2425 39TH ST ROYELL MANUFACTURING INC 2019-12-09

3. OFFSITE ANALYSIS AND MITIGATION This project proposes to construct approximately 3,240 sf of new plus replaced impervious surfaces. The following table summarizes the new/replaced PGIS proposed for the entire improvements associated with this application (both on-site and off-site). PGIS (sf) NPGIS (sf) Total (sf) Existing N/A N/A - Replaced 0 0 0 New - -New+ Replaced 3,240 0 3,240 Total Impervious 3,240 0 3,240 The new/replaced 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 areas on-site contribute runoff to the area of improvements. The only area that is tributary to the area of improvements is from the west within the City R/W. This area currently flows east and the new improvements will be graded to ensure these flows can pass over the new improvements un-impeded and into the downstream area to the east. B. DOWNSTREAM ANALYSIS Runoff from the proposed area of improvements on-site currently is collected by the on- site drainage system and conveyed to a pond and then discharged east into the BNSF R/W. The new on-site improvements (approximately 450 sf) of replaced impervious surfaces will now drain to the south in to the City R/W before turning east and draining into the BNSF R/W. The off-site improvements within the existing City R/W currently drains east into the BNSF R/W and this will not change because of this project. Permission was not obtained from BNSF to enter their R/W and walk alongside the R/R tracks so the entire downstream system could not be visited at this time. However, during a previous permit application for this property permission was granted and the downstream system was walked and observed at that time. In September of 2012 a downstream site visit was performed in this area and the following was observed: Once the runoff enters the BNSF R/W to the east it continues to flow east until it is collected by a ditch system along the west side of the R/R tracks. The ditch system flows south for approximately 1,800 feet before entering a large wetland system. The culvert connecting the ditch to the wetland system east of the R/R tracks was not found but it was clear based on the water surface elevation on the east and west side of the tracks that the two areas were connected either via a culvert or through a free flow of water through the R/R ballast. Royell Manufacturing 15-0714 July 2017 Page 5