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Factors Contributing to Bank Stability in Channelized, Alluvial streams
Abstract
Bank failure is a common fluvial process and can be a pervasive fluvial response to natural and anthropogenic disturbances. Previous research has identified causes and types of bank failure, but the conditions that lead to the cessation of bank failure remain poorly explained. This research examines differences between banks with active bank failure and banks that exhibit evidence of past bank failure that ceased (dormant) throughout three West Tennessee (USA) rivers to provide insight into the processes that cause bank failure to end. Bank characteristics were observed at 68 sites, and data from 55 banks were used to create a logistic regression model. Bank characteristics entered into the model included: vegetative cover, failure location, bar association, bank material, channel width‐to‐depth (w/d) ratio, and average bank angle. Results of the logistic regression suggest that bank angle best explains (p = 0.31 and odds ratio = 8.2) the difference between banks with active and dormant bank failure. Interestingly, vegetative cover and bank material composition, which have been found to be important in bank stabilization by previous researchers, were not significant predictors of bank stability according to the logistic regression model. These results suggest that in absence of drastic differences in bank material resistance (bedrock vs sediment): (1) spatial patterns of bank failure at the system‐scale will be diffuse, (2) bank stability can require a multiple decades, and (3) the potential for vegetation to stabilize banks may be limited in some alluvial systems because of positive feedbacks created by repeated human disturbance. Copyright © 2012 John Wiley & Sons, Ltd.
... In Table 3, the construction types identified on the German and Czech sides are classified in terms of their resistance. Magilligan, 1992, LfU, 1996, Oplatka, 1998, Knighton, 1999, Gerstgraser 2000a& 2000b, Schillinger, 2001, SMUL, 2005, Julian & Torres, 2006, Nachtnebel, 2008, Feldmann, 2009, MŽP CR, 2009, Krapesch, Hauer, & Habersack, 2011, LANUV NRW, 2012, Sin, Thornton, Cox, & Abt, 2012, Vocal Ferencevic & Ashmore, 2011, Buraas et al., 2014, Langhammer, 2014, EFIB, 2015, Marchi et al., 2016, MacBroom 2017 No Based on a further literature screening (Davis & Harden, 2014;Gerstgraser, 2000aGerstgraser, & 2000bHopkinson & Wynn-Thompson, 2016;Klösch et al., 2018;Kolb, 1979;Magilligan, 1992;Park, Kim, Park, Jo, & Kang, 2016;Sin et al., 2012), we identified flow velocity and shear stress to be governing impact variables. A number of articles also rely on the impact variables of absolute and specific stream power presented earlier in Section 3.2. ...
In addition to their ecological importance, rivers and streams have always been used in diverse ways by humans, resulting in the development of settlements and their connected built environments along many of the world's watercourses. During heavy rainfall, buildings, traffic infrastructure and water-related infrastructure are exposed to potential hazards in the form of (flash) floods. In contrast to near-natural watercourses, anthropogenically modified channels in urban areas are particularly susceptible to damage by flooding. Previous damage assessments have highlighted the need to forecast such damage to watercourses in order to identify critical areas and justify the selection and expansion of adaptation measures. Within the scope of the current study, we have developed a method based on the hydro-morphological properties of watercourses to make transferable estimates of the economic damage potential based on ecologically-relevant parameters. Using a scale-specific cause-effect analysis, we have identified characteristics of the watercourse type and adjacent structures as well as construction-related properties of reinforcements that can increase the damage potential during flooding. In this way, we are able to show that several influencing factors determine the vulnerability of watercourses: in addition to the specific longitudinal gradient and size (macroscale) of various watercourse types, damage-relevant boundary conditions in watercourse sections (mesoscale) and the resistance of typical bed and bank constructions are also important, reflecting the specific structural conditions. Taking rivers in Germany and the Czech Republic as case studies, in the following, we review the local identification of critical areas and describe the necessary data management. The presented “Hydro-morphological based Vulnerability Assessment-Concept (HyVAC)” can contribute to the flood damage prevention at watercourses by utilizing existing basic data to the greatest possible extent and thus is suitable for preliminary investigations according to the EC Flood Risk Management Directive.
... Nardi et al. (2013) revealed the links between the near-bank shear stress and bank erodibility. Davis and Harden (2014) examined the characteristics of stream banks with active and dormant bank failure in three alluvial streams to determine the factors involved in the cessation of bank failure and bank stabilization. Relatively, researches on the interaction between bank failure and riverbed evolution are rather limited and most of them rely on numerical models ( Duan, 2005;Amiri-Tokaldany et al., 2007;Chen and Duan, 2008;Nardi and Rinaldi, 2010;Wang et al., 2010;Nasermoaddeli, 2011;Sahoo et al., 2011). ...
Bank erosion is a typical process of lateral channel migration, which is accompanied by vertical bed evolution. As a main sediment source, the failed bank soil may directly cause the increase of sediment concentration and considerable channel evolution in a short time. The paper presents an experimental study on non-cohesive and cohesive homogenous bank failure processes, influence of the failed bank soil on bank re-collapse, as well as the interaction between bank failure and near-bank bed evolution due to fluvial hydraulic force. A series of experiments were carried out in a 180° bend rectangular flume. The results reveal the iteration cycle between bank erosion and bed deformation: undercutting of the riverbank, slip failure of the submerged zone of the bank, as well as cantilever failure of the overhang, failed bank soil staying at bank toe temporarily or hydraulic transportation, exchange between the failed bank soil and bed material, bed material load being re-transported either as bed load or as suspended load, and bed deformation. Same as bank failure, the mixing of failed bank soil and bed material is more severe near the curved flow apex. Moreover, non-cohesive bank failure tends to occur near the water surface while cohesive bank failure near the bank toe. For non-cohesive dense (sandy) soil, the bank erosion amount and residual amount of failed bank soil on the bed increase with the near-bank velocity or bed erodibility. But for cohesive soil, only bank erosion amount follows the above rule. The results are expected to provide theoretical basis for river management and flood prevention.
