When rivers are denied the space to meander due to levees, rock revetments, or other impediments, many beneficial river services are diminished. Setback levees increase channel capacity for carrying floodwaters. Once a levee is setback, the river may begin to meander and this poses a challenge to implementing riparian restoration on the floodplain.
Along many major rivers, levees have been constructed close to the edge of the river channel, which maximizes the amount of land protected by a levee. By placing levees close to the channel, rivers become more effective conduits for drainage. It can also maximize the use of surrounding lands, even in times of high water levels.
However, levees close to the channel can create a set of problems and challenges. Because they greatly narrow the area available to transport floods, they do work to rapidly flush floodwaters and sediments through the system – but this means that the levees are exposed to high-velocity water along their “wet” side. This can result in erosion and high maintenance costs. In many places, the growing list of sites needing repair has outstripped the maintenance budget, resulting in levees that are more likely to fail during a flood (Leavenworth 2004; American Society of Civil Engineers 2009).
Levees close to a river also dramatically restrict the area of floodplain that benefits from periodic connections with the river and constricts the ability of the river to meander and create new river- floodplain habitats. Because of the vulnerability to erosion mentioned above, these levees often require armouring to prevent erosion and meandering, further diminishing the natural habitat values of the river’s edge, which is generally the most biologically valuable habitat. Also, while levees may prevent flooding at one location, they may increase the risk of flooding upstream and/or downstream of the levees. Moving levees back away from the channel - often called “setback levees” - can alleviate these problems.
Benefits
Setback levees increase channel capacity for carrying floodwaters. By increasing conveyance through a section of river, setback levees can relieve “bottleneck” points on a river where floodwaters would tend to back up and potentially cause flooding.
While levees close to the channel are exposed to deep, high-velocity water during floods, setback levees are less frequently exposed to floodwaters because of the increased channel capacity. Further, because flow over floodplains is generally much shallower and slower than rivers, when setback levees are exposed to floodwaters they are less vulnerable to erosion
Co-benefits
In addition to flood-management benefits, setting levees back increases the area of floodplain exposed to periodic inundation from the river, thus increasing the variety of benefits from river-floodplain connectivity. The expanded area on the “wet side” of the levee provides greater room for the channel to meander and create floodplain habitat features, such as wetlands and forests. During overbank flooding, floodwaters spread out on floodplains and, due to slower water velocities on the floodplain, much of the sediment in transport is deposited there. Because nutrients such as phosphorous are largely adsorbed to sediment particles, this deposition can reduce the loads of sediment and some nutrients in rivers and thus improve water quality for downstream water bodies, such as estuaries and near-shore marine habitats (Noe and Hupp 2005). Biogeochemical processes within floodplain wetlands, such as denitrification, can also reduce nitrogen loads in river water (Burt and Pinay 2005; Valett et al. 2005).
During overbank flooding, a portion of floodwaters can percolate into the shallow groundwater. Portions of the reconnected floodplain can continue to be used for agriculture, with crop selection varying by expected inundation frequency.
Costs
The primary costs for levee setbacks are the removal and construction of levees and, potentially, the purchase of title or easements on the reconnected floodplain. If a levee needs to be replaced or rebuilt anyhow, then the primary costs are for the difference in land area no longer protected by a levee and now prone to periodic flooding. Because the reconnected floodplain can provide habitat and other benefits, conservation funding can be combined with flood-management funding to implement these projects. For example, funds for river restoration were committed to a proposed levee setback project on the Sacramento River in California, USA (Opperman et al. 2011).