Drainage Report and Supplement 82625

SUMMARY OF DRAINAGE CALCULATIONS The drainage calculations are intended to document that the stormwater runoff quantity provisions of the Ordinance are met, the drainage pipe sizes proposed are adequate, and to provide the carrying capacity of the existing open riprap channel in Nicholas Avenue as requested by the Township Engineer. The reader will notice that the computer modeling output was prepared by Stonefield Engineering & Design LLC. I engaged Stonefield to assist with computer modeling. Stonefield is also serving as the Professional Planning Consultants on this project. Stonefield performed the calculations at my direction and their output was reviewed and approved by me. The flow paths for the times of concentration for Existing Drainage Area E-1 and Proposed Drainage Area P-1 vary somewhat while the flow paths and times of concentration for Existing Drainage Area E-2 and Proposed Drainage Area P-2 are virtually identical. The stormwater runoff from the existing and proposed drainage areas was calculated for the 2-, 10-, and 100-year return period storms for the required 2020 rainfall totals and for the required projected 2050 rainfall totals. The runoff curve numbers for the existing conditions were assigned with the assumption that all existing surfaces are in “good” condition. To demonstrate compliance with the runoff quantity requirements the existing and proposed runoff hydrographs were compared in a “curve under curve” analysis for each of the required return periods. In all instances the proposed runoff hydrographs remain “under” the existing runoff hydrographs. Therefore, the project complies with the requirements. We believe that the runoff hydrographs for the proposed conditions may be less than the calculated hydrographs indicate. The runoff curve number for the proposed sand riding arena was kept somewhat high to provide a factor of safety in the calculations, especially for the lower return period storm events. The 3” depth of stone below the invert of the perforated drains in the riding arena will still hold water in the stone voids after the stored water above the inverts has drained out. The Soil Survey data indicates that the lowest expected infiltration rate for the existing soils will be 0.06 inches per hour. If the 3” stone depth below the drain inverts was actually 100% water, it would drain within 50 hours. The calculations

expected infiltration rate for the existing soils will be 0.06 inches per hour. If the 3” stone depth below the drain inverts was actually 100% water, it would drain within 50 hours. The calculations also indicate that the proposed pipe sizes for the leader drain system and for the 15” parking lot drain are more than adequate to carry runoff from the 25 year return period storm. The capacity of the riprap channel was evaluated at a flow depth of 15 inches, even though the depth of the channel is deeper. A flow depth of 15 Inches was chosen because the outlet pipe is a 15” diameter pipe. The calculated flow capacity for the channel is 47.68 c.f.s. which is obviously greater than the capacity of the downstream 15” pipe. Page 3.