Compound Channels for Stream Restoration and Flood Management in Urban and Urbanizing Watersheds: Case Studies of Tassajara Creek, CA, and Lower Silver Creek, CA

Watershed urbanization can increase flashiness of flood peaks, increase magnitudes of floods (due to increased impervious area), and alter sediment transport dynamics in stream channels. In addition, flood control infrastructure confines increased flood peaks within artificially narrow corridors. Together, these alterations to urban and urbanizing streams have resulted in channel incision and disconnection of channels from floodplains. One form of stream restoration that has developed largely in urban watersheds has been the construction of compound channels. In the compound channel restoration approach, incised and / or confined channels are replaced with wider compound channels that have a "functional floodplain" (i.e. a flat, depositional surface that is inundated by high flows) adjacent to a "low flow channel" (i.e. an open water channel that is regularly reorganized by peak flows and is allowed to change with succession of aquatic and riparian vegetation). The primary objectives of these projects have been to maintain or improve high flow conveyance and restore floodplain functions that allow complex aquatic and riparian habitat to develop and persist, thereby improving geomorphic and ecological conditions. Recent compound channel projects on Tassajara Creek (2001), in an urbanizing area of Alameda County, CA, and lower Silver Creek (2003), in a fully urbanized area of Santa Clara County, CA illustrate successes, failures, and lessons learned from this increasingly applied approach to multi-objective stream restoration and flood management. We conducted post project appraisals (PPAs) of these two projects by analyzing design documents, design drawings, as-built surveys, available physical and ecological data from past monitoring and modeling activities, and new monitoring and modeling data conducted for this study to systematically assess performance with respect to geomorphic, ecological, and high flow conveyance objectives shared by these two projects. In 2006, both projects had successfully met high flow conveyance objectives. Both projects also improved geomorphic and ecological conditions relative to pre-project conditions. However, performance with respect to geomorphic and ecological objectives varied both within and between projects. In general, we documented better performance with respect to geomorphic and ecological objectives for the Tassajara Creek project, primarily due to greater land availability for the project than in the fully urbanized lower Silver Creek project area. We also determined that the elevation of the functional floodplain above the low flow channel bed and the width of the functional floodplain relative to the low flow channel width influenced performance of these compound channel projects with respect to geomorphic and ecological objectives.