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HomeMy WebLinkAboutL_Drainage_Assessment_Lot14_ForestTerrace Forest Terrace – Lot 14 Page 1 of 4 December 13, 2021 D R A I N A G E A S S E S S M E N T PROJECT: FOREST TERRACE – LOT 14 PERMIT #: ADDRESS: 2611 UNION AVE NE RENTON, WA 98059 PARCEL #: 0423059142 DATE: DECEMBER 13, 2021 This drainage assessment accompanies the site development plan as a part of the building permit application for the development of Lot 14 of within the recent Forest Terrace Plat (to be recorded). The Forest Terrace Plat is a 25-lot single family development currently under construction within the City of Renton. Project Summary The project proposes to construct a new single-family residence with attached garage and associated driveway and utilities. The site is subject to a Simplified Drainage Review as less than 5,000 square feet of total new and replaced impervious surface are proposed (reference Renton Surface Water Design Manual (Manual), Figure 1.1.2.A). Compliance will be demonstrated through the implementation of a small site drainage plan and erosion and sediment control (TESC) plan as detailed in Appendix C of the Manual Existing Conditions The project is located on Lot 14 (9,360 square feet) within the to be recorded plat of Forest Terrace, which is zoned R4. Access to the lot is from NE 27th Pl to the south. Recent plat construction has provided the lot with utility connections to the plat infrastructure, such as domestic water, sanitary sewer, and stormwater service for conveyance of roof and footing drain runoff to the Forest Terrane Detention Vault. The property is roughly a parallelogram in shape, measuring approximately 71.5-feet in the east- west direction and approximately 131- and approximately 136.5-feet in the north-south direction. The lot is graded such that there is not much vertical relief across the site, however; there is a wall and a 2:H1:V slope in the northeast corner of the lot which provides a grade transition of approximately 16 feet. The lot is clear of vegetation and trees. There are no known critical areas on or adjacent to the proposed project. Forest Terrace – Lot 14 Page 2 of 4 December 13, 2021 Proposed Site Improvements The proposal is to construction a new 2,572 sf (roof area) house along with a 560 sf driveway, and 67 sf of walkway. A total of 3,199 sf of impervious surfaces are proposed for a total lot coverage of 34.18%. The remaining site pervious areas will be stabilized with vegetation. Proposed Stormwater Management The project lot is smaller than 22,000 square feet, thus is subject to the Small Lot BMP Requirements according to Section 1.2.9.2.1 of the Manual. The following is a feasibility analysis of available stormwater BMPs according to the individual lot BMP requirements. The onsite BMPs were applied in the order of preference as listed below in the BMP Feasibility Discussion. In support for evaluation of BMPs that would utilize site soils for infiltration, a geotechnical study by Terra Associates has been completed for the Forrest Terrace plat, dated July 19, 2017. The infiltration feasibility analysis in Terra Associate’s report indicate that the soils onsite are not a suitable receptor soil for discharge of development stormwater due to the shallow glacial till content below grade. It is of the opinion of Terra Associates that conventional stormwater detention with controlled release to the drainage basin be used to manage development stormwater. Because this site is created by a previous subdivision, a BMP discussion for the Forest Terrace plat has been provided in the approved Technical Information Report for the plat improvements dated July 14, 2020, approved September 11, 2020. This report provides guidance on the assumed impervious area tributary to the plat’s detention vault and other BMP conditions that may be placed on the Forrest Terrace project. A total of 4,000 sf of impervious surface was set as each individual lot’s maximum to be sent to the plat’s detention facility. No further covenants have been placed on the on the lot as a part of the plat improvements. BMP feasibility Discussion (1.2.9.2.1 Small Lot BMP Requirements) 1) To address the requirements for mitigation of target impervious surfaces (new and replaced), the applicability and feasibility of full dispersion was considered first. Due to the size of the property and location of the existing house, there is insufficient space to provide the required 100-foot flow path segment. Therefore, full dispersion is not feasible. 2) Full infiltration for rooftop runoff was considered next. According to the geotechnical study as discussed above by Terra Associates, the site soils will not support infiltration BMPs or facilities. 3) For target surfaces not addressed by items 1 and 2 above, full infiltration, limited infiltration, bioretention, and permeable pavement were considered. The site is not conclusive to infiltration BMPs due to the shallow glacial till found across property, thus the listed BMPs are determined to be infeasible for the proposed improvements. 4) All target surfaces not mitigated by items 1, 2, and 3 above must implement basic dispersion per Appendix C, Section C.2.4. Basic dispersion was evaluated for the new rooftop runoff. Based on the property’s topography and the location of the proposed residence, there is insufficient space to Forest Terrace – Lot 14 Page 3 of 4 December 13, 2021 provide the required 25-foot flow path segment or provide a vegetative flow path on slopes less than 15 percent that is needed for a dispersion trench. Basic sheet flow dispersion was also evaluated for the proposed driveway and walkways according to Section C.2.4.5. Basic sheet flow dispersion requires, at a minimum, a 10-foot- long vegetative flow path for surface water to drain through at slopes less than 15 percent. Both the driveway and the walkways have adjacent vegetation of adequate length to flow water through. Therefore, this BMP has been implemented for the front driveway and walkway totaling 627 sf. 5) BMPs must be implemented, at minimum, for an impervious area equal to at least 10% of the site/lot for site/lot sizes up to 11,000 square feet. For Lot 14 of the Forrest Terrace plat, the area of BMP implementation is: (9,360 sf lot) (0.10) = 936 sf. To implement BMPs on the minimum 936 sf of impervious areas required, a restricted footprint BMP per Section C.2.9.6. of the Manual will be implemented along with the driveway sheet dispersion discussed in #4 above. Because the 3,199 sf of proposed impervious surface is less than the 50% max impervious allowed by zoning, the project is eligible for a reduced footprint credit. Therefore, the area of target impervious surface credit as mitigated is: (9,360 sf lot) (0.50) = 4,680 sf 4,680 sf – 4,371 sf = 309 sf (Restricted Footprint Credit The impervious area managed by basic dispersion is 627 as discussed in #4. The total impervious area managed by BMPs on site is: 309 sf + 627 sf = 936 sf 6) The soil moisture holding capacity of new pervious surfaces must be protected in accordance with the soil amendment BMP. Soil amendments are proposed to the new pervious areas to mitigate for lost moisture holding capacity where compaction or removal of some or all of the underlying soils has occurred. Soil amendment specifications have been provided within the Temporary Erosion and Sediment Control Plan. 7) The proposed connection of roof downspouts to the local drainage system will be via a perforated pipe connection as detailed in Appendix C, Section C.2.11 of the Manual. This perforated pipe connection consists of a 2-foot by 10-foot gravel trench with a perforated pipe installed lengthwise through the trench. Forest Terrace – Lot 14 Page 4 of 4 December 13, 2021 Temporary Erosion and Sediment Control In order to prevent erosion and trap sediments within the project site, the following BMPs will be used as shown in the ESC details on the TESC plan.  Clearing limits will be marked by fencing or other means on the ground.  The driveway will be constructed and graveled immediately. A rocked construction entrance will be placed at the end of the driveway.  Runoff will not be allowed to concentrate, and no water will allowed to point discharge onto the slopes.  Silt fencing will be placed along the slope contours at the downslope limits of clearing.  Mulch will be spread over all cleared areas of the site when not being worked. Mulch will consist of dried straw and chipped site vegetation.