most effective over the long term in transporting
sediment. This concept has formed the basis for a large body of literature (and occasional controversy)
over the subsequent five decades relating to the relationships between these flow frequencies and
principal channel dimensions (e.g., bankfull stage, width-to-depth ratio), and the application of these
relationships to stream design and restoration, as well as prediction and control of hydromodification
impacts. Much of the controversy has related to the use of a single event (“dominant discharge” or
“bankfull flow”) as the basis for such applications, with the implicit assumption that control for that
single discharge will result in commensurate channel changes regardless of the distribution of flow
frequencies and flow durations over a wider range of discharges.
More recently, the concept of a range of moderately frequent, “geomorphically significant” flows that
transport the majority of the sediment over the long term (King County 1990, Bledsoe 2002, Roesner
and Bledsoe 2003) was proposed to replace the focus on a single event. The geomorphically significant
flow range is considered to be the most influential in determining channel form, as this collective group
Page-16
of flows typically does the most “work” on the channel boundary over engineering time scales.
Controlling changes to the frequency of flows within this range is therefore critical to reducing impacts
to stream morphology, and is the scientific basis for the flow-duration control criteria discussed in the
following sections. A flow-duration criterion aims to match the pre-development volumes, durations,
and frequencies of this critical range of sediment transporting flows
over a period of many decades. Even this concept, however, relies
on the implicit assumption that infrequent large events, no matter
how dramatic their effects, typically occur “too infrequently” to
reset channel morphology and habitat over the timescales of
concern in meeting regulatory requirements. These events are
typically managed through traditional flood control practices as
opposed to hydromodification management.
2.6.4 Applicability to California Streams
The traditional concepts of dynamic equilibrium in streams and geomorphically significant flows,
discussed above, derive largely from studies on perennial streams in humid areas. An important
question is: to what extent do these concepts apply to managing hydromodification impacts to streams
within arid and semi-arid areas (such as large portions of California, and particularly the southern and
eastern regions)? In such climate regions, precipitation is highly variable, with low annual totals and
episodic, large events. Many streams are ephemeral or intermittent and located in a setting of
extremely high sediment production associated with erosive geology resulting from high rates of
tectonic uplift, sparse vegetative cover and frequent fires (Graf 1988, Stillwater Sciences 2007). These
streams are often characterized by multi-thread sand-bed channels that are inherently unstable and
readily respond to changes in flow conditions. In the ephemeral streams described by Bull (1997), for
example, the natural behavior is one of alternating periods and locations of aggradation and
degradation, varying both temporally and spatially. In such “episodic” streams, the vast majority of
sediment may be moved by extreme, highly infrequent events. The importance of understanding the
role of episodic events has been emphasized for semi-arid and arid fluvial systems (e.g., Wolman and
Gerson 1978, Brunsden and Thornes 1979, Yu and Wolman 1987). The latter authors reviewed concepts
of frequency and magnitude in geomorphology research and noted that episodic behavior hinges on
frequency of episodic events relative to the time required to return to an equilibrium” channel form.
Episodic behavior is more prevalent where the average long-term disturbance is low but the year-to-
year variability is high, a characteristic of arid and semi-arid climates.
Although the morphology of arid and semi-arid streams may be more strongly influenced by extreme
events under natural conditions, hydromodification has nevertheless been shown to cause rapid and
significant physical changes in such California streams (Trimble 1997, Coleman et al. 2005, Hawley and
Bledsoe 2011). Such dramatic responses to the effects of urbanization on relatively frequent flows,
often over periods of a decade or less, have profound implications for aquatic life and physical habitat.
Despite the flashy streamflow regimes, high sediment supplies, and steep gradients of many streams in
the region, the responses of California streams are controlled by the same physical processes as those in
A flow-duration management
approach aims to match the pre-
development volumes, durations,
and frequencies of this critical
range of sediment transporting
flows over a period of many
decades.
Page-17
other regions that have been studied more extensively. As such, the key controls of stream response
can be identified and managed to mitigate the chronic effects of hydromodification between infrequent
extreme events. However, it is always advisable to ensure that the application of tools and approaches
for prediction and assessment should be based on reference data and empirical models (where
applicable) drawn from stream types that are similar in both hydrologic and geomorphic characteristics.
2.6.5 Factors Determining Extent of Impacts
The extent and nature of impacts to stream morphology and habitat from a given change in runoff and
sediment supply vary widely, depending on the channel geometry, longitudinal slope, channel material
type(s) and size(s), and the type and density of channel vegetation (Center for Watershed Protection
2003, Roesner and Bledsoe 2003). For example, increased flows within a deep, narrow channel may
result in significantly higher shear stresses at the bed; this same increase in a wide, shallow channel may
become predominantly overbank flow, with less effect on bed shear stress. Where all other factors are
equal, fewer impacts would be expected where flows have access to broad overbank areas (i.e.,
floodplains) during relatively common floods (Segura and Booth 2010), channel materials are more
resistant, and stabilizing riparian vegetation is present. Conversely, where erosion and bank instability
result in the loss of vegetation reinforcement, a positive feedback response may cause erosion to be
accelerated. Furthermore, the relative erosive resistance of bed and bank materials will influence the
extent of lateral versus vertical channel adjustments (Simon and Rinaldi 2006, Simon et al. 2007). For
example, if bank resistance is lower than bed resistance, then the channel will tend to widen rather than
deepen.
The extent of impacts will also depend on the stream's
physiographic context and spatial and temporal patterns of
urban development within the watershed (Konrad and
Booth 2005). Large-scale studies of hydrologic responses to
urbanization (Chin 2006, Poff et al. 2006) also highlighted
the regional variation in these responses and reinforced the
need to understand local watershed and channel
characteristics when managing hydromodification impacts.
The presence of road crossings and other infrastructure can
provide local grade control and create sediment
bottlenecks which often translate to exacerbated erosion in the immediately downstream areas.
An additional consideration relates to the pre-development balance between sediment and streamflow,
which is dependent on precipitation patterns, the location of a stream reach within the watershed, the
associated sediment behavior of that reach (i.e., production, transport or deposition zone), and local
rates of sediment production.
While many of these factors may be quantified for a given time and location, stream systems are
enormously complex both spatially and temporally. The existence of physical thresholds and feedback
systems can cause an incremental change to result in a disproportionately large response (Schumm
1977, 1991). Furthermore, there may be significant temporal lags between the point in time at which
The extent and nature of impacts to
stream morphology and habitat from a
given change in runoff and sediment
supply vary widely, depending on the
channel geometry, longitudinal slope,
channel material type(s) and size(s), and
the type and density of channel
vegetation, and the spatial and temporal
patterns of urban development
Page-18
land use is altered and when channel impacts are observed
(Trimble 1995, 1997). In recognition of these effects and the
associated uncertainty, predictive models and management
tools may present results in terms of probabilities or within the
context of a risk-based approach, as discussed further in this
document. Such effects also have substantial implications for
the design of assessment and monitoring programs.
2.6.6 Impacts on Other Types of Receiving Waters
Although outside the scope of this document, hydromodification impacts to other water body types are
recognizable and should be the subject of additional research and future consideration.
Wetlands, Estuaries, and Coastal Ecosystems. Urbanization can alter water quality, quantity and
sediment delivery to wetlands and sensitive coastal ecosystems. Urbanization has led to loss or
degradation of wetlands and estuaries as a result of 1) draining and conversion to agriculture (Dahl,
1997); 2) upstream alterations to flow and sediment regimes that can change the magnitude, frequency,
timing, duration, and rate of change of estuarine salinity, turbidity, freshwater flooding, freshwater
baseflow, and groundwater recharge dynamics (Azous and Horner 2001); and 3) contaminated runoff
from urban areas (Paul and Meyer 2001, J Brown et al. 2010). Urbanization may also lead to coastal
erosion in circumstances where reservoir sediment trapping or post-development decreases in sediment
yield reduce the sediment supply to the coast (Pasternack et al. 2001, Syvitski et al. 2005).
Alluvial Fans. Alluvial fans are dynamic landforms that are under increased development pressure in
recent decades, particularly in the expanding cities of the American West. Upstream urbanization, and
the resultant flashier flow regime, shortens the time available for infiltration and groundwater recharge
in alluvial fans. Furthermore, development on fans themselves results in channel straightening and/or
construction of concrete flood conveyance channels that also reduce or eliminate infiltration. The
reduction in infiltration amplifies the flood risk further downstream. Additionally, alluvial fans may be
more vulnerable than other landscapes to channel instability resulting from hydromodification, because
they lack intrinsic geologic controls on channel gradient, and commonly have little vegetation or bank
cohesion to provide stability in the purely alluvial deposits (Chin 2006).
2.6.7 Influence of Scale
The ability to detect impacts from land-use changes depends upon the spatial and temporal scale at
which they are measured. Issues of hydrograph timing and the relative size of the storm system with
respect to the watershed area may confound relationships at larger spatial scales. Furthermore, a
number of fluvial geomorphic features that are commonly used as metrics of geomorphic condition are
scale-dependent. For example, width-depth ratio, tendency toward braiding, and channel depth relative
to stable bank height all commonly increase downstream. Other factors, such as the influence of
vegetation, depend on protrusion relative to width and rooting depth relative to bank height. The
There may be significant
temporal lags between the
point in time at which land use
is altered and when channel
impacts are observed.
Page-19
temporal scale over which channel changes occur will be influenced by precipitation variability, in
addition to the many physical factors already discussed.
