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HomeMy WebLinkAboutP_Written_Drainage_Assessment_Lot04_190125_v2CP|H Page 1 of 2 TECHNICAL MEMORANDUM DATE: January 25, 2019 TO: Single-Family Stormwater Reviewer, City of Renton Building Department FROM: Andrew Stephenson, EIT Jamie Schroeder, PE Cc: Patrick Danner (DR Horton) PROJECT: Meadowvue Subdivision – Lot 4 13833 156th Ave SE, Renton, WA 98059 CPH Project No. 0035-15-012 SUBJECT: Drainage Assessment – Lot 4 Building Permit Drainage Assessment Lot 4 – Meadowvue Subdivision This memorandum is provided for the Simplified Drainage Review Submittal. Lot 4 is one of the fourteen parcels from the Meadowvue subdivision, zoned R-4, located in the southeast portion of the City of Renton. The street address of the Meadowvue project site is 13833 156th Ave SE (King County tax parcel no. 1463400060). More generally, the site is located in the NE ¼ of the SW ¼ of Section 14 of Township 23 North, Range 5 East, in King County, Washington. The house and yard are proposed in the middle of the lot and occupy a vast portion of the 9,909 square-foot lot area. The northern quarter of the lot is the backyard, which has a slope of 2% from the foundation wall to the yard drain, where a 2:1 slope starts at the three-foot landscape wall and continues until the clearing and grading line located near the property boundary. The driveway and walkway will be approximately 811sf and 97sf, respectively. The roof area of the house is 2,713sf. A maximum of 700sf conveys to each basic dispersion splashblock and conveys to a yard drain where the mitigated runoff then conveys to the perforated pipe trench prior to the existing storm drainage system within the public utility easement in the front yard. Gravel-filled dispersion trenches are determined not feasible to mitigate the entire roof area due to limited pervious area. The dispersion splashblocks and perforated pipe trench have been sized to mitigate the entire roof area over a wide area to minimize oversaturated soils and is shown on the storm drainage plan. There are no critical areas on-site or adjacent to the property, therefore limitations due to steep slope hazards and wetland buffers were not a factor. The lot is comprised of a single drainage basin, as all roof runoff discharges at a single point, being at the southwest corner of the lot. Roof runoff is dispersed via splashblocks and conveys to a yard drain connected to underground pipes that discharges at the southwest corner of the lot, whereas pavement and pervious surface runoff travels primarily as sheet flow through vegetated flow paths to the discharge point at the southwest corner of the lot, where an existing storm drainage system conveys lot runoff to an existing storm pond at the southwest corner of the site. Storm drainage facilities for the Meadowvue Subdivision were constructed per Civil Construction Permit No. LUA17-000106 in accordance with the 2017 City of Renton Surface Water Design Manual (RSWDM), which is based on the 2016 King County Surface Water Design Manual (KCSWDM) with some 11431 Willows Road NE, Suite 120 Redmond, WA 98052 Phone (425)285-2390 Fax (425)285-2389 www.cphconsultants.com PR17000137 B19002102 V2 REVISION Received: 05/31/2019 / aalexander BUILDING DIVISION APPROVED BUILDING DIVISION THIS SET OF APPROVED PLANS MUST BE ON THE JOB AT ALL TIMES DURING CONSTRUCTION. THIS BUILDING IS NOT TO BE OCCUPIED UNTIL AFTER FINAL INSPECTION BY THE RENTON BUILDING DIVISION AND RECEIPT OF CERTIFICATE OF OCCUPANCY. APPROVAL SHALL NOT BE CONSTRUED AS THE AUTHORITY TOVIOLATE OR CANCEL ANY PROVISIONS OF CODES, ORDINANCES,OR OTHER REGULATIONS ENFORCED BY THIS CITY. DATE: 06/10/2019 CP|H Page 2 of 2 modifications to reflect City of Renton specific requirements. The approved TIR assumed 40% impervious area per lot (3,964sf for Lot 4) in the design of the downstream conveyance system and detention and water quality pond facility. The proposed Lot 4 total impervious area is 3,790sf. The lot is still subject to individual lot BMPs and since the lot is less than 22,000sf, making it subject to the Small Lot BMP requirements, requiring that on-site BMPs must be applied as specific in Appendix C, section C.1.3.1. The following notes are a summary of the BMP selection process, per KCSWDM and RSWDM, for the Lot 4 storm drainage plan: • Full Dispersion is not feasible due to limited undisturbed native vegetation and insufficient flow paths. • Full Infiltration is not feasible due to poor soil infiltration characteristics. Infiltration tests were performed at four onsite infiltration test pits. Results of the test pits are summarized in section 5.7 of the geotechnical report performed by Terra Associates provided in Appendix A of the TIR for Meadowvue Subdivision. In summary, a measured infiltration rate of 0 in/hr was observed at all infiltration test pit locations. • Limited Infiltration facilities such as drywells, rain gardens, bioretention facilities, and permeable pavement were determined to be not feasible, again, due to poor native soil infiltration characteristics and the fact that the RSWDM does not allow underdrains for these systems. • Basic Dispersion in the form of splashblocks and vegetated flow paths graded with positive slopes to a yard drain connected by cleanouts and solid pipes to a perforate pipe connection has been determined feasible prior to runoff discharge at the lot stub to the existing stormwater conveyance system approved with the construction permit. Basic Dispersion in the form of gravel-filled dispersion trenches are determined not feasible to mitigate the entire roof area due to limited pervious area. Dispersion trenches concentrate more runoff flow over a smaller pervious area and therefore more likely to induce muddy soil conditions compared to splashblocks which disperse runoff over a larger pervious area thereby reducing the frequency of oversaturated soils. Per RSWDM section 1.2.9.2.1, the lot is less than 11,000sf, making it subject to the requirement per Appendix C of the RSWDM that an impervious area equal to 10% of lot area is mitigated by a BMP. Per RSWDM C.2.4.2, a maximum roof area of 700sf per splashblock has been used and shown. Four (4) splashblocks can mitigate the entire roof area by means of basic dispersion over vegetated flowpaths and is shown accordingly on the storm drainage plans. • Prior to the lot stub connection, a perforated pipe trench is a second individual lot BMP in a final attempt to reduce roof runoff. The minimum required length is 10 linear feet per 5,000sf of impervious area, per RSWDM section C.2.11.1. Mitigated runoff conveyed to the yard drain that is not dispersed by splashblock and sheet flow dispersal conveys to the perforated pipe trench. All impervious areas are mitigated by a BMP (3,790sf), which is less than this threshold impervious area. Designed as 10 linear feet, the perforated pipe trench has been shown to the extent feasible for the home product and maintaining a usable yard. This completes the BMP evaluation required by the 2017 RSWDM, with the BMP requirement met. • The soil moisture holding capacity of new pervious surfaces shall be protected in accordance with the soil amendment BMP as detailed in Section C.2.13 of the RSWDM. The Storm Drainage Plan summarizes the areas mitigated by the splashblock and sheet flow dispersal, perforated pipe trench, and pipe conveyance system connecting the downspouts to the existing lot stub. The downstream storm drainage detention/water quality facility provides the required detention and water quality treatment for the Meadowvue Subdivision project.