543 VIEW RIDGE DR 2022-03-14 - Laserfiche WebLink

6. PRESERVATION OF NATURAL DRAINAGE SYSTEM The runoff from the NPGIS improvements will be dispersed on -site. By not grading or compacting the majority of the pervious surfaces to remain after construction in the area of disturbance a maximum amount of site runoff from the developed project will be absorbed and infiltrated into the existing on -site soils. This will mimic the existing conditions by maintaining the existing direction and condition of flow from this site. 7. ON -SITE STORMWATER MANAGEMENT The soils for this project were found to be medium sands. Since infiltration is not feasible due to soil conditions and site slopes, dispersion of the NPGIS and PGIS may be used if infiltration is not feasible. This project proposes to construct 7,802 sf of new plus replaced impervious surfaces. Dispersion of the NPGIS will be done using downspout splash blocks and a level spreader trench. The proposed roof surfaces total approximately 5,966 sf. Four downspout splash blocks are proposed. Each downspout will not be allowed to collect runoff from more than 700 sf of roof area. The splash blocks will disperse runoff away from the proposed SFR's in the northwesterly direction. The remaining roof surfaces total approximately 3,166 sf and will be dispersed via a level spreader trench. Level spreader trenches serving up to 700 sf of NPGIS area shall be 10 feet long by 2 feet wide. For NPGIS greater than 700 sf, the trench length shall be calculated at a rate of 10 feet per 700 sf of NPGIS, per section 12.3.5 of the City of Everett 2010 SMM. The trench length shall not exceed 50 feet. The trench will be 50 feet long by 2 feet wide. A total of approximately 1,848 sf of PGIS is proposed on -site for this project. The runoff generated by the proposed impervious driveway surfaces will be dispersed on -site as sheet flow. Dispersed flows will have a minimum 10 foot vegetated flow path before flows leave the project parcel. Finally, any area of the site not proposed to be covered with impervious surfaces but graded or compacted will be treated to meet the criteria for DOE BMP T.5.13. Please refer to the engineering plans for the locations of the soil amendments and additional details. 8. RUN-OFF TREATMENT As noted before this project proposes to install a total of 1,848 sf of new PGIS to serve this project based on the Owner/Applicants plans. Per COE 2010 Stormwater Manual "Minimum Requirements 6: Runoff Treatment — Requirements" a project is required to provide runoff treatment if: (a) Projects in which the total effective, pollution -generating impervious surface (PGIS) is 5, 000 square feet or more in a threshold discharge area of the project, or Pignataro SFR 15-1008 March 2016 Page 9 Revised June 2016