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<br />the trapezoidal breach. Failures due to overtopping of the dam and/or <br />failures in which the breach bottom does not erode to the bottom of the <br />reservoir may also be analyzed by specifying an appropriate "a" parameter <br />which is the elevation of the reservoir water surface elevation when breach <br />formation commences minus the final breach bottom elevation (i.e., "a" is <br />the depth to which the breach cuts). <br /> <br />The model uses a single equation to determine the maximum breach <br />outflow and the user is required to supply the values of four variables for <br />this equation. These variables are: 1) the surface area of the reservoir; <br />2) the depth to which the breach cuts; 3) the time required for breach <br />formation; and 4) the final width of the breach. (Note: For "pre-event" <br />analyses, the user must estimate the last three variables above. To assist <br />in this estimation, the following table of default values is provided.) <br /> <br />. <br /> <br />Table l--Defau1t Values for Breach Description <br /> <br />tf <br /> <br />minutes <br /> <br />Time of failure <br /> <br />Default <br /> <br />Height of dam <br /> <br />3 x breach depth (earth dams) <br />1/4 - 1/2 dam width (concrete <br />gravity dams) <br />Entire dam width (concrete <br />arch dams) <br /> <br />a/3 (earth dams) <br />c..- a/1000 (concrete dams) <br /> <br />a <br /> <br />Units <br />ft <br />ft <br /> <br />Description <br />Depth to which breach cuts <br />Final breach width <br /> <br />Value <br /> <br />Br <br /> <br />-::::::-' <br /> <br />Once the maximum outflow at the dam has been computed, the depth of <br />flow produced by this discharge may be determined based on the geometry of <br />the channel immediately downstream of the dam, the Manning "n" (roughness <br />coefficient) of the channel and the slope of the downstream channel. This <br />depth is then compared to the depth of water in the reservoir to find <br />whether it is necessary to include a submergence correction factor for <br />tai1water effects on the breach outflow (i.e., to find whether the water <br />downstream is restricting the free flow through the breach). This <br />comparison and (if necessary) correction allows the model to provide the <br />most accurate prediction of maximum breach outflow which properly accounts <br />for the effects of tailwater depth downstream of the dam. <br /> <br />;f< <br /> <br />2.2 Channel Description <br /> <br />The river channel downstream of the dam to the specified routing point <br />is approximated as a prismatic channel by defining a single cross section <br />(aQ avera~e section that incorporates the geometric properties of all <br />intervening sections via a distance weighting technique) and fitting a <br />mathematical function that relates the section's width to depth. This <br />prismatic representation of the channel allows easy calculation of flow area <br />and volume in the downstream channel which is required to accurately predict <br />the amount of peak flow attenuation. <br /> <br />. <br /> <br />Approximating the channel as a prism requires three steps. First, <br />topwidth vs. depth data must be obtained from topographic maps or survey <br /> <br />4 <br />