Figure 2-3 - uploaded by Zan Rubin
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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).
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... One aim of hydrological restoration projects in urban watersheds is to mitigate chronic channel erosion by facilitating geomorphic recovery in the receiving stream network (e.g. Stein et al. 2012). We define geomorphic recovery as a return to geomorphic equilibrium (i.e. ...
Stream channel erosion, enlargement, and habitat degradation are ubiquitous in urban watersheds with conventional stormwater management that increase channel-eroding flows relative to undeveloped watersheds. Hydrologic-based restoration aims to discharge a more natural flow regime via stormwater management interventions. Whether such interventions facilitate geomorphic recovery depends, in part, on the degree to which they restrict discharges that would otherwise contribute to channel erosion. Erosion potential (E), the ratio of post-developed to predeveloped sediment transport capacity, provides a simplified, mechanistic framework to quantify the relative influence of stormwater interventions on the geomorphic effectiveness of the flow regime. This paper compiles ca. five years of data following stormwater-based interventions in three distinct settings in the United States and Australia to demonstrate how the E framework can elucidate the role of hydrologic restoration interventions in facilitating trajectories of geomorphic recovery (or lack thereof). In a previously developed watershed with unstable streams, substantial reductions in E in one stream coincided with a trajectory of geomorphic recovery, whereas the control stream without E-reducing interventions exhibited continued instability. Furthermore, a stream downstream of a greenfield development that optimized their stormwater control measures to match the sediment transport capacity of the predeveloped regime (E = 1) was able to maintain a recovery trajectory in a legacy-impacted setting that is otherwise highly susceptible to hydromodification. Streambed material size, channel evolution stage, and the hydrogeomorphic setting also likely affect the level of E reduction necessary to promote geomorphic recovery, with coarser-grained and over-widened streams potentially needing less reduction than finer-grained and more entrenched channels. Although available space and funding will limit the ability to fully reduce E in previously developed watersheds, these case studies underscore the value of using stormwater control measures to maximize reductions in E if geomorphic stability is a goal of stormwater interventions.
... One aim of hydrological restoration projects in urban watersheds is to mitigate chronic channel erosion by facilitating geomorphic recovery in the receiving stream network (e.g. Stein et al., 2012). We de ne geomorphic recovery as a return to geomorphic equilibrium (i.e. ...
Stream channel erosion, enlargement, and habitat degradation are ubiquitous in urban watersheds with conventional stormwater management. Hydrologic-based restoration aims to discharge a more natural flow regime via stormwater management interventions. Whether such interventions facilitate geomorphic recovery depends, in part, on the degree to which they restrict discharges that would otherwise contribute to channel erosion. Erosion potential (E), the ratio of post-developed to predeveloped sediment transport capacity, provides a simplified, mechanistic framework to quantify the relative influence of stormwater interventions on the geomorphic effectiveness of the flow regime. This paper compiles ca. five years of data following stormwater-based interventions in three distinct settings in the United States and Australia to demonstrate how the E framework can be used to elucidate the role of hydrologic restoration interventions in helping to facilitate trajectories of geomorphic recovery (or lack thereof). In a previously developed watershed with unstable streams, substantial reductions in E coincided with a trajectory of geomorphic recovery, whereas our case study that did not reduce E between the study periods exhibited continued instability. Furthermore, a greenfield study site that used the E framework to optimize their SCMs to match the sediment transport capacity of the predeveloped regime (E = 1) was able to maintain a recovery trajectory in a legacy-impacted setting that is otherwise highly susceptible to hydromodification. Although available space and funding will limit the ability to fully reduce E in previously developed watersheds, these case studies underscore the mechanistic value of using stormwater controls to maximize reductions in E if geomorphic stability is a goal of stormwater interventions. Streambed material size and channel evolution stage also likely affect the level of E reduction necessary to promote geomorphic recovery, with coarser-grained and/or over-widened streams potentially needing less reduction than finer-grained and/or more entrenched channels.
... Hydrologic mitigation may, therefore, need to be paired with sediment supply augmentation (e.g. bypass around structures or direct replenishment) (Houshmand et al., 2014;Russell et al., 2019a) or coarse sediment source protection ( Stein et al., 2012) to encourage recovery. In a perfect world, flow mitigation would perfectly mimic the natural flow regime for a given catchment through a combination of infiltration, evaporation, reuse and detention strategies. ...
