Stream restoration that allows for self-adjustment can increase channel-floodplain connectivity

Streams are often “restored” to reduce sediment loading using one or a combination of practices such as livestock exclusion, riparian plantings, and/or bank reshaping and stabilization. Direct comparisons of how these methods affect stream processes, including channel-floodplain connectivity, over time are essential to informing restoration design. (Channel-floodplain connectivity is the ability of a stream to exchange water, sediment, and nutrients with its floodplain at high flows.) To investigate the impact these stream restoration practices have had on channel-floodplain connectivity, we developed a 2-D HEC-RAS hydraulic model for 3 restoration treatments along an urban and agriculturally impacted stream in southwest Virginia, United States. All 3 treatments excluded cattle in 2009. The farthest upstream treatment, Treatment 1, had no other intervention while the other two, Treatments 2 and 3, were regraded and stabilized, then replanted with native species (completed May 2010). The overhanging banks of Treatment 2 were regraded to a slope of 3:1, while those of Treatment 3 had a flat inset floodplain cut into the bank before sloping the banks at 3:1. During the 11-year monitoring timeline, prior work showed the streambanks in Treatment 1 migrated through both outer bank erosion and inner bank deposition with the autogenic creation of inset floodplains, while Treatments 2 and 3 had minimal bank adjustment. The adjusted geometry of Treatment 1 provided higher floodplain volume, channel-floodplain exchange flows, and flow moving across the floodplain than Treatments 2 and 3. Treatment 3 showed some metrics of higher connectivity than Treatment 2, but there was not uniform agreement between metrics. While the hydraulic analysis indicates a higher channelfloodplain connectivity in Treatment 1, active management of Treatments 2 and 3 has reduced the bank erosion rate and accelerated the riparian forest regrowth, providing other benefits including increased shading, wood supply, and vegetation diversity.