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Principal Spillway: Determine the required diameter for the principal <br /> spillway(riser pipe). The diameter shall be the minimum necessary to pass <br /> the site's 15-minute, 10-year flowrate. If using the Western Washington <br /> Hydrology Model (WWHM), Version 2 or 3, design flow is the I0-year(1 <br /> hour) flow for the developed(unmitigated) site,multiplied by a factor of <br /> 1.6. Use Figure 4.2.21 to determine this diameter(h= I-foot). Note:A <br /> Permanent control struchire may be used instead of a temPoraly riser. <br /> Emergency Overflow Spillway: Determine the required size and design <br /> of the emergency overflow spillway for the developed 100-year peak flow <br /> using the method contained in Volume III. <br /> Dewatering Orifice: Deternine the size of the dewatering orifice(s) <br /> (minimum I-inch diameter)using a modified version of the discharge <br /> equation for a vertical orifice and a basic equation for the area of a circular <br /> orifice. Determine the required area of the orifice with the following <br /> equation: <br /> _ A.,.(2 h)os <br /> A` 0.6x3600Tgo,5 <br /> where Ao — orifice area(square feet) <br /> As = pond surface area(square feet) <br /> h = head of water above orifice(height of riser in feet) <br /> T = dewatering time(24 hours) <br /> g — acceleration of gravity (32.2 feet/second2) <br /> Convert the required surface area to the required diameter D of the orifice: <br /> D=24x " =13.54x A <br /> The vertical,perforated tubing connected to the dewatering orifice must be <br /> at least 2 inches larger in diameter than the orifice to improve flow <br /> characteristics. The size and number of perforations in the tubing should <br /> be large enough so that the tubing does not restrict flow. The orifice <br /> should control the flow rate. <br /> • Additional Design Specifications <br /> The pond shall be divided into two roughly equal volume cells by a <br /> permeable divider that will reduce turbulence while allowing <br /> movement of water between cells. The divider shall be at least one- <br /> half the height of the riser and a minimum of one foot below the top of <br /> the riser. Wire-backed, 2-to 3-foot high, extra strength filter fabric <br /> supported by treated 4"x4%can be used as a divider. Alternatively, <br /> staked straw bales wrapped with filter fabric(geotextile)may be used. <br /> If the pond is more than 6 feet deep, a different mechanism must be <br /> proposed. A riprap embankment is one acceptable method of <br /> f7ohnne II T-Gnistruction Stowniwer Pollrrlion Prevention-August 2012 <br /> 4-109 <br />