For streams draining urban catchments, sediment transport capacity is the key driver of physical impacts including bed sediment removal and channel incision. The main unanswered question is the relative role of flow alteration compared to sediment supply in influencing sediment transport capacity. With this objective, we computed sand and gravel bed sediment transport capacity using the Wilcock and Ken-worthy two-fraction bedload transport relation for nine streams in catchments covering a gradient of urbanisation. Computations were done for typical natural bed surface material, based on conditions in the least urban study streams. We compared transport capacity distributions and cumulative transport capacity over one-year between the streams. Transport capacity was up to three orders of magnitude higher in urban streams than in forested-catchment streams. This was driven overwhelmingly by the urbanisation-induced alterations to the flow regime, with only minor feedback from channel form changes. Transport capacity was two to three orders of magnitude greater than measured bedload transport in all but the least urban streams. This excess bedload transport capacity mobilises and removes bed sediment, produces channel incision and enlargement and reduces channel complexity. Rebalancing transport capacity with sediment supply therefore requires significant flow mitigation towards pre-urban conditions. Other responses, which may theoretically help to regain this balance-channel widening, grade control, increasing roughness, sediment augmentation-are either inappropriate or only feasible following flow mitigation measures.
... It is this sediment limitation that, coupled 652 with dramatically increased transport capacity, contributes to bed sediment scour and channel 653 Given this context, sediment supply limitations are likely to remain a constraint to stream channel 671 recovery, even under better hydrological management regimes. It seems likely, therefore, that source 672 control measures will need to be paired with coarse-grained sediment bypass or specific 673 replenishment arrangements to allow stream substrates and disturbance regimes to return to more 674 natural conditions (Stein et al., 2012). A major design challenge exists in developing such bypass 675 arrangements while keeping harmful and potentially contaminated urban fine-grained sediment out 676 37 of streams (Taylor and Owens, 2009). ...
Coarse-grained sediments supplied to a stream, in concert with the flow regime, play an important role in channel form and functioning, but are poorly understood in urban catchments. Improved knowledge of coarse-grained (>0.5 mm) sediment sources and supply rates will underpin strategies to mitigate impacts of urbanization on streams. We quantified key hillslope (i.e. non-channel) sources of sediment in urban areas by monitoring coarse-grained sediment yields from nine street-scale stormwater catchments over one year. From our observations, we developed a suburban hillslope sediment budget and a conceptual model of the response of hillslope coarse-grained sediment supply to different levels of urbanization. Coarse-grained sediment supply from the urban land surface was substantial. The highest unit-area yields came from infill construction sites (2800 kg/ha/yr), followed by gravel surfaces (740 kg/ha/yr), grass/mulch surfaces (84 kg/ha/yr), then impervious surfaces (21 kg/ha/yr), with the latter still producing yields far above background conditions. In typical suburban catchments grass and mulch surfaces and construction areas were key sources, with gravel and impervious surfaces making smaller contributions. Small source areas were important, for example construction produced 32% of sediment from 0.5% of the area. Connectivity of sediment sources to impervious surfaces, and hence to drainage systems, was important in driving sediment yields. Our conceptual model indicates that hillslope coarse-grained sediment supply increases with urbanization from natural to suburban conditions as connectivity increases, then declines with higher levels of urbanization as sources become scarcer. Impervious surfaces provide sources and supply pathways of coarse sediment, but also increase sediment transport capacity, causing severely supply-limited conditions and reducing the persistence of bed sediments in streams. When reducing hydrological connectivity to address the urban flow regime, consideration should be given to maintaining coarse-grained sediment supply through bypass or replenishment arrangements, to help reduce stream degradation and maintain form and functioning.
... Poor management of water resources has resulted in substantial loss of biodiversity, fisheries, and other amenities around the world ( Vorosmarty et al. 2010;Richter et al. 2006;Cardinale et al. 2012). As a result, most jurisdictions in California and many other states are required to address the effects of hydromodification as conditions of either a municipal stormwater permit or a statewide general permit ( Stein et al. 2012). ...
... Most models have not been able to satisfactorily address the issues of screening (i.e., identifying the extent of hydromodification at a site) and predicting hydromodification. Therefore, rather than relying on a single model, a toolbox with multiple models used simultaneously to assess the complex hydromodification responses is usually recommended to the management community ( Stein et al. 2012;Stein and Bledsoe 2013). An improvement in the ability to predict sites that are susceptible to hydromodification can en- hance watershed planning and management, such as disconnecting imperviousness and implementing low-impact development to control runoff. ...
