Authors: C. van Rees, M. Chambers, A. Catalano, D. Buhr, A. Mansur, D. Hall, A. Nelson, R. Hawley, B. Bledsoe and N. Nibbelink.

Abstract: Nature-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.

Authors: M. Chambers, R. Lammers, A. Gupta, M. Bilskie and B. Bledsoe.

Abstract: 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.

Authors: M. Chambers, C. van Rees, B. Bledsoe, D. Crane, S. Ferreira, D. Hall, R. Lammers, C. Landry, D. Nelson, M. Shudtz and B. Suedel.

Abstract: 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.

Authors: R. Lammers, M. Chambers and B. Bledsoe.

Abstract: Relocating 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.