Levee Setback Research Articles

An interdisciplinary overview of levee setback benefits: Supporting spatial planning and implementation of riverine nature‐based solutions

AbstractNature‐based solutions (NbS, and related concepts like natural infrastructure, Ecosystem‐based Adaptation, and green infrastructure) are increasingly recognized as multi‐benefit strategies for addressing the critical sustainability challenges of the Anthropocene, including the climate emergency and biodiversity crisis. Mainstreaming NbS in professional practice requires strategic, landscape‐level planning integrating multiple sources of benefits and their synergies and trade‐offs. Levee setbacks (LS) are among the best‐studied riverine NbS with recognized benefits for flood risk management, drought resilience, water quality management, recreational opportunities, and ecological restoration for biodiversity. Although awareness of the multifarious benefits of LS as forms of Natural Capital is growing, implementation remains ad‐hoc and opportunistic. To address this critical implementation gap for one major example of NbS, we review and synthesize literature across diverse disciplines to provide an overview of the primary social, economic, and ecological mechanisms that affect the co‐benefit delivery of LS projects. Next, to make this information relevant to NbS practitioners, we link these mechanisms to spatial metrics that can be used to approximate the relative magnitude of project benefits and costs across these mechanisms. Finally, we highlight examples of key synergies and trade‐offs among benefits that should be considered for LS planning. This synthetic approach is intended to familiarize readers with the diverse potential benefits of LS, and provide an understanding of how to select and prioritize potential sites for further study and implementation. Synergies and trade‐offs among important benefit drivers abound, and social equity concerns will be paramount in ensuring the successful implementation of LS and other NbS in the future.This article is categorized under: Engineering Water > Sustainable Engineering of Water Engineering Water > Planning Water Water and Life > Nature of Freshwater Ecosystems

Assessing the IUCN global standard for nature-based solutions in riverine flood risk mitigation

The International Union for Conservation of Nature (IUCN) published their Global Standard for Nature-based Solutions (NbS) in an effort to further a common understanding and successful application of NbS. Our objective is to analyse the applicability of and considerations and advancements in using the IUCN Standard, as very few studies have examined and reflected on its actual application. As method, we applied the IUCN Standard to three case studies of river restoration projects with a focus on flood risk mitigation: (1) Eddleston Water Project, (2) “Room for the River” Deventer Project, and (3) Missouri River Levee Setback Project. Rather than evaluating the case studies itself, we evaluated the outcome to find the strong and weak points of the IUCN Standard. The gathered data (publicly accessible documents, conducted interviews with experts and stakeholders) was analysed and showed the role of the number of documents and interviews available. This determined the outcome of the IUCN assessment. The consultation of project experts has appeared to be an essential step in the data collection, while stakeholder interviews and field visits were less important, but did increase the degree of substantiation and the ease of data collection, respectively. Although restricted by a limited evaluation of flood risk mitigation studies, using the IUCN Standard for an ex-post assessment can provide credibility to project processes (e.g. stakeholder engagement and adaptive management), reveal project strengths and weaknesses, and provide opportunities for the comparison of projects. Hence, the IUCN Standard aptly evaluates process-based aspects of Nature-based Solutions for riverine flood risk mitigation.

Modeling the flood protection services of levee setbacks, a nature-based solution

Levee setbacks are an intuitive nature-based solution that can improve the flood protection services provided by levees and help rehabilitate levee-stressed ecosystems. Their application in professional practice is in part limited by a lack of guidance on how to size setbacks to balance different interests in floodplain land use. To help address this application barrier, we demonstrate how flood hazard and levee failure risk reduction may be modeled with a numerical hydraulic model of a leveed river in the Midwestern USA, as well as how to calculate scaling relationships between levee failure risk reduction and setback size. Our results suggest setbacks can greatly reduce the severity of flood hazards, for example, by alleviating flow bottlenecks and lowering flood heights. Our results also suggest that improvements in levee reliability scale non-linearly with setback size and exhibit diminishing returns. We conclude with a discussion of design insights and cautions to support the application of setbacks in practice.

