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<br /> <br /> <br /> <br />16 <br />blockages, corrosion, or structural damage that <br />may affect performance. <br /> For cases where it is not possible for an air <br />vent to have direct connection to the <br />atmosphere, such as for control gates located <br />in outlet works tunnels, air demand must be <br />supplied by an air duct above the free surface <br />of the flowing water, and the hydraulic design <br />should ensure flow never rises to the level of <br />the air duct. <br /> <br /> <br />It is also important to point out that there are several <br />outlet works hydraulic flow issues that are commonly <br />misattributed to insufficient air vent size, but are <br />actually associated with inadequate hydraulic design or <br />operations errors. These include surging, structural <br />damage due to filling the pipe too rapidly, and bi- <br />stable flow in the conduit. <br />References (with Links where available) <br /> Air-Water Flow in Hydraulic Structures, A Water Resources Technical <br />Publication, Engineering Monograph No. 41, United States Department <br />of the Interior, Water and Power Resources Service, by Henry T. Falvey, <br />Engineering and Research Center, Denver, CO, December 1980. <br /> Cavitation in Chutes and Spillways, A Water Resources Technical <br />Publication, Engineering Monograph No. 42, United States Department <br />of the Interior, Bureau of Reclamation, by Henry T. Falvey, Research <br />Engineer, Denver, CO, April 1990. <br /> Air Demand in Free Flowing Gated Conduits, D. Peter Oveson, A thesis <br />submitted in partial fulfillment of the requirements for the degree of <br />Master of Science, Civil and Environmental Engineering, Utah State <br />Engineering, Logan, Utah, 2008. <br /> Determining Air Demand for Small- to Medium-Sized Embankment Dam <br />Low-Level Outlet Works, Journal of Irrigation and Drainage Engineering, <br />American Society of Civil Engineers, B.P. Tullis, and J. Larchar, December <br />2011. <br />Hydraulic Design of Reservoir Outlet Works, EM 1110-2-1602, U.S. Army <br />Corps of Engineers, October 15, 1980 ; together with HDC 050-1 and HDC- <br />050-2. <br />Air vent sizing example using method from <br />the 1980 publication Air-Water Flow in <br />Hydraulic Structures: <br /> <br />Given: <br /> Conduit diameter = 2 feet <br /> Maximum water depth in conduit <br />corresponding to 75% gate opening = 1.5 feet <br /> Volume flow rate of water (Qw) = 50 ft3/s <br /> <br />Calculate: <br /> <br /> <br /> = <br /> = 3.14 ft2 <br />Awp = 2.53 ft2 (obtained from table typically found in <br />hydraulic textbooks that provides numerical values for <br />area, wetted perimeter, and hydraulic radius for a <br />partially filled circular pipe) <br /> <br />( <br /> <br />) <br /> <br /> = <br /> = 0.24 <br />Qa = 0.24 * Qw = 0.24*50 ft3/s = 12 ft3/s <br />Setting maximum velocity at 100 ft/s, <br />A=Q/V = (12 ft3/s)/(100 ft/s) = 0.12 ft2 =17.3 in2 <br /> <br />Dpipe = √ <br /> = √ <br /> = 4.7 inches <br />Increase Dpipe to commonly available pipe size of 6 <br />inches. <br /> <br /> <br />Figure 4: Typical outlet works air vent for a small dam