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3. OFF-SITE ANALYSIS AND MITIGATION <br /> This development is expected to add approximately 1,864 sf of new NPGIS on-site. <br /> Because this project is proposing more than 200 sf and less than 5,000 sf of impervious <br /> surfaces, and is in thecombined sewer area of the city, minimum requirements 1-5 and 7 <br /> are triggered for this,project. The following table summarizes the new PGIS and NPGIS <br /> proposed for the entire project. <br /> PGIS NPGIS Total <br /> (sf) (sf) (sf) <br /> Existing (To be removed) 0 0 0 <br /> Existing (To be replaced) 0 0 0 <br /> Existing (To remain) 71 1,158 1,229 <br /> New+ Replaced 0 1,864 1,864 <br /> Total Site Impervious 71 3,022 3,093 <br /> Net New Impervious 0 1,864 1,864 <br /> The new impervious surfaces will be mitigated using on-site stormwater management <br /> BMPs including a bioretention area and soil management. <br /> B. UPSTREAM ANALYSIS <br /> During the site visit it was observed that no significant off-site area appears to drain <br /> towards the project area.It was clear that all off-site flows will pass through or around this <br /> site and remain that way during construction. Based on the location of the proposed <br /> improvements the existing runoff from the upstream area will continue to flow unaffected <br /> around this site after the project has been constructed. <br /> C. DOWNSTREAM-ANALYSIS <br /> Runoff from the proposed disturbance area of the existing site sheet flows north and west <br /> and across the respective property lines. Runoff from both directions is caught into a catch <br /> basin in the roadway just northwest of the project parcel. Runoff then follows the city <br /> stormwater conveyance system north approximately 650 feet, until reaching 19th St, and <br /> turning east. The stormwater system runs downhill to the east a further 650 feet. At this <br /> point the downstream analysis was concluded. <br /> Based..on.the fact..that..the new impervious.surfaces will be infiltrated, and that this project <br /> proposes to increase the total impervious surfaces on the site by less than 2,000 sf, the <br /> developed flows are expected to be lower than in the existing conditions. During the site <br /> visit and review of the downstream system, no portion of the system appeared to be <br /> inadequate. <br /> Diedrich Addition 21-0224 <br /> Sept 2021 Page 6 <br />