Actively incising Lower Tassajara Creek in Dublin, California, was restored as a compound channel in 1999-2000 to mitigate incision and provide flood conveyance capacity to reduce flood risk to an adjacent greenfield residential development. The project benefitted from a wide floodplain corridor set aside during development, allowing a rough vegetated overbank channel with a design capacity to carry the 100-year flood. Several years of cross-sectional channel surveys conducted under an informal monitoring program from 2000 to 2006 indicated that active incision was largely attenuated following the restoration, with some minor incision still evident and low-flow channel complexity just beginning to develop. This research evaluates Lower Tassajara Creek 20 years after its restoration, investigating ongoing channel evolution, incision and aggradation, and floodplain reconnection, and discussing channel conditions in the context of the catchment land use history. I repeated and overlayed a series of cross-section surveys to detect channel incision and overbank deposition, documented field observations of channel condition, and cross-referenced against LiDAR terrain data to detect depositional landforms on the floodplain. I also developed a one-dimensional hydraulic model to better understand current flood conveyance capacity, using channel roughness back-calculated using measured high water marks from a recent storm. Preliminary results suggest overbank deposition has occurred in some locations, particularly in the upper reaches, while shallow slopes and fine sediment in the low flow channel throughout the project indicate a depositional environment. While active incision appears to have halted, entrenched conditions persist in the reach with a deeper low-flow channel sized to carry the 5-year flow. There may be opportunities to improve floodplain connection by adding woody debris to push more frequent flows out of the low flow channel, but additional hydraulic model validation and testing will be required to determine where flood conveyance capacity is sufficient to support such an intervention.

Actively incising Lower Tassajara Creek in Dublin, California, was restored as acompound channel in 1999-2000 to mitigate incision and provide flood conveyancecapacity to reduce flood risk to an adjacent greenfield residential development. Thecompound channel design incorporated wide floodplain terraces, planted with nativeriparian and upland vegetation. Prior geomorphological and ecological studies conductedin the first decade after the restoration project suggested that the project hadsuccessfully halted channel incision and that riparian vegetation was developing. I builtupon the last vegetation study in 2008, recreating the photo monitoring points andresurveying the established vegetation transects for the Tassajara Creek project’snorthern reach. I also used remote sensing to quantify changes in vegetation cover overthe last decade, finding a 63% increase in vegetation cover. Both field and remote sensinganalyses indicated continued tree canopy growth and maturation of the riparianecosystem in this restored urban stream.