V4 11 Stormwater Report Rev 3 1 26 2023

WATER QUANTITY BASIN #1A — 1.508 CF Detention Basin #1A is a subsurface detention basin with 1,508 cubic feet of storage. The system is comprised of 2 rows of 48" diameter HDPE pipe, each 60 feet in length. The bottom of the basin is at elevation 418.00, along with 1 — 4" x 6.5" orifice. Lastly, a 4' weir is placed at an elevation of 421.40. BASIN #1B — 3,042 CF Detention Basin #1B is a subsurface detention basin with 3,042 cubic feet of storage. The system is comprised of 3 rows of 48" diameter HDPE pipe, each 70 feet in length. The bottom of the basin is at elevation 404.00, along with 2 — 3" x 6" orifices. Lastly, a 4! weir is placed at an elevation of 407.15. BASIN #2 — 16,898 CF Detention Basin #2 is a surface basin with 16,898 cubic feet of storage. The bottom of the basin is set at elevation 414.00 along with 1 — 8" x 15" rectangular orifice. 2— 15” x 21” orifices will be placed at elevation 416.00, and a 4' weir is placed at elevation 418.00. It should be noted that the 100-year storm event reaches an elevation of 417.96, just below the elevation of the weir. The top of the basin is set at elevation 419.00, which allows for 1 foot of freeboard. BASIN #3 — 13,250 CF Detention Basin #3 is another surface basin with 13,250 cubic feet of storage. The bottom of the basin is set at elevation 416.00, where 1 — 6” x 8" orifice will also be placed. 1 — 17” x 16" orifices will be placed at elevation 417.30, and finally a 4' weir will be placed at elevation 420.00. It should be noted that the 100-year storm event reaches an elevation of 419.95, just below the elevation of the weir. The top of the basin is set at 421.00, which allows for 1 foot of freeboard. POST-DEVELOPMENT RUNOFF Using the information described above, the peak flows during the 2-, 10-, and 100-year storm events were calculated. Table 5 below summarizes the peak flows during each storm event. (See attached Hydraflow Output for Proposed Conditions Hydrographs, CN Values, and Time of Concentration Calculation.) Table 5 — Proposed Peak Runoff Rates Target . Storm Event Site Proposed Site Runoff Runoff 2 Year 14.859 cfs 14.72 cfs 10 Year 39.811 cfs 38.24 cfs 100 Year__| 86.688 cfs 86.61 cfs ¢, *°