Modeling the effects of levee setbacks on flood hydraulics

AbstractRelocating levees further back from river channels to increase river–floodplain connection can reduce flood stages and provide a host of co‐benefits. Modeling case studies show the significant potential of large levee setbacks for reducing flood stages; however, the difficulty of comparing between these case studies limits our understanding of how the hydraulic effects of setbacks vary in different settings. We filled this research gap by systematically modeling the hydraulic effects of setbacks across a range of river and flood conditions. We used unsteady, 1D Hydrologic Engineering Center‐River Analysis System models to quantify changes in flood stage, channel velocity, and sediment transport capacity for various setback sizes with different river slopes, widths, floodplain roughness, and flood sizes (peak flows) and durations. Setbacks reduce flood stages within the setback, as well as up‐ and downstream. Channel velocity and sediment transport capacity both increased upstream and decreased within the setback. Channel slope, flood size, and flood duration had the largest influence on hydraulic changes. There are diminishing returns in hydraulic effects with increasing setback size. These results can help guide the design and prioritization of levee setback projects and help set reasonable expectations for the scale of changes to flood hydraulics relative to the size of the reconnected floodplain.

Incorporating the riverscape into models of river–floodplain function

Rivers and their flooded alluvial plains integrate physical, biological, and human processes at the scale of continents. Despite their ecological and economic values, these complex ecosystems are poorly understood and highly modified by humans. A primary problem is that most research in fluvial ecosystems has been conducted in small streams and then scaled up to rivers. Furthermore, the point where a stream transitions into a river is not well understood. Although many conceptual models exist, the role that large river–floodplain complexes play within these frameworks is lacking. These models focus on flooding as a temporary reset to river ecosystems, but floodplains and rivers may continue to interact long after floodwaters recede. We revisit the concept of the riverscape, a unique mosaic of perennially interacting wetland and channel habitats that have unique ecological properties during both non-flood and flooding periods relative to the small tributary streams within the riverscape network. This strong bidirectional interaction within low-lying alluvial plains may define large rivers. To determine whether a riverscape is indeed a useful unit of study for river ecology, conservation, and restoration, baseline conditions with measurable, comparable metrics, such as primary and secondary production need to be established. Responses of these metrics to multiple stressors and restoration such as levee setbacks, wetland mitigation, and dam removals will inform both basic models of riverscape function and future management actions. Because humans currently affect nearly all aspects of the environmental structure and function of riverscapes, human perceptions of riverscape value and threat need to be considered as a fundamental component of riverscape ecology.

Nature-based solutions for leveed river corridors

The conceptual framework for nature-based solutions (NbS) is well developed, however realizing the potential of NbS at scale and in widespread professional practice in infrastructure systems depends on overcoming operational challenges rooted in the historical policies and engineering practices of the action agencies capable of implementation. In this article, we explore levee setbacks as a NbS for improving the sustainability of leveed river corridors within the context of the United States (US) and its primary action agency of flood risk management, the Army Corps of Engineers (USACE). By identifying the social and environmental consequences of historical levee management and linking these consequences with historical policies and engineering practices, we highlight knowledge gaps, challenges and opportunities for progress with NbS. We also briefly discuss USACE’s decision-making processes for infrastructure investments and the valuation of ecosystem services as it pertains to operationalizing setbacks in practice. We then develop a case study of a recent setback on the Missouri River to showcase how USACE overcame implementation challenges. Lessons from past levee corridor management in the US, and USACE’s current corrective actions, may help foster understanding of how to overcome operational challenges in the implementation of setbacks in other social and political contexts.

Show-Me Resilience: Assessing and Reconciling Rural Leaders' Perceptions of Climate Resilience in Missouri.

