Restoring riffle-pool structure in an incised, straightened urban stream channel using an ecohydraulic modeling approach

Abstract

Streams in urban and urbanizing watersheds are impacted by altered runoff hydrology and sediment yields, floodplain modifications, and channel relocations. One morphological response to urbanization is degradation of riffle-pool sequences. Pools and riffles are fundamental mesohabitat units where many lotic biota have evolved to occupy preferentially, and support diverse biological communities. Restoring self-maintaining pool-riffle structures is essential to the ecological rehabilitation of urban streams when lost. However restoring these structures can be problematic in straightened urban streams constrained by civil infrastructure preventing channel re-meandering. The project goal was to utilize geomorphic, hydraulic, and ecological principles to rehabilitate a study reach in a confined geomorphic setting with a primary emphasis on developing a stable riffle design that improves habitat patch dynamics. A 270-m tree-lined study site was selected on Beaver Creek, Knox County, East Tennessee. Through experimental 3D and 2D hydraulic modeling, a riffle-pool design was developed consisting of removing trees at expanded channel locations, placing 3.8-cm gravel substrate for the riffle bed, and deepening the pool prior to riffle entrance. Riffle-pool maintenance processes of the proposed design included occurrence of shear stress reversal between low- and high-flows, and high-flow acceleration-deceleration from pools to riffles. Cobble was interspersed on riffle surfaces for leaf pack formation. Root wads were positioned at bank locations potentially vulnerable to erosion. River2D provided useful design information to assess pre-construction channel stability and habitat quality. In this case study, an ecohydraulic modeling approach to urban stream restoration is described. Construction of four riffle-pool structures was completed in March 2012, and a geomorphic survey completed in April 2013 observed the riffle structures remained stable even with the project site experiencing eight bankfull events. Post-construction monitoring has shown that the unique design for planform-constrained urban channels has promise for increasing hydraulic habitat diversity and improving biotic integrity in these stressed environments.