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<br />~ <br />..1 <br />c_J <br />--\ <br />-'_.~:, <br /> <br />Even if the boundary geometry of a river is well defined, n-values <br />used for the river should be considered subject to question unless <br />discharge-stage data are available at two or more locations in the <br />study reach to assist in verifying that the roughness values are <br /> <br />correct. <br /> <br />'. <br />, <br /> <br />Meaninq of n-Values <br /> <br />0: <br />I, <br /> <br />If water surface elevations and the associated discharges are <br /> <br />known, then t1anning's "n" can be calculated. flo~lever, n-values com- <br />puted in thls manner reflect more than just friction loss even though <br />separate evaluations are made for other losses such as contraction, <br />expansion and bend losses. In alluvial channels, n-values from cal- <br />ibration studies incorporate the effects of changes in bed form and <br /> <br />in cross section area due to scour and deposition. Both water tem- <br />perature and sediment load affect bed forms. In overbanks. n-values <br />account for energy losses due to trees and growth, where actually a <br />form loss is involved. Finally. n-values will vary from one computa- <br />tion scheme to the next and should be calibrated accordingly. <br />It is particularly important to appreciate the meaning of <br />n-values when evaluating flood profiles through urban areas. In <br />determining n-values through residential areas, it is desirable to <br />assign n-values to streets, lawns, and sidewalks and to deduct the <br />space occupied by buildings from available cross-sectional area. The <br />same net effect can be accomplished by manipulating n-values alone. <br />For example. if a cross section in a residential area is about 50 <br /> <br />4.09 <br />