4624 SEAHURST AVE 2018-01-02 MF Import

OMEGA ENGINEERING, INC. May 28. 2013 City of Everett 3200 Cedar Street Everett. WA 98201 2829 Rockefeller Avenue - Everett, WA 98201 t 425.387.3820 f 425.259.1958 RE: Drainage Letter for Salz SFR - Revision to Thompson SP Drainage Plan. Dear Staff This letter is intended to provide the City with the minor changes to the drainage plan for Lot 3 approved with the Thompson SP 11.001. The original SP design was also prepared by Omega Engineering, Inc so we are basing the proposed revision to address the changes to the drainage plan based on the proposed building permit application This drainage plan for the Thompson SP proposed a future building area of approximately 3,200 sf on Lot 3 along with some PGIS to be dispersed using a level spreader trench along the west side of the future GI-R. Per section 8 of the approved drainage report for the Thompson SP project the total impervi� is area directed to the leve spreader would be 3,67P sf and the design trench length of 53-feet was shown on the plans However, the building area for the Salz building permit application is larger then the 3,200 sf originally assumed so the length of the level spread must be adjusted to address the charge. Per Section 8 of the approved plans the length of the trench was sized based on a ratio of 700 sf of building/pavement per 10-feet of trench The proposed roof area and driveway pad area in front of the garage for the Salz SFR will be 4,785 sf which is an increase of 1.100 sf over the original assumption Therefore the trench length required to mitigate the improvements for the Salz SFR will be 63-feet. This assumes the majority of the driveway will remain pervious except for the final 20-feet in front of the garage Under the Thompson SP the viability of the level spreader located west of the SFR above the steep slopes was evaluated by the project geotech and found to be safe assuming the vegetation on the slope was retained and the other recommendations of the geotechnical report were followed. The proposed drainage revision for the Salz SFR only proposes to modify the length of the trench and no other factors so the remainder of the geotechnical recommendations will be met. Finally, in reviewing the drainage report for the Thompson SP the original project with the reduced impervious area on Lot 3 was found to increase the peak runoff rates from this project by 0.02 cfs Therefore, it is reasonable to assume that by increasing the impervious area on Lot 3 that the increase in the prcject peak runoff rate will remain less than 0.1 cfs and this change will not affect the conditions for the original SP construction approval. All other aspects of the original constriction plans and requirements for erosion control and soil amendments should still be met during the construction of the building improvements on Lot 3. Any questions or concerns regarding the soil conditions during construction or at the time of final stabilization should be directed to this office or the project Geotech as appropriate. Final