Laserfiche WebLink
<br />The three parts of the momentum equation represent the total mo- <br />mentum flux in the constriction expressed in terms of the channel pro- <br />perties and flow depths upstream, within and downstream of the con- <br />stricted section. If each part of this equation is plotted as a func- <br />tion of the water depth. three curves are obtained (Figure 1) represent- <br />ing the total momentum flux in the constriction for various depths at <br />each location. The desired solutions (water depths) are then readily <br /> <br />available for any class of flow. The momentum equation is based on a <br />trapezoidal section and therefore requires a trapezoidal approximation <br />of the bridge opening. A logic diagram for the momentum calculation <br />is shown in Figure 2. <br />Class A low flow occurs when the water surface through the bridge <br />is above critical depth, i.e., subcritica1 flow. The bridge routine <br />uses the Yarnell equation for this class of flow to determine the <br />change in water surface elevation through the bridge. As in the momentum <br />calculations, a trapezoidal approximation of the bridge opening is <br />used to determine the areas. <br /> <br />H3 a 2K (K + lOw - 0.6) (0 + 1504) V~ /2g <br /> <br />~ere, <br /> <br />H3 = drop in water surface in feet from upstream to downstream <br />sides of the bridge <br /> <br />K <br /> <br />= pier shape coefficient <br />. ratio of velocity head to depth downstream from the bridge <br /> <br />~ <br /> <br />o <br /> <br />a obstructed area <br />total unobstructed area <br /> <br />V3 = velocity downstream from the bridge in feet per second <br /> <br />5 <br />