Rural areas of the United States play a vital role in coping with, adapting to and mitigating climate change, yet they often lag urban areas in climate planning and action. Rural leaders-e.g., policymakers, state/federal agency professionals, non-profit organization leadership, and scholars - are pivotal for driving the programs and policies that support resilient practices, but our understanding of their perspectives on climate resilience writ large is limited. We conducted semi-structured interviews with 23 rural leaders in Missouri to elucidate their conceptualizations of climate resilience and identify catalysts and constraints for climate adaptation planning and action across rural landscapes. We investigated participants' perceptions of the major vulnerabilities of rural communities and landscapes, threats to rural areas, and potential steps for making rural Missouri more resilient in the face of climate change. We found that most rural leaders conceptualized climate resilience as responding to hazardous events rather than anticipating or planning for hazardous trends. The predominant threats identified were flooding and drought, which aligns with climate projections for the Midwest. Participants proposed a wide variety of specific steps to enhance resilience but had the highest agreement about the utility of expanding existing programs. The most comprehensive suite of solutions was offered by participants who conceptualized resilience as involving social, ecological, and economic systems, underscoring the importance of broad thinking for developing more holistic solutions to climate-associated threats and the potential impact of greater collaboration across domains. We highlight and discuss a Missouri-based levee setback project that was identified by participants as a showcase of collaborative resilience-building.

Quantifying the effects of surface conveyance of treated wastewater effluent on groundwater, surface water, and nutrient dynamics in a large river floodplain

Restoration and reconnection of floodplain systems provide multiple societal and ecosystem benefits, while providing municipalities the opportunity to attempt alternative approaches to maintain infrastructure protection and function. In some restored floodplains, treated wastewater effluent discharge is redirected over land instead of directly into rivers to allow natural flow and infiltration, to facilitate restoration designs such as levee setback, and to provide additional freshwater to floodplain ecosystems. However, indirect discharge of treated effluent over land may pose risks to surface and groundwater when pollutants like excess nutrients enter the floodplain and undergo transformation. We investigated the consequences for groundwater and surface water quality when effluent was redirected as open water channels over a floodplain surface. In this study, seasonal floodplain nutrient concentrations in groundwater and surface water were observed for more than 5 years as a floodplain and wastewater treatment plant underwent a major restoration project that included river-floodplain reconnection with levee setback and redirection of effluent discharge from a river channel to open flow across the restored floodplain. Nutrient loading to the surrounding floodplain groundwater and surface water was observed, but based on measures of hydrological connectivity, groundwater flow paths, and biogeochemistry, nutrients from the effluent moved within the floodplain with minimal effect to the surrounding floodplain water quality. We did not find evidence of substantial additional processing that could replace advanced nutrient treatment in this system, however we did observe evidence of diverse nutrient processes that may support enhanced retention if treatment channels were designed to enhance these processes. We suggest that indirect discharge of high quality treated effluent in a restored floodplain is a viable alternative to direct discharge into a river when groundwater flow directs that discharge to habitats where minimal nutrient sensitivity is expected.

Adapting to sea level rise: Emerging governance issues in the San Francisco Bay Region

San Francisco Bay, the largest estuary on the Pacific Coast of North America, is heavily encroached by a metropolitan region with over 7 million inhabitants. Urban development and infrastructure, much of which built over landfill and at the cost of former baylands, were placed at very low elevations. Sea level rise (SLR) poses a formidable challenge to these highly exposed urban areas and already stressed natural systems.“Green”, or ecosystem-based, adaptation is already on the way around the Bay. Large scale wetland restoration projects have already been concluded, and further action now often requires articulation with the reinforcement of flood defense structures, given the level of urban encroachment. While levee setback, or removal, would provide greater environmental benefit, the need to protect urban areas and infrastructure has led to the trial of ingenious solutions for promoting wetland resilience while upgrading the level of protection provided by levees.We analyzed the region’s environmental governance and planning structure, through direct observation, interviews with stakeholders, and study of planning documents and projects. We present two examples where actual implementation of SLR adaptation has led, or may lead to, the need to revise standards and practices or require uneasy choices between conflicting public interests.Among the region’s stakeholders, there is an increasing awareness of the risks related to SLR, but the institutional arrangements are complex, and communication between the different public agencies/departments is not always as streamlined as it could be. Some agencies and departments need to adapt their procedures in order to remove institutional barriers to adaptation, but path dependence is an obstacle. There is evidence that more frank and regular communication between public actors is needed. It also emphasizes the benefits of a coordination of efforts and strategies, something that was eroded in the transition from central-government-led policies to a new paradigm of local-based adaptive governance.

