Graham Place Subdivision Stormwater Report July 2026 <br />PAGE 3 OF 6 <br /> <br />runoff generated during larger storm events, detain outflow rates to pre-construction levels, and provide <br />treatment of runoff from the water-quality storm event by using the imported growing medium layer and rain <br />garden planting scheme. Stormwater facilities are designed in accordance with requirements set in the ESMM <br />(2025). The table below summarizes the physical characteristics of the rain garden: <br />Table 2: Filtration Rain Garden Open Storage Area Characteristics. <br />Top of Open Storage Elev. (ft) 423.00 <br />Top of Open Storage Area (sf) 2,841 <br />Bottom of Open Storage Elev. (ft) 420.00 <br />Bottom of Open Storage Area (sf) 1,245 <br />Maximum Side Slopes 3:1 <br />Lowest Overflow Grate Elevation (ft) 420.50 <br />Open-Storage Depth (ft) 3.0 <br />Bottom Width (ft) 29.0 <br />Bottom Length (ft) 42.9 <br />Bottom Slope (%) 0.00 <br /> <br />Although the rain garden’s open storage is 3’ deep, the top of the facility is proposed to be offset from adjacent <br />pedestrian paths with a flat distance between them, and the landscaped side slopes will be constructed at a <br />maximum slope of 3:1. Therefore, it is not proposed to install fencing around the facility. <br /> <br />In addition to the open storage area provided above, the facility is proposed to have 12” of growing medium <br />per the ESMM, as well as a 12” rock chamber layer to provide storage for larger storm events. The rock chamber <br />will also contain underdrain pipes due to low infiltration rates measured on site as mentioned above. The <br />following table summarizes storage capacities of the facility: <br /> <br />Table 3: Rain Garden Volume Capacities. <br />Open Storage Capacity <br />– 100% Void Space (cf) <br />Growing Medium Capacity <br />– 10% Void Space (cf) <br />Rock Chamber Capacity <br />– 35% Void Space (cf) <br />Total Storage <br />Capacity (cf) <br />6,003 125 436 6,563 <br /> <br />The solid lid catch basin just downstream of the rain garden is proposed to include a small 1/3 HP pump to work <br />with the site’s existing grading while still being able to tie into the existing public system with gravity flow. The <br />facility’s overflow structures will consist of an atrium dome grate atop a 4” standpipe at the 420.50’ elevation, <br />as well as an atrium dome grate atop another 4” standpipe at the 422.00’ elevation. A 6” underdrain perforated <br />piping system will be installed within the rain garden’s rock chamber to route water out of the facility and into <br />the existing downstream system on site. For more information, refer to the plan set prepared by A&O <br />Engineering included with this tentative subdivision submittal. <br />The previous site improvements included an underground stormwater pipe system that routes to the existing <br />15” storm main within Cal Young Rd. Discharge from the proposed rain garden will route to this system via an <br />extension of this underground piping within the property boundary.