15 FERN RD 2017-10-30 - Laserfiche WebLink

3. OFF-SITE ANALYSIS AND MITIGATION This site proposes to construct approximately 4,615 sf of new impervious surfaces. The following table summarizes the new PGIS and NPGIS proposed. PGIS (sf) NPGIS (sf) Total (sf) Existing TBR 1,946 1,467 3,413 New + Replaced 1,160 3,035 4,195 Total Site Impervious 1,160 3,035 4,195 Net New -786 1,568 782 The new impervious surfaces will be mitigated using on-site stormwater management BMPs including downspout splash blocks and soil management. A. UPSTREAM ANALYSIS During the site visit it was observed that there was no area which potentially contributes runoff to this site. The existing roadway to the south drains to the vegetated right-of-way and appears to be collected by a very ill-defined ditch. This ditch appears to drain east within the right-of-way. It was clear that all off-site flows will pass through or around this site and will remain that way after 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 Existing Conditions Runoff from the proposed disturbance area of the existing site appears to sheet flow in the easterly direction and leaves the site over the eastern property line. Runoff appears to continue to drain in the northeasterly direction through private properties toward the intersection of Tyee Rd and E Beech St. Based on information available on MapEverett, runoff appears to be collected by a catch basin found at this intersection. The catch basin appears to pipe runoff east to East Fork Pigeon Cr, No. 2. See Figure 5 for a depiction of the project basin and downstream path. Some runoff appears to drain to the very ill-defined shallow ditch located along the north side of Fern Rd within the right-of-way. The vegetated surfaces south of the southern property line appear to drain east to a catch basin found within the right-of-way in the shoulder in front on the adjacent parcel to the east. This catch basin appears to pipe runoff east. Runoff appears to continue east via an existing stormwater system consisting of pipes, catch basins and open ditches and eventually discharges to East Fork Pigeon Cr, No. 2, approximately 800 feet east of the project site. Based on the fact that the new NPGIS runoff will be dispersed on-site and partially infiltrated into on-site soils and that this project only proposes to increase the total impervious surfaces on the site by 782 sf the developed flows are expected to be slightly more than the existing conditions. During the site visit and review of the downstream system no portion of the system appeared to be inadequate. 15 Fern Rd Duplex 16-0302 REVISED: June 2016 Page 5