1031 51ST PL SW 2023-06-23 - Laserfiche WebLink

1 I e/Dd O' '�� 2707 Wetmore Avenue • 41. Everett, WA 98209 ENGINEERING, INC. t 425.387.3820 f 425-259-1958 April 7, 2021 City of Everett Permit Services �- 1 � D 3200 Cedar Street r Everett, WA 98201 0 .. .. .._.._. .f�. _..._._. _ APR 9 202.1 J CITY OF EVERETT RE: Drainage Letter—Seahurst SP Lot 6 Permit Services B2102-062 Dear Staff, This letter is intended to provide confirmation that the proposed site plan for Lot 6 of the Seahurst SP project (Permit No. 1906-021) has been reviewed by Omega Engineering, Inc. (engineer of record) to verify the proposed improvements under the building permit application for this project meet the design requirements provided in the approved drainage report for this project and the site areas do not exceed the amount allowed in the approved drainage report for the SP project. Seahurst SP Drainage Overview The approved Seahurst SP was designed at the time of permit application/issuance to meet the 2014 DOE Surfacewater Management Manual for Western Washington (SWMMWW) as well as Chapter 4 of the City of Everett Design and Construction Standards and Specifications in place at that time. The proposed on-site impervious surfaces (roof and driveway) were designed. to fully infiltrate on-site up to the 100-year storm event. The City has since adopted the 2019 DOE SWMMWW which is in place at the time of building permit application. With regard to site drainage design the 2019 manual much like the 2014 manual encourages LID BMPs to be implemented when found to be feasible. So, the use of roof downspout infiltration and permeable pavement for on-site impervious surfaces is still included as stormwater mitigation options in the 2019 manual. Finally, Post-Construction Soil Quality and Depth (BMP T5.13) will still be required to be applied to the disturbed/landscaped portions of the parcel at the end of the project. Design Site Areas The total on-site proposed impervious surfaces for the Seahurst SP, including lots 3-6, totaled 11,695 sf (0.27 acres) with an additional 2,900 sf for work within the R/W including curb, sidewalk and driveway approaches. This can be found in Sections 3 & 9 of the approved Seahurst Drainage report. The site plan for the Lot 5 building permits calculates that there is 3,114 sf of total impervious surfaces proposed which is a combination of 2,168 sf (roof), and 946 sf (driveway). For the driveway 490 sf is on-site and permeable pavement and the remaining 456 sf is in the R/W. So the total on-site impervious surfaces proposed for Lot 5 is 2,638 sf.