In a new paper published this month in WIREs Water, a team including partners from the University of Georgia Institute for Resilient Infrastructure Systems and US Army Corps of Engineers’ Engineering With Nature® program review the myriad of benefits that have been associated with an increasingly popular nature-based solution: levee setbacks.
A levee is an embankment structure placed alongside a river to prevent flooding. Conventional riverine infrastructure, such as traditional levees, often reduce the floodplain around a river, causing ecological harm and flood drainage issues. Levee setbacks are exactly what they sound like: by moving a levee further back from the riverbank, thereby widening the floodplain and riparian zone, managers can improve flood protection and conserve more of the riverine ecosystem. Levee setbacks (LS) may also support water quality and supply, as well as habitat restoration.
Although LS are a very well-studied form of natural infrastructure (NI), implementation of this solution is still sparse– as the paper describes, the “reactive manner” in which they are often placed has resulted in missed opportunities for the full array of benefits they might bring. The team of researchers decided to address this gap between knowledge and implementation by consolidating the literature around LS benefits and highlighting key mechanisms that affect co-benefit delivery.
The paper is organized by three spatial scales: watershed, reach (river subsections and tributaries) and local (includes land parcels). The authors evaluate flood risk management, water quality, water supply, biodiversity conservation, and other socioeconomic and equity considerations at each of these scales. They then review the synergies and tradeoffs between multiple benefits–patterns like how location affects outcomes–to show that levee setbacks do not inherently come with a long list of co-benefits.
To show this in practice, the authors highlight three examples of such trade-offs in LS projects. In short, researchers and practitioners have found that:
- While larger LS provide more space for floodwater storage, groundwater infiltration, denitrification, and sedimentation processes, they are more expensive.
- Although LS can be placed to divert water to nearby populated areas or valuable infrastructure, this may interrupt the ecological communities the water is diverted from.
- Sediment deposition in the channel caused by an LS could reduce navigable channel depths and necessitate additional engineering, such as dredging, to maintain navigation routes.
The bottom line of this paper: while levee setbacks are a promising form of natural infrastructure, the current state of implementation is not handling them like the powerful nature-based solution they are. Levee setbacks, like all engineering projects, must be planned carefully and with intentional prioritization of specific co-benefits to have the impacts they are reported to have.
Check out the paper here: An interdisciplinary overview of levee setback benefits: Supporting spatial planning and implementation of riverine nature-based solutions
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