These scale considerations, as well as previous discussion of factors influencing stream response, are
important when determining the choice of both management tools and monitoring approaches. It is
generally much easier to predict the direction of response than the magnitude. Accurate, detailed
predictions of response are difficult to make, and they are generally only possible when applied to
specific locations, using extensive data input, to answer very specific questions; even then they are
subject to uncertainty. Policies or assessment methods aimed to address a range of streams and
geographic conditions are better suited to probabilistic approaches that explicitly acknowledge
uncertainty, as described further in subsequent sections.
2.7 Impacts on Fluvial Riparian Vegetation
Stream channel form and stability is closely linked with the ecology of instream and floodplain habitats
(Figure 2-7). Spatial and temporal distributions of plant communities are tied to moisture availability
and seasonality. The ability of vegetation to stabilize soils,
trap sediments, and reduce flow velocities (Sandercock et al.
2007) can create positive feedback that promotes further
vegetation establishment and enhancement of these
stabilizing features. This can result in a strong influence on
channel geometric features, specifically channel narrowing
(Anderson et al. 2004). The change in frequency of overbank
flows resulting from channel incision will also affect riparian
processes, including nutrient transfer and seed dispersal. For example, it is believed that Tamarix
dominance over native species along Western US rivers would be less extensive if not for anthropogenic
alteration of streamflow regimes (most recently supported by Merritt and Poff (2010)).
Impacts to stream biota may occur
through the alteration of habitat
structure and habitat dynamics caused
by hydrologic and geomorphic changes,
as well as directly from hydrologic
alteration.
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Figure 2-7. Land use changes, hydrology, geomorphology and ecology are closely and complexly
interrelated. (Adapted from Palmer et al. 2004).
Vegetation changes not only are a result of morphological impacts but also can result directly from
changes in streamflow. These findings continue to be supported by recent studies; for example,
increases or decreases in baseflow or changes to the seasonal availability of water can determine the
extent and type of riparian vegetation capable of thriving in that environment (White and Greer 2006).
Vegetation changes can have cascading effects on indigenous fauna that require native plants for food
or nesting (Riley et al. 2005). Channel incision can also result in phreatic draining of adjacent wetland
and floodplain habitats and result in loss of key riparian species (Scott et al. 2000).
2.8 Impacts on In-Stream Biota
As shown in Figure 2-7, impacts to stream biota may occur through the alteration of habitat structure
and habitat dynamics caused by hydrologic and geomorphic changes, as well as directly from hydrologic
alteration. (The term biota is used here to refer to a range of non-plant species including algae,
macroinvertebrates, amphibians, fishes, etc.) Because of these relationships, the condition of in-stream
biota is considered to reflect the effects of all other impacts and has been recommended as an
integrative measure of stream health (discussed further in Section 5).
Studies continue to build on Poff et al. (1997), who highlighted the importance of the “natural flow
regime” and its variability as critical to ecosystem function and native biodiversity. Streamflow pattern
or “regime” interacts with the geomorphic context to control the physical and biological response of
streams to hydromodification. The basic characteristics of streamflow regimes are typically described in
five ways: magnitude, frequency, duration, timing, and rate of change. There is a large body of science
Page-21
linking one or more of these five elements of flow regimes to geomorphic processes, physical habitat,
and ecological structure and function. A few examples of linkages with physical habitat are provided in
Table 2-1; these linkages describe the mechanisms by which flow changes can impact stream ecology
through morphological alterations.
Table 2-1. Examples of Relationships between Flow Regime Attributes and Physical Habitat
Characteristics (adapted from Roesner and Bledsoe 2002).
Flow Attribute
Example Relationships with Physical Habitat
Magnitude
Determines extent to which erosion/removal thresholds for substrate, banks,
vegetation, and structural habitat features are exceeded
Determines whether floodplain inundation/exchange occurs
Habitat refugia may become ineffective during extreme events
Frequency
Flashiness can affect potential for recovery of quasi-equilibrium channel forms
between events, bank stability, and streambank/riparian vegetation assemblages
Frequency of substrate disturbance can act as a major determinant of fish
reproductive success and benthic macroinvertebrate abundance and composition
Duration
Determines the impact of a threshold exceeding event, e.g., scour depths
Urbanization frequently increases the duration of geomorphically effective flows
which also affect bank vegetation establishment and maintenance
Extended durations of high suspended sediment concentrations can act as chronic
and acute stressors on fish communities
Timing
The temporal sequence of flow events affects channel form and stability as
geomorphic systems may be “primed” for abrupt changes.
Stream biota may use flow timing as a life-cycle cue
Predictability of flow can affect utilization of habitat refugia
Rate of Change
Affects bank drainage regimes (bank stability) and sedimentation processes, e.g.,
re-suspended fine sediment concentrations during storm hydrographs,
embeddedness, armoring
Rapid drawdown can result in stranding of instream biota
Rise and fall rates control riparian water table dynamics and seedling recruitment
The mechanisms of such impacts are also well detailed by Center for Watershed Protection (2003); for
example, increased flows are related to a reduction in habitat diversity and simplification of habitat
features such as pools; this in turn reduces the availability of deep-water cover and feeding areas.
Many studies support the conclusion that stream biota are also directly impacted by altered flow
regimes, independent of channel instability and erosion. Konrad and Booth (2005) identified four
hydrologic changes resulting from urban development that are potentially significant to stream
ecosystems: increased frequency of high flows, redistribution of water from baseflow to stormflows,
Page-22
increased daily variation in streamflow, and reduction in low flow. They caution that ecological benefits
of improving physical habitat and water quality may be tempered by persistent effects of altered
streamflow and sediment discharge, and that hydrologic effects of urban development must be
addressed for restoration of urban streams. Walsh et al. (2007) concluded that low-impact watershed
drainage design was more important than riparian revegetation with respect to indicators of
macroinvertebrate health. Bioengineered bank stabilization can also have positive effects on habitat
and macroinvertebrates, but it cannot completely mitigate impacts of urbanization with respect to
stream biotic integrity (Sudduth and Meyer 2006). Walters and Post (2011) and Brooks et al. (2011)
found impacts to benthic macroinvertebrates due to upstream water abstractions, including reductions
in total biomass of insects and reductions in abundance respectively.
2.9 Conclusions
Alterations in streamflow and sediment transport as a result of land use change can have severe impacts
on streams. Common responses include changes in water balance, surface and near-surface runoff
timing and magnitude, groundwater recharge, sediment delivery and transport, channel enlargement,
widespread incision, and habitat degradation. The extent and consequences of these impacts depend
on stream type, watershed context, and local controls on channel adjustment; as such, stream
responses to hydromodification are complex and difficult to predict with any precision. Due to the
direct impacts of streamflow modification on vegetation and biota, channel morphology cannot be the
sole measure of hydromodification impacts. Thus, mitigation efforts that are narrowly focused on
channel stability may be insufficient for sustaining key ecological attributes. Likewise, reach-scale
stabilization of streams will not necessarily result in the return of comparable habitat quality and
complexity (Henshaw and Booth 2000, Roesner and Bledsoe 2003). Hydromodification management
should be considered in the context of an overall watershed-scale strategy that targets maintenance and
restoration of critical processes in critical locations in the watershed. Furthermore, it is imperative that
monitoring and adaptive management be focused on achieving desired objectives for aquatic life and
overall stream “health” in addition to simply measures of geomorphic response.
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3. FRAMEWORK FOR HYDROMODIFICATION MANAGEMENT
3.1 Introduction and Overview
The current approach to managing hydromodification impacts on a project-by-project basis is not
sufficient to protect beneficial uses of streams. This section outlines a comprehensive, alternative
framework that begins with watershed analysis and uses the results to guide the site-based
management decisions that are the current focus of most hydromodification management strategies. It
also recommends the implementation of a compensatory mitigation program in support of
hydromodification management objectives identified in the watershed analysis. Figure 3-1 summarizes
this approach and illustrates how current site-based management relates to the larger framework.
Figure 3-1. Framework for Integrated Hydromodification Management.
Monitoring
Watershed Management Actions
Stream Restoration
Floodplain Management
Flow and Sediment Management
New Development Site Controls and
Mitigation Requirements
On-site Actions
Off-site Actions
Other Entities or Programs
New Development Site Analysis
Watershed Hydromodification Management
Opportunities/Constraints
Management Objectives
Framework for Determining Site Control Requirements
Valuation Method for Mitigation
Watershed Analysis/Mapping
Watershed Characteristics and Processes
Current Land Use and Stream Conditions
Past Actions/Legacy Effects
Proposed Future Actions/Changes in Land Use
Page-24
This section discusses the details of the integrated framework proposed in Figure 3-1. Key features of
this comprehensive approach to hydromodification management are:
Hydromodification management needs to occur primarily at the watershed scale. The
foundation of any hydromodification management approach should be an analysis of existing
and proposed future land use and stream conditions that identifies the relative risks,
opportunities, and constraints of various portions of the watershed. Site-based control
measures should be determined in the context of this analysis.
Clear objectives should be established to guide management actions. These objectives should
articulate desired and reasonable physical and biological conditions for various reaches or
portions of the watershed. Management strategies should be customized based on
consideration of current and expected future channel and watershed conditions. A one-size-
fits-all approach should be avoided.
An effective management program will likely include combinations of on-site measures (e.g.,
low-impact development techniques), in-stream measures (e.g., stream habitat restoration),
and off-site measures. Off-site measures may include compensatory mitigation measures at
upstream locations that are designed to help restore and manage flow and sediment yield in the
watershed.
Management measures should be informed and adapted based on monitoring data. Similarly,
monitoring programs should be designed to answer questions and test hypotheses that are
implicit in the choice of management measures, such that measures that prove effective can be
emphasized in the future (and those that prove ineffective can be abandoned).
