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<br />Agricultural Overbanks <br /> <br />When evaluating the roughness characteristics of agricultural overbanks, <br />all the previously described factors except meandering need to be considered. <br />Guidelines for estimating n values for agricultural overbanks are given in <br />table 6. In evaluating overbank roughness with respect to minimum, normal, <br />and maximum conditions, the following need to be considered. Removal of dense <br />natural vegetation will decrease roughness, Leveling and regrading for culti- <br />vation, irrigation, and drainage may smooth out natural surface irregulari- <br />ties. Construction of roads, irrigation ditches, and laterals may obstruct <br />flow. Type and orientation of crops with respect to the direction of flow can <br />significantly affect n values. Row crops parallel to the direction of flow <br />offer less resistance to flow than field crops or row crops planted transverse <br />to the direction of flow. Roughness characteristics can be significantly <br />affected by the depth of flow. General guidelines on changing n values with <br />depth of flow are given in table 7. <br /> <br />It may be necessary to revise initial values after a preliminary <br />hydraulic calculation has been made and depth of flow has been more accurately <br />determined. Several computer models enable the user to input a varying n <br />value with-depth relation. Probable flood-season, crop-growth conditions need <br />to be considered when selecting n values. <br /> <br />Because it is difficult or impossible to predict farming practices--which <br />fields will lie fallow, what types of crops will be planted--selection of <br />roughness coefficients needs to be based on typical growing-season conditions, <br /> <br />Table 7.--Sample values of n for agricultural overbank areas under <br />various stages for the average growing season <br />[From Chow, 1959] <br /> <br />Flood-plain cover1 <br /> <br />Depth of <br />water, in feet <br /> <br />Corn <br /> <br />Pasture <br /> <br />Meadow <br /> <br />Small <br />grains <br /> <br />Brush and <br />waste <br /> <br />Less than 1--- 0,06 0.05 0.10 <br />1 to 2-------- .06 .05 ,08 <br />2 to 3-------- .07 .04 .07 <br />3 to 4-------- .07 .04 .06 <br />More than 4--- .06 .04 .05 <br /> lFrom studies on the Nishnabotna River, Iowa. <br /> <br />0.10 <br />.09 <br />.08 <br />,07 <br />.06 <br /> <br />0.12 <br />.11 <br />.10 <br />.09 <br />.08 <br /> <br />41 <br />