This article looks at studies on how to use business continuity management for Hong Kong’s virtual banks in order to reduce customer information risks, so as to maintain business sustainability. Firstly, the development of virtual banks in Hong Kong were investigated, the laws and regulations and regulatory policies of Hong Kong and the Mainland were benchmarked, and the main risks that may occur and be harmful to the bank business sustainability were analyzed. Considering the characteristics of virtual banks, the main concerns of public customers about the IT risks of virtual banks through questionnaire surveys were collected and analyzed. Moreover, the importance of business continuity management to virtual banks was drawn. Secondly, in the case studies, via understanding the overall situation of WeBank, its performance during the COVID-19 pandemic, and the regulations of the Monetary Authority of Singapore, the practice standards of virtual banks in business continuity management were further clarified. At the end, three suggestions for virtual banks in Hong Kong were put forward to reduce customer information security risks through business continuity management, thereby maintaining its business sustainability.
Reservoir bank collapse, as a typical geological hazard, severely affects the geomorphic evolution and safety of residential sites. In this study, a case of Tankeng Reservoir, which is a large reservoir in eastern China is used to analyze a three-dimensional evolution mechanism and deformation characteristics of reservoir bank collapse by years of field investigation, underwater topography measurements and monitoring and corresponding physical modeling experiment. The results show that 90% of the bank collapsed within six years, and a full collapse occurred within nine years for the coarse-grained soil bank. In addition, a three-dimensional progressive pattern for the bank collapse evolution process and a new parameter of stable angles were proposed for first time, and this parameter has practical significance for further scientific calculations of the width and length of bank collapse.
Bank slope collapse is a kind of natural phenomenon which commonly existed on both sides of alluvial plain rivers. The mechanism of bank collapse is complex, and it is an interdisciplinary frontier research subject. The collapse of the bank slope will lead to the instability of river regime and frequent changes of erosion and siltation, which will cause great harm to river regulation and people's livelihood. Through review of river bank soil collapse at home and abroad, it is concluded that the main influencing factors of river bank soil collapse are the action of water flow and the soil structure of river bank. In addition, the stability of river bank and the numerical simulation of river bank collapse are also studied by scholars. In view of the above research results, the deficiencies of the current research are pointed out and the research directions that should be followed in the future are put forward.
Bank collapse is a sudden change of lateral channel migration, along with the riverbed deformation. The paper presents a study on the process and influence factor of bank collapse and bed evolution of homogeneous soil through a series of experiments carried out in a bend flume. Experimental results reveal a repeat process as follows: Bank collapse after accumulative basal erosion, the failed bank soil remaining at the bank-toe temporarily but soon afterwards being disintegrated and transported by intensified turbulent flow nearby, the coarser soil disintegrated from the bank or the fallen block being dragged to the convex bank downstream as bed load while the finer parts being transported by flow as suspended load, near-bank current structure being adjusted due to bank collapse and bed evolution. The research also reveals that the magnitude of the bank collapse or the riverbed erosion are mainly in proportion to the near-bank velocity and scouring duration, but reverse to the cohesion of material. The amount of bank collapse is larger than that of bed erosion when the channel suffers from erosion. The range of the named relative rate of bed erosion is about 0.40~0.92, decreasing as the amount of bank erosion.
An accurate estimation of riverbank erosion rates is critical for the evaluation of the past, present, and future sediment regime of river systems. Understanding these relationships allows watershed managers and regulators to prioritize river restoration and contaminated site remediation projects. In this dendrogeomorphic study, changes in the anatomy of tree roots exposed between 1 and 31 years were used to estimate the average annual erosion rates of riverbank sediments on a large river in Michigan, USA. Exposed root samples from diffuse and ring-porous hardwoods, together with buried ones as controls, were analyzed. Differences in the arrangement, size and frequency of vessels, fiber diameter, cell wall thickness, growth ring width, and scarring between the exposed and the buried samples were used to identify the first year of root exposure. Results of the regression analysis between the average annual erosion rate and the Bank Erosion Hazard Index (BEHI) indicated that the more recently exposed roots (less than 7 years in this study) explained more of the variance (R2 = 0.67) than when all samples were included (R2 = 0.38). Although the average erosion rates for long periods can be accurately determined from the dating of exposed tree roots, attempts to relate these rates using the BEHI for longer periods are less successful, as BEHI values can vary considerably over time as the riverbank erodes. Consequently, when using exposed tree roots to develop regression equations and erosion rate curves for the estimation of erosion rates based on BEHI scores, it is necessary to use roots that were recently exposed. Copyright © 2013 John Wiley & Sons, Ltd.
Bank retreat processes and mechanisms are discussed. These depend on bank material characteristics. It is shown that the basal end point control links bank and channel processes regardless of the bank characteristics. The influence of bank material characteristics, bank stability and the concept of basal end point control on the evolution of regime hydraulic geometry in a straight channel are demonstrated. (from paper)
Alteration of sediment dynamics is a common effect of human disturbance in fluvial systems. The longevity of human-induced changes and the implications of altered sediment dynamics for system-wide responses to disturbance are not entirely understood. In this study, I calculated sediment entrainment potential (τbf /τcr) for the D50 particle mea- sured for 34 reaches located in three alluvial streams currently adjusting to historic distur- bance, including channelization and land clearance, that have large in-channel sediment storage features. Analysis of the spatial variability of τbf /τcr values within the three study streams suggests that most study reaches are located in active transport zones, with the exception of three reaches in two of the study streams that appear to be located within sediment storage zones. The results of this research suggest that there is great potential for in-channel sediment storage features to be reworked and to serve as secondary sources of sediment well into the future. (Key words: in-channel alluvial benches, sediment entrain- ment potential, channelization.)