Hydromodification is a serious management concern in semiarid regions and is expected to become worse with land use and climate change. Potential stream channel responses range from increased or decreased sediment loads, incision, and dramatic nonequilibrated channel enlargements. The prevalence of hydromodification, particularly in semiarid regions, creates a need for new predictive tools that can support decisions aimed at reducing or mitigating hydromodification effects in a large geographical region. Existing models to screen and predict hydromodification are limited in terms of performance and require time and data. This paper examines three nonlinear learning algorithms—support vector machines, artificial neural networks, and random forests—for predicting changes in stream channel morphology as an indicator of hydromodification. The authors explore the ability of each algorithm to rank the important variables that explain the degree of channel response for streams located in Southern California. Results suggest that variables pertaining to the stream bank morphology, such as bank angle, maximum bank height, and bottom widths, rank high in their ability to predict hydromodification. Among the three algorithms, random forest performance is robust compared to artificial neural networks and support vector machines, given its ability to accommodate
small data sample sizes and minimal data preprocessing. The study shows that for complex responses, such as hydromodification of stream channels, preprocessing will continue to be a necessary step for nonlinear algorithms. With proper guidance, there is potential for using nonlinear algorithms to assess stream reaches vulnerable to hydromodification and inform management decisions to manage streamflow and runoff in Southern California.
... Geological/topographic context, phase of urbanization, and sediment caliber (e.g. fine or coarse-grained) all influence changes to sediment load, sediment size distribution and diversity (Hawley et al., 2013a;Stein et al., 2010;Vietz, 2013). Given this understanding of response, sediment management (and investigations) must be targeted for contexts and sediment characteristics. ...
The urban stream syndrome is an almost universal physical and ecological response of streams to catchment urbanization. Altered channel geomorphology is a primary symptom that includes channel deepening, widening and instability. While the common approach is to treat the symptoms (e.g. modifying and stabilizing the channel), many stream restoration objectives will not be achieved unless the more vexing problem, treating the cause, is addressed in some way. Research demonstrates that the dominant cause of geomorphic change in streams in urban catchments is an altered flow regime and increase in the volume of stormwater runoff. Thus, managers can choose to treat the symptoms by modifying and controlling the channel to accommodate the altered flow regime, or treat the cause by modifying the flow regime to reduce the impact on channel morphology. In both cases treatments must, at the least, explicitly consider hydrogeomorphology—the science of the linkages between various hydrologic and geomorphic processes—to have a chance of success. This paper provides a review of recent literature (2010 to early 2015) to discuss fluvial hydrogeomorphology in the management of streams subject to urbanization. We suggest that while the dominant approach is focused on combating the symptoms of catchment urbanization (that we refer to as channel reconfiguration), there is increasing interest in approaches that attempt to address the causes by using stormwater control measures at a range of scales in the catchment (e.g. flow-regime management). In many settings in the oft-constrained urban catchment, effective management of stream morphology may require multiple approaches. To conclude, we identify five research areas that could inform urban hydrogeomorphology, one of the most challenging of which is the extent to which the volume of excess urban stormwater runoff can be reduced to mitigate the impact on stream geomorphology.
... Stream channel responses to urbanization can vary widely depending on geomorphic and watershed setting, making it necessary to avoid one size fits all management approaches and to develop improved management practices (Bernhardt and Palmer, 2007; Chin and Gregory, 2009; Bledsoe et al., 2012; Shoredits and Clayton, 2013 ). Most hydromodification management solutions consist of site-based runoff control rather than longterm management strategies that integrate a regional or watershedscale approach (Stein et al., 2012) ...
... Implementing such a scheme in conjunction with low impact development (LID) technologies that promote on-site water reuse, infiltration, and slow runoff at the source would likely prove to be more beneficial for both stability and ecologic function. Until the efficacy of such an approach in maintaining stream integrity is empirically demonstrated (sensu Poff et al., 2010), post-construction monitoring and adaptive management may be warranted (Stein et al., 2012). ...
Sediment in urban stormwater systems creates a significant maintenance burden, while a lack of coarse-grained bed sediment in streams limits their ecological value and geomorphic resilience. Gravel substrates, for example, provide benthic habitat yet are often scoured from the channel bed only to end up in a detention basin or treatment wetland. This dual problem of both ‘too much’ and ‘too little’ coarse-grained sediment reflects a watershed sediment budget that is profoundly altered. We developed a conceptual urban coarse-grained (> 0.5 mm) sediment budget across three domains: hillslopes (urban land surfaces), the built stormwater network and stream channels. We then quantified key sources, sinks and storages for a suburban case study, using a combination of hillslope and in-channel monitoring, and interrogation of local government records. Around 36% of the sediment supplied to the stormwater network reached the catchment outlet, a level of sediment delivery much higher than observed in similar-sized natural catchments. The remainder was deposited in the sediment cascade and either stored, or extracted and removed from the catchment (e.g. material deposited in sediment ponds and gross pollutant traps). Conventional urban drainage networks are characterised by high hillslope sediment supply and low storage, resulting in efficient sediment delivery. Channel erosion, deposition in (and extraction from) pipes and channels, and floodplain deposition are small compared to sediment transport through the cascade. An understanding of the sediment budget of urban headwater catchments can provide stormwater and waterway managers with the information they need to address specific sediment problems such as sedimentation in stormwater assets and geomorphic recovery of urban streams.
