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<br />~reeboard <br /> <br /> <br />I <br />------------ <br /> <br /> <br />____ Design flood level <br /> <br /> <br />- <br />.J:. <br />Cl <br />"Qj <br />J: <br /> <br />/. <br /> <br />."",:'; <br />), <br /> <br />.. <br /> <br />'--- Width of 3 to 5 times height -I <br /> <br />Ground level <br /> <br />Earth levees require significant space because of their width. <br /> <br />Levees and Walls <br /> <br />The principal purpose of levees <br />and floodwalls is to confine flood- <br />waters to the stream and a selected <br />portion of the floodplain. Seawalls <br />serve much the same purpose in stopping <br />ocean or lake waves. Each of these <br />measures protects only the area <br />immediately behind it and only against <br />the selected flood height for which it <br />was designed. <br /> <br />Levees are normally constructed <br />of earth. The width of a levee' s base <br />mus t be several time s greater than the <br />levee's height for stability. As a <br />result, levees more than a few feet in <br />height require that significant space <br />be available between the stream and the <br />property to be protected. Significant <br />space may also be required for setting <br />levees back far enough from the channel <br />to provide adequate flow carrying <br />capacity. Levees are best suited for <br />use along large rivers where their <br />construction is less likely to be a <br />severe encroachment on the floodplain. <br />Floodwalls and seawalls are usually <br />constructed of concrete or steel and <br />take up far less room. They are more <br />suitable for use in congested areas. <br /> <br />A principal advantage of levees, <br />floodwaUs and seawalls is the flexi- <br />bility they offer to protect either a <br />specific site or a larger area. Unlike <br />dams and reservoirs, they can be used <br />to protect a single community or a <br /> <br />portion of a community. However, as <br />with dams and reservoirs, they some- <br />times create a false sense of security <br />about the protection provided. Floods <br />exceeding the level for which levees <br />and walls are designed can cause <br />disastrous losses of life and property. <br /> <br />Levees and floodwalls may <br />increase flooding in other areas unless <br />designed as part of a comprehensive <br />program. Blocking off a flood's natural <br />course on one side of a stream normally <br />causes more widespread flooding on the <br />other side. Containing a flood between <br />levees or walls on boti sides of a <br />stream can create a bottleneck that <br />raises flood levels in upstream areas <br />and increases the speed of water flow <br />immediately downstream. Seawalls in <br />coastal areas also have disadvantages. <br />They are highly susceptible to erosion <br />and, if not properly placed, may also <br />increase erosion in adjacent areas. <br /> <br />Levees and walls often have other <br />undesirable environmental aspects such <br />as blocking access to and views of the <br />stream or ocean, or destruction of <br />wildlife habitat. For communities <br />straddling a river, high levees or <br />walls may make expensive construction <br />necessary to permit travel between <br />sections of the community. <br /> <br />Levees, floodwalls J and seawalls <br />can also block natural drainage from <br />the protected area to the stream or <br />ocean. Preventing flooding of the low <br /> <br />32 <br />