The purpose of this article is to provide researchers, editors, and readers with a set of guidelines for what to expect in an article using logistic regression techniques. Tables, figures, and charts that should be included to comprehensively assess the results and assumptions to be verified are discussed. This article demonstrates the preferred pattern for the application of logistic methods with an illustration of logistic regression applied to a data set in testing a research hypothesis. Recommendations are also offered for appropriate reporting formats of logistic regression results and the minimum observation-to-predictor ratio. The authors evaluated the use and interpretation of logistic regression presented in 8 articles published in The Journal of Educational Research between 1990 and 2000. They found that all 8 studies met or exceeded recommended criteria.
Channelization of rivers can alter channel morphology and sediment dynam- ics for decades and as a result, channelization is considered to be one of the most pro- found forms of human-induced disturbance in fluvial systems. Previous research conducted in large alluvial rivers suggests that spatial patterns of geomorphic adjustment processes vary with distance from the channelized reach. Few studies have investigated spatial patterns of geomorphic processes in smaller, tributary streams, which differ to larger systems in a variety of ways. I examined spatial patterns of widening and incision in three tributary streams with similar land use history and geology, using w/d ratios calcu- lated from field measurements of channel morphology. Spatial patterns of w/d ratios sug- gest that channel incision dominates two of the tributaries, while widening dominates a third tributary. These results suggest that because tributary streams are smaller in area than larger rivers, one type of geomorphic adjustment process can dominate the entire water- shed. The dominance of channel widening or incision throughout a system results in large amounts of sediment being produced throughout the system at one time. (Key words: geo- morphic adjustment, channelization, tributaries.)
Recent studies of river bank erosion in three catchments in the UK have been characterized by the persistent occurrence of negative erosion-pin results. The cause of these negative recordings is considered with reference to field data from the Afon Trannon, Nant Tanllwyth and River Arrow, and to a laboratory study of freeze–thaw and desiccation processes. It seems that there is potential for, and in some cases evidence of, a number of different circumstances that generate negative results, but none of these alone is sufficient to explain all incidents. Factors considered include: deposition of sediment during high flows; soil fall from the upper parts of the bank on to lower erosion pins; loosening of the soil surface and expansion/contraction of the soil mass with fluctuations in temperature and moisture content; movement of the erosion-pin within the bank and human interference. Each has its own implications for the use of erosion pins.
Further issues arise when including negative data in subsequent data analysis, and it is demonstrated that attempts to correlate erosion rates with hydro-meteorological data in order to ascertain causes of erosion will be influenced by the way in which negative data are handled. It is thus suggested that any study of river bank erosion using erosion pins should state whether or not negative data were obtained, and if so, how they were included in data analysis. Failure to include this information could lead to comparison of mean erosion rates that reflect bank processes very differently.
The studies presented here offer a clear example of the value of ‘anomalous’ field data: results which do not appear to fit expected patterns can reveal as much about the processes in operation as those that do. Copyright
The aim of this paper is to demonstrate how subaerial processes vary with the silt–clay content of river bank soil and to consider this variation in the context of erosion observed in the field. In particular, spatial and temporal variations in erosion, interactions with other bank erosion processes and the implications for bank morphology are explored.A number of soil blocks of known silt–clay content were subjected to a series of freeze–thaw and drying–wetting cycles. Changes in the dimensions of the soil blocks were recorded and the mass of soil ‘eroded’ from each block together with maximum size of eroding aggregates were measured. Limitations to this methodology are acknowledged, including a limited ability to replicate the forces to which a river bank may be subject, restrictions imposed by the available equipment in producing temperature and humidity cycles which reflect those occurring in the field, and issues of scale.The results indicate that river banks with high silt–clay contents are the most susceptible to erosion by subaerial processes. High susceptibility may influence spatial variations in bank erosion processes and rates, including downstream changes in the effectiveness and significance of subaerial erosion. A ‘vertical zoning’ of processes on a river bank is recognised, whereby the upper part of the bank is subject to subaerial erosion and the lower part to fluvial erosion. Given that a high silt–clay content tends to indicate increased susceptibility to subaerial erosion but increased resistance to fluvial erosion, this vertical zoning may have implications for bank morphology.
This study identifies and assesses the controls on hydraulic erosion of cohesive riverbanks on a 600-m reach of an urban ephemeral stream with active bank erosion. We examined hydraulic bank erosion by separating estimated bank shear stress into four properties: magnitude, duration, event peak, and variability. The values of these independent variables were used as a bank erosion context at three transects. Stepwise regression showed that the event peak (maximum peak) of excess shear stress best predicts cohesive bank erosion at the two transects with moderate critical shear stresses (1.93–4.08 N/m2), while the variability (all peaks) of excess shear stress best predicts erosion at the transect with low critical shear stress (0.95 N/m2). These results suggest that the amount of hydraulic erosion of cohesive riverbanks is dictated by flow peak intensities. Finally, the results of this study were combined with results from previous bank erosion studies to produce a conceptual model for estimating bank erosion rates based on their silt–clay content.
Channelization has increased energy conditions along main stems and tributaries and initiated systematic trends in channel adjustment. Gradation processes and adjustment trends are a function of the magnitude and extent of an imposed disturbance on a stream channel and the location of the adjusting reach in the fluvial network. Degradation at a site is described by power-decay equations. A six-stage, semiquantitative model of channel evolution in disturbed channels is developed by quantifying gradation trends, by interpreting process-response relations during stages of bank-slope development, and by interpreting changes over space as changes over time. -from Author
Reviews the processes of river bank erosion i.e. fluvial entrainment and weakening and weathering. Considers the mechanics of failure for non-cohesive and cohesive banks and notes field observations from gravel bed rivers in Wales, U.K.. Discusses various types of ship failure and reviews field observations. Considers erosion and failure of composite banks. Discusses implications for design of bank protection; toe protection; and river regulation. Defines basal endpoint control. Discussion of the paper, pages 259-271, includes notes on bank erosion in Arctic areas. (C.J.U.)