Hydromodification potentially affects all downstream receiving waters; therefore, there
generally should be no areas exempted from hydromodification management plans. However,
the variety of types and conditions of receiving waters should result in a range of requirements.
This also means that objectives, and the management strategies employed to reach them, will
need to acknowledge pre-existing impacts associated with historical land uses.
Implementation of this approach will likely require changes in the current administration of
hydromodification management plans statewide, both in the development and promulgation of
regulations by the State and Regional Water Boards and in the administration and execution of those
regulations by local jurisdictions (Table 3-1). In the short term, municipalities will need to broaden the
approaches to on-site management measures and expand monitoring and adaptive management
programs based on the tools described in this document. In the long term, regulatory agencies will need
to develop watershed-based programs that allow for implementation of management measures in the
locations and manner that will have the greatest impact on controlling hydromodification effects. A
A watershed-based approach to hydromodification management will allow integration of objectives with
related programs such as water quality management, groundwater management, and habitat management
and restoration through mechanisms such as Integrated Regional Water Resources Management Plans.
Page-25
watershed-based approach will also allow the integration of hydromodification management objectives
with related programs such as water quality management, groundwater management, and habitat
management and restoration through mechanisms such as Integrated Regional Water Resources
Management Plans.
Table 3-1. Recommendations for implementation of watershed-based hydromodification
management, organized by the scale of implementation and the time frame in which useful results
should be anticipated.
Time Frame
Programmatic: State and
Regional Water Boards
Local: City and
County Jurisdictions
Short-term
(<10 years)
Define the watershed context for local
monitoring (at coarse scale)
Evaluate whether permit requirements are
making positive improvements
Evaluate whether specific projects/
regulations are meeting objectives
Identify the highest priority action(s) to take
Long-term
(1+ decades)
Define watershed context and setting
benchmarks for local-scale monitoring (i.e.,
greater precision, if/as needed)
Demonstrate how permit requirements can
improve receiving-water “health,” state-wide
(and change those requirements, as needed)
Evaluate and demonstrate whether actions
(on-site, instream, and watershed scale)
are improving receiving-water conditions
Assess program cost-effectiveness
Identify any critical areas for resource
protection
3.2 Background on Existing Strategies and Why They are Insufficient
Current hydromodification approaches and strategies, such as flow and sediment-control basins, have
been long-recognized as insufficient to fully address hydromodification impacts (e.g., Booth and Jackson
1997, Maxted and Horner 1999). Present understanding of the causes and effects of urbanization
suggest that such approaches must be expanded to include integrated flow and sediment management
at the watershed scale, along with stream corridor/floodplain restoration (NRC 2009).
Flow management has its origins in flood-control basins intended to reduce peak discharge through
stormwater detention (Dunne and Leopold 1978). A key shortcoming of these approaches for
hydromodification management is that they do not address (and may exacerbate) cumulative erosive
forces on the receiving channel because they trap sediment and release sediment-starved water to
downstream areas. Simple detention can increase the frequency and duration with which channels are
exposed to erosive effects (McCuen and Moglen 1988, Bledsoe et al. 2007), resulting in an increase in
the downstream impacts of hydromodification.
Since the late 1980’s in parts of the US, hydromodification management plans began to explore “flow-
duration” control standards as a way to address this shortcoming. These standards require that the
post-project discharge rates and durations may not deviate above the pre-project discharge rates and
Page-26
durations by more than a specific (and typically quite small) percent, across a broad range of discharges
at and above the presumed threshold of instream erosion and sediment transport, as averaged over a
multi-year period of measured (or simulated) record. This approach is a dramatic improvement over
earlier methods, although it does not adequately address the issues of sediment deficit associated with
urbanization (Chin 2006). In addition, current flow-duration standards do not fully account for the
effects of flow alteration on in-stream habitat and biological functions (e.g., they do not address the
seasonality of peak flows, rates of hydrograph rise and recession, low-flow magnitude and duration) and
therefore may not be protective of all beneficial uses of downstream waterbodies.
Current strategies are also insufficient with respect to how municipal stormwater permits apply
hydromodification standards. Currently, development triggers are established to determine if a project
is subject to the standards. These triggers are generally specified by either project land use type in
conjunction with size, or by project size alone (e.g., 20 units or
more of single family residential housing, or 10,000 square feet
or more of new impervious area). The exemption of many small
projects from hydromodification controls can result in
cumulative impacts to downstream waterbodies (see Booth and
Jackson, 1997, for an example from western Washington of the
cumulative effects of a small-project exemption); a move to
include LID requirements that apply to all projects, regardless of
size, is a positive development to begin to address this issue.
There is usually also an exemption for projects discharging to
hardened channels or waterbodies; however these exemptions
may not be supportive of future stream restoration possibilities,
and do not address the impacts of hydromodification on lentic and coastal waterbodies (as yet not fully
understood). A further limitation of the current permit structure is that there is no consideration of
project characteristics such as position within the watershed, sensitivity of the receiving stream reach,
or level of coarse sediment production on the proposed project site. Finally, current programs rely
solely on regulating new development and re-development to prevent hydromodification impacts
without addressing pre-existing conditions which may limit the effectiveness of future management
actions.
When flow-control measures of whatever regulatory standard have failed to protect streams from
erosion, hydromodification “management” typically consists of bank or channel armoring, drop
structures, and other hard engineering approaches. Although these methods may reduce local
hydromodification impacts, it is typically at the expense of other in-stream or riparian functions or
beneficial uses. For example, channel armoring can reduce habitat and water conservation functions
and services by direct habitat removal, increased bed scour, and decreased connectivity between the
channel and its floodplain. In addition to loss of biological and physical stream function, many armoring
solutions degrade or fail over time because they address only the localized channel instability rather
than the overarching processes that led to the instability (Kondolf and Piegay 2004). For example, drop
structures constructed to stabilize a specific channel reach will tend to shift downstream the
Shortcoming of current
hydromodification standards that may
limit their effectiveness include the
exemption of many small projects,
which can result in cumulative
impacts to downstream waterbodies,
and the reliance solely on regulating
new development and re-
development without addressing pre-
existing conditions which may limit
the effectiveness of future
management actions.
Page-27
consequences of an insufficient sediment loadthe reach immediately upstream of the drop structure
is “protected,” but that immediately downstream is degraded even more severely. In extreme cases,
the structure itself can be undermined by downstream erosion and headcutting that is exacerbated by
the sudden shift in velocity and associated eddy effects (i.e., hydraulic jump) that often occurs
downstream of grade stabilization (Chin 2006). Bank armoring can also fail due to being undermined by
erosion at the toe of slope, which can lead to scour (Figure 3-2). In both cases, structural failures often
lead to a sequence of incremental increases in the size and extent of the structural solution in an
attempt to continually repair increasing channel degradation. In extreme cases, catastrophic failure of
bank or grade stabilization can lead to sudden and dramatic changes in channel form, which can be
associated with devastating loss of habitat, infrastructure, and property.
Figure 3-2. Undermining of grade control and erosion of banks downstream of structures
intended to stabilize a particular stream reach. Left photo is looking upstream at drop structure;
right photo is looking downstream from the drop structure.
3.3 Development of Comprehensive Hydromodification Management Approaches
The goal of hydromodification management should be to protect and restore overall receiving water
conditions, by maintaining or reestablishing the watershed processes that support those conditions, in
the face of urbanization. Achieving these goals will require that hydromodification management
strategies operate across programs beyond those typically regulated by NPDES/MS4 requirements.
Successful strategies will need to be developed, coordinated, and implemented through land-use
planning, non-point source runoff control, and Section 401 Water Quality Certifications and Waste
Discharge Requirement programs in addition to traditional stormwater management programs. Thus,
all levels of the regulatory frameworkfederal, state, and localwill need to participate in developing
such a program, with program development occurring mainly through regulatory and resource
protection agencies and program implementation occurring mainly through local jurisdictions.
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As shown in Figure 3-1, watershed-scale hydromodification management should include all of the
following key elements:
Watershed-wide assessment of the condition of key
watershed processes, to understand the natural functioning
of the watershed and what has been (or is at risk of being)
altered by urbanization.
Watershed-wide assessment of hydromodification risk, to
categorize areas based on the likelihood of
hydromodification impacts and to identify opportunities for
restoration or protection of key reaches or sub-basins.
Appropriate management objectives for various stream reaches and/or portions of the
watershed.
Process for selecting management actions and mitigation measures for project sites and stream
reaches.
Monitoring program that is consistent with the goals of the HMP so that information generated
can be used to improve the HMP over time.
3.4 Watershed Mapping and Analysis Identification of Opportunities and Constraints
Watershed analysis should be the foundation of all
hydromodification management plans. Analysis should
identify the nature and distribution of key watershed
processes, existing opportunities and constraints in order to
help prioritize areas of greater vs. lesser concern, areas.
“Watershed analysis” has several steps, of which the first is
mapping. Mapping may occur at the watershed or regional
(i.e., multiple watersheds) scale. Mapping should include
data layers to facilitate the following analyses. Most of these
data layers are freely available as online. Further information
on analysis tools is provided in the next section. These maps
should be designed for iterative updates over time as new
information becomes available:
Dominant watershed processes analysis of topography (10-m digital elevation model),
hydrology, climate patterns, soil type (NRCS soil classifications) and surficial geology can be used
to identify the location and type of dominant watershed processes, such as sediment source
areas and areas where infiltration is important or where overland flow likely dominates. This
can provide a template for the eventual design of management measures that correspond most
The goal of hydromodification
management should be to
protect and restore overall
receiving water conditions, by
maintaining or reestablishing the
watershed processes that
support those conditions, in the
face of urbanization.
