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1 <br />Principal Spillway: Determine the required diameter for the principal spillway (riser <br />pipe). The diameter shall be the minimum necessary to pass the site's 15-minute, 10- <br />year flowrate. If using the Western Washington Hydrology Model (WWHM), Version 2 or <br />3, design flow is the 10-year (1 hour) flow for the developed (unmitigated) site, multiplied <br />by a factor of 1.6. Use Figure II-4.2.21 Riser Inflow Curves (p.394) to determine this dia- <br />meter (h = 1-foot). Note: A permanent control structure may be used instead of a tem- <br />porary user. <br />Emergency Overflow Spillway: Determine the required size and design of the emer- <br />gency overflow spillway for the developed 100-year peak flow using the method con- <br />tained in Volume III. <br />Dewatering Orifice: Determine the size of the dewatering orifice(s) (minimum 1-inch dia- <br />meter) using a modified version of the discharge equation for a vertical orifice and a <br />basic equation for the area of a circular orifice. Determine the required area of the orifice <br />with the following equation: <br />AS(2h)o.5 <br />`4° 0.6 x 3600Tgo.5 <br />where <br />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 7r=13.54 x v/'A---,- <br />The vertical, perforated tubing connected to the dewatering orifice must be at least 2 <br />inches larger in diameter than the orifice to improve flow characteristics. The size and <br />number of perforations in the tubing should be large enough so that the tubing does not <br />restrict flow. The orifice should control the flow rate. <br />. Additional Design Specifications <br />The pond shall be divided into two roughly equal volume cells by a permeable <br />divider that will reduce turbulence while allowing movement of water between <br />cells. The divider shall be at least one-half the height of the riser and a minimum of <br />one foot below the top of the riser. Wire -backed, 2- to 3-foot high, extra strength fil- <br />ter fabric supported by treated 4"x4"s can be used as a divider. Alternatively, <br />2014 Stormwater Management Manual for Western Washington <br />Volume 11 - Chapter 4 - Page 395 <br />