Suspended sediment transported in river systems has the ability to degrade water quality and impart negative impacts downstream as material is deposited. This study examines the significance of valley wall slumping as a source of both suspended sediment and phosphorus in the Blue Earth River. Several streambanks on the Blue Earth River in south central Minnesota were repeatedly surveyed from 1997 to 2000 to determine annual rates of streambank slumping. Volumetric changes between digital elevation models from successive topographic surveys were converted into changes in mass, and, subsequently, annual erosion rates were calculated based on surface area. An erosion rate constant was derived based on surface area of surveyed sites and applied to an inventory of eroding escarpments along the entire river to estimate the contribution of streambank slumping to the total suspended sediment (TSS) and total phosphorus (TP) loads in the Blue Earth River. Erosion rates for surveyed sites ranged from 544 to 3,995 t ha-1 yr-1 (242 to 1,782 tons ac-1 yr-1), with an average rate of 2,154 t ha-1 yr-1 (961 tons ac-1 yr-1). The derived erosion rate constant for slumping sites was 0.23 t m-2 yr-1 (0.024 tons ft-2 yr-1). Between 75,804 and 106,213 t (83,536 to 117,047 U.S. tons) of sediment is delivered to the river annually from all eroding escarpments inventoried, with between 28,047 and 39,299 t (30,908 and 43,307 U.S. tons) transported by flow as TSS. Streambank slumping accounts for 31% to 44% of the TSS load at the mouth of the Blue Earth River. The percentage of the TP load originating from streambank slumping is estimated to be from 7% to 10%, with annual contributions of 12 to 17 t P (14 to 19 U.S. tons).
Riparian buffers are promoted for water quality improvement, habitat restoration, and
stream bank stabilization. While considerable research has been conducted on the effects of
riparian buffers on water quality and aquatic habitat, little is known about the influence of riparian
vegetation on stream bank erosion.
The overall goal of this research was to evaluate the effects of woody and herbaceous riparian
buffers on stream bank erosion. This goal was addressed by measuring the erodibility and critical
shear stress of rooted bank soils in situ using a submerged jet test device. Additionally, several soil,
vegetation, and stream chemistry factors that could potentially impact the fluvial entrainment of soils
were measured. A total of 25 field sites in the Blacksburg, Virginia area were tested. Each field site
consisted of a 2nd-4th order stream with a relatively homogeneous vegetated riparian buffer over a
30 m reach. Riparian vegetation ranged from short turfgrass to mature riparian forest. Multiple linear
regression analysis was conducted to determine those factors that most influence stream bank
erodibility and the relative impact of riparian vegetation.
Study results indicated that soil erosion is a complex phenomenon that depends primarily on soil bulk
density. Freeze-thaw cycling, soil antecedent moisture content, the density of roots with diameters of 0.5 mm to 20 mm, and the interaction of soil pore water and stream water had a significant impact on
soil erodibility and critical shear stress, depending on the soil type. Riparian vegetation had multiple
significant effects on soil erodibility. In addition to reinforcing the stream banks, the streamside
vegetation affected soil moisture and altered the local microclimate, which in turn affected freezethaw
cycling.
This study represents the first in situ testing of the erodibility of vegetated stream banks; as such, it
provides a quantitative analysis on the effects of vegetation on stream bank erosion, relative to other
soil physical and chemical parameters. It also represents the first measurements of the soil erosion
parameters, soil erodibility and critical shear stress, for vegetated stream banks. These parameters
are crucial for modeling the effects of riparian vegetation for stream restoration design and for water
quality simulation modeling.
Erosion pins were installed on 32 banks in five tributaries of the Little River, eastern Tennessee, to quantify the contributions of streambanks to stream sediment loads and better understand the processes of streambank erosion. In the first two years of monitoring, erosional losses from streambanks were readily measurable, with a median loss of 1 cm bank-1. Streambanks lost material through subaerial erosion processes as well as by mass movements and removal of sediment by hydraulic forces. Although the highest median erosion rates occurred below the water line, erosion also occurred on the upper banks. Initial results demonstrate that channel bank instability is widespread and channel banks are potentially important sources of sediment in these tributaries. Two observations made in this study, of streambank erosion independent of hydraulic forces and of bank undercutting active during drought, call attention to gaps in knowledge that affect models of streambank processes and efforts to improve water quality in headwater streams.
This article reviews previous investigations into gully and badland research and discusses processes and definitions in the context of existing research in southeast Spain. The theory of badland development is summarized and definitions are proposed which draw on previous work and continuing studies. The processes influencing gully and channel head morphology are then discussed including overland flow, hillslope processes, pipe initiation and enlargement, mass failures and the magnitude and frequency distribution of storm events. Finally, modelling of badland landscapes is discussed. The article highlights that much detailed research has been carried out on badlands, but long-term rates of gully development are not well understood. There are also gaps in our understanding of pipe network formation and collapse. In the short term theoretical modelling may provide the way forward and a direction for more holistic investigations.
Hundreds of miles of streams in West Tennessee have been channelized or otherwise modified since the turn of century. After all or parts of a stream are straightened, dredged, or cleared, systematic hydrologic, geomorphic, and ecologic processes collectively begin to reduce energy conditions towards the premodified state. One hundred and five sites along 15 streams were studied in the Obion, Forked Deer, Hatchie, and Wolf River basins. All studied streams, except the Hatchie River, have had major channel modification along all or parts of their courses. Bank material shear-strength properties were determined through drained borehole-shear testing (168 tests) and used to interpret present critical bank conditions and factors of safety, and to estimate future channel-bank stability. Mean values of cohesive strength and angle of internal friction were 1.26 pounds per square inch and 30.1 degrees, respectively. Dendrogeomorphic analyses were made using botanical evidence of channel-bank failures to determine rates of channel widening; buried riparian stems were analyzed to determine rates of bank accretion. Channel bed-level changes through time and space were represented by a power equation. Plant ecological analyses were made to infer relative bank stability, to identify indicator species of the stage of bank recovery, and to determine patterns of vegetation development through the course of channel evolution. Quantitative data on morphologic changes were used with previously developed six-stage models of channel evolution and bank-slope development to estimate trends of geomorphic and ecologic processes and forms through time.
