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<br />5, Floodplains <br /> <br />a, Floodplain width affects the length of the highway crossing, <br />composed of the approach embankment(s) and the bridge. <br /> <br />b, For longer highway crossings, there are usually more bridge <br />components exposed to the flow. <br /> <br />6. Natural levees <br /> <br />a, Form during overbank flow conditions. <br /> <br />b. When well developed, natural levees tend to limit lateral <br />migration. <br /> <br />7, Apparent incision <br /> <br />a, Determined by bank height at normal stage relative to width. <br /> <br />b, Incised streams tend to be fixed in position with slow lateral <br />migration rates, except for western arroyos with high, vertical <br />and clearly unstable banks. <br /> <br />8, Channel boundaries <br /> <br />a. Channel boundaries may be alluvial, semialluviial, or <br />nonalluvial. <br /> <br />b, Alluvial channels may be defined as channels that are <br />formed in materials that have been and can be transported <br />by water, Nonalluvial channels may be defined as channels <br />in bedrock or in very large material (cobbles or boulders). <br /> <br />2.6 <br />