Graham Place Subdivision Stormwater Report July 2026 <br />PAGE 4 OF 6 <br /> <br />Hydraulic Calculations <br />HydroCAD software was utilized to analyze the treatment, detention, and flood control capabilities of the <br />proposed storm facility. Below are some important parameters utilized in modeling the proposed stormwater <br />infrastructure: <br /> <br />Runoff Method: Santa Barbara Urban Hydrograph (SBUH) <br />Development Area: 1.199 acres total <br />Curve Numbers: 98: Water Surface, HSG D <br />98: Paved Parking, HSG D <br />98: Unconnected Roofs, HSG D <br />80: >75% Grass Cover, Good, HSG D <br />Storm Type: Type 1A 24-Hour <br />Storm Events: Water-Quality = 1.40 inches <br />Flood Control (10-Year) = 4.46 inches <br /> <br />A Flowmaster report was prepared and is included in the Appendix of the report that analyzes the proposed <br />stormwater pipe sizes to show that each pipe has capacity for the design storm (10-Year). <br /> <br />Although the property has two different hydrologic soil group classifications, HSG D in the northern area and <br />HSG B in the southern area, the development areas modeled within the storm analysis were assumed to be HSG <br />D given the 0 in/hr measured infiltration rates. See the basin map in the Appendix of this report for more <br />information. <br />The following table compiles the hydraulic calculation results from the stormwater model: <br /> <br />Table 4: Hydraulic Elevations of Proposed Filtration Rain Garden. <br />Flood <br />Elev. (ft) <br />Open Storage <br />Bottom Elev. (ft) <br />Lowest <br />Overflow Grate <br />Elev. (ft) <br />Lowest Catch <br />Basin Rim Elev. <br />Upstream (ft) <br />Water-Quality <br />Peak HGL (ft) <br />10-Year Peak <br />HGL (ft) <br />423.00 420.00 420.50 424.46 420.19 422.38 <br /> <br />Per Table 4 above, the proposed rain garden meets the requirements prescribed by the ESMM. The facility is <br />not expected to overtop during the flood control storm event and thus protects surrounding infrastructure and <br />residences during the 10-year design storm. <br />The catch basin just downstream of the facility is proposed to include small 1/3 HP pump to empty the rain <br />garden between stormwater events due to the difference in existing grades on site and the tie-in elevation to <br />the public main. The other catch basin rims within the proposed drive aisle draining to the facility were kept <br />higher than the facility’s top bank elevation to limit surcharging through the grates even if the pump were to <br />fail. The invert elevation of the tie-in pipe that routes to Cal Young Rd sits lower than the top of bank, ensuring <br />runoff would likely exit to the public storm system before overtopping the facility and flooding future structures <br />on site.