Floodplain restoration increases hyporheic flow in the Yakima River Watershed, Washington

Hyporheic exchange between a river channel and its floodplain region assists in mediating processes such as nutrient removal and temperature regulation. Floodplain restoration in the form of levee setbacks are often carried out to improve the hyporheic exchange. In this study Light Detection and Ranging (LiDAR) data were used along with the head data from observation wells and stage data from rivers to setup and calibrate a groundwater model for 458 km2 of area within Gap to Gap reach of the Yakima River, WA. This area has witnessed several efforts of floodplain restoration in the form of levee setbacks. The groundwater model was used to quantify hyporheic flow emerging from the Yakima River in steady and transient states during pre-restoration (using LiDAR data of 2008) and post-restoration period (after levee setback using LiDAR data of 2013). The comparison of results from the model runs during pre and post-restoration periods showed that the length of the pathlines increased after levee setback for both steady and transient state model simulations. The largest increase of about 62 m was noticed in the month of September 2014 (pre: 398 m and post: 460 m). The study also showed that the direction of the flow changed following levee setback, expanding the area for hyporheic flux exchange between surface and groundwater. The model run during transient state also suggested that pathlines were longer during drier months compared to wet months. Overall, the study showed that levee setbacks improved the hyporheic connection between surface and groundwater in the Yakima floodplain which demonstrates that levee setback can provide a valuable hydrologic tool to restore ecosystem processes in previously leveed rivers.

Tradeoffs of strategically reconnecting rivers to their floodplains: The case of the Lower Illinois River (USA)

During the latter half of the 19th Century and first half of the 20th Century, the Illinois River was heavily altered through leveeing off large portions of its floodplain, draining wetlands, and the construction of dams and river-training structures that facilitated navigation. As a result of these alterations, flood stages continue to rise, increasing flood risk and threatening to overtop levees along the La Grange Segment (LGS) of the Illinois River. Over the last two decades, more emphasis has been placed on reconnecting portions of floodplains to rivers in order to solve the long-term problem of rising flood heights attributed to continual heightening of levees to provide flood protection. Multiple studies have suggested that strategically reconnecting larger portions of the LGS could result in more sustainable floodplain management. However, the true costs and benefits of reconnecting the floodplain are not known. We use a novel hydrodynamic, geospatial, economic, and habitat suitability framework to assess the tradeoffs of strategically reconnecting the Illinois River to its floodplain in order to decrease flood risk, improve floodplain habitats, and limit the costs of reconnection. Costs include building-associated losses, lost agricultural profits, and levee removal and construction costs. Tested scenarios demonstrate that while flood heights and environmental benefits are maximized through the most aggressive levee setbacks and removals, these scenarios also have the highest costs. However, the tradeoff of implementing lower-cost scenarios is that there is less flood-height reduction and less floodplain habitat available. Several individual levee districts have high potential for reconnection based on limiting potential damages as well as providing floodplain habitat. To implement large-scale strategic floodplain reconnection, costs range from $1.2–$4.3 billion. As such, payments for ecosystem services will likely be necessary to compensate landowners for decreased long-term agricultural production and building losses that result in flood-reduction benefits and increased floodplain habitat.

