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has been strengthened at least t"vice since then after other major dam failures or near failures, and has been used as model stare legislation for the revie\"I, inspection, certification, and maintenance of non federal dams. As of 1989, 31 states had statutory authority to perform all of these functions, and only two had no statutory authority at all. The states had a collective 1989 budget for dam safety of $17,668,552. The Association of State Dam Safety Officials, which was organized in 1984, has become a major influence in improving state regulation of dams. Dikes, Levees, and Floodwalls Dikes, including levees and flood.......alls, can be thought of as dams built roughly parallel to a stream rather than across its channel. or parallel to the shorelines of lakes, oceans, and other water bodies. Levees are generally con- All estimnted 25,000 miles oj b:~m and .Jlooda'(1lls have bul'/. built rwlianu;w. Thy can be L't7)' ifftcthe in redll~jl1g .flood ws.m. altlwugh areas behind levm and .floodu.alls 7lU~y ri,k greaifr than rwmwl flood dal7W.ge FbJodu'(111, H.bterloo, Iowa structed of earth, floodwalls of masonry or steel. Levees v....ere probably the first structures built for flood control by European immigrants to ='lorth America. The'first levee in the I\-1ississippi Valley \vas constructed at >Jew Orleans in 1717. Levees are the most common type offload control works. Although they ca~"be effective in reducing flood losses, a large percentage of private or locally built levees and flood.......alls provide a low kvel of protection suitable only for agricultural purposes or are poorly designed and maintained. Levee or floodwall overtopping or failure is involved in approximately one-third of all flood disasters. Areas behind levees and tlood\',:alls may be at risk of greater than normal flood damage for several reasons. .L\."1any floodplain residents in those areas believe that they are protected from floods and do not think it necessary to take proper precautions. Development may also continue or accelerate based on expected flood protection. A levee breach or floodwall failure, like a dam break, can release a large wave of flood waters with high velocity. After a breach, the downstream portion of the levee system may also act like a dam, catching and prolonging flooding of the once*protected area. The Corps has designed and constructed about 10,500 miles of levces and floodwalls, most of \',,'hich have been assigned to non federal sponsors for operation and maintenance after construction. The Federal Emergency :\1an- agement Agency has established minimum design, operation, and maintenance standards for levees that, for insurance purposes, must be met in order to be credited with providing protection against a 1 % annual probability flood. The Tennessee Valley Authority mvns and inspects 37 saddle dams and levees and treats them \\lith the same criteria as regular dams, including inspections, instrumentation, and maintenance. Thirteen states have special regulations governing the construction of levees. THE CLASSIFICATION OF DAM FAIWRE RISK Classification of tlu risk if potentw.l dam failure is based on the severit}' qf potentw.l impact rather than tlu structural safety oj a dam. Dams ma)' be rif very sound corzsfrurtion but classified as "high hazard" if their failure, Mwever unlikely, could result in catastroPhic loss oj life. Lower risk classifications indude dams that pose a ''signijUant hazard"for which failure is estirnated to resull in large proper!;' loss; and those that are "low hazard, "jar which failure is expected to result in minimal proper~y loss. TIu failure if several dams during tlu 1970s led to flu evaluation and repair if numerous unsai"e dams in the United Slates. LEVEES IN THE UNITED STATES About 1,000 communitus (5.5% qf.floodprone communities) haue If/;US that protect from 1% annual probability floods; the Imglh oj these slruc- lures is about 9.000 miln and the)' protect about 5,000 square miles of tand. - 37