Processes of riverbank erosion involve fluvial entrainment, mass failure and subaerial erosion. The role of freeze-thaw action in these erosion processes was investigated along a small mountain stream in a region with seasonal frost. Riverbank profiles, horizontal erosion, soil temperature, frost depth and soil water content were monitored over 20 months including two winters. Diurnal and annual freeze-thaw action directly triggers subaerial erosion and indirectly contributes to mass failure and fluvial entrainment. The rate of subaerial erosion reaches a maximum during the thawing period, reinforced by increasing water content and decreasing soil hardness. The subaerial erosion, which occurs uniformly along the riverbank and recurs every year, contributes to progressive deepening of a notch developed above the water level as both preparatory and erosive processes. The overdeepening of the notch decreases bank stability, followed by occasional mass failure immediately after the thawing period and periodic fluvial entrainment during the summer flood period. The annual amount of subaerial erosion is comparable to that of fluvial entrainment. These observations demonstrate that freeze-thaw erosion plays a fundamental role in erosion of riverbanks subject to deep seasonal frost. Copyright © 2006 John Wiley & Sons, Ltd.
The majority of sediment leaving catchments may be from streambank failure. Seepage erosion of unconsolidated sand above a restrictive layer is an important erosion process in incised streams that leads to streambank failure by undercutting banks. The objective of this study was to determine the impact of soil properties on seepage erosion and the resulting streambank failure. Seepage flow and sediment concentrations were measured in situ at eight locations along the banks of a deeply incised stream in northern Mississippi. Using field observations as a guide, the soil profile conditions of a shallow (45 cm) streambank, consisting of 30 cm of topsoil, a 10 cm conductive layer, and a 5 cm restrictive layer, were mimicked in laboratory lysimeter experiments to quantify the hydrologic properties controlling seepage erosion and bank failure under a 40 cm head. The time to flow initiation and the flow rate were linearly related to the slope of the restrictive layer. Seepage erosion began within minutes of flow initiation and resulted in substantial (3 to 34 cm) undercutting of the bank. Sediment concentrations of seeps were as high as 660 g l−1in situ and 4500 g l−1 in the lysimeters. Sediment concentrations were related to the layer slope, thereby indicating the importance of detailed site characterization. The USDA-ARS Streambank Stability model demonstrated the increase in instability of banks due to undercutting by seepage erosion, but failed to account for the sediment loss due to sapping for stable banks and overestimated the sediment loads for failed banks. Published in 2006 by John Wiley & Sons, Ltd.
To investigate the role of pore water pressures in the stability of a streambank, a series of tensiometers and piezometers was installed in a bank of the Sieve River, Tuscany, Italy. Fluvial entrainment at the bank toe was monitored by repeated cross-profiling, erosion pins and marked pebbles. Fluctuations in matric suction measured at the tensiometers reflected the overall response of pore water pressures to rainfall, evapotranspiration, rising and drawdown of the river stage, and variations in water table. An expression was derived for the safety factor with respect to mass movement of the upper bank, incorporating the failure criterion for unsaturated soils and the normal Mohr–Coulomb criterion for saturated conditions. Variations in matric suction have important effects on the stability of the streambank. During low-flow periods, the shear strength term due to the matric suction allows the bank to remain stable at a steep angle. However, during rainfall and flow events, reduction in matric suction and increase in unit weight of the material from vertical and lateral infiltration may be sufficient to trigger a mass failure, without development of significant positive pore water pressures. During the rising limb of high-flow events, the factor of safety increases as a consequence of the stabilizing confining pressure of the water in the river, despite a reduction in matric suction. During drawdown in the river, when the suction values are still low and the confining pressure in the river decreases to zero, the factor of safety falls to lower values than those experienced prior to the runoff event. Measurements of fluvial entrainment reveal that, although the processes, mechanisms and the frequency of retreat of basal and upper bank zones differ significantly, the amount of retreat at the bank toe due to fluvial erosion is comparable to that of the upper portion of the bank due to mass failure. Copyright © 1999 John Wiley & Sons, Ltd.
Where and when do natural rivers become unstable? To answer this question, we visually estimated bank-failure extent in 100-m increments along 180 km of riverbanks in three watersheds of the northern Yellowstone ecosystem. The riverbank data reveal precise power-law relationships between the number of bank failures of a given size throughout each watershed and the magnitude of those bank failures. The slopes of log-log graphs (i.e., the exponent t) of bank-failure magnitude versus failure frequency in alluvial reaches vary from 1.07 to 1.44, while t for all reaches combined (alluvial, colluvial, and bedrock) varies from 1.18 to 1.53, suggesting that lower-gradient, alluvial streams are more susceptible to large bank failures. Cellular automata simulations of riverbanks show similar power-law failure relationships, as do bank-erosion data from a long-term independent dataset from another location. These power-law structures can be interpreted as the spatial signal of a self-organized critical (SOC) system, in which local instabilities function to generate broader-scale order. SOC systems are considered to be at the ''edge of chaos,'' where local processes interact to make prediction of specific failure events impossible, although probability distribution prediction of the magnitude and spatial frequency of those events is possible. A critical structure of this sort is to be expected in bank failures along a stream given a nonlinear diffusive system such as a drainage basin. If riverbanks are, in fact, part of a critical system, then long-term local or watershed-wide stability is an unlikely or even impossible engineering or restoration goal. The existence of criticality in natural stream settings suggests that local human alterations designed to increase channel stability, while changing the local frequency of small failures, will only encourage an increase in the magnitude of system-wide, low-frequency large failures. A restoration or stabilization effort will not eliminate the bank instability. Instead, it will transfer that instability to neighboring riverbank areas. Key Words: bank stability, fluvial geomorphology, self-organized criticality, stream restoration, width adjustment. U nderstanding patterns and processes of stream bank failure is critical to predicting and explain-ing changes in river form, provides key informa-tion for river-hazard assessment, and helps guide stream management and restoration. ''Bank failure,'' a phrase we use to indicate all forms of bank erosion and mass wasting, is generated by a number of different processes that operate locally to create a complex mosaic of bank erosion and stability along the stream system. Although the individual processes controlling bank failure in different locales have been well defined (e.g., Lawler 1992, 1993), our understanding of the basin-wide distribution of bank failure and stability is still in its infancy. Few geographic studies have looked at basin-wide distributions of bank failure per se (Lawler et al. 1999), focusing instead on watershed-wide distributions of lateral migration and overall stream cross-sectional changes as a function of disturbances such as agriculture (e.g., Knox 1977), loss of vegetative cover (Graf 1979), climate change (Graf 1987), channelization (Simon 1989), dams (Williams and Wolman 1984), and urbanization (Trimble 1997). Although the changes in channel cross-sections and the controlling processes documented by these and other researchers are related to bank failure, they are not direct measures of that phenomenon. Furthermore, the channel changes documented in previous studies are all based on a limited number of measurements along the stream, which potentially miss the depth of detail and pattern that could be revealed by a continuous sample along an entire stream length. In this article, we document bank failure along 180 km of stream length in the northern Yellowstone ecosystem, examine how well these patterns fit within the theoretical context of self-organized criticality, and discuss some of the practical implications of these findings.
