1023 100TH ST SW 2022-05-12 - Laserfiche WebLink

• 3. OFF-SITE ANALYSIS AND MITIGATION This residential development is expected to add an assumed 13,540 sf on-site for future private road, driveways and roofs. However, the total effective impervious surfaces is 4,222 sf (see section 7 and 9 of this report). The runoff from the roof surfaces on lots 1 and 2 will be directed to individual rain gardens • proposed on each lot. Due to-site constraints, rain gardens are infeasible for the upper lots 3-5. The driveways and private road will be constructed of permeable pavement. Therefore, these surfaces are considered ineffective. Runoff from the roof surfaces on lots 3, 4 and 5 will be tightlined to perforated stub-out connections, therefore these are the only impermeable surfaces proposed for this project. B. UPSTREAM ANALYSIS There are no significant off-site areas that drain toward the proposed project area. 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. C. DOWNSTREAM ANALYSIS Runoff from the proposed disturbance area of the existing site sheet flows to the southeast over vegetated surfaces towards 100th St SW. Runoff appears to leave the site and flows to the right-of-way of 100th St SW. Runoff is the collected by the existing stormwater conveyance system located on the north side of the road. Runoff is collected by a catch basin which directs runoff via a 12" pipe to the east within the right-of-way: Runoff appears to go through a series of catch basins and pipes to the east, passing beneath Holly Dr. Runoff is then directed south, beneath the roadway and flows down the west side of Olivia Park Rd via a combination of pipes, catch basins and open ditches. Runoff appears to eventually outfall to a low depression area west of Olivia Park Rd, approximately 1,800 feet from the project site. Per the City's GIS map, this low depression area drains to the south and eventually combines with Swamp Creek. See Figure 4 for Downstream Basin. 6