2207 GIBSON PL 2022-10-28 - Laserfiche WebLink

1. PROJECT OVERVIEW This document is intended to provide engineering information necessary to support the new garage and additional driveway building and construction permit applications to the City of Everett on this lot. The site is located at 2207 Gibson PI, Everett, WA 98203, refer to Figure 1 for a vicinity map. The gross lot area is approximately 0.59 acres. The existing site contains a SFR, asphalt and concrete driveway and landscaping. The existing SFR was recently permitted and will remain along with the existing driveway. This project will add to the existing driveway and construct a detached garage south of the SFR. The average slope of the site within the proposed area of disturbance is flat and averages less than 3% down to the north. Steep slopes are present to the north of the lot, the top of the steep slopes are located in the northern portion of the parcel, between the SFR and the BNSF RNV. The toe of the slope is located off-site on the BNSF R/W. The existing driveway takes access from a 30' ingress, egress and utility easement serving this parcel and a two-lot short plat to the east. Access to the site will be maintained off the existing driveway as it currently exists, to the southwest. This project proposes to construct approximately 2,702 sf of new/replaced impervious surfaces. Due to the size of the project and that it is an expansion of the earlier site improvements, minimum requirements 1-9 apply to all of the new impervious surfaces for this project along with all the disturbed pervious surfaces. Included on the following page is a copy of the MR flow chart out of the COE Drainage Manual showing how the applicable minimum requirements were determined. This proposal does not meet any of the land-use criteria that require specific source control for the final use and a SWPPP has been prepared to address the runoff from the site during construction. Refer to Sections 4 and 5 for a detailed discussion. Figures 2 &3 are provided to show the existing and developed basins and runoff conditions for the two lots. The runoff from the proposed disturbance area, in the existing conditions, spreads over the flat landscaping as sheet flow. The topography of the site directs the runoff from the disturbance area over the steep slopes to the north of the lot. In the developed conditions runoff generated by the proposed impervious surfaces will be collected and conveyed to an existing conveyance system installed as part of the short plat to the east. This conveyance system was sized to accommodate this parcel and directs runoff over the bank with a dispersal tee discharging onto the BNSF R/W at the base of the slope. The on-site soils have also been reviewed by Julian Liu, a geotechnical engineer. The on- site soils were found to be Esperance sands, with the water table not encountered at a depth of 31.5 feet below grade and an infiltration rate of 4.5 in/hr. Refer to Appendix 'B' of this report, to view Julian's findings and results. The USDA Web Soil Survey was also used to identify the on-site soils, which identified the soils as gravelly sandy loam. Refer to Figure 4 to view a soils map. However due to the proximity to the bluff to the north it was determined by the City of Everett that infiltration would not be feasible for this project and that is the reasoning behind the direct discharge to the base of the bluff. The existing tight lined drainage design serving this project was laid out per the COE 2010 Stormwater Management Manual. This project will be designed to meet the 2019 SWMMWW(DOE). The downstream system for this project is comprised of sheet flow over existing flat landscaping, with collection in a piped conveyance system as noted above that discharges Langus Garage 15-0403-L March,2022 Page 3