Page-29
closely to the pre-development conditions, which support processes that promote long-term
channel health. The Central Coast Hydromodification Control Program (the “Joint Effort”; see
Booth et al. 2011) provides an example of this type of analysis.
Existing stream conditions At a minimum the National Hydrography Database (NHD) can
provide maps of streams and lakes in the watershed. Additional information on stream
condition should be included to the extent that it is available. This could include major bed
material composition, channel planform, grade control locations and condition, and
approximate channel evolution stage. These maps can also be used to conduct general stream
power evaluations.
Current (Past) and anticipated future land use - Current land use and land cover plus proposed
changes due to general or specific plans. Historical information on past land use practices or
stream conditions should be included if it is readily available. Classified land cover (NLCD 2006)
is available from the Multi-Resolution Land Characteristics Consortium (MRLC).
Potential coarse and fine sediment yield areas methods such as the Geomorphic Land Use
(GLU) approach (Booth et al. 2010) can be used that to estimate potential sediment yield areas
based on geology, slope and land cover.
Existing flood control infrastructure and channel structures maps should include major
channels, constrictions, grade control, etc. that affect water and sediment movement through
the watershed. Any available information on water quality, flood control or hydromodification
management basins should also be included.
Habitat both upland and in-stream and riparian habitat should be mapped to help determine
areas of focus for both resource protection and restoration. This may be based on readily
available maps such as the National Wetlands Inventory and National Land Cover Database,
aerial photo interpretation, or detailed local mapping.
Areas of Particular Management Concern these may include sensitive biological resources,
critical infrastructure, 303(d) listed waterbodies, priority restoration areas or other locations or
portions of the watershed that have particular management
needs.
Economic and social opportunities and constraints
comprehensive watershed management includes consideration
of opportunities for improving community amenities associated
with streams, economic redevelopment zones, etc. Details on
this are beyond the scope of this paper, but emphasize the
need to include planning agencies in the development of
hydromodification management plans.
Substantial resources will be
necessary to implement a
watershed analysis approach;
therefore, opportunities for
joint funding and leveraging of
resources should be
vigorously pursued.
Page-30
Watershed analysis will be challenging especially for smaller municipalities with limited resources or
where their jurisdiction only encompasses a portion of the watershed. Substantial resources will be
necessary to implement this approach; therefore, opportunities for joint funding and leveraging of
resource should be vigorously pursued. A cooperative approach should replace the current fragmented
efforts among regions and jurisdictions. Furthermore, the State and Regional Water Boards should
support completion of these maps and common technical tools as the foundation for future
hydromodification management actions.
3.5 Defining Management Objectives
Results of the watershed analysis should be used to
determine the most appropriate management actions for
specific portions of the watershed. Management strategies
should be tailored to meet the objectives, desired future
conditions, and constraints of the specific channel reach being
addressed.
Decisions should be based on considerations of areas suitable
for specific ecosystem services, opportunities, and constraints
as described above. Management objectives may be aimed at
reducing effects of proposed future land use or mitigating for
the effects of past land use, and they may apply to stream
reaches or upland areas. Potential management objectives
for specific stream reaches may include: protect, restore, or
manage as a new channel form.
The specific manifestation of each of these strategies will
differ by location, based on constraints of the stream,
watershed plan objectives, etc. Decisions about appropriate
objectives will need to consider current and future
opportunities and constraints in upland, floodplain, and in-
stream portions of the watershed. General definitions are
provided below as a starting point for case-specific
refinement.
3.5.1 Protect
This approach consists of protecting the functions and services of relatively unimpacted streams in their
current form through conservation and anti-degradation programs. This strategy should not be used if
streams are degraded, or nearing thresholds of planform adjustment or changes in vegetation
community. This strategy may apply following natural disturbances such as floods depending on the
condition of the stream reach and the ability for natural rehabilitation to occur (due to how intact
Management strategies should be
tailored to meet the objectives, desired
future conditions, and constraints of the
specific channel reach being addressed.
Objectives for specific stream reaches
may include:
Protect
Restore
Manage as a new channel form
Page-31
watershed processes are). The goal of this strategy is not to create an artificial preserve (such as a
created stream running through an urban park) but rather a naturally function river system. Fully
channelized systems are not considered in this framework. Examples of specific actions include:
Preserving intact channel systems through easements, restrictions, covenants, etc. This should
be considered in the watershed context to ensure adequate connectivity with upstream and
downstream reaches of similar condition, and to ensure that the watershed processes
responsible for creating and maintaining instream conditions will persist.
Providing appropriate space for channel processes to occur (e.g., floodplain connectivity).
Establishing transitional riparian and upland buffer zones that are protected from encroachment
by infrastructure or development.
3.5.2 Restore
There are many definitions of “restoration”. For the purposes of this document, restoration is
considered re-establishing the natural processes and characteristics of a stream. The process involves
converting an unstable, altered, or degraded stream corridor, including adjacent riparian zone (buffers),
uplands, and flood-prone areas, to a natural condition. In most cases, restoration plans should be based
on a consideration of watershed processes and their ability to support a desired stream type. The
watershed analysis discussed above should be used to determine how and where watershed process
should be protected or restored in order to best support stream and stream-corridor restoration. This
process should be based on a reference condition/reach for the valley type and includes restoring the
appropriate geomorphic dimension (cross-section), pattern (sinuosity), and profile (channel slopes), as
well as reestablishing the biological and chemical integrity, including physical processes such as
transport of the water and sediment produced by the stream’s watershed in order to achieve dynamic
equilibrium. Design of restoration structural elements must be based on existing and anticipated
upstream land uses, and reflect the modified hydrology resulting from these uses. Restoration should
apply to streams that are already on a degradation trajectory where there is a reasonable expectation
that a more stable equilibrium condition that reflects previously existing conditions can be recreated
and maintained via some intervention. Creating a stream system that differs from “natural conditions”
is not considered restoration. All elements of the protection” strategy should also be included once the
restoration actions are complete. Examples of specific actions include:
Floodplain and in-stream measures that restore natural channel form consistent with current
and/or anticipated hydrology and sediment yield. Examples include recontouring, biotechnical
slope stabilization, soft-grade control features (e.g., woody debris).
Revegetation of stream banks and beds, including removal of invasive species.
Preserving intact channel systems through easements, restrictions, covenants, etc. This should
be considered in the watershed context to ensure adequate connectivity with upstream and
downstream reaches of similar pristine condition.
Page-32
Providing appropriate space for channel processes to occur (e.g. channel migration at allowable
levels, floodplain connectivity, and development of self-sustaining riparian vegetation).
Establishing transitional riparian and upland buffer zones that are protected from encroachment
by infrastructure or development.
3.5.3 Manage as New Channel Form
Once a stream channel devolves far enough down the channel evolution sequence, it is extremely
difficult to recover and restore without substantial investment of resources. If critical thresholds in key
structural elements, such as planform or bank height, are surpassed, streams should be allowed to
continue progressing toward a new stable equilibrium condition that is consistent with the current
setting and watershed forcing functions, if such progress does not pose a danger to property and
infrastructure. Substantial alteration of flow or sediment discharge, slope or floodplain width may make
it improbable that a stream can be restored to its previous condition. In such circumstances, it may be
preferable to determine appropriate channel form given expected future conditions and “recreate” a
new channel to match the appropriate equilibrium state under future conditions. For example, a multi-
thread braided system may not be the appropriate planform based on new runoff and sediment
pattern; instead, a single-thread channel or step-pool structure may be a more appropriate target.
Examples of specific actions include:
In-channel recontouring or reconstruction of channel form.
Floodplain recontouring or reconstruction that improves connectivity with the channel.
In extreme circumstances based on channel condition, position in the watershed, etc. this may
involve hardening portions of the channel and focusing “mitigation” measures at off-site
measures at a different part of the watershed. Off-site mitigation can be informed by
“hydromodification risk mapping”.
Re-establishing longitudinal connectivity for sediment transport and ecological linkages.
Preserving intact channel systems through easements, restrictions, covenants, etc. This should
be considered in the watershed context to ensure adequate connectivity with upstream and
downstream reaches of similar pristine condition.
Providing appropriate space for channel processes to occur (e.g. floodplain connectivity).
Establishing transitional riparian and upland buffer zones that are protected from encroachment
by infrastructure or development.
Several authors have previously noted that in urban systems, natural channel state often can no longer
be sustained under changed hydrological conditions. Thus, different management goals are probably
appropriate for watersheds at varying stages of development (Booth, 2005) and at varying degrees of
adjustment (Chin and Gregory 2005). In this context, identifying which channels are suitable for
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protection, restoration, or alternative channel form can be used to guide restoration and management
efforts (Booth et al. 2004).
Upland objectives should be established to support management objectives for stream reaches. These
objectives will have direct implications and will influence site-specific control requirements (discussed
below). Potential management objectives for upland areas may include:
Conserve open space for infiltration: Infiltration reduces the magnitude and duration of runoff
to the stream channel and allows flow to re-enter the stream through diffuse overland flow,
shallow subsurface flow, or groundwater recharge. This in turn reduces the work (energy) on
the channel bed and banks and helps promote stability.
Conserve open space for stream buffers: Buffers allow many of the same infiltration processes
discussed above to occur. In addition, they provide space for channel migration and overbank
flow, both of which function to reduce energy and allow the channel to better withstand
potentially erosive forces associated with high flow events.
