Environmental Impacts of Levees and their Mitigation

Over the last few decades, the construction of riverine levee systems to protect communities from flooding has come under fire from environmental groups because it can result in the elimination of the stream channel’s connectivity with its floodplain. Undisturbed floodplains provide energy dissipation and flood storage for the stream during high water events, reducing velocities and providing areas for sediment deposition. Most construction of levees involves the deepening, widening, and/or straightening of the adjacent channel to obtain hydraulic capacity. When the channel is modified, problems arise due to the disruption of stream equilibrium. If the cross sectional area of the channel is increased, the velocity is reduced and large deposits of sediment are created within the channel. The deposits may drastically reduce the cross sectional area of the channel, increasing the velocity and resulting in the erosion of streambank and streambed material. The velocity of the channel will also increase when a natural meandering channel is straightened, also resulting in erosion. In addition to the environmental damages caused by channel modification, adverse impacts to the project’s flood protection capabilities are likely. Large sediment deposits reduce the capacity of the levee project, increasing the risk of overtopping. Erosion of streambanks has the potential to cut into the levee embankment itself. Many levee owners are not able to provide proper and timely maintenance in removing the sediment bars and repairing erosion. Obtaining permits to remove sedimentation is often challenging. The Pennsylvania Department of Environmental Protection (PADEP) is taking steps to implement natural stream structures into levee channels to control sedimentation and reduce erosion. Several recent projects have consisted of the installation of these structures within levee channels, but they are not without challenges. The structures do not restore the floodplain and the reduced water storage and energy dissipation remains. An area of concern is whether or not natural stream structures could withstand the higher velocities that will occur in levee channels. PADEP plans to discuss the merits of the environmental groups’ claims and will continue to explore solutions that provide both flood protection and environmental enhancement to its streams.