Few studies have considered downstream changes in bank erosion rates and variability along single river systems. This paper reports some preliminary results of an intensive and direct field monitoring exercise of bank erosion rates on 11 sites along 130 km of the 3315 km2 Swale-Ouse river system in northern England over a 14·5 month period. Data were collected at active sites using grid networks of erosion pins read at c. 18–30 day intervals and bank-line resurveys.
Erosion rates were relatively high for a river of this scale: spatially averaged bank erosion magnitudes over the 14·5 months varied from 82·7 mm to 440·1 mm, although at one highly mobile reach retreat of 1760 mm was recorded over 4 months. Bank erosion rates tended to peak in mid-basin, possibly because of an optimum combination there of high stream powers and erodible bank materials, as predicted theoretically by Lawler (1992, 1995). The piedmont (upland–lowland transition) zone was especially active. Graphical erosion representations for specific periods, however, showed that bank retreat was often highly localized within individual sites.
Strong seasonal variations in erosion rate were also observed with a significant winter (December–March) peak. A novel finding, however, was the apparent downstream increase in the length of the erosion ‘season’, with measurable retreat occurring at the lower sites from September to July. This is interpreted as a reflection of a richer mix of bank erosion processes at the downstream sites, where mass failure, fluid entrainment and weathering processes are all active, with each process group having its own, but overlapping, temporal (seasonal) domain. Copyright © 1999 John Wiley & Sons, Ltd.
Pore water pressures (positive and negative) were monitored for four years (1996–1999) using a series of tensiometer-piezometers at increasing depths in a riverbank of the Sieve River, Tuscany (central Italy), with the overall objective of investigating pore pressure changes in response to flow events and their effects on bank stability.
The saturated/unsaturated flow was modelled using a finite element seepage analysis, for the main flow events occurring during the four-year monitoring period. Modelling results were validated by comparing measured with computed pore water pressure values for a series of representative events. Riverbank stability analysis was conducted by applying the limit equilibrium method (Morgenstern-Price), using pore water pressure distributions obtained by the seepage analysis.
The simulation of the 14 December 1996 event, during which a bank failure occurred, is reported in detail to illustrate the relations between the water table and river stage during the various phases of the hydrograph and their effects on bank stability. The simulation, according to monitored data, shows that the failure occurred three hours after the peak stage, during the inversion of flow (from the bank towards the river). A relatively limited development of positive pore pressures, reducing the effective stress and annulling the shear strength term due to the matric suction, and the sudden loss of the confining pressure of the river during the initial drawdown were responsible for triggering the mass failure.
Results deriving from the seepage and stability analysis of nine selected flow events were then used to investigate the role of the flow event characteristics (in terms of peak stages and hydrograph characteristics) and of changes in bank geometry. Besides the peak river stage, which mainly controls the occurrence of conditions of instability, an important role is played by the hydrograph characteristics, in particular by the presence of one or more minor peaks in the river stage preceding the main one. Copyright
Riverbank retreat along a bend of the Cecina River, Tuscany (central Italy) was monitored across a near annual cycle (autumn 2003 to summer 2004) with the aim of better understanding the factors influencing bank changes and processes at a seasonal scale. Seven flow events occurred during the period of investigation, with the largest having an estimated return period of about 1·5 years. Bank simulations were performed by linking hydrodynamic, fluvial erosion, groundwater flow and bank stability models, for the seven flow events, which are representative of the typical range of hydrographs that normally occur during an annual cycle. The simulations allowed identification of (i) the time of onset and cessation of mass failure and fluvial erosion episodes, (ii) the contributions to total bank retreat made by specific fluvial erosion and mass-wasting processes, and (iii) the causes of retreat. The results show that the occurrence of bank erosion processes (fluvial erosion, slide failure, cantilever failure) and their relative dominance differ significantly for each event, depending on seasonal hydrological conditions and initial bank geometry. Due to the specific planimetric configuration of the study bend, which steers the core of high velocity fluid away from the bank at higher flow discharges, fluvial erosion tends to occur during particular phases of the hydrograph. As a result fluvial erosion is ineffective at higher peak discharges, and depends more on the duration of more moderate discharges. Slide failures appear to be closely related to the magnitude of peak river stages, typically occurring in close proximity to the peak phase (preferentially during the falling limb, but in some cases even before the peak), while cantilever failures more typically occur in the late phase of the flow hydrograph, when they may be induced by the cumulative effects of any fluvial erosion. Copyright © 2008 John Wiley & Sons, Ltd.