Conserve open space for coarse sediment production: Course sediment functions to naturally
armor the stream bed and reduce the erosive forces associated with high flows. Absence of
coarse sediment often results in erosion of in-channel substrate during high flows. In addition,
coarse sediment contributes to formation of in-channel habitats necessary to support native
flora and fauna.
Encourage development on poorly-infiltrating soils: The difference between pre and post
development runoff patterns is less when development occurs on soils that have low infiltration
rates and functioned somewhat like paved surfaces. Focusing development on these areas
reduces changes in hydrology associated with transition to developed land uses.
Encourage urban infill: Urban infill reduces the effect on watershed processes by concentrating
development on previously impacted areas. This reduces disruption of hydrology and sediment
process compared to developing on open space or other natural areas.
3.6 Selecting Appropriate Management Objectives
The combination of expected force acting on the stream channel (in terms of higher flow and less
sediment) and estimated resistance (in the form of channel and floodplain condition) can be used to
inform selection of an appropriate management objective for a specific stream reach, as shown in Figure
3-3. This figure represents a conceptual approach to selecting
appropriate management objectives, in which modifications to
runoff and sediment are compared against stream reach
conditions. By weighing these factors within the context of
watershed opportunities, constraints and resources,
management objectives and specific actions can be
determined. More complete decision support systems or
guidance will need to be developed for individual
Selection of appropriate management
objectives should consider changes to
runoff and sediment, and existing
stream reach conditions, within the
context of watershed opportunities,
constraints and resources.
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hydromodification management plans that account for other considerations such as upstream and
downstream conditions, cost, infrastructure constraints, availability of floodplain area for restoration,
presence of downstream sensitive resources, etc. All decisions should be made in the context of the
watershed position of a project site relative to existing opportunities and constraints as discussed above.
A number of tools are available to be used in conjunction with watershed mapping to inform this
prioritization process. For example, GLU mapping (Booth et al. 2010) and hydromodification risk
mapping can be used to assign high, medium or low ratings to watershed resistance (i.e., susceptibility
to change). Similarly, field based tools such as the hydromodification screening tool (Bledsoe et al.
2010) or European tools such as Fluvial Audit or River Habitat Survey can be used to assign a rating of
high, medium or low at the reach scale. In addition to geomorphic assessments, habitat assessments
such as the California Rapid Assessment Method (CRAM; Collins et al. 2008) or biological evaluations via
an index of biotic integrity (IBI; e.g., Ode et al. 2005) should be used as measures of biological condition
to provide a more complete stream assessment. The next section provides an overview of
hydromodification assessment and prediction tools, as well as further details on specific tools to support
the selection of management objectives.
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Figure 3-3: Example of a hydromodification management decision-making process.
3.7 Framework for Determining Site-Specific Control Requirements
Once the watershed analysis is complete and opportunities,
constraints and management objectives have been identified
for both upland areas and stream reaches, a framework
should be developed for site-specific project analyses and
control requirements. The level of detail required for the
analysis of proposed projects should be based on a
combination of factors including project size, location within
the watershed, and point of discharge to receiving waterbody.
The HMP should specify how these factors will be evaluated
within the context of the identified management objectives to
determine analysis requirements. The HMP should also
ideally contain scalable BMP designs (based on conservative assumptions and consistent with prevailing
watershed conditions) that can be applied by small projects where appropriate to avoid overly
burdensome requirements for site-specific analysis. The framework should include the following
components:
A set of standard on-site management measures/BMPs that should apply to all projects; no
projects should be exempted from these measures as they will have broader water quality
benefits beyond helping to control the effects of hydromodification. These management
actions consist of reducing the effects of urbanization on catchment runoff and sediment yield.
On-site management measures should attempt to reduce excess runoff, maintain coarse
sediment yield (if possible) and provide for appropriate discharge to receiving streams to
support in-stream biological resources. In some cases, common features or facilities may be
able to accommodate these objectives. In other cases, separate features or facilities will be
necessary to deal with distinct objectives. On-site measures should generally be applied in all
cases as allowed by site-specific geotechnical constraints, with specific management practices
informed by the watershed processes most important at particular locations in the watershed,
as well as by the nature of downstream receiving waters:
o Low impact development (LID) practices.
o Disconnecting impervious cover through infiltration, interception, and diversion.
o Coarse sediment bypass through avoidance of sediment yield areas or measures that
allow coarse sediment to be discharged to the receiving stream.
o Flow-duration control basins to reduce runoff below a threshold value.
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Specification of the level of analysis detail and design requirements for the project, depending
on project location, discharge point, and project size. Levels of analysis and design
requirements may include:
o Application of scalable, standardized designs for flow control based on site-specific soil
type and drainage design. The assumptions used to develop these scalable designs
should be conservative, to account for loss of sediment and uncertainties in the analysis
and our understanding of stream impacts.
o Use of an erosion potential metric, based on long-term flow duration analysis and in-
stream hydraulic calculations. Guidelines should specify stream reaches where in-
stream controls would and would not be allowed to augment on-site flow control.
o Implementation of more detailed hydraulic modeling for projects of significant size or
that discharge to reaches of special concern to understand the interaction of sediment
supply and flow changes.
o Analysis of the water-balance for projects discharging into streams with sensitive
habitat. This may include establishment of requirements for matching metrics such as
number of days with flow based on the needs of species present.
Guidelines for prioritization of on-site or regional flow and sediment control facilities.
Watershed analysis will help identify opportunities for regional flow or sediment control
facilities, which may help to mitigate for existing hydromodification impacts.
Appendix A provides detailed guidance on the appropriate application of tools to meet site control
requirements.
3.8 Off-site Compensatory Mitigation Measures
In some cases, on-site control of water and sediment will not
be sufficient to offset the effects of hydromodification on
receiving waters. In these cases, off-site compensatory
mitigation measures will be necessary (similar to the concepts
used in the Section 401/404 permitting programs). Off-site
measures could be implemented by project proponents or
through the use of regional mitigation banks or in-lieu fee
programs.
Off-site mitigation may be necessary for several reasons:
Off-site measures may be more effective at
addressing effects or at achieving desired management goals.
This may be particularly true for sites near the bottom of a
watershed where upstream measures may be preferred
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Off-site measures may be necessary to supply compensation for residual project impacts where
on-site measures are limited by site constraints or solutions are beyond the scope of what can
be accomplished on an individual site.
Off-site measures may be necessary where accomplishing specified management objectives is
not practical using on-site measures alone. Off-site measures may be desired to remedy legacy
effects of prior land use or to achieve desired beneficial uses.
Performance monitoring and adaptive management must be a part of compensatory mitigation given its
inherent uncertainty.
The location and type of mitigation should be determined in the
context of the watershed analysis and should account for the
size and nature of the impact, location in the watershed, pre-
existing conditions in the watershed, and uncertainty associated
with the success of the proposed mitigation actions. In some
cases these measures may be near the project site (e.g.,
restoring a stream reach downstream of the project site), but in
other cases the off-site mitigation may be in the form of in-lieu
fee or “mitigation bank” type contributions to a project located
in a different portion of the watershed (e.g. upstream grade
control, protection of sediment source areas). Such off-site
mitigation relatively far from the site will only be possible if
conducted in the context of an overall watershed plan, as
discussed above. Off-site measures may include:
Stream corridor restoration
Purchase, restoration and protection of floodplain/floodway habitat
Purchase and/or protection of critical sediment source or transport areas
Regional basins or other retention facilities
Upstream or downstream natural/bio-engineered grade control
Retrofit or repair of currently undersized structures (e.g. culverts, bridge crossings)
Removal or hydrologically disconnecting impervious surfaces
A valuation method will be necessary for assigning appropriate mitigation requirements in light of the
anticipated impacts of hydromodification on receiving streams. The valuation method should be
developed by the State Water Board.
To support the management approaches discussed above, HMPs should provide general guidance for
application of models and other tools based on the questions being asked and the desired outcomes of
In cases where on-site control of
water and sediment will not be
sufficient to offset the effects of
hydromodification on receiving
waters, off-site compensatory
mitigation measures will be necessary.
Implementation of this approach will
require that the State Water Board
develop a valuation method to help
determine appropriate off-site
mitigation requirements in light of the
anticipated impacts of
hydromodification on receiving
streams.
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the HMP. Models can also be used to help communicate levels of uncertainty in particular management
actions and to guide restoration / in-channel management actions. Modeling and other tools are
discussed in detail in Section 4 and Appendices A and B.
Finally, management endpoints should articulate the desired
physical and biological conditions for various reaches or
portions of the watershed. To the extent possible, these
desired conditions should be expressed in numeric, quantifiable
terms to avoid ambiguity. Additionally, since regulatory
strategies will invariably rely on quantifiable measures to
determine whether stormwater management actions achieve
these desired conditions, identifying appropriate numeric
objectives will support determinations of regulatory
compliance. As desired physical and biological watershed conditions are expressed in quantifiable terms
to the extent possible, a similar need would apply to site control requirements. Control measures
should be linked to, a) a desired condition (or goal), b) the parameter(s) that best define that condition,
and c) quantifiable measures that serve to evaluate performance of the control measure. Direct
measures (e.g., volume of runoff to be retained) as well as indirect or surrogate measures (IBI scores)
are appropriate if they are quantifiable.
Management endpoints should
articulate the desired physical and
biological conditions for various
reaches or portions of the watershed.
To the extent possible, these desired
conditions should be expressed in
numeric, quantifiable terms to avoid
ambiguity.
