The perfect storm : flow through a restored compound channel : Tassajara Creek, Dublin, CA : assessment of the roughness, flow, floodplain conveyance, and compound channel capacity of the restoration of Tassajara Creek from the high-water marks of a 20-year storm
In 1999, Alameda County completed the restoration of a 1-mile stretch of Tassajara Creek in Dublin, California. The project created a compound channel with a low-flow channel capacity of Q5 in the upper and middle reaches, Q2 in the lower reach, and a natural floodplain terrace along all reaches to accommodate the design-estimated 100- year flood of 5,200 cfs. Downstream of the restoration reach is a trapezoidal concrete channel. On December 30th and 31st of 2005, a 20-year storm with a cumulative rainfall of 3.56 inches passed over the Tassajara Creek watershed, generating flows that overtopped the low-flow banks of Tassajara Creek, providing an opportunity to assess flow capacity of the compound channel configuration. We conducted long profile and cross-section surveys along the entire restoration reach, and the first 100 feet of the concrete channel. Using the Manning Equation in a HEC-RAS steady flow model, we used the geometry of the concrete channel and the elevation of the high water marks to estimate the peak flow from the storm as 1,500 cfs. We then back-calculated the roughness coefficient (Manning’s "n") for each compound channel cross-section by matching model water surface elevations to observed elevations. We also compared the design 2-year and 5-year low flow channel water surface elevations to modeled water surface elevations using our calibrated roughness values. Finally, we compared the calculated flow capacity to the original design estimations, and we determined that the compound channel successfully accommodates the 100-year flood, although at one cross section with only 1 foot of freeboard.