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<br />Table 5.--Base values of n for modified channels--Continued <br /> <br />Type of channel and description <br /> <br />n value <br /> <br />Minimum <br /> <br />Normal <br /> <br />Maximum <br /> <br />B. Excavated or dredged channels--Continued <br />2, Earth, winding and sluggish: <br />a. No vegetation ----------------- <br />b. Grass, some weeds ------------- <br />c. Dense weeds or aquatic plants <br />in deep channels ------------ <br />d. Earth bottom and rubble sides-- <br />e, Stony bottom and weedy banks -- <br />f. Cobble bottom and clean sides-- <br /> <br />3. Drag-line excavated or dredged: <br />a. No vegetation ----------------- <br />b. Sparse brush on banks --------- <br /> <br />4. Rock cuts: <br />a. Smooth and uniform ------------ <br />b. Jagged and irregular ---------- <br />5. Channels not maintained, weeds and <br />brush uncut: <br />a. Dense weeds, high as depth of <br />flow ------------------------ <br />b. Clean bottom, brush on sides -- <br />c. Dense brush, high stage ------- <br /> <br />0.023 <br />.025 <br /> <br />.030 <br />.028 <br />.025 <br />.030 <br /> <br />.025 <br />.035 <br /> <br />.025 <br />.035 <br /> <br />.050 <br />.040 <br />.080 <br /> <br />0.025 <br />.030 <br /> <br />0.030 <br />.033 <br /> <br />,035 <br />.030 <br />,035 <br />.040 <br /> <br />.040 <br />.035 <br />.040 <br />,050 <br /> <br />.028 <br />.050 <br /> <br />.033 <br />.060 <br /> <br />,035 <br />.040 <br /> <br />.040 <br />.050 <br /> <br />,080 <br />.050 <br />.100 <br /> <br />.120 <br />.080 <br />.140 <br /> <br />An important aspect of selecting the base roughness coefficients for a <br />constructed channel is a realistic appraisal of whether and how periodic <br />maintenance will be done. While design values of n for newly constructed <br />channels may be very small, the roughness may increase with time and may <br />lead to significant changes in water-surface elevations, unless periodic <br />maintenance such as mowing grass-lined channels, weed control, repair of <br />broken concrete or rock linings, and removal of debris is done. <br /> <br />OVERBANK FLOW RESISTANCE <br /> <br />In general, overbank flow resistance is affected by factors similar to <br />those affecting channel flow resistance, and an evaluation of the roughness <br />characteristics should take all factors into consideration. The relative <br />effect of these factors may, however, vary significantly between main channel <br />and overbank roughness and between various types of overbanks. To some <br />extent, roughness coefficients in overbanks can be evaluated in a manner sim- <br />ilar to that of channel roughness. Generally, very little verified data are <br />available for overbank roughness. Factors affecting overbank roughness and <br />procedures for estimating roughness coefficients for natural, agricultural, <br />and urban flood plains are described in the following sections. <br /> <br />38 <br />