421 ROCKEFELLER AVE 2023-05-31 - Laserfiche WebLink

3. OFFSITE ANALYSIS A MGAON This entire project proposes to constructND approximatelyITI TI 3,942 sf of new and replaced impervious surfaces. The following table summarizes the new PGIS and NPGIS proposed for the entire project. PGIS NPGIS Total (sf) (sf) (sf) Existing (To be removed) 0 0 0 Existing (To be replaced) 0 0 0 Existing (To remain) 0 0 0 New + Replaced 1,297 2,645 3,942 Total Site Impervious 1,297 2,645 3,942 Net New Impervious 1,297 2,645 3,942 The new impervious surfaces will be mitigated using on-site stormwater management BMPs including a combined detention system, and soil management. A. UPSTREAM ANALYSIS During the site visit it was observed that there a small off-site area to the east that appeared to drain toward the proposed project area. It was clear that all off-site flows will pass through or around this site and remain that way during construction. Based on the location of the proposed improvements the existing runoff from the upstream area will continue to flow unaffected around this site after the project has been constructed. B. DOWNSTREAM ANALYSIS Runoff from the proposed disturbance area of the existing site sheet flows west/southwest and across the southwestern property lines. Runoff continues to sheet flow south approximately 200 feet and into a detention pond located on the Legion Memorial golf course. Out of the pond, runoff flows north through a control structure and into a combined storm/sewer main that flows southeast approximately 1,800 feet through the Legion Memorial golf course, at which point the downstream analysis was concluded. Based on the fact that runoff from the new impervious surfaces will be stored and released at a rate less than the site runoff in the existing conditions, the downstream system will not be affected by this project. During the site visit and review of the downstream system, no portion of the system appeared to be inadequate. Legion Lots 10& 11 20-0214 April 2020 Revised:April 2021 Page 5