Graham Place Subdivision Stormwater Report July 2026 <br />PAGE 2 OF 6 <br /> <br />soils infiltrate at a rate less than 2 in/hr. Therefore, infiltration is not proposed to be an outlet option for <br />stormwater discharge on site. For more information, refer to Core Geoengineering’s geotechnical report within <br />this report’s Appendix. <br /> <br />The static groundwater depth ranges from 23 ft – 40 ft bgs based on local well log data as stated in the <br />geotechnical report. The rain garden’s open storage bottom is roughly 4 ft below bgs. If groundwater is <br />encountered during construction of the storm facility, the facility’s design will be updated to include an <br />impermeable liner for groundwater to not surcharge up through the facility. <br /> <br />Site topography indicates minimal slope from the south side of Basin #4 to the north side of Basin #1. The <br />property is 1.420 acres; however, the proposed development area draining to the storm facility is only 1.199 <br />acres. See the attached basin map at the end of this report for more information on basin locations. Given the <br />0 in/hr measured infiltration rate, the existing conditions were modeled with an HSG D classification even <br />though there were multiple hydrologic soil groups present on site per Web Soil Survey. Below is a table <br />summarizing the parameters used for analyzing the existing condition: <br /> <br />Table 1: Existing Site Physical Characteristics. <br />Land Type & Description Total Acreage Curve Number TC (mins) <br />Brush, Fair, HSG D 0.992 77 <br />13.1 Unconnected roofs, HSG D 0.057 98 <br />Paved parking, HSG D 0.122 98 <br /> <br />The time of concentration above comes from NRCS equation 15-9 (below) for maximum sheet flow lengths of <br />less than 100 feet and greater than 0.10 ft flow depths: <br /> <br />𝒍=𝟎𝟎𝟎√𝑺 <br />𝒏 <br /> <br /> where: <br /> n = Manning’s roughness coefficient <br /> l = limiting length of flow (ft) <br /> S = slope (ft/ft) <br /> <br />Using the equation above while assuming a grassed flow surface and an ambient slope of 1.40% along the <br />longest water path, the resulting maximum sheet flow length is 29 ft, which gives a time of concentration of 9.5 <br />minutes. Using the same land slope, a velocity factor of 7.0 ft/sec for short grass pastures, and a resulting flow <br />length of 181 ft on the same flow path as used before, the additional time of concentration through shallow <br />concentrated flow is 3.6 minutes. Together these values add to 13.1 minutes total. <br /> <br />Post Construction <br />To work with the proposed site layout and existing grading, it is proposed to route stormwater runoff from <br />basins #1 - 4 into the proposed filtration rain garden. The facility is designed to provide adequate storage for