3505 UPLAND AVE 2020-09-08 - Laserfiche WebLink

• 3. OFFSITE ANALYSIS AND MITIGATION Total Short Plat: This short plat proposes to construct approximately 12,545 sf of New+ Replaced impervious surfaces. The following table summarizes the PGIS and NPGIS proposed. PGIS (sf) NPGIS (sf) Total (sf) Existing 1 ,712 1,760 3,472 Removed 1,636 295 1,931 Replaced 76 1 ,465 1,541 New 4,078 6,926 11,004 Total On-Site Impervious 4,154 8,391 12,545 at Project Completion The new and replaced impervious roof surfaces on each lot for this project will be mitigated using on-site stormwater management BMPs including sheet flow dispersion for driveways, soil management will be used for permeable areas. All runoff from the new/replaced impervious surfaces along with the landscaping along the south side of the future SFRs will be conveyed to the proposed detention facility prior to release from the site via the existing pipe conveyance system that drains to the northwest. A. UPSTREAM ANALYSIS During the site visit it was observed that due to the site grades and surrounding development/improvements that the areas contributing runoff to the project site was from the roadways to the south and east as well as the private 12 inch concrete culvert that drains into the drainage ditch along the eastern property line. Said ditch does not drain well and is subject to filling beyond the capacity of the ditch. Said ditch spills over onto the landscaping-of the project site and sheet flows towards the stream located on the adjacent - parcel to the west. One of the requirements of this project is to improve the conveyance system, to properly convey the upstream runoff to the existing stormwater conveyance system located within the right of way of Upland Ave, in front of Lot 1. It was clear that all other off-site flows will be allowed to pass through or around this site and will remain that way after construction. B. DOWNSTREAM ANALYSIS Runoff from the proposed disturbance areas of the three lots sheet flows west over flat to moderate landscaped slopes, and leaves the project site over the western property line. On the adjacent landscaped lot a 12 inch PVC storm sewer pipe outfalls into the beginning of a ditch/stream. Runoff from the project site drains into this stream. Said stream continues to flow as an open ditch through several parcels for approximately 700 feet until it is captured by a catch basin located on the south side of Mukilteo Blvd. Said catch basin- directs asindirects the runoff west through a series of pipes and catch basins for approximately 800 feet. At this point the runoff is conveyed down a steep slope via a corrugated polyethylene pipe of.unknown diameter. Said pipe outfalls to a stream located at the base of the steep slope. This stream is a non-fish bearing stream, flows northwest for approximately 850 feet and drains directly to Puget Sound. Based on the on/off site observations the Puget Uplands Short Plat 14-0108 June,2016 Page 5