Strategic floodplain reconnection for the Lower Tisza River, Hungary: Opportunities for flood-height reduction and floodplain-wetland reconnection

Summary During the late 19th Century, the Tisza River’s vast floodplain-wetland system was largely disconnected by levees, facilitating “reclamation” for agriculture and resulting in an estimated loss of over 90% of historical wetlands. While levees have been successful in preventing catastrophic flooding for a century, Lower Tisza flood stage records have been set repeatedly during the last 15 years. The decrease in the Tisza’s current floodway carrying capacity has reduced the flood-protection level of the Tisza’s aging levee system. Recently in Hungary, “Room for the River” policies have gained more prominence. To explore the possibilities of a room for the river approach along the Lower Tisza, we assess eight potential floodplain-reconnection scenarios between Csongrad, Hungary and the Hungary–Serbia border. A novel framework using hydrodynamic and geospatial modeling was used to perform planning-level evaluations of the tradeoffs between floodplain-reconnection scenarios and enhancement of the existing levee system. The scenarios evaluated include levee removal and levee setbacks to strategically reconnect significant historical wetlands while reducing flood levels. Scenario costs and human population impacts are also assessed. Impacts of reconnecting the Lower Tisza floodplain are compared to heightening levees, the prevailing strategy over the previous century. From a purely construction-cost perspective, heightening Lower Tisza levees is potentially the most cost-effective and politically expedient solution (i.e., impacts the least number of people). However, levee-heightening does not solve the long-term problem of reduced flood conveyance, which has been attributed to aggradation and increased floodplain roughness, nor does it result in wetland reconnection or enhancement of other floodplain ecosystem services. The suite of reconnection options we evaluate provides engineers, planners, and decision makers a framework from which they can further evaluate potential flood-risk reduction options. At least three of the eight reconnection scenarios (setting the western levee back, 1500-m, and 2000-m setbacks) along the Lower Tisza demonstrate that floodplain-wetland reconnection is possible while achieving the objectives of minimizing impacts on human populations and reducing flood heights.

A historical examination of the Corps of Engineers and natural valley storage protection: the economics and politics of ‘green’ flood control

Abstract Between 1972 and 1994, the US Army Corps of Engineers undertook five studies in New England evaluating the benefits and costs of protecting natural valley storage (NVS) areas—natural reservoirs—for flood mitigation. Only along the Charles River did benefits outweigh costs. Analysis of the studies finds that the costs of large-scale land acquisition will often exceed the sole benefits of avoided flood damages. To generate net benefits, there must be significant amounts of NVS lands still undeveloped, development pressure on those lands, and downstream areas that would sustain large damages. The NVS studies also raised questions of whether the Corps should be involved in land acquisition, and whether regulating land use could substitute for purchasing land. Note, these findings do not apply to other forms of natural flood-risk reduction, such as levee setbacks, green infrastructure for stormwater management, and multi-purpose projects, which have different economic and institutional contexts.

The Economics and Politics of 'Green' Flood Control: A Historical Examination of Natural Valley Storage Protection by the Corps of Engineers

Between 1972 and 1994, the New England Division of the US Army Corps of Engineers undertook five studies evaluating the benefits and costs of protecting natural valley storage (NVS) areas as a flood mitigation strategy in various watersheds. NVS lands function as natural reservoirs, temporarily storing floodwaters. In only one case—along the Charles River in Massachusetts—were the benefits found to outweigh the costs. Along the Charles, the Corps acquired approximately 8,500 acres of floodplain land to keep as open space in perpetuity. This paper reviews the five studies in detail to inform ongoing interest in green approaches to flood control. The analysis finds that large-scale land acquisition to contain major riverine flood events is difficult to justify by avoided flood damages alone. For such a project to generate net benefits, there must be significant amounts of NVS lands still undeveloped, substantial development pressure on those lands, and downstream areas that would sustain high levels of damage in the event of a flood. Perhaps more importantly, however, these studies raise two fundamental institutional questions: (1) Should the Corps, or the federal government more broadly, be involved in land acquisition? (2) Should regulating land use be preferred over purchasing NVS land? The economic and political issues uncovered in the historic examination of these five studies suggest an explanation for the current focus on other approaches to green flood control, such as multipurpose projects, levee setbacks, and green infrastructure to manage stormwater.

