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IOl3. 09 <br />• MANNIIQG'S n VALUES <br />An important part of dealing with the hydraulics of a channel is the selection <br />or estimation of n values associated unth various conditions. In work involving <br />natural streams it is common practice to divide the total cross section into at least <br />two parts, the channel and floodplain, and to select n values for each part. Where <br />conditions are sufficiently different, the cross section should be divided into three <br />or more parts and an n value selected for each. <br />Following a systematicprocedureu~illresult in theestimationof more reliable <br />n values. This procedure should include two phases; first, the selection of a basic <br />value of n for astraight, uniform, smooth channel in the natural materials involved; <br />second, through critical consideration of five specific factors, select modifying <br />values for each factor and add to the basic value to obtain the estimated value of <br />n for the actual channel. In considering each factor more critical judgment will <br />be exercised if it is kept in mind that any condition that causes turbulence results <br />in a greater value of n; i. e. , each factor should be judged on the basis of capability <br />of inducing turbulence and retarding flow. <br />Tr:e estimation of n values involves the follo~«ng steps: <br />1. Selection of the basic n value from the following: <br />• . <br />Kin of channel Basic n <br />Channels in earth 0.020 <br />Channels cut in rock 0. 025 <br />Channels in fine gravel 0. 024 <br />Channels in coarse gravel 0.028 <br />2. Selection of the modifying n for the degree of irregularity of roughness of <br />the channel sides and bottom: <br />Degree of irreaular~ Modifying n <br />Smooth 0. ODO <br />Minor 0. 005 <br />Moderate 0. Ol0 <br />Severe 0.020 <br />3. Selection of the modif}ping n for variations in size and shape of cross section: <br />C~racter of sharp ch~i~~ in <br />sizes and shape of cross section ModiS~~in~ n <br />Gradual changes 0.000 <br />~• Occasional clianl;es 0.005 <br />Frequent and ~+~ide variations 0.010 to <br />in size or shape 0. Ol5 <br />