DCCM was contracted by the USACE San Francisco District to complete a hydraulic analysis and conceptual design for a water control structure on Artesian Slough.  The San Jose – Santa Clara Regional Wastewater Facility (SJRWF) discharges effluent to the upper end of Artesian Slough, which terminates in South San Francisco Bay. An engineered levee is proposed to cross Artesian Slough and a gated outfall structure is required at the levee crossing to allow SJRWF effluent discharges to pass while protecting areas behind (south) the levee during extreme high tide events.

A 2D HEC-RAS model was constructed representing the existing site conditions and calibrated to observed water level data above and below the SJRWF weir. A 1D model was also developed and calibrated for rapid simulations of various closure culvert designs. With-project conditions (WPC) 2D and 1D models were developed. The 1D WPC model was utilized to perform the closure design analysis. Sea-level rise (SLR) estimates were computed for the area based on the Coyote Creek tide gage and the USACE high SLR curve. The models were run for current and year-50 tidal signals, adjusted by the SLR curve.  A total opening area of 210 square feet was selected to minimize the increase in water surface elevations upstream of the structure while considering cost-efficiency.

The conceptual design was informed by the hydraulic analysis and looking ahead to maximize cost efficiency, be readily constructable, and minimize maintenance procedures and costs. It consists of six 72-inch by 70-inch culverts for a total opening of 210 square feet. The conceptual design has two sets of 3 culverts spaced closely together near each wing of the closure, with staggered outlets turned 90-degrees inward, which improves construction and maintenance efficiency.

The conceptual design was translated to the 2D HEC-RAS model and run for six cases of varied tidal conditions, SJRWF outflow, and sea level. The water level results from the 2D HEC-RAS model were modified for areas above closure structure to account for the 90-degree bends recommended in the conceptual design. The water level, velocity, and shear stress results from the cases tested in the 2D model of the conceptual design were used to make conclusions regarding closure operations and erosion potential.