Two diverse fluvial systems show that with time, channels adjust such that the rate of energy dissipation is minimized. One fluvial system, characterized by high relief and coarse-grained sediment, was subjected to an explosive volcanic eruption; the other system, characterized by low relief and fine-grained sediment, was subjected to dredging and straightening. Study of the expenditure of kinetic- and potential-energy components of total-mechanical energy provide an energy-based rationale of the interdependency between processes and forms during channel evolution. Spatial and temporal trends of aggradation and degradation are similar although relative amounts of aggradation in the high-energy system are greatly enhanced by the deposition of large amounts of eroded bank material from upstream reaches. Degradation accompanied by widening is the most efficient means of energy dissipation because all components of total-mechanical energy decrease with time. Widening dominates energy dissipation in the coarse-grained system to offset increases in hydraulic depth caused by incision. In the low-energy fine-grained system, channel adjustment and energy dissipation are dominated by vertical processes because of low relative values of kinetic energy, and because eroded bank sediment is transported out of the drainage basin and does not aid in downstream aggradation, energy dissipation, or channel recovery.Specific energy is shown to decrease nonlinearly with time during channel evolution and provides a measure of reductions in available energy at the channel bed. Data from two sites show convergence towards a minimum specific energy with time. Time-dependent reductions in specific energy at a point act in concert with minimization of the rate of energy dissipation over a reach during channel evolution as the fluvial systems adjust to a new equilibrium.
In this article a statistical multivariate method, i.e., rare events logistic regression, is evaluated for the creation of a landslide susceptibility map in a 200 km2 study area of the Flemish Ardennes (Belgium). The methodology is based on the hypothesis that future landslides will have the same causal factors as the landslides initiated in the past. The information on the past landslides comes from a landslide inventory map obtained by detailed field surveys and by the analysis of LIDAR (Light Detection and Ranging)-derived hillshade maps. Information on the causal factors (e.g., slope gradient, aspect, lithology, and soil drainage) was extracted from digital elevation models derived from LIDAR and from topographical, lithological and soil maps. In landslide-affected areas, however, we did not use the present-day hillslope gradient. In order to reflect the hillslope condition prior to landsliding, the pre-landslide hillslope was reconstructed and its gradient was used in the analysis. Because of their limited spatial occurrence, the landslides in the study area can be regarded as “rare events”. Rare events logistic regression differs from ordinary logistic regression because it takes into account the low proportion of 1s (landslides) to 0s (no landslides) in the study area by incorporating three correction measures: the endogenous stratified sampling of the dataset, the prior correction of the intercept and the correction of the probabilities to include the estimation uncertainty. For the study area, significant model results were obtained, with pre-landslide hillslope gradient and three different clayey lithologies being important predictor variables. Receiver Operating Characteristic (ROC) curves and the Kappa index were used to validate the model. Both show a good agreement between the observed and predicted values of the validation dataset. Based on a qualified judgement, the created landslide susceptibility map was classified into four classes, i.e., very high, high, moderate and low susceptibility. If interpreted correctly, this classified susceptibility map is an important tool for the delineation of zones where prevention measures are needed and human interference should be limited in order to avoid property damage due to landslides.
Sometimes regional meteorological anomalies trigger different types of mass movements. In May 1998, the western Black Sea region of Turkey experienced such a meteorological anomaly. Numerous residential and agricultural areas and engineering lifelines were buried under the flood waters. Besides the reactivation of many previously delineated landslides, thousands of small-scale landslides (mostly the earthflow type) occurred all over the region. The earthflows were mainly developed in flysch-type units, which have already presented high landslide concentrations. In this study, three different catchments — namely Agustu, Egerci, and Kelemen — were selected because they have the most landslide-prone geological units of the region. The purposes of the present study are to put forward the spatial distributions of the shallow earthflows triggered, to describe the possible factors conditioning the earthflows, and to produce the shallow earthflow susceptibility maps of the three catchments. The unique condition units (UCU) were employed during the production of susceptibility maps and during statistical analyses. The unique condition units numbered 4052 for the Agustu catchment, 13,241 for the Egerci catchment and 12,314 for the Kelemen catchment. The earthflow intensity is the highest in the Agustu catchment (0.038 flow/UCU) and lowest in the Egerci catchment (0.0035 flow/UCU). Logistic regression analyses were also employed. However, during the analyses, some difficulties were encountered. To overcome the difficulties, a series of sensitivity analyses were performed based on some decision rules introduced in the present study. Considering the decision rules, the proper ratios of UCU free from earthflow (0) / UCU including the earthflow (1) for the Agustu, Egerci and Kelemen catchments were obtained as 3, 6, and 5, respectively. Also, a chart for the proper ratio selection was developed. The regression equations from the selected ratios were then applied to the entire catchment and the earthflow susceptibility maps were produced. The landslide susceptibility maps revealed that 15% of the Agustu catchment, 8% of the Egerci catchment, and 7% of the Kelemen catchment have very high earthflow susceptibility; and most of the earthflows triggered by the May 1998 meteorological event were found in the very high susceptibility zones.
A large and geographically diverse data set consisting of meandering, braiding, incising, and post-incision equilibrium streams was used in conjunction with logistic regression analysis to develop a probabilistic approach to predicting thresholds of channel pattern and instability. An energy-based index was developed for estimating the risk of channel instability associated with specific stream power relative to sedimentary characteristics. The strong significance of the 74 statistical models examined suggests that logistic regression analysis is an appropriate and effective technique for associating basic hydraulic data with various channel forms. The probabilistic diagrams resulting from these analyses depict a more realistic assessment of the uncertainty associated with previously identified thresholds of channel form and instability and provide a means of gauging channel sensitivity to changes in controlling variables.