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4. OVERVIEW OF ASSESSMENT AND PREDICTION TOOLS
4.1 Introduction
The previous section discussed a number of potential actions for managing hydromodification impacts.
These ranged from high-level watershed-scale characterization to the site-specific design of a proposed
development. This section provides an overview of the current and emerging assessment and
prediction tools available to inform these management actions. An organizing framework helps explain
the appropriate application of these tools, as well as their strengths and weaknesses. Specific tools that
support the selection of management objectives are also discussed. Examples of “suites” of tools that
are commonly used together to predict stream responses and formulate management prescriptions for
channels of varying susceptibility are presented in Appendix B. Appendix A provides detailed guidance
on the appropriate application of tools to meet site control requirements.
Municipalities are the primary audience for this section, as they select and incorporate these tools into
their HMPs. However, the State and Regional Water Boards should be aware of the overall capabilities,
appropriate uses, and gaps in our current toolbox. The development of new and improved tools should
ideally be coordinated at the State level for optimum cost effectiveness and widest applicability. The
table below identifies the key actions necessary at both the programmatic and local level to
address the considerations discussed above, within the context of the goals of the framework
described in Section 3.
Table 4-1. Recommendations for the application and improvement of tools in support of the
proposed management framework.
Time Frame
Programmatic: State and
Regional Water Boards
Local: City and
County Jurisdictions
Short-term
(<10 years)
Develop quality control and standardization
for continuous simulation modeling
Perform additional testing and demonstration
of probabilistic modeling for geomorphic
response
Pursue development of biologically- and
physically-based compliance endpoints
Work cooperatively with adjacent
jurisdictions to implement hydromodification
risk mapping at the watershed scale
Implement continuous simulation modeling
for project impact analysis
Long-term
(1+ decades)
Improve tools for sediment analysis and
develop tools for sediment mitigation design
Develop tools for biological response
prediction
Improve tools for geomorphic response
prediction
Expand use of probabilistic and statistical
modeling for geomorphic response
Apply biological tools for predicting and
evaluating waterbody condition
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4.2 Background
In the context of hydromodification, tools and models are typically used to help answer one or more of
the following questions involving an assessment of natural and human influences at various spatial and
temporal scales:
How does the stream work in its watershed context?
Where is the stream going? For example, have past human actions induced channel changes?
What are the effects on sediment transport and channel form? What is the magnitude of
current and potential channel incision following land use conversion?
How will the stream likely respond to alterations in runoff and sediment supply?
How can we manage hydromodification and simultaneously improve the state of the stream?
Previous sections have underscored the variability and complexity of relationships among land use, the
hydrologic cycle, and the physical and ecological conditions of stream systems. It follows that the
process of assessing stream condition and predicting future conditions is highly challenging and subject
to uncertainty. Therefore it is important to understand the inherent strengths and limitations of the
available tools, especially with respect to prediction uncertainty and how it is expressed for various
tools. Considerable judgment is needed to choose the appropriate model for the question at hand. In
addition to prediction uncertainty, considerations in choosing the right model for a particular application
include appropriate spatial and temporal detail, cost of calibration and testing, meaningful outputs, and
simplicity in application and understanding (NRC 2001; Reckhow 1999a,b).
Figure 4-1. Organizing Framework for understanding hydromodification assessment and
management tools.
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4.3 Organizing Framework
Figure 4-1 presents an organizing framework by which to understand the available tools that may be
applied in support of hydromodification management and policy development. Tools fall into three
major categories: descriptive tools, mechanistic and empirical/statistical models that are used
deterministically, and probabilistic models/predictive
assessments with explicitly quantified uncertainty. The
organizing framework relates these categories to the types of
question the tools are designed to answer, specifically:
characterization of stream condition, prediction of response,
establishment of criteria/requirements, or evaluation of
management actions. The framework also characterizes the
tools according to the following features: intensity of resource
requirements (i.e., data, time, cost), and the extent to which uncertainty is explicitly addressed.
Subsequent sections of this section discuss each of the three major categories in turn, highlighting
examples of specific tools within each category.
Tools required to support the management framework presented in Section 3 include watershed
characterization and analysis tools and project analysis and design tools. The level of resolution that is
required will depend on the point in the planning process. At early stages, descriptive tools will be
sufficient, but more precise tools will be required toward the design phase. Currently, most projects
rely solely on deterministic models. However, given the uncertainty associated with predicting
hydromodification impacts, probabilistic models should be incorporated into analysis and design,
particularly where resource values or potential consequences of impacts are high.
4.3.1 Descriptive Tools
Descriptive tools include conceptual models, screening tools, and characterization tools. These tools are
used to answer the question: What is the existing condition of a stream or watershed? Although
descriptive tools are not explicitly predictive, they can be used to assess levels of susceptibility to future
stressors by correlation with relationships seen elsewhere. The application of some type of descriptive
tool, such as a characterization tool, is almost always necessary before applying a deterministic model.
In particular, descriptive tools can aid in understanding the key processes and boundary conditions that
may need to be represented in more detailed models.
Conceptual Models. A conceptual model, in the context of river systems, is a written description or a
simplified visual representation of the system being examined, such as the relationship between
physical or ecological entities, or processes, and the stressors to which they may be exposed.
Conceptual models have been used to describe processes in a wide range of physical and ecological
fields of study, including stream-channel geomorphology (Bledsoe et al. 2008). For example, Channel
Evolution Models (CEMs) are conceptual models which describe a series of morphological configurations
of a channel, either as a longitudinal progression from the upper to the lower watershed, or as a series
at a fixed location over time subsequent to a disturbance. The incised channel CEM developed by
Given the uncertainty associated
with predicting hydromodification
impacts, probabilistic models should
be incorporated into analysis and
design, particularly where resource
values or potential consequences of
impacts are high.
Page-42
Schumm et al. (1984) is one of the most widely known conceptual models within fluvial geomorphology.
This CEM documents a sequence of five stages of adjustment and ultimate return to quasi-equilibrium
that has been observed and validated in many regions and stream types (ASCE 1998, Simon and Rinaldi
2000). The Schumm et al. (1984) CEM has been modified for streams characteristic of southern
California, including transitions from single-thread to multi-thread and braided evolutionary endpoints
(Hawley et al., in press).
Conceptual models also include planform classifications of braided, meandering and straight, and other
general geomorphic classifications, which categorize streams by metrics such as slope, sinuosity, width-
to-depth ratio, and bed material size. The qualitative response model described by Lane’s diagram
(1955), and discussed earlier in this report, is also a conceptual model.
Characterization Tools. Examples of characterization tools include baseline geomorphic assessments,
river habitat surveys, and fluvial audits. A fluvial audit uses contemporary field survey, historical map
and documentary information and scientific literature resources to gain a comprehensive understanding
of the river system and its watershed. Fluvial audits, along with watershed baseline surveys are a
standardized basis for monitoring change in fluvial systems. These types of comprehensive assessments
are comprised of numerous, more detailed field methodologies, such as morphologic surveys, discharge
measurements, and estimates of boundary material critical shear strength through measurements of
resistance (for cohesive sediments) or size. Baseline assessments may also draw on empirical
relationships such as sediment supply estimation models.
Screening Tools. Screening tools can be used to predict the relative severity of morphologic and
physical-habitat changes that may occur due to hydromodification, as a critical first step toward tailoring
appropriate management strategies and mitigation measures to
different geomorphic settings. However, assessing site-specific
stream susceptibility to hydromodification is challenging for
several reasons, including the existence of geomorphic
thresholds and non-linear responses, spatial and temporal
variability in channel boundary materials, time lags, historical
legacies, and the large number of interrelated variables that can
simultaneously respond to hydromodification (Schumm 1991,
Trimble 1995, Richards and Lane 1997).
Despite the foregoing difficulties, the need for practical tools in stream management have prompted
many efforts to develop qualitative or semi-quantitative methods for understanding the potential
response trajectories of channels based on their current state. For example, predictors of channel
planform can be used to identify pattern thresholds and the potential for planform shifts (e.g., van den
Berg 1995, Bledsoe and Watson 2001, Kleinhans and van den Berg 2010).
In addition, regional CEMs (discussed above) can partially address the needs of the hydromodification
management community by providing a valuable framework for interpreting past and present response
trajectories, identifying the relative severity of potential response sequences, applying appropriate
Screening tools can be used to
predict the relative severity of
morphologic and physical-habitat
changes due to hydromodification,
as a critical first step toward tailoring
appropriate management strategies
and mitigation measures to different
geomorphic settings.
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models in estimating future channel changes, and developing strategies for mitigating the impacts of
processes likely to dominate channel response in the future (Simon 1995).
More recent screening-level tools for assessing channel instability and response potential, especially in
the context of managing bridge crossings and other infrastructure, have borrowed elements of the CEM
approach and combined various descriptors of channel boundary conditions and resisting vs. erosive
forces. For example, Simon and Downs (1995) and Johnson et al. (1999) developed rapid assessment
techniques for alluvial channels based on diverse combinations of metrics describing bed material, CEM
stage, existing bank erosion, vegetative resistance, and other controls on channel response. Although
based on a strong conceptual foundation of the underlying mechanisms controlling channel form, these
specific examples are either overly qualitative with respect to the key processes, or developed with
goals and intended applications (e.g., evaluating potential impacts to existing infrastructure such as
bridges or culverts) that differ from what is needed by current hydromodification management
programs.
SCCWRP has recently proposed a general framework for developing screening-level tools that help
assess channel susceptibility to hydromodification, and a new region-specific tool for rapid, field-based
assessments in urbanizing watersheds of southern California (Booth et al. 2010, Bledsoe et al. 2010).
