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<br />Meandering (m) <br /> <br />In selecting the value of m, the degree of meandering depends on the <br />ratio of the total length of the meandering channel in the reach being <br />considered to the straight length of the channel reach. The meandering <br />is considered minor for ratios of 1.0 to 1.2, appreciable for ratios of <br />1.2 to 1.5, and severe for ratios of 1.5 and greater. According to Chow <br />(1959), meanders can increase the n values as much as 30 percent where <br />flow is confined within a stream channel. The meander adjustment should <br />only be considered when the flow is confined to the channel. There may <br />be very little flow in a meandering channel when there is flOOd-plain <br />flow. <br /> <br />METlDDS FOR ASSIGNING n VALUES FOR FIOOD PLAINS <br /> <br />As stated earlier, it is usually necessary to determine roughness <br />values for channels and flood plains separately. The makeup of a flood <br />plain can be quite different from that of a channel. The physical shape <br />of a flood plain is different from that of a channel and the vegetation <br />covering a flood plain is typically different from that found in a <br />channel. The procedure given in the following paragraf*1s is designed for <br />determining an n value for flood plains. <br /> <br />Modified Channel Method <br /> <br />By altering Cowan's (1956) procedure that was developed for esti- <br />mating n values for channels, the following equation can be used to <br />estimate n values for a flood plain. <br /> <br />where n = (nb + nl + n2 + n3 + n4)m <br /> <br />(6) <br /> <br />nb = a base value of n for the flood plain's natural <br />bare soil surface, with nothing on the surface, <br />nl = a value to correct for the effect of surface <br />irregularities on the flood plain, <br />n2 = A value for variations in shape and size of the <br />flOOd-plain cross section, assumed to equal 0.0, <br />n3 = a value for obstructions on the flood plain, <br />~ = a value for vegetation on the flood plain, <br />m = a correction factor for sinuosity of the flood <br />plain, equal to 1.0. <br /> <br />Using equation 6, the roughness value for the flood plain is determined <br />by selecting a base value of nb for the natural bare soil surface of <br />the flood plain and adding adjustment factors due to surface irregularity, <br />obstructions, and vegetation. The selection of an nb value is the same <br />as outlined for channels in the previous section. A description of the <br />major factors follows, and table 3 gives n value adjustments. The adjust- <br />ment for cross-section shape and size is assumed to be 0.0. The cross <br /> <br />13 <br />