Loading...
HomeMy WebLinkAboutR_DrainageAssessmentReport_VoyagerEast32nd_240420_v1Soil Report/ Storm Drainage Assessment For Single Family Residence 1015 N 32nd St. Renton, WA 98056 Parcel No.: 3342101557 Prepared for: Karan Singh Voyager Development LLC 4/18/2024 Prepared by: Bruce S. MacVeigh, P.E. 14245 59th Avenue S. Tukwila, WA 98168 206-571-8794 bruce_engr91@comcast.net (*Note – Per Simplified Review, a TIR format is not required. The discussion document is termed an “Assessment” with relatively informal format.) Project Overview Parcel No.: 3342101557 Address: 1015 N 32nd St. Renton WA 98056 Existing Site and Drainage The purpose of this report is to evaluate the soils on the above property for general stability, erosion potential and suitability for on site stormwater infiltration. The 5,100 square foot lot is located in Renton on the south side of N 32nd St. in north Renton. Currently the lot has been cleared of the previous single family home by the previous owner. There is an existing paved driveway along the east side of the site which will also be removed and replaced with a new driveway. A new single family residence with an attached garage with an approximate impervious area of 2,913 square feet (including roof areas with overhang, walkway, patio, and driveway) will be constructed. There are single family residences to the north, south, east and west of the parcel. The property does not currently have a stormwater management system as there are currently no structures on the property. The site is not documented within any sensitive environmental area or critical aquifer recharge zone per King County iMap. A Geotechnical Analysis was done by Gary A. Flower of Geological and Geotechnical Consulting on February 5, 2023. Based on the results of the of the infiltration trench testing conducted by Mr. Flowers the preferred method of stormwater management is a tightline connection to the City of Renton Stormwater System that runs along the South side of N 32nd St. The driveway, walkway and patio will use Basic Sheet Flow onto adjacent lawn area. The site is served by public sewer and water. Based on the topography of the site, no significant earthwork should be required to allow standard crawlspace construction. Proposed New Residence: Impervious Surfaces – House: 2,850 Sf. Driveway: 380 Sf. Walkway: 36 Sf. Total: 3,266 Sf. Total area of Land Disturbing Activity: 5,100 square feet (entire parcel to be disturbed) Converted Native Pervious: 0 Square feet (there is no existing native vegetation on the site; all vegetated areas are presently lawn and landscaped areas from existing house.) Since this lot is smaller than 22,000 square feet, it is subjected to Small Lot BMP requirements in Appendix C which specify that the target impervious surface is equivalent to the roof area, including overhang, and driveway. Soils Evaluation The soils evaluation was conducted by Mr. Flowers on February 5th 2023. The analysis determined that on site infiltration is not feasible due to soil conditions. 5 exploration pits were completed on site. Please reference Geotechnical report for Geotech Engineer recommendations on Site Drainage. Proposed Site and Drainage The Geotechnical report provided by Mr. Flowers concluded that infiltration on site is negligible. Dispersion is also not an option for the property given the lack of suitable flow path availability. The proposed project impervious areas will be required to drain to the fronting site street system. The existing system is an existing 12” concrete pipe flowing downward to the west. A new Type 1 Catch basin will need to be installed connecting site drainage to city system. The drainage from the site will have an installed Perforated Pipe Connection in its front yard. All site drainage will pass through the connection prior to release to the street system. A type 1 catch basin on site with a downturned elbow will prevent floating debris and silt from entering the Perforated Pipe Connection trench. The use of the perforated pipe connection is the highest method of flow control feasible for this site. The use of a Perforated Pipe Connection requires no further engineering evaluation for its installation. All foundations and retaining walls should be protected by footing drains with outlet. Note that any permeable retaining walls or rockeries would not require a footing drain. Flow Control BMPs The flow control best management practices were considered int eh following order: Full Dispersion – Full Dispersion is not feasible. Unable to achieve the minimum vegetated flow paths due to the small size of parcel and lack of native vegetation. Basic Dispersion – Basic sheet flow dispersion is feasible for the driveway. This BMP is used for this project. It is not feasible for the roof runoff due to the limited area available required flow path lengths. Full Infiltration – Full infiltration is not feasible. The soils make on – site infiltration of site runoff infeasible. Basic Infiltration – Basic infiltration is not feasible. The soils make on site infiltration of site runoff infeasible. Rain Garden – The use of a raingarden for stormwater mitigation is infeasible due to the constraints of the site as open space on the lot is limited. Permeable Pavement: - Permeable Pavement is not proposed for the residence. The soils make Permeable Pavement infeasible. Rainwater Harvesting – Rainwater harvesting methods would be fairly large nd therefore difficult to site on the lot due to space constraints. The landscaped area that would receive irrigation is also fairly small. Considering the costs and benefits rainwater harvesting is not practical for this site. Vegetated Roof – A Vegetated roof is not proposed ort his residence. Reduced Impervious Surface Credit – Not applicable to this site. Note that the driveway uses Basic Sheet Flow Dispersions BMP and manages 380/3266 = 11.6% of site impervious area. Use of wheel strips is unnecessary since the driveway uses acceptable Basic Sheet Flow Dispersion. The foundation cannot be reduced. There are no decks which can be made open grid pervious. There can be no Native Growth Area or Tree Retention since these do not exist in this site. As stated in the original version of this report, Reduced Impervious Surface Credits are not available for this site. Perforated Pipe Connection – Aside from the Basic Sheet flow Dispersion for the driveway used fort his project, the only remaining available and acceptable stormwater management BMP is a Perforated Pipe Connection to be used to manage roof runoff. Erosion and Sediment Controls In order to prevent erosion and trap sediment within the project site, the following BMPs will be used approximately as shown on the ESC plan: Clearing Limits will be marked by fencing or other means on the ground. Topsoil will be stockpiled in a designated area to be reapplied after the completion of construction A rocked construction entrance will be placed at the location of the driveway at the beginning of construction and used throughout construction. Cleared or disturbed areas will be seeded or mulched. Runoff will not be allowed to concentrate and no water will be allowed to point discharge Silt fencing will be placed along slope contours at the downslope limit of clearing. Mulch will be spread over all cleared areas of the site when they are not being worked. Mulch will consist of air dried straw and chipped site vegetation. Conclusions and Recommendations The proposed site will be required to drain directly the fronting street drainage system. The site is infeasible for all LID BMP’s except for Basic Sheet flow Dispersion for the driveway used for this project. The roof runoff must be Perforated Pipe Connection with drainage to the existing street system. Temporary Erosion and Sediment Control measures will be implemented by installing a silt fence, construction entrance and catch basin protection on N 32nd St.