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<br />The difference between the flood damage with the bridge in place <br />and the flood damage under natural conditions is the damage caused by <br />backwater flooding. The depth-damage relation for types of land use is <br />used to estimate the damage. For example, water 1 foot deep in a house <br />valued at $20,000 might cause $4,550 damage while 2 ft could cause <br />$6,300 damage. The difference between the water-surface elevation and <br />the initial damage elevation is the depth of water at that point. The <br />depth-damage relation is used to obtain the average value of the damage. <br />Depth-damage relations have been developed by FEMA (Flood Emergency <br />Management Agency) (F. V. Reilly, 1978, written commun.), the Corps of <br />Engineers (see Sumrall, 1970), and others as a result of damage surveys <br />and preplanning activities. The classification of structures and land <br />uses are being refined by FEMA as claims experience develops. As these <br />damage values are based on actual experience, it is recommended that <br />FEMA data be used to compute flood damage. <br /> <br />One computational procedure was developed by Sumrall (1970, 1972). <br />In this approach, the value of the structure and the floor elevation are <br />the input data. Damage is a percentage of the structural value and <br />varies with the depth of flooding above the floor. Building contents are <br />specified as a percentage of the structural value, but for estimating <br />purposes are taken as 50 percent of the structural value. Water-surface <br />elevation frequency information is obtained from the hydraulic analysis. <br /> <br />The computations of depth damage are conceptually simple and can be <br />hand calculated. However, they are also laborious and the use of a <br />computer program is recommended. <br /> <br />Backwater damage (fig. 4) will increase with embankment elevation <br />for a constant bridge length because more water is forced to flow under <br />the bridge and less is allowed to flow over the embankment. However, as <br />bridge length increases more water flows under the bridge, relieving the <br />backwater. Therefore, backwater damages decrease as bridge length <br />increases and increase as embankment elevation increases. For a given <br />discharge, when all flow goes through the bridge, no additional backwater <br />increase will occur. <br /> <br />Traffic Interruption.--When the road is out of service either by <br />overtopping or structural failure, the difference between the cost of <br />driving a vehicle on the primary detour and the usual route is considered <br />to be part of the risk involved in the design. These running costs are <br />available from the state transportation agency. Considerable knowledge <br />of the road is required to compute running costs. Necessary data includes <br />the traffic volumes, value of time lost, vehicle running costs, basic <br />geometries of the section of road, unit accident costs, length of the <br />section, transition costs owing to changes in speed along the route, and <br />delay costs. These data are used in doing cost benefit analyses for <br />road improvements. <br /> <br />20 <br />