The criteria used to assign susceptibility ratings are designed to be repeatable, transparent, and
transferable to a wide variety of geomorphic contexts and stream types. The assessment tool is
structured as a decision tree with a transparent, process-based flow of logic that yields four categorical
susceptibility ratings through a combination of relatively simple but quantitative input parameters
derived from both field and GIS data. The screening rating informs the level of data collection,
modeling, and ultimate mitigation efforts that can be expected for a particular stream-segment type and
geomorphic setting. The screening tool incorporates various measures of stream bed and bank
erodibility, probabilistic thresholds of channel instability and bank failure based on regional field data,
integration of rapid field assessments with desktop analyses, and separate ratings for channel
susceptibility in vertical and lateral dimensions.
An example of a specific analysis component that predicts changes in post-development sediment
delivery, and that can be applied within this screening tool framework, is a GIS-based catchment
analyses of “Geomorphic Landscape Units (GLUs). A GLU analysis integrates readily available data on
geology, hillslope, and land cover to generate categories of relative sediment production under a
watershed’s current configuration of land use. Those areas subject to future development are
identified, and corresponding sediment-production levels are determined by substituting developed
land cover for the original categories and reassessing the relative sediment production. The resultant
maps can be used to aid in planning decisions by indicating areas where changes in land use will likely
have the largest (or smallest) effect on sediment yield to receiving channels.
Effective screening tools for assessing the susceptibility of streams to hydromodification necessarily rely
on both field and office-based elements to examine local characteristics within their broader watershed
context. Proactive mapping of flow energy measures (e.g., specific stream power) throughout drainage
networks has the potential to complement field-based assessments in identifying hotspots for channel
Page-44
instability and sediment discontinuities as streamflows change with land use. Such analyses may
partially guide subsequent field reconnaissance; however, this approach also has limitations in that
some geomorphic settings are inherently difficult to map using widely available digital elevation data. In
particular, maps of stream power in narrow entrenched valleys and low gradient valleys (ca. <1%) with
sinuous channels should be carefully field-truthed and used with a level of caution commensurate with
the accuracy of the input data.
Moreover, spatial variability in channel boundary materials and form cannot be accurately mapped at
present using remotely sensed data. Thus, boundary materials and channel width are typically assumed
in watershed-scale mapping efforts, thereby introducing potential inaccuracies. Coupling desktop
analysis with a field-based assessment when using such an approach can help resolve variation in site-
specific features such as the erodibility of bed and bank materials, channel width, entrenchment, grade
control features, and proximity to geomorphic thresholds.
4.3.2 Mechanistic and Empirical/Statistical Models with Deterministic Outputs
Mechanistic/deterministic models are simplified mathematical representations of a system based on
physical laws and relationships (link to next). Empirical/statistical models use observed input and output
data to develop relationships among independent and dependent variables. Statistical analyses
determine the extent to which variation in output can be explained by input variables. Both types of
models are typically used to generate a single output or
answer for a given set of inputs. These tools can be used to
help answer such questions as: What are the expected
responses in the stream and watershed given some future
conditions? What criteria should be set to prevent future
hydromodification impacts? However, hydromodification
modeling embodies substantial uncertainties in terms of
both the forcing processes and the stream response.
Deterministic representations of processes and responses
can therefore mask uncertainties and be misleadingly precise, unless prediction uncertainty is explicitly
characterized as described later in this section.
Hydrologic Models are used to simulate watershed hydrologic processes, including runoff and
infiltration, using precipitation and other climate variables as inputs. Some models, such as the
commonly-used HEC-HMS, can be run for either single-event simulations or in a continuous-simulation
mode which tracks soil moisture over months or years. Other hydrologic models that are commonly
used for event-based and continuous simulation modeling include HSPF and SWMM. It is widely
accepted that continuous simulation modeling, rather than event-based modeling, is required to assess
long term changes in geomorphically-significant flow events (Booth and Jackson 1997; Roesner et al.
2001).
Several HSPF-based continuous simulation models have been developed specifically for use in
hydromodification planning. These include the Western Washington Hydrology Model (WWHM) and
Although valuable, deterministic
representations (such as those derived
from continuous simulation modeling) of
processes and responses can mask
uncertainties and be misleadingly precise
unless prediction uncertainty is explicitly
characterized.
Page-45
the Bay Area Hydrology Model (BAHM). Hydromodification Management Plans (HMPs) in Contra Costa
County, San Diego County and Sacramento County have developed sizing calculators for BMPs based on
modeling done using HSPF models. To illustrate the point about uncertainly in mechanistic models,
HSPF contains approximately 80 parameters, only about 8 of which are commonly adjusted as part of
the calibration process.
Hydraulic Models are used to simulate water-surface profiles, shear stresses, stream power values and
other hydraulic characteristics generated by stream flow, using a geometric representation of channel
segments. The industry standard hydraulic model is the HEC River Analysis System (HEC-RAS).
Coupled Hydrologic and Hydraulic Models represent a valuable tool in hydromodification management.
Because the streamflow regime interacts with its geomorphic context to control physical habitat
dynamics and biotic organization, it is often necessary to translate discharge characteristics into
hydraulic variables that provide a more accurate physical description of the controls on channel erosion
potential, habitat disturbance, and biological response. For example, a sustained discharge of 100 cfs
could potentially result in significant incision in a small sand bed channel but have no appreciable effect
on the form of a larger channel with a cobble bed. By converting a discharge value into a hydraulic
variable (common choices are shear stress, or stream power per unit area of channel relative to bed
sediment size), a “common currency” for managing erosion and associated effects can be established
and applied across many streams in a region. Such a common currency can improve predictive accuracy
across a range of stream types. As opposed to focusing on the shear stress or stream power
characteristics of a single discharge, it is usually necessary to integrate the effects of hydromodification
on such hydraulic variables over long simulated periods of time (on the order of decades) to fully assess
the potential for stream channel changes. By using channel morphology to estimate hydraulic variables
across a range of discharges, models like HEC-RAS provides a means of translating hydrologic outputs
from continuous simulations in HEC-HMS, SWMM, or HSPF into distributions of shear stress and stream
power across the full spectrum of flows.
Sediment Transport Models such as HEC-6T, the sediment transport module in HEC-RAS, CONCEPTS,
MIKE 11 and FLUVIAL12, use sediment transport and supply relationships to simulate potential changes
in channel morphology (mobile boundary) resulting from imbalances in sediment continuity. This means
that hydraulic characteristics are calculated as channel form and cross-section evolve through erosion
and deposition over time. Such models have high mechanistic detail but are often difficult to apply
effectively. Although it is not a mobile boundary model, the SIAM (Sediment Impact Analysis Method)
module in HEC-RAS represents an intermediate complexity model designed to predict sediment
imbalances at the stream network scale and to describe likely zones of aggradation and degradation.
Statistical Models use descriptive tools and empirical data to develop relationships that quantify the risk
of specific stream behaviors. For example, Hawley (2009) developed a statistical model to explain
variance in channel enlargement based on measures of erosive energy and channel features such grade
control and median bed sediment size. Such models often include independent variables based on input
from the mechanistic models described above; however, a key difference is that statistical models do
not explicitly represent actual physical processes in their mathematical structure. Instead, these models
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simply express the observed correlations between dependent and independent variables. Like
mechanistic models, the output from these models is commonly treated as precise results in
management decisions, despite the fact that predictions from most statistical models could be readily
(and more accurately) expressed in terms of confidence intervals with a range of uncertainty.
Probabilistic/Risk-based Models integrate many of the tools discussed above, using modeled changes in
hydrology as input to hydraulic models, which in turn provide input to various types of statistical models
to predict response. However, the predictions are not represented as deterministic outputs, instead,
the range of (un)certainty in the likelihood of the predicted response
is explicitly quantified. Although not commonly used for
hydromodification management at this time, there are well
established models based on these principals currently in use in
other scientific disciplines. An example of a probabilistic approach
that has been used for hydromodification management is a logistic
regression analysis that was used to produce a threshold “erosion
potential metric” that can be used to quantify the risk of a degraded
channel state. More details on this approach are provided in
Appendix B.
Risk-based modeling in urbanizing streams provides a more scientifically defensible alternative to
standardization of stormwater controls across stream types. A probabilistic representation of possible
outcomes also improves understanding of the uncertainty that is inherent in model predictions, and can
inform management decisions about acceptable levels of risk.
Predictive Tools for Habitat Quality and Stream Biota. The tools discussed above focus on physical
stream impacts; however, as discussed in the preceding chapter, it is recognized that maintenance of
stream “stability” does not necessarily conserve habitat quality and biological potential. In general, the
knowledge base for biota/habitat associations is not generally adequate to allow for prediction of how
whole communities will change in response to environmental alterations associated with urbanization.
Making such predictions deterministically requires a thorough knowledge of species-specific
environmental responses, as well as an adequate (accurate) characterization of habitat structure and
habitat dynamics (both of which are modified by urbanization). However, recent studies have
demonstrated that the effects of hydrologic alterations induced by urbanization on selected stream
biota can be quantitatively described without a full mechanistic understanding, using stressor-response
type relationships and empirical correlations from field-measured conditions (Konrad and Booth 2005,
Konrad et al. 2008, DeGasperi et al. 2009).
