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CHAPTER 4: AREAS BEHIND UNSAFE OR INADEQUATE LEVEES THE HAZARD An estimated 25,000 miles of levees hme beell built natiollwide. Levees are the most common type of flood control works. They do reduce flood losses. However, results of the U.S. Army Corps of Engineers Ilon-fed- eral dam inspection program sllggest that a large percelltage of private or locally built levees provide a low level of protection or are poorly designed and mailltained. Some pril'ately-bllilt levees may have had 110 design standards at all. Over time, a levee's history -- and the limita- tions on its protective capability -- are easily forgotten. Levee overtop- ping or failllre is involved in approximately one-third of all flood dis- asters. Levees provide only partial protection from flood problems for several reasons: 1. Many levees (emergency, agricultural) are designed to provide protection only from smaller floods (e.g., 5-15 year flood frequencies) or a specific flood even l. 2. Only a portion of all earthen levees built with crown clcvations at the computcd 100-year flood elevation can provide the expectcd protection because of changing hydrologic conditions and the possibility of structural failure beforc overtopping. 3. Areas behind levccs are often subject to severe internal drainage problcms. The cxclusion of flood waters also scrvcs to retain storm water runoff. Surfacing ground watcr may be another problem. For cxamplc, containment of the Colorado River flows within thc Icvec has rcsulted in high water table levels along hundreds of milcs of lcvces in Arizona. This high watcr tablc has damaged crops and municipal facilities, including sewer and watcr supply lines and streets, although thcre is no dircct river flooding. D-I