Laserfiche WebLink
<br />'Iable 4.--<Jutline and example of procedures for determining n values <br />for a hypothetical channel and flood plain--COntinued <br /> <br />Flood-Plain ROughness (steps 14-23)--COntinued <br />Subsection 3 (cotton field)--COntinued <br /> <br />Step <br /> <br />Item to be determined or <br />operation to be performed <br /> <br />Factors on which decisions are <br />based and the results. <br /> <br />15 <br /> <br />Assign n by evaluation of bcundary <br />roughness only. <br /> <br />~thod to be used in. <br />assigning n <br /> <br />16 <br /> <br />Subdivision of flood plain <br /> <br />No division of flood plain is <br />necessary. <br /> <br />17 <br /> <br />Roughness factors <br /> <br />ROughness factors to be considered <br />are surface irregularity and <br />vegetation. <br /> <br />18 <br /> <br />Table 1 gives a base nb value for <br />firm earth of 0.020-0.030. USe <br />0.025. <br /> <br />Base nb <br /> <br />19 <br /> <br />Adjustment factors <br /> <br />Irregularity is moderate with <br />furrows parallel to flow on flood <br />plain, nl = 0.010 (table 3) . <br />Vegetation is cotton crop, depth of <br />flow about equal to height of <br />vegetation, n4 = 0.040 (table 3) . <br /> <br />N::Jt applicable. <br /> <br />20 <br /> <br />no <br /> <br />21 <br /> <br />Vegetation density of <br />representative sample area <br /> <br />Not applicable. <br /> <br />22 <br /> <br />n for flood plain <br /> <br />n = (nb + nl + n2 + n3 + n4)m <br />n = (0.025+0.01+0+0+0.040)1.00 <br />n = 0.075 <br /> <br />23 <br /> <br />Ree and Crow (1977, p. 39-40) <br />assigned cotton fields an n value <br />of about 0.08. <br /> <br />Compare with other <br />flood plains <br /> <br />59 <br />