Monitoring performance of a novel approach to channel design for environmentally- sensitive stormwater management: Lower Silver Creek, California

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Compound channel designs are increasingly being applied in urban watersheds to achieve both stormwater conveyance and ecosystem protection and restoration goals. A compound channel project was constructed on Lower Silver Creek in 2003, and a detailed monitoring program has been implemented to assess project performance and to improve future similar channel designs. Initial monitoring results identified unexpected incision of the low flow channel that occurred during the first high flow after construction but then stabilized during subsequent high flows. Monitoring results also suggest that while geomorphic and habitat objectives have not been fully achieved, it is likely that they will be achieved if current channel evolution trajectories continue. Stormwater conveyance objectives have been achieved. This monitoring effort offers insights regarding how compound channels can be successfully integrated into stormwater management programs so that both conveyance and ecological goals are achieved.

INTRODUCTION
In the U.S., interest in and resources devoted to restoration of ecological, aesthetic, and recreational values of degraded river channels is extremely high (Bernhardt et al. 2005), and stormwater managers are increasingly adopting ecologically-based approaches to channel design instead of traditional engineering approaches that rely heavily on structures placed in and along channels to control natural processes (Palmer et al. 2005; NRC 1992; Evans 1991; Williams 1990). Little more than thirty years ago when the National Environmental Policy Act (NEPA) and the California Environmental Quality Act (CEQA) were just starting to be implemented, stormwater conveyance channel management consisted almost exclusively of channel simplification through channelization, removal of large wood and riparian vegetation, and armoring of banks with rock and concrete (Brown 2000). The focus that NEPA and CEQA provided for environmental concerns in the planning of stormwater conveyance improvement and the resulting interest of the communities changed the perspectives of many flood agencies to start considering multi-purpose projects that incorporated not only protection and mitigation of the environment, but also restoration.

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