2702 59TH ST SW 2024-07-05 - Laserfiche WebLink

DRAINAGE REPORT Miller Residence Drain DEVELOPED SITE AND RUNOFF CONDITIONS The developed site conditions are based on the Civil Plan Set, located in APPENDIX A. June 2023 A standard dispersion trench with notched board has been proposed along the structural retaining wall (see APPENDIX C) to mitigate the stormwater runoff and limit instability and erosion affecting the parcels southern slope. A site visit was performed to visually inspect and assess site conditions. See Figure 2, Figure 3, and Figure 4 for existing conditions of the site. DCG/Watershed worked along with the geotechnical (Nelson Geotechnical Associates) and structural (DCG/Watershed) engineers to propose a dispersion trench along the southwest side (down slope) of the proposed structural retaining wall. Any stormwater runoff generated from uncollected impervious and pervious surfaces shall sheet flow towards the dispersion trench and eventually bubble to the top of the drain rock discharge via sheet flow in an unconcentrated manner into existing vegetation to the southwest. The dispersion trench will be separated into three different lengths of 15 linear feet (LF), 57 LF, and 44 LF, to fit the topography of the site and to ensure unconcentrated dispersion. The conservative potential contributing basin area was modeled early in the design to include the existing house, deck, concrete walkway and landscape, totaling approximately 6,098 square feet (0.140 acres). The was modeled using the 2012 Western Washington Hydrologic Model (WWHM), a continuous event modeling software program using long-term recorded precipitation data. The resulting 100-year peak flow rate was 0.095 cubic feet per second (cfs). All storm event peak flows from the preliminary model run are shown in Table 2. See APPENDIX F for the WWHM summary print out. The results from the WWHM model are conservative as the SFR roof downspouts will not be realigned to the dispersion trench and shall continue to be tightlined to the City of Everett detention vault. Table 2 Contributing Basin Storm Event Flow Results Event Peak Q cfs 2 year 0.036 5 year 0.049 10 year 0.059 25 year 0.072 50 year 0.083 100 year 0.095 2.0 EXISTING CONDITIONS SUMMARY SITE TOPOGRAPHY The topography of the project site is generally steep across the entire property. The general stormwater flows to the southwest with existing contours. EXISTING DRAINAGE PATTERNS Much of the project site is covered in landscape surfacing surrounding the existing SFR and concrete walkways. Most of the storm water generated on site is naturally discharged to the steep slope at the southern side of the parcel. A site visit was performed by the geotechnical engineer confirming the SFR DCG/Watershed 20230616 Miller Drainage Report FINAL Page 4