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<br />0011u;~ <br /> <br /> <br />Figure 6. &9 Thompson Creek, Colorado. May. 19n. Nine Months After Rash Flood lhal Took 139 Uves. <br />(Photo: R P1anl. <br /> <br />Stream channels seldom follow the <br />centerline of a floodplain. Even in con- <br />stricted mountain valleys, the channel <br />veers from side to side, leaving a flood. <br />plain first on one side and then on the <br />other. In broader floodplains, rivers <br />meander, developing loops which wander <br />back and forth like an undulating snake <br />(Fig. 5). This is the result of energy <br />being exerted at times of high flow. <br />scouring and eroding material on the <br />Quter side of each curve where the cur. <br />rent moves faster, while simultaneously, <br />sediment is being deposited on the inner <br />side where the current is slower. Over <br />time, this action causes the stream chan- <br />nel to shift drastically within the overall <br />floodplain, which remains relatively <br />static. This shifting of the channel plays <br />havoc with property boundaries and po. <br />litical jurisdictions which are drawn ....ith <br />reference to its centerline. Furthermore, <br />the common use of stream channel cen. <br />terlines as political boundaries leads to <br />the split of floodplain authority on op- <br />posite sides of the stream channel, which <br />raises problems we will consider later. <br /> <br />4. Kinds of Floods <br /> <br />As noted above, floods are broadly <br />classified as coastal or riverine, with <br />coastal flooding normally being caused <br />by an exceptionally high tide known as <br />a storm surge, and riverine flooding <br />being caused by excess surface drainage <br />within a particular watershed. Flooding <br />in coastal estuaries, at the mouths of <br />rivers, may involve both coastal and <br />riverine flooding. <br />Both coastal and riverine floods are <br />classified in terms of magnitude accord- <br />ing to their estimated periods or return. <br />Thus, floods are referred to as "annual," <br />"IO-year," "50-year," "IOO-year," and <br />so forth, according to how often a flood <br />of a particular magnitude may be ex- <br />pected to recur-the greater the flood, <br />the longer the recurrence interval. This <br />of course doE'S not mean that a "100- <br />year" flood will not recur for a century. <br />In any year there is a probability of one <br />percent that a flood of that magnitude <br />\.\-ill happen. Some floodplain managers <br />prefer the term "one percent flood It to <br /> <br />8 <br />