6402 HIGHLAND DR 2022-10-28 - Laserfiche WebLink

3. OFFSITE ANALYSIS AND MITIGATION This entire project proposes to construct approximately 3,487 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) 859 653 1,512 Existing (To be replaced) 0 804 804 Existing (To remain) 0 0 0 New.+ Replaced 718 2,769 3,487 Total Site Impervious 718 2,769 3,487 Net New Impervious 718 1,965 2,683 The new impervious surfaces will be mitigated using on-site stormwater management BMPs including sheet flow dispersion, perforated stubout connection and soil management. A. UPSTREAM ANALYSIS During the site visit it was observed that no significant off-site area appears to drain towards the 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 east/northeast and across the northeastern property lines. Runoff then flows towards Highland Dr and is caught into a storm drain inlet. The stormwater conveyance system then flows north to 63rd St SE, and continues in an eastern direction. At this point the downstream analysis was concluded. Based on the fact that the new impervious surfaces will either be dispersed on-site or tightlined to the existing drainage system and that this project proposes to increase the total:..impervious.„surfaces:_on. the—site..by:.�less,-than_.5,000 sf, the developed flows- are expected to be slightly greater than in the existing conditions. During the site visit and review of the downstream system, no portion of the system appeared to be inadequate. Kedrich SFR 20-0724 August 2020 Page 5