Assessment of Missouri River floodplain invertebrates during historic inundation: implications for river restoration

Floodplain connectivity is important to aquatic organisms in large rivers. Anthropogenic alterations regulating the Missouri River have limited connectivity and negatively affected native fauna. Determining the biological response to rare inundation events may be important when considering potential restoration options on a regulated river; thus, we assessed benthic invertebrate and zooplankton communities at three floodplain sites during a historic Missouri River high-water event. Chironomid larvae dominated during most sampling trips and densities were often highest during initial sampling trips with lower densities as high water persisted. Similar trends were evident for rotifer, cladoceran, and copepod densities. Nonmetric multidimensional scaling also showed relatively high dissimilarity of densities between early and late sampling trips for benthic invertebrate and zooplankton communities. As such, short-term inundation may be more beneficial to Missouri River benthic invertebrate (mainly chironomid larvae) and zooplankton production than more prolonged inundation lasting a month or more. Furthermore, restoration projects may be designed at elevations allowing more short-term inundation, which would likely benefit native fishes with additional spawning, nursery, and foraging habitat. Levee setbacks may be an effective restoration option for increasing the amount of habitat available for short-term inundation while potentially providing socioeconomic, flood-risk reduction benefits by enhancing flow conveyance.

Integrative Method for Quantifying Floodplain Habitat

AbstractRestoring functional, self-sustaining floodplain habitats requires explicit consideration of the flow dynamics that once created healthy floodplain ecosystems. This paper describes a new method for quantifying the benefits of river projects by linking the spatial and temporal characteristics of floodplains to define the functional habitat they create. Combining standard hydrologic and hydraulic analysis, habitat can be quantified using area-duration-frequency (ADF) curves for several durations and across multiple frequencies of flood occurrence. From this, a value can be created for expected annual habitat (EAH) analogous to expected annual damages (EAD) used in flood risk analysis. To illustrate this method, a modeling case study was conducted on the Lower San Joaquin River in California. The case study shows that a levee setback to restore floodplain connectivity could benefit splittail fish populations but not rearing salmonids. The development of ADF curves and EAH values is a transparent and ...

Evaluation of levee setbacks for flood-loss reduction, Middle Mississippi River, USA

Summary One-dimensional hydraulic modeling and flood-loss modeling were used to test the effectiveness of levee setbacks for flood-loss reduction along the Middle Mississippi River (MMR). Four levee scenarios were assessed: (1) the present-day levee configuration, (2) a 1000 m levee setback, (3) a 1500 m levee setback, and (4) an optimized setback configuration. Flood losses were estimated using FEMA’s Hazus-MH (Hazards US Multi-Hazard) loss-estimation software on a structure-by-structure basis for a range of floods from the 2- to the 500-year events. These flood-loss estimates were combined with a levee-reliability model to calculate probability-weighted damage estimates. In the simplest case, the levee setback scenarios tested here reduced flood losses compared to current conditions for large, infrequent flooding events but increased flood losses for smaller, more frequent flood events. These increases occurred because levee protection was removed for some of the existing structures. When combined with buyouts of unprotected structures, levee setbacks reduced flood losses for all recurrence intervals. The “optimized” levee setback scenario, involving a levee configuration manually planned to protect existing high-value infrastructure, reduced damages with or without buyouts. This research shows that levee setbacks in combination with buyouts are an economically viable approach for flood-risk reduction along the study reach and likely elsewhere where levees are widely employed for flood control. Designing a levee setback around existing high-value infrastructure can maximize the benefit of the setback while simultaneously minimizing the costs. The optimized levee setback scenario analyzed here produced payback periods (costs divided by benefits) of less than 12 years. With many aging levees failing current inspections across the US, and flood losses spiraling up over time, levee setbacks are a viable solution for reducing flood exposure and flood levels.

