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found to be released through the riser structure,the volume of rainfall upon the developed <br /> site will be considered to have exceeded the system's infiltration and detention capacities, <br /> thereby identifying a failure in design. A properly designed and most efficient drainage <br /> system will use the infiltration and detention facilities to their maximum capacities <br /> without noting any water released through the theoretical riser used in the model. <br /> WWHM also be used to estimate 100-year runoff rates generated from the new driveway <br /> area in order to determine how much area will be needed to infiltrate 100% of these <br /> waters. <br /> Using WWHM three models were created to demonstrate the performance of the (1) <br /> Infiltration trench to infiltrate roof runoff, (2) Determine the necessary area required to <br /> infiltrate runoff from the proposed driveway area, and (3) Determine the runoff rates <br /> generated from the 470 sf of new sidewalk proposed along the project frontage. <br /> House Basin: <br /> Using an infiltration rate of 6.33 inches per hour as allowed within the attached <br /> geotechnical report prepared by Geotest for this project, it has been determined that it <br /> will take 46 if of 2-foot wide x 2-feet deep infiltration trench having a bottom elevation <br /> set at 3-feet below surface is necessary to infiltrate 100% of the post-developed runoff <br /> waters generated from the new 3,376 sf(0.77-ac) house roof area. As noted within the <br /> WWHM view information provided with this memo for the House it is noted that 0 cfs is <br /> released via the riser used in the modeling, indicating 100%infiltration has been achieved <br /> using the proposed trench. Refer to WWM I view and Report for the House attached <br /> with this memo for more detailed analysis information. <br /> Proposed Infiltration Trench(tight-lined to downspouts): <br /> 46'long x 2' deep x 2' deep Drainrock filled trench(bottom @ 3-feet below ex. grade) <br /> Void Ratio: 0.33 <br /> Infiltration: 6.33 in/hr(per the geotechnical report prepared by Geotest dated 08/30/17) <br /> Since the approved runoff model methods note that the entire runoff file is infiltrated up <br /> though the 100-year storm, the house is considered to be "ineffective" per (per the <br /> definition of"Effective Impervious Surface"(DOE Manual Volume I-Appendix G). <br /> The attached Memo prepared by Geotest Engineers notes that the proposed infiltration <br /> trench is extremely unlikely to negatively impact any surrounding foundations at its <br /> proposed location 11-feet from the north property line. <br /> Driveway Basin: <br /> As an alternative way to look at the driveway dispersion/infiltration provided, the use of <br /> WWHM allows us to calculate the 100-year flow frequency runoff that will be generated <br /> from the 1,190 sf(0.028 acre) of driveway area to be dispersed. Based on this analysis <br /> the 100-year storm event runoff values increase from 0.0016 cfs during the predeveloped <br /> (forest) condition up to 0.0171 cfs for the post developed (concrete) condition. This <br /> 0.0155 cfs increase in runoff that is anticipated from the conversion of 0.028 acres from <br /> forested condition to concrete driveway can easily be infiltrated onto the undeveloped <br /> P:lProjectsl170201DOCStDrainage Memo 10.26.17.doe 2 <br />