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Structural techniques 10 TlWdifJ' dumnelJ ami control stream flow indwk the constructi(J1l if diversionary walls and gabions-prefabricated baskts if rock within u;ire cages wed 10 stabilize banks Cif;.' Creek Ca'!}'on, Salt Lake Cil)', Utah. AN ALTERNATIVE APPROACH TO STORMWATER MANAGEMENT The cost to communities of darmlges caused b)' stomw)ater flooding and investment in cost(y chan- nelization and other conduits can sometimes be reduced through different approaches w swr17UJ.;ater management. In Arizontl, for example, the larger, rapidly urbanizing communities alllulVt some form of stomuvater management requirements for /U'w dLvelopment. All oj the wrger communities in the sto.te's two urban counties, which iru:ludL 77% of the state's population, regulate the development oj watersheds. ON-SITE DETENTION: A MULTIPURPOSE TOOL Priru:ipal on-site detention measures iru:{ude restrict- ing land clearing, crrating impervious areas, and prot'iding jor temporar)' storage of some or all of the rurwff jrom a property. )\1an)' urban communities ha~'e begun to recogniu that areas devoted to storm- water maTUlgmunt represent a significant portion oj their open space land and opportunities jor urban recreation and wildl~fe protection. Shallow grass- covered basins can bl! used as athletic fields, parkmg lots, or for other purposes during dr;' periods and as detention basins after storms. Equipping roofs or parking {ols jor temporaril;' storing at least a part of the water tlult falls c,n them, dLsigning streets in hilly areas to prevent rapid runqff, iruorporating small retention basins into londscaping, using rad- jilled pits to catch gutter runofJ, and using pave- nunts t/wt let water seep through into the ground belnw all slow runrfJ - 38 Channel Alterations Channel alterations increase the flow.carrying capacity of a stream's channel and thereby reduce the height of a flood. The various types of altera- tions include straightening, deepening, or widening the channel, removing debris, paving the channel, raising or enlarging bridges and culverts, and removing dams and other obstructions. Channel alteration is widely practiced by state and local governments to control flooding by rapidly conveying storm runoff through populated locales to downstream areas. The Corps and the Soil Conservation Serv.ice also undertake channel alterations. The Corps projects typically lie on larger streams and rivers, while Soil Consenr3tion Service works mostly in smaller streams on the upper portions of watersheds. The Soil Conservation Service has provided assistance in the construction of 10,700 miles of open channels. The use of channel modifications has decreased primarily because of the potentially adverse environmental impacts. Alternative designs are now devel- oped that include less straightening of channels, employ more gradual slopes, and use natural vegetation or riprap rather than concrete-lined channels. This minimizes destruction of fish and wildlife habitat, helps maintain water quality, and avoids undesirable downstream impacts. High Flow Diversions Diversions intercept flood waters upstream of a damage-prone or con- stricted area and convey them around it through an artificial channel or a designated flow~way. Diversions may either completely reroute a stream or collect and transport only excessive or potentially damaging flows. A negative aspect of such diversions is the false sense of security that may prevail in the protected areas along with a lack of awareness that the floodway actually exists. Several high flow diversions have been constructed along the Mississippi River. Excess water has also been temporarily diverted from the Great Salt Lake to an evaporation basin to prevent lakeshore flooding. Stormwater Management Stormwater management is the removal of water that falls directly onto properties as opposed to flood water that flows onto the property from upstream sources or an ocean surge. Stonnwater networks have historically been con- structed in urban and agricultural areas to remove these waters. Generally, the stormwater system removes the excess rainfall over a period of days and the temporary ponding floods only low-lying buildings and roads. A signifi- cant problem occurs when an agricultural zone '\fith an adequate stormwater system is urbanized. Large areas are paved with roofs, roads, and parking, con- tributing to additional runoff. Often, shopping centers and other develop- ments are placed on natural drainageways. The pre-existing stormwater net- work becomes inadequate for its ne",- urban use. Localized flooding then occurs. In an alternative approach often used in new developments today, runoff may be retained on the site, within a regional system, and total runoff within a watershed may be managed so that discharges from different units reach the main channel at different times to reduce peak flows downstream. Natural drainage systems may be used instead of concrete-lined channels or enclosed pipes. ~1any local ordinances OO\\f require a zero-increment runoff for new development, making such on-site detention a necessity. . Shoreline Protection Quasi-natural methods such as beach nourishment or artificial sand- dune building are often used to attempt to restore an eroding beach as well as protect development. Long reaches of shore can be protected by artificial nour- ishment at a relatively low cost per linear foot. In addition, nourishment can widen a beach and increase its recreational value. A ,..,rell-known beach nourishment project is the 10.5 miles of beach restoration in Dade County, Florida, which includes ~1iami Beach. However, these methods provide only temporary solutions to chronic long-term erosion caused by the diminishing supply of sediment in the littoral system. They also require periodic renourish-