A wetland/habitat design plan was created to mitigate impacts caused by the elimination of wetlands during creation of a retail project and associated drainage improvements in Sacramento, CA. These impacts included loss of habitat of the federally-listed threatened giant garter snake (Thamnophis couchi gias).
7Q10 performed a detailed hydrologic analysis of the mitigation site. HEC-RAS was used to develop two hydrologic scenarios: (1) a scenario for the existing site conditions, to establish surface water profiles under 100-year flood conditions; and (2) a scenario for the design plan which included lowering the site elevation to the proposed design conditions, breaching the hibernacula levee to increase the areal and temporal extent of the perennially wet marsh lands, and building giant garter snake hibernacula on the intact levee above the level of the site 100-year floodplain determined in the baseline scenario. Of particular interest in this modeling exercise was the impact of the construction of the hibernacula on the elevation of the floodplain, if any. The predictive numerical model indicated that the impact on floodplain elevation caused by the proposed design would be less than 0.01 foot and therefore, the addition of one-acre hibernacula necessary for giant garter snake winter habitat at the site would not have measurable effect on flood stage.