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8. FLOW CONTROL (MR #7) <br /> The thresholds for flow control within the combined sewer area are as follows: <br /> • Projects that have a total of 200 square feet or more of net new impervious <br /> surface, or <br /> • Projects that convert '/. acres or more of vegetation to lawn or landscape. <br /> The site design for the improvements is for approximately 388 sf of new and replaced hard <br /> surfaces, including off-site improvements. This project will result in more than 200 square <br /> feet of net new impervious surface, so this project must meet the 'Flow Control <br /> Performance _Standard". The .existing .and proposed conditions were modeled and <br /> compared using WWHM12 software. Per the combined storm sewer design and <br /> construction standards and specifications chapter 6-10.1(1), the Washington Department <br /> of Ecology approved modelling program (WWHM), shall be performed using a 5-minute <br /> timestep. Therefore the 5-minute time step was used in this analysis. <br /> As per the interim Public Works Policy #2-11 the existing site will not be modeled as <br /> forested land cover but rather the existing site conditions. <br /> Using WWHM software the basin was analyzed for the existing flow conditions for the <br /> disturbed basin only. This data was then used as the matching criteria required in the <br /> sizing of the detention pipe. <br /> For this project the existing .soils were-found to be -light brown sandy loam to 21 inches <br /> below grade, before hitting compacted till hardpan. Due to the shallow depth of permeable <br /> soils, a typical infiltration trench is infeasible. However, a standard dispersion trench would <br /> have increased the flow from the site, which would be infeasible due to the combined <br /> storm and sewer standards which do not allow for increased runoff from existing <br /> conditions. Therefore, a dispersion trench was designed to infiltrate into the hardpan itself. <br /> A standard infiltration rate of 0.1 inches/hour for glacial till was used to size the facility. <br /> Additionally, the trench was designed such that it has a storage capacity of approximately <br /> 53 cubic feet. This storage coming from the 35% porosity of the 2' x 7.5' x 10' trench <br /> volume. By having the trench at ground level uncovered, overflow capability is provided if <br /> the storage volume is not found to be sufficient in a large storm event. The trench was <br /> sized with the Western Washington Hydrology Model to generate no more than the <br /> existing site runoff. The results from the model are shown below. <br /> Lalone Addition 20-0902 <br /> • October2020 Page 11 <br />