Quantifying Activated Floodplains on a Lowland Regulated River: Its Application to Floodplain Restoration in the Sacramento Valley

We describe a process and methodology for quantifying the extent of a type of historically prevalent, but now relatively rare, ecologically-valuable floodplains in the Sacramento lowland river system: frequently-activated floodplains. We define a specific metric the “Floodplain Activation Flow” (FAF), which is the smallest flood pulse event that initiates substantial beneficial ecological processes when associated with floodplain inundation. The “Activated Floodplain” connected to the river is then determined by comparison of FAF stage with floodplain topography. This provides a simple definition of floodplain that can be used as a planning, goal setting, monitoring, and design tool by resource managers since the FAF event is the smallest flood and corresponding floodplain area with ecological functionality—and is necessarily also inundated in larger flood events, providing additional ecological functions. For the Sacramento River we selected a FAF definition to be the river stage that occurs in two out of three years for at least seven days in the mid-March to mid-May period and "Activated Floodplains" to be those lands inundated at that stage. We analyzed Activated Floodplain area for four representative reaches along the lower Sacramento River and the Yolo Bypass using stream gauge data. Three of the most significant conclusions are described: (1) The area of active functional floodplain is likely to be less than commonly assumed based on extent of riparian vegetation; (2) Levee setbacks may not increase the extent of this type of ecologically-productive floodplain without either hydrologic or topographic changes; (3) Within the Yolo Bypass, controlled releases through the Fremont Weir could maximize the benefits associated with Activated Floodplain without major reservoir re-operation or grading. This approach identifies a significant opportunity to integrate floodplain restoration with flood management by establishing a FAF stage metric as an engineering design criterion alongside the commonly-used 100-year flood stage for flood hazard reduction.

Up or Out?—Economic-Engineering Theory of Flood Levee Height and Setback

Levee setback location and height are important issues in flood levee system design and modification. This paper derives an economic-engineering theory of the optimal trade-off of levee setback for height both for original and redesigned flood levees, demon- strating the interconnection of levee setback, height, costs and risks, and economically optimal design. These analyses assume stationary flood hydrology and static ratios among damageable property value, unit construction cost, and land price. The economic trade-off of levee setback for height depends on economic cost and benefit and hydraulic parameters, and only indirectly on flood frequency and economic damage parameters. The redesign rules derived in this paper indicate conditions where existing levees should be raised or moved in response to changes in conditions. Numerical examples illustrate the results. This paper demonstrates several ideas and theory for economic flood levee system planning and policy rather than providing guidelines for direct design practice.

Modeling the influence of river rehabilitation scenarios on bed material sediment flux in a large river over decadal timescales

A stochastic flood generator and calibrated sediment transport formulae were used to assess the decadal impact of major river rehabilitation strategies on two fraction bed material sediment flux and net storage, first‐order indicators of aquatic riverine habitat, in a large river system. Model boundary conditions were modified to reflect the implementation of three major river rehabilitation strategies being considered in the Sacramento River Valley: gravel augmentation, setting back of levees, and flow alteration. Fifty 30‐year model simulations were used to compute probabilities of the response in sediment flux and net storage to these strategies. Total annual average bed material sediment flux estimates were made at six gauged river cross sections, and ∼60 km reach‐scale sediment budgets were evaluated between them. Gravel augmentation to improve spawning habitat induced gravel accumulation locally and/or downstream, depending on the added mixture. Levee setbacks to recreate the river corridor reduced flow stages for most flows and hence lowered sediment flux. Flow alteration to mimic natural flow regimes systematically decreased total annual average flux, suggesting that high‐magnitude low‐frequency transport events do not affect long‐term trends in bed material flux. The results indicate that each rehabilitation strategy reduces sediment transport in its target reaches and modulates imbalances in total annual bed material sediment budgets at the reach scale. Additional risk analysis is necessary to identify extreme conditions associated with variable hydrology that could affect rehabilitation over decades. Sensitivity analysis suggests that sorting of bed material sediment is the most important determinant of modeled transport and storage patterns.