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<br />. <br /> <br />4 <br /> <br />II. CHARACTERISTICS OF SAND-BED CHANNELS <br />2.l General <br />An alluvial river (or sand-bed channel) generally is continually changing <br />its position and shape as a consequence of hydraulic forces acting on its <br />bed and banks and related biological forces interacting with these physical <br />forces. These changes may be slow or rapid and may result from natural envi- <br /> <br />ronmental changes or from changes caused by man's activities. When a river <br /> <br />channel is modified locally, the change frequently causes alterations in <br /> <br />channel characteristics both up and downstream. The response of a river <br /> <br />to man-induced changes often occurs in spite of attempts to control the river <br /> <br />environment. <br /> <br />Natural and man-induced changes in rivers frequently set in motion <br /> <br />responses that can be propagated for long distances. In spite of the complexity <br /> <br />of these responses, all a1uvia1 rivers are governed by the same basic forces, <br /> <br />but to varying degrees. It is necessary that river system design be based <br /> <br />on adequate knowledge of: (1) geologic factors, including soil conditions; <br />(2) hydrologic factors, including possible changes in flow and runoff, and <br />the hydrologic effects of changes in land use; (3) geometric characteristics <br />of the stream, including the probable geometric alterations that developments <br />will impose on the channel; (4) hydraulic characteristics such as depth, <br />slope, velocity of streams, sediment transport, and the changes that may be <br />expected in these characteristics in space and time; and (5) ecological/ <br />biological changes that will result from physical changes that may in turn <br />induce or modify physical changes. <br />Documented effects of river development, flood control measures, and <br /> <br />channel structures built during the last century have proven the need for <br /> <br />considering immediate, delayed, and far-reaching effects of alterations man <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br />