The Use of Geospatial Hydraulic Analysis for the Characterization of Habitat Impacts on Wide, Braided Rivers

The Santa Clara River (Los Angeles and Ventura Counties), draining one of the largest watersheds in Southern California (1,634 sq-mi [4,232 sq-km]), is a wide, braided, non-sinuous system with a high percentage of riparian habitat along its course. Future development in the watershed has the potential to alter hydraulics within the River, significantly modifying or disrupting native habitat. An estimate of the possible impacts of future development on hydraulics, and subsequently on riverine habitat, is important for successful sustainable development within the River corridor. It is understood that hydraulic factors in a river provide a significant contribution to the quantity and type of habitat in the river channel. To evaluate the hydraulic impacts associated with specific, hypothetical channel modifications existing and proposed (Project) conditions hydraulic numerical modeling is used to calculate existing and Project velocities and water depths, for the 2-year through 100-year discharges. Habitat surveys were also made of the Project river reaches. Hydraulic impacts of the Project's floodplain modifications and impacts to the riparian habitats was determined using numerical model output integrated with habitat survey data with GIS software. Potential for erosion within the River can be evaluated by reviewing changes to flow velocities with respect habitat type. Hydraulic impacts were found to vary in magnitude by location and discharge in the River. GIS analysis indicates that change in floodplain areas experiencing velocities above and below a 4 fps (1.2 mps) threshold is expected. The 4 fps (1.2 mps) limit is recognized as a practical boundary between where particular species maintain the ability to resist the mechanical forces of flow. In the present study the largest reductions in floodplain acreage with flows in excess of 4.0 fps (1.2 mps) would be on land presently used for agricultural purposes.