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<br />r <br /> <br />Modified Channels <br /> <br />Flow resistance in modified channels can vary significantly depending on <br />whether the stream is in its natural state or has been modified by man. The <br />roughness characteristics of modified or constructed channels depend on the <br />type of modification or construction. For those channels that retain some or <br />most of their natural characteristics, all pertinent factors described in the <br />preceding section need to be considered. The roughness coefficient for such <br />stream channels needs to be selected according to the procedure described in <br />that section, <br /> <br />Many streams have had their natural characteristics modified; channel <br />designs are needed for modifying natural and existing conditions by <br />realignment--straightening, dredging, and lining with natural or manmade <br />materials. Generally, these channels are modified for esthetic reasons, for <br />reduction of overbank flooding, for increasing conveyance within a limited <br />right-of-way, or for prevention of erosion, Considerable verification data <br />have been compiled for constructed channels, and the data from table 5 may be <br />used to estimate the base n value, Minimum, normal, and maximum ranges of n <br />values for each type of channel are included in table 5, <br /> <br />Table 5.--Base values ot n tor modi tied channels <br />[Modified from Aldridge and Garrett, 1973] <br /> <br />Type of channel and description <br /> <br />n value <br /> <br />Minimum <br /> <br />Normal <br /> <br />Maximum <br /> <br />A. Lined or built-up channels <br />1. Concrete: <br />a, Finished ---------------------- <br />b. Unfinished -------------------- <br /> <br />2. Gravel bottom with sides of: <br />a. Formed concrete --------------- <br />b. Random stone in mortar--------- <br />c, Dry rubble or riprap----------- <br />3. Vegetal lining -------------------- <br /> <br />B, Excavated or dredged channels <br />1. Earth, straight and uniform: <br />a, Clean, after weathering ------- <br />b. Gravel, uniform section, clean- <br />c, With short grass, few weeds --- <br /> <br />37 <br /> <br />0.011 <br />.014 <br /> <br />.017 <br />.020 <br />.023 <br />.030 <br /> <br />,018 <br />.022 <br />.022 <br /> <br />0.015 <br />.017 <br /> <br />0.016 <br />.020 <br /> <br />.020 <br />,023 <br />.033 <br /> <br />.025 <br />.026 <br />.036 <br />.500 <br /> <br />.022 <br />.025 <br />.027 <br /> <br />.025 <br />,030 <br />.033 <br />