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<br />similar figures are given for the flood of March <br />1938. It is to be expected that for floods of <br />equivalent magnitude, monetary damage would <br />be greater in 1969 than in 1938 because of the' <br />greater degree of development in 1969 and the <br />lower purchasing power of the dollar. The <br />lesser damage in Los Angeles County in 1969 is <br />conclusive evidence that flood-control measures <br />taken since 1938 were effective in evidence that <br />flood-control measures taken since 1938 were <br />effective in protecting areas that have been <br />occupied or fully developed for at least the last <br />30 years. <br /> <br />EFFECT OF URBAN SPRAWL <br />ON FLOOD DAMAGE <br /> <br />Damage in areas of urban sprawl is of two <br />types. The first type is damage by inundation. <br />Encroachment on natural flood plains, in the <br />absence of facilities for the storage and <br />conveyance of floodwaters, exposes homes and <br />other structures to the damaging effect of <br />rampaging streams carrying large sediment and <br />debris loads. Urban development has encroached <br />on alluvial cones, which in their virgin state are <br />highly permeable. Urbanization almost <br />waterproofs the ground surface, with the result <br />that storm runoff from urbanized areas on the <br />cones is increased in volume and intensity and is <br />virtually dumped on lower-lying developed <br />areas. <br /> <br />The second type of damage in areas of urban <br />sprawl is that which occurs as a result of <br />landslides or mudflows during storm periods. <br />The steep flanks of many mountains and <br />foothills in south-coastal California are unstable <br />when saturated by heavy rainfall and often slide, <br />particularly under stresses associated with the <br />construction of buildings and roads. It is not <br />enough that one consult an experienced <br />engineering geologist before building on or near <br />such slopes, because a safely designed structure' <br />may be endangered by subsequent improper <br />design and building practices by others in the <br />vicinity. Appropriate studies on a broader scale <br />should be made before development, and if <br />adequate zoning and (or) building codes are not <br />established, prospective developers and residents <br /> <br />BIO <br /> <br />should at least be made aware of the potentiaJ... <br />hazard. . <br /> <br />Urban development that takes place too close <br />to the head of an alluvial cone, in the absence of <br />an upstream debris basin, exposes structures to <br />mudflows. A mudflow is a viscous mixture of <br />water, sediment, and rock debris that often has <br />the consistency of wet concrete. <br />Water-transported rock debris has been <br />deposited on alluvial cones from time <br />immemorial, and the cones, in fact, have been <br />naturally built by that process. The distance the <br />large material is carried downslope on a cone is, <br />to a large degree, dependent on the amount of <br />flowing water available to supply the energy for <br />transport. Under natural conditions the cone is <br />permeable and streamflow decreases in its course <br />down the cone. When the streamflow decreases <br />sufficiently the coarser material comes to rest. <br />Development that extends far up the cone <br />provides an impermeable surface that prevents <br />the seepage of large amounts of water, and the <br />streamflow reaching the developed area will b.e <br />undiminished as it travels downstream, or ma <br />even be augmented by local runoff from the <br />impermeable area. When that occurs, the "mud" <br />and rock debris that reaches the developed area <br />will be carried downstream until the slope of the <br />cone becomes too flat to support such transport, <br />or until the water has spread far enough laterally <br />to effectively reduce its energy for further <br />transport. The situation is often aggravated by <br />brush fires in the mountains that consume the <br />erosion-retarding vegetation and thereby <br />increase the potential for damage by sediment <br />and debris. During the 1969 flood, however, it <br />was only in the Glendora-Azusa area that recent <br />brush fires were responsible for significantly <br />increased production of sediment and debris. <br />That area received intense precipitation in <br />January. In other areas where recent burns had <br />occurred, as in the Little Tujunga basin, erosion <br />rates were not significantly increased because <br />precipitation intensities there were moderate in <br />January. <br /> <br />1 <br />1 <br />j <br />I <br /> <br />. <br /> <br />. <br /> <br />Because debris damage is usually accompanie. <br />by water damage, it is not possible t <br />differentiate the monetary damage attributable <br />