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<br /> <br />impacts on flood forecasts. For example, a broken levee system <br />upstream can reduce the flood threat downstream. Flood debris <br />can get caught on bridge supports reducing the channel capacity <br />and resulting in upstream flooding. Local officials must monitor <br />weather Service reports and remember that flood forecasts are <br />dynamic and subject to change. <br /> <br />In an attempt to reduce the flood threat faced by a community, <br />some areas have constructed massive flood control projects. <br />Common flood control works include levees, dikes, and flood <br />walls. Flood control projects that prevent the natural inundation of <br />the floodplain can have drastic effects on the waterway. <br /> <br />Notes <br /> <br />A levee is a mounded structure built of clay, soil or rock to <br />increase the height of the natural river bank. If the river is <br />confined by levees and dikes, the total volume of water that must <br />flow past a given point is forced to do so in a more narrow zone. <br />This results in increasing the height and velocity of the water flow <br />in the channel. The water heights in a confined levee channel are <br />higher than those that would be normally expected in an unrestrict- <br />ed natural channel and floodway. . <br /> <br />..... Levee Channel ~ <br />I I <br /> <br /> <br /> <br />I <br />I <br />I <br /> <br />.. <br /> <br />Naturall-looat'lam <br /> <br />~I <br /> <br />In addition the safe capacity of the river may decrease when it <br />is confined by levee systems. During the Summer of 1993, there <br />were extensive and devastating floods along the upper Mississippi <br />. River. Records suggest the total water flow through this flood <br />zone was far below previous record volumes although flood <br />heights reached record levels. <br /> <br />If a levee were to breach, the flood depth on the breech side is <br />likely to be higher than if the water had been allowed to naturally <br />inundate the entire flood plain. In addition, the violence of a levee <br />breach can cause more damage than would the slowly rising waters <br />of the natural inundation process. A levee breech will also provide <br />little time for evacuation and public protection. <br /> <br />Page 1 - 8 <br />