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<br />Chapter 2. Protecting the Natural Functions <br />of the Floodplain <br /> <br /> <br />Floods are nature's way of shaping the landscape, <br />enriching the soil, and providing habitat for \vildlife. <br />\Vhen floodv.raters spill over river banks and people <br />lose their lives and homes, floods become a disaster. But <br />flooding in and of itself is not disastrous; it is very <br />naturaL Flooding occurs in different forms and levels of <br />intensity in every part of the United States. <br />There are five primary types or sources of floods: <br />riverine flooding; flooding from stonnwater runoff, <br />urban drainage, and high groundVlo'ater levels; coastal <br />flooding; ground failure; and fluctuating lake levels. This <br />report deals primarily ,"vith riverine and coastal flooding. <br />Riverine flooding is the most common type of <br />flooding. It occurs when there is an increase in the <br />volume of water in a river channel, and the river <br />overflmvs onto the ;;tdjacent floodplain. (Floodplains are <br />defined and discussed in more detail in the follmving <br />section.) Riverine floods usually are the result of <br />prolonged rainfall. Recent riverine floods include those <br />in June 1993 along the Mississippi River and its <br />trjbutaries and those that ,"vere the result of rapid <br />snowmelt in April 1997 in North Dakota and <br />Minnesota. <br />Other less common types of riverine flooding include <br />alluvial fan flooding and ice jams. Alluvial fans exist <br />mostly in dry mountainous regions of the vVest. They are <br />deposits of rock and soil that have eroded from mountain- <br />sides and accumulated on valley floors in a fan-shaped <br />pattern. Flood\vaters on alluvial fans travel at high <br />velocities and carry more dehris than typical riverine <br />flood\'vaters because the water gathers speed and debris as <br />it travels dmvll steep mountainsides. Flood\vaters on fans <br />are not confined to a single channel on the fan, and the <br />water velocity and debris create nev..T channels and paths <br />as ,"vater travels to the valley floor. These factors make <br />alluvial fan flooding diH1cult to predict and avoid. <br />lee jam flooding occurs mostly in the upper Mid\vest <br />and Ne\v England states. The most common ice jams <br />happen during the spring tha\v vvhen rising waters in <br />streams and rivers break the existing ice cover into <br />floating masses that get lodged at bridges or other <br />abutments, creating dams. \'\laters first rise rapidly <br />behind the jam and upstream and then move rapidly <br />do\vnstream \-vhen the jam breaks loose. <br /> <br />The often-used term "flash flood" is not specific to <br />any exact type of flood. The term refers to any flood in <br />\vhich there is a rapid rise in \vater, high water velocity, <br />and a large amount of debris. Flash floods can be <br />caused by steady rain over a long period or after short <br />intense thunderstorms. <br /> <br /> <br />WHAT IS A FLOODPLAIN? <br />Floodplains ;;tre lowlands adjacent to rivers, streams, <br />oceans, lakes, or other water bodies that have been or <br />may be inundated with water. Floodplains are formed <br />as moving \t'v'ater carves channels out of the landscape <br />and deposits sand, silt, and other material to form rich <br />soils. Sediment deposits and 100,v-lying areas around the <br />edges of channels result in wetlands, areas that arc <br />ahvays or periodically inundated \vith water (Protecting <br />Floodplai" Resollrces 1996,5). The size of the floodplain <br />depends largely on topography. The flat terrain in areas <br />along major rivers or on the coastal plain results in \-vide <br />floodplains. Floods in these areas typically involve <br />shn....-moving floodwaters that may inundate the <br />floodplain for days or vveeks at a time. l'....fountainous or <br />hilly areas have narrow, confined channels in which <br />floodwaters tend to move quickly and disperse <br />downstream more rapidly than in flat areas. <br />A floodplain is a part of a larger \-vatershed. A <br />watershed is a region or area that drains into a <br />particular river or other water body. \Vatershcds are <br />separated from each other by ridges or divides and <br />often contain an interconnected system of floodplains in <br />their lower elevations. (See Figures 2-1 and 2-2.) <br />\'Vatersheds can encompass thousands of acres and <br />can cross numerous political jurisdictions. Activities <br />that disrupt one part of the vvatershed, such as land <br />clearing and development, can have multiple effects on <br />other parts of it, just as development \-vithin a floodplain <br />can affect areas both downstream and upstream. In <br />general, \\'hat's good for the watershed is good for the <br />floodplain and vice versa. <br /> <br />FLOODPLAIN RESOURCES <br />There are three main categories of floodplain <br />resources and functions: water resources, biological <br />resources, and societal resources. <br /> <br />11 <br />