In moving beyond a narrow focus on linkages between flow alteration and channel instability, scientific
understanding of hydrologic controls on stream ecosystems has recently led to new approaches for
assessing the ecological implications of hydromodification. The essential steps in developing
quantitative “flow-ecology relationships” have been recently described in the Ecological Limits of
Hydrologic Alteration (ELOHA) process (Poff et al. 2010), a synthesis of a number of existing hydrologic
techniques and environmental flow methods. ELOHA provides a regional framework for elucidating the
Risk-based modeling in
urbanizing streams provides a
more scientifically defensible
alternative to standardization of
stormwater controls across
stream types, and can inform
management decisions about
acceptable levels of risk.
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key hydrologic influences on biota of interest, and translating that understanding into relationships
between hydromodification and biological endpoints that can be used in management decision making.
This requires a foundation of hydrologic data provided by modeling and/or monitoring, and sufficient
biological data across regional gradients of hydromodification. Although hydrologicecological response
relationships may be confounded to some extent by factors such as chemical and thermal stressors,
there are numerous case studies from the US and abroad in which stakeholders and decision-makers
reached consensus in defining regional flow standards for conservation of stream biota and ecological
restoration (Poff et al. 2010; http://conserveonline.org/workspaces/eloha).
4.3.3 Strengths, Limitations and Uncertainties
The Organizing Framework shown in Figure 4-1 shows the applicability of the three major categories of
tools in support of various management actions. This section addresses a range of issues relating to
strengths, limitations and uncertainty of the tools discussed above. Detailed analysis of individual
models is beyond the scope of this document, but EPA/600/R-05/149 (2005) contains an extensive
comparison of functions and features across a wide range of hydrologic and hydraulic models.
General Considerations. The well-known statistician George Box famously said that “all models are
wrong, some are useful.” The usefulness of a model for a particular application depends on many
factors including prediction accuracy, spatial and temporal detail, cost of calibration and testing,
meaningful outputs, and simplicity in application and understanding. There is no cookbook for selecting
models with an optimal balance of these characteristics. Models of stream response to land-use change
will always be imperfect representations of reality with associated uncertainty in their predictions. In
addition to the prediction errors of standard hydrologic models, common limitations and sources of
uncertainties include insufficient spatial and/or temporal resolution, and poorly known parameters and
boundary conditions. Ultimately, the focus of scientific study in support of decision making should be
on the decisions (or objectives) associated with the resource and not on the model or basic science.
Each model has limitations in terms of its utility in addressing decisions and objectives of primary
concern to stakeholders. Prediction error, not perception of mechanistic correctness, should be the
most important criterion reflecting the usefulness of a model (NRC 2001; Reckhow 1999a,b). The
predictive models discussed above may be thought of as predictive scientific assessments; that is, a
flexible, changeable mix of small mechanistic models, statistical analyses, and expert scientific judgment.
Region-Specific Considerations. Because all models are vulnerable to improper specification and
omission of significant processes, caution must be exercised in transferring existing models to new
Explicit consideration, quantification, and gradual reduction of model uncertainty will be
necessary to advance hydromodification management.
The uncertainty inherent to hydromodification modeling underscores the need for carefully
designed monitoring and adaptive management programs.
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regional conditions. For example, mobile boundary hydraulic models are mechanistically detailed but
not generally well-suited to many southern California streams given the prevalence of near-supercritical
flow, braiding and split flow (Dust 2009). In addition, bed armoring and channel widening resulting from
both fluvial erosion and mass wasting processes are key influences on channel response in semi-arid
environments. These processes are not well-represented and constrained in current mobile boundary
models. Accordingly, the appropriateness of existing models for addressing a particular
hydromodification management question should be empirically tested and supported with regionally
appropriate data from diverse stream settings.
Managing Uncertainty. To date, hydromodification management has generally relied on oversimplified
models or deterministic outputs from numerical models that consume considerable resources but yield
highly uncertain predictions that can be difficult to apply in management decisions. Numerical models
are nevertheless an important part of the hydromodification toolbox, especially in characterizing
rainfall-response over decades of land-use change. It is challenging to rigorously quantify the prediction
accuracy of these mechanistic numerical models; however, their utility of can be enhanced by
addressing prediction uncertainties in number of ways (Cui et al. 2011). Candidate models can be
subjected to sensitivity analysis to understand their relative efficacy for assessment and prediction of
hydromodification effects. Moreover, it should also be demonstrated that selected models can
reasonably reproduce background conditions before they are applied in predicting the future. Modeling
results that are used in relative comparisons of outcomes are generally much more reliable than
predictions of absolute magnitudes of response.
Hydromodification modeling embodies substantial uncertainties in terms of both the forcing processes
and stream response. Deterministic representations of processes and responses can mask uncertainties
and can be misleading unless prediction uncertainty is explicitly quantified. Errors may be transferred
and compounded through coupled hydrologic, geomorphic, and biologic models. Accordingly, explicit
consideration, quantification, and gradual reduction of model uncertainty will be necessary to advance
hydromodification management. This points to two basic needs. First, there is a need to develop more
robust probabilistic modeling approaches that can be systematically updated and refined as knowledge
increases over time. Such approaches must be amenable to categorical inputs and outputs, as well as
combining data from a mix of sources including mechanistic hydrology models, statistical models based
on field surveys of stream characteristics, and expert judgment. Second, the uncertainty inherent to
hydromodification modeling underscores the need for carefully designed monitoring and adaptive
management programs, as discussed in Section 5.
A risked-based framework can provide a more rational and transparent basis for prediction and
decision-making by explicitly recognizing uncertainty in both the reasoning about stream response and
the quality of information used to drive the models. Prediction uncertainty can be quantified for any of
the types of models described above; however, some types are more amenable to uncertainty analysis
than others. For example, performing a Monte Carlo analysis of a coupled hydrologic-hydraulic model is
a very demanding task. A simple sensitivity analysis of high, medium, and low values of plausible model
parameters is much more tractable and still provides an improved understanding of the potential range
of system responses. Such information can be subsequently integrated with other model outputs and
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expert judgment into a probabilistic framework. For example, Bayesian probability network approaches
can accommodate a mix of inputs from mechanistic and statistical models, and expert judgment to
quantify the probability of categorical states of stream response. Such networks also provide an explicit
quantification of uncertainty, and lend themselves to continual updating and refinement as information
and knowledge increase over time. As such, they have many attractive features for hydromodification
management, and are increasingly used in environmental modeling in support of water quality
(Reckhow 1999a,b) and stream restoration decision-making (Stewart-Koster et al. 2010).
Sediment Supply. As described above, a reduction in sediment supply to a stream may result in
instability and impacts, even if pre- and post-land use change flows are perfectly matched. Thus, there
is a need to develop management approaches to protect stream channels when sediment supply is
reduced, and to refine and simplify tools to support these approaches. This continues to prove
challenging because, the effects of urban development on sediment supply in different geologic settings
are not well understood and poorly represented in current models. As a starting point, models used to
analyze development proposals that reduce sediment supply could be applied with more protective
assumptions with respect to parameters and boundary conditions (inflowing sediment loads). Effects of
altered sediment supply on stream response could be addressed in a probabilistic framework by
adjusting conditional probabilities of stream states to reflect the influence of reductions in important
sediment sources due to land use change.
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5. MONITORING
“Monitoring” can cover a tremendous range of activities in
the context of stormwater management in general, and of
hydromodification in particular. For example, the NPDES
Phase 2 general permit for California (SWRCB, 2003
(www.swrcb.ca.gov/water_issues/.../stormwater/.../final_ms
4_permit.p...), National Pollutant Discharge Elimination
System (NPDES) General Permit No. CAS000004, p. 11) notes
that the objectives of a monitoring program may include:
Assessing compliance with the General Permit.
Measuring and improving the effectiveness of
stormwater management plans.
Assessing the chemical, physical, and biological
impacts on receiving waters resulting from urban runoff.
Characterizing storm water discharges.
Identifying sources of pollutants.
Assessing the overall health and evaluating long-term trends in receiving water quality.
These objectives span multiple goals, ranging from verifying of compliance, evaluating effectiveness,
characterizing existing conditions, and tracking changes over time. Each would likely require different
monitoring methods, duration of measurement, and uses of the resulting data (Table 5-1). This
variability emphasizes what we consider the key starting point of any monitoring program: to answer
the questions, “What is the purpose of monitoring? How will the data be used?” Even secondary
considerations can exert great influence over every aspect of the design of a monitoring program: “How
quickly do you need to have an answer?” And, perhaps most influential of all, “What are the resources
available to provide that answer?”
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Table 5-1. The recommended purpose(s) of monitoring associated with hydromodification control
plans, organized by the scale of implementation and the time frame in which useful results should
be anticipated.
Time Frame
Programmatic: State and
Regional Water Boards
Local: City and
County Jurisdictions
Short-term
(<10 years)
Define the watershed context for local
monitoring (at coarse scale)
Evaluate whether permit requirements are
making positive improvements
Evaluate whether specific projects/
regulations are meeting objectives
Identify the highest priority action(s) to take
Long-term
(1+ decades)
Define watershed context and setting
benchmarks for local-scale monitoring (i.e.,
greater precision, if/as needed)
Demonstrate how permit requirements can
improve receiving-water “health,” state-wide
(and change those requirements, as needed)
Evaluate and demonstrate whether actions
(on-site, instream, and watershed scale)
are improving receiving-water conditions
Assess program cost-effectiveness
Identify any critical areas for resource
protection
5.1 The Purpose of Monitoring
In the context of hydromodification assessment and management, we propose three interrelated
purposes for monitoring that will guide the discussion and recommendations in this section:
Characterizing the conditions of receiving waters downstream of urban development (including
any trends in those conditions over time).
Evaluating the effectiveness of hydromodification controls at protecting or i<