4326 CRESCENT AVE 2025-07-14 - Laserfiche WebLink

Acre Short Plat Stormwater Site Plan SECTION 8 -- PERMANENT STORMWATER CONTROL PLAN Per Section 3.1.1 of the COESWMM, projects that must comply with MRs #6, #7, and/or #8 shall also include the following sections and subsections as applicable. A. Off-Site Analysis A field inspection was performed on Wednesday, July 22, 2020 on a sunny day with a temperature of approximately 70 degrees. The site is undeveloped and is heavily covered with shrubs, trees, and various grasses. On-site Drainage Basin The existing topography of the site has slopes ranging from 17 to 50 percent over most of its area that are directed from west to east. Steeper slopes exist in the southeast corner (up to 50% slope) and level out in the northeast and southwest corners. The site is comprised of one drainage basin with surface runoff travelling primarily as sheet txnd shallow concentrated flows towards the Crescent Avenue frontage. Downstream Basin There are no defined drainage ditches or conveyance systems in place in Crescent Avenue immediately in front of the project site.There are also no signs of flooding or ponding,indicating that most runoff is contained on site or infiltrates in the flatter areas along the Crescent frontage. Crescent Avenue gradually slopes from south to north. There are several catch basins and culverts that collect any additional surface runoff and discharge to the combined sewer system. Upstream Basin The project site is bordered by Federal Avenue to the west which contains its own collection and conveyance system to divert flows away from the site. Two other properties located along the western property line are sloped towards the project site over an area on Lot#3 that is to remain undisturbed. Runoff I will continue as it does today in this location to the property directly north. i i B. Pre-Developed Hydrology The hydrologic analysis of the runoff conditions for this project is based on drainage characteristics such as basin area, soil type, and land use (i.e., pervious vs. impervious).The Western Washington Hydraulic Model 2012 (WWHM) software was used to evaluate the storm water runoff conditions and mitigation measures for the project site and to design the on-site flow control facilities. The following is a summary of the results of the analysis and the proposed drainage facility improvements for this project. The existing site conditions are shown in Figure 2 of the Appendix. Historic site (i.e., fully forested) conditions were considered in the analysis of the pre-developed conditions for all on-site targeted.surfaces. WWHM software was used to model the site hydrology and calculate runoff peak rates in accordance with COESWMM standards. The existing land use conditions summarized in Table 8.2 were used as the pre- developed site conditions for the WWHM model. C. Developed Hydrology The project proposes roadway, storm drainage, and utility connections in support of the three new single- family lots. Developed conditions were modeled using the developed land use coverages. The developed land use conditions displayed in Table 8.1 were used as the developed site conditions for the WWHM model. October 15, 2021 Page 16