Laserfiche WebLink
<br />flow and orifice (pressure) flow are omitted because these are found in <br />many standard texts. The methods used to determine the controlling type <br />of flow or combination of flows are also described and illustrated. <br />Shortcomings of the bridge routines are emphasized in the hope that <br />further development of practical methods for predicting flow through <br />bridges will be stimulated. <br />METHODS A VAIIABLE <br />Several procedures are available for computing energy losses through <br />bridges including the criteria set forth by the US Army Engineer Waterways <br />Experiment Station (reference 3), the Bureau of Public Roads (reference 4). <br />and the US Geological Survey (reference 5). All of these procedures are <br />applicable primaril,y to conditions of low flow control, When the free <br />surface of the water is obstructed by the bridge, pressure flow or a <br />combination of pressure flow and weir flow can exist. Little information <br />is available for computing losses under these conditions, although proce- <br />dures for pressure flow and weir flow through flood control conduits and <br />spillways are well established. Unfortunatel,y, the various methods <br />available do not give comparable answers for a fixed set of conditions. <br />Technical procedures are not available for accurately determining flow <br />conditions that would cause a bridge to fail. Current procedures do not <br />take into account the local scour that occurs at high flows, although <br />this factor should be evaluated when possible. Provision for debris and <br />trash obstructions in the bridge opening are o:f'ten ignored in the compu- <br />tations, although these can produce substantial increases in the upstre8lll <br />water. . surface elevation. <br /> <br />i <br /> <br />2 <br />