1622 E MARINE VIEW DR WHOLE SITE 2022-02-01

Principal Spillway: Determine the required diameter for the principal • spillway (riser pipe). The diameter shall be the minimum necessary to pass the site's 15-minute, 10-year flowrate. If using the Western Washington Hydrology Model (WWHM), Version 2 or 3, design flow is the 10-year (1 hour) flow for the developed (unmitigated) site, multiplied by a factor of 1.6. Use Figure 4.2.21 to determine this diameter (h = 1-foot). Note: A permanent control structure may be used instead of a temporary riser. Emergency Overflow Spillway: Determine the required size and design of the emergency overflow spillway for the developed 100-year peak flow using the method contained in Volume III. Dewatering Orifice: Determine the size of the dewatering orifice(s) (minimum 1-inch diameter) using a modified version of the discharge equation for a vertical orifice and a basic equation for the area of a circular orifice. Determine the required area of the orifice with the following equation: As (2h) o.s A° 0.60600Tgos where Ao = orifice area (square feet) As = pond surface area (square feet) h = head of water above orifice (height of riser in feet) T = dewatering time (24 hours) • g = acceleration of gravity (32.2 feet/second2) Convert the required surface area to the required diameter D of the orifice: D = 24x A° =13.54x A° The vertical, perforated tubing connected to the dewatering orifice must be at least 2 inches larger in diameter than the orifice to improve flow characteristics. The size and number of perforations in the tubing should be large enough so that the tubing does not restrict flow. The orifice should control the flow rate. Additional Design Specifications The pond shall be divided into two roughly equal volume cells by a permeable divider that will reduce turbulence while allowing movement of water between cells. The divider shall be at least one- half the height of the riser and a minimum of one foot below the top of the riser. Wire -backed, 2- to 3-foot high, extra strength filter fabric supported by treated 4"x4"s can be used as a divider. Alternatively, staked straw bales wrapped with filter fabric (geotextile) may be used. If the pond is more than 6 feet deep, a different mechanism must be proposed. A riprap embankment is one acceptable method of . Volume II — Construction Stormwater Pollution Prevention - August 2012 4-109