630 57TH ST SW 2023-06-23 - Laserfiche WebLink

• 3. OFF-SITE ANALYSIS AND MITIGATION This site proposes to construct approximately 6,676 sf of new plus replaced impervious surfaces. *The following table summarizes the new PGIS and NPGIS proposed. PGIS NPGIS Total (sf) (sf) (sf) Existing (TBR) 0 0 0 New+ Replaced 3,170 3,506 6,676 Total Site Improvements 3,170 3,506 6,676 Net New 3,170 3,506 6,676 The new impervious surfaces will be mitigated using stormwater management BMPs including sheet flow dispersion and soil management. A. UPSTREAM ANALYSIS During the site visit it was observed that no offsite area drained toward the project area of disturbance. The stream/wetland flows entering the site form the southwest will pass through or around this site and remain that way after construction. B. DOWNSTREAM ANALYSIS 0 Runoff from the proposed disturbance area of the existing site sheet flows south-southeast. Runoff exits the site over the south property line. This runoff is intercepted by an existing stream/wetland system on the site to the south. This creek flows to the east and crosses the southeast corner of.this parcel. Once the creek leaves the site it turns north within a deep well-defined ravine area that is densely vegetated. The site runoff remains within the creek/ravine to the 'h.mile downstream point. Dr SE. Runoff sheet • th Dr SE and appear frontage of the project site. The pipe appears to direct runoff to the north, however the th Dr SE), however a Based on the fact that this project proposes to construct less than 5,000 sf of impervious surfaces (for each of the two underlying parcels) and that the new roof surfaces will be dispersed on-site or adjacent to the existing wetland/stream system, developed flows are expected to be slightly higher than in the existing conditions. See Figure 5 for a depiction of the project basin and downstream path. Seahurst SP 18-1015 ' May'2020 Page 5