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<br />PRESSURE FLOW <br /> <br />The method used in the computer program to determine the ftXistence <br /> <br />of pressure flow is to compare the energy grade line elevations required <br /> <br />to pass a given discharge based on low-flow control and pressure-flow <br /> <br />control. The higher energy elevation represents the controlling type <br /> <br />of flow, as shown on Figure 5. The delineation between pressure flow <br /> <br />and the cOIIlbination of pressure and weir flow is determined by comparing <br /> <br />the energy grade line elevation for pressure flow alone to the elevation <br /> <br />of the top of roadway. If this pressure flow energy grade line is above <br /> <br />the top of the roadway elevation, then a combination of pressure flow <br /> <br />and weir flow is assumed. The basic formula for pressure flow is: <br /> <br />J~ <br />Q . A k (8) <br /> <br />where: <br /> <br />Q D The discharge for pressure flow <br /> <br />A . The cross sectional area of the submerged opening <br /> <br />H = The head under the bridge, nonnally mea.sured from the upstream <br /> <br />energy grade line to the downstream tailwater <br /> <br />k . The tota.J. loss coefficient, including the velocity head <br /> <br />conversion from static energy to kinetic energy, friction loss, <br /> <br />and other minor losses <br /> <br />10 <br />