Landslides can be caused by storms and earthquakes. Most logistic regression models proposed in recent years have been targeted at rainfall-induced landslides. In areas such as Taiwan, where landslides can be triggered by typhoons (tropical cyclones) and earthquakes, a rainfall-induced model is insufficient because it provides only a partial explanation of landslide occurrence and overlooks the potential effect of earthquakes on typhoon-triggered landslides. This study used landslides triggered by a major earthquake and a typhoon prior to the earthquake to develop an earthquake-induced model and a typhoon-induced model. The models were then validated by using landslides triggered by three typhoons after the earthquake. According to the results, typhoon-triggered landslides tended to be near stream channels and earthquake-triggered landslides were more likely to be near ridge lines. Moreover, a major earthquake could still affect the locations of typhoon-triggered landslides 6 years after the earthquake. This study therefore demonstrates that an earthquake-induced model both sheds light on the environmental factors for triggering landslides, and augments a rainfall-induced model in its predictive capability in areas such as Taiwan.
Implementation of the EU Water Framework Directive (2000/60/EC) has forced regulatory authorities to take an integrated view of the link between water quality and catchment management. In Northern Ireland, concern over progressive habitat degradation of one of the Province's prime salmon rivers (the River Bush) provided the stimulus for a sediment monitoring programme. The aims of this study were to quantify instream sediment loads and to identify sources of fine sediment feeding into the river channel. This information would then be used to justify changes in the way the catchment's resources were managed. Sediment loads were assessed at four sampling stations on the River Bush system over a one-year period (between 18/07/02 and 24/07/03) using sediment collection tubes and pit-type traps. These ranged between 0.196 and 4.98 t yr− 1 for bed load and 1.70 and 102 t yr− 1 for suspended load. A combination of desktop studies (erosion potential mapping), monitoring (visual observations, bank erosion magnitude and yield) and analytical work (sediment fingerprinting) was then applied in order to elucidate links between catchment sediment erosion and downstream sediment delivery. Bank erosion was highest in regions of the catchment with the least cohesive bank materials during high flow conditions. Livestock poaching and peak flows caused damage to banks at a localised scale and led to selective patches of bare land being susceptible to further erosion, augmenting the sediment load (approximately 2% of the annual suspended sediment and 60% of the annual bed load). Drainage maintenance work (60% and 30%), forest clearfell (1% and 2%) and ploughed land (37% and 8%) also contributed to the annual suspended sediment and bed load, respectively. Nine key actions were suggested in order to improve habitat quality in relation to sediment supply in the River Bush. These included wetland restoration; prohibiting drainage maintenance work; strategies to control conifer plantation felling; reducing bare ground; livestock access restrictions; construction site management; macrophyte clearance; employment of a river warden and systematic dissemination of project recommendations to the general public to generate community involvement. It was essential that these actions were both cost effective and practical in order to maximise catchment stakeholder uptake.
Several mechanisms contribute to streambank failure including fluvial toe undercutting, reduced soil shear strength by increased soil pore-water pressure, and seepage erosion. Recent research has suggested that seepage erosion of noncohesive soil layers undercutting the banks may play an equivalent role in streambank failure to increased soil pore-water pressure. However, this past research has primarily been limited to laboratory studies of non-vegetated banks. The objective of this research was to utilize the Bank Stability and Toe Erosion Model (BSTEM) in order to determine the importance of seepage undercutting relative to bank shear strength, bank angle, soil pore-water pressure, and root reinforcement. The BSTEM simulated two streambanks: Little Topashaw Creek and Goodwin Creek in northern Mississippi. Simulations included three bank angles (70° to 90°), four pore-water pressure distributions (unsaturated, two partially saturated cases, and fully saturated), six distances of undercutting (0 to 40 cm), and 13 different vegetation conditions (root cohesions from 0·0 to 15·0 kPa). A relative sensitivity analysis suggested that BSTEM was approximately three to four times more sensitive to water table position than root cohesion or depth of seepage undercutting. Seepage undercutting becomes a prominent bank failure mechanism on unsaturated to partially saturated streambanks with root reinforcement, even with undercutting distances as small as 20 cm. Consideration of seepage undercutting is less important under conditions of partially to fully saturated soil pore-water conditions. The distance at which instability by undercutting became equivalent to instability by increased soil pore-water pressure decreased as root reinforcement increased, with values typically ranging between 20 and 40 cm at Little Topashaw Creek and between 20 and 55 cm at Goodwin Creek. This research depicts the baseline conditions at which seepage undercutting of vegetated streambanks needs to be considered for bank stability analyses.
Limited information exists on one of the mechanisms governing sediment input to streams: streambank erosion by ground water seepage. The objective of this research was to demonstrate the importance of streambank composition and stratigraphy in controlling seepage flow and to quantify correlation of seepage flow/erosion with precipitation, stream stage and soil pore water pressure. The streambank site was located in Northern Mississippi in the Goodwin Creek watershed. Soil samples from layers on the streambank face suggested less than an order of magnitude difference in vertical hydraulic conductivity (K(s)) with depth, but differences between lateral K(s) of a concretion layer and the vertical K(s) of the underlying layers contributed to the propensity for lateral flow. Goodwin Creek seeps were not similar to other seeps reported in the literature, in that eroded sediment originated from layers underneath the primary seepage layer. Subsurface flow and sediment load, quantified using 50 cm wide collection pans, were dependent on the type of seep: intermittent low-flow (LF) seeps (flow rates typically less than 0·05 L min-1), persistent high-flow (HF) seeps (average flow rate of 0·39 L min-1) and buried seeps, which eroded unconsolidated bank material from previous bank failures. The timing of LF seeps correlated to river stage and precipitation. The HF seeps at Goodwin Creek began after rainfall events resulted in the adjacent streambank reaching near saturation (i.e. soil pore water pressures greater than -5 kPa). Seep discharge from HF seeps reached a maximum of 1·0 L min-1 and sediment concentrations commonly approached 100 g L-1. Buried seeps were intermittent but exhibited the most significant erosion rates (738 g min-1) and sediment concentrations (989 g L-1). In cases where perched water table conditions exist and persistent HF seeps occur, seepage erosion and bank collapse of streambank sediment may be significant.
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