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section of a flood plain is generally subdivided where there are abrupt changes in the shape of the flood plain. The adjustment for meandering is assumed to be l. 0, because there may be very 1 ittle flow in a meander- ing channel when there is flood-plain flow. In certain cases where the roughness of the flood plain is caused by trees and brush, the roughness value for the flood plain can be determined by measuring the "vegetation density" of the flood plain rather than directly estimating from table 3. This is discussed under "Vegetation Density Methods". Adjustment Factors for Flood Plains Surface Irregularities (~).--Irregularity of the surface of a flood plain causes an increase in the roughness of the flood plain. Such physical factors as rises and depressions of the land surface and sloughs and hummocks increase the roughness of the flood plain. A hummock can be defined as a low mound or ridge of earth above the level of an adjacent depression. A slough is a stagnant swamp, marsh, bog, or pond. Shallow water depths, accompanied by an irregular ground surface in pastureland or brushland and by deep furrows perpendicular to the flow in cultivated fields, can increase the n values by as much as 0.02. Obstructions (ng) .--The roughness contribution of some obstructions on a flood plain, such as debris deposits, stumps, exposed roots, logs, or isolated boulders, cannot be measured directly but must be considered. Table 3 lists values of roughness for obstructions at different percent- ages of occurrence. Vegetation (n4) .--Visual observation, judgment, and experience may be used in selecting adjustment factors for the effects of vegetation from table 3. An adjustment factor for tree trunks and other measureable obstacles is described in the next section. Although it is relatively easy to measure the area occupied by tree trunks and other major vegetation, it is much more difficult to measure the area occupied by vegetation such as low vines, briars, grass, and crops. Mjustments of the roughness factor due to these types of vegetation can be determined by using table 3. In the case of open fields and cropland on flood plains, several references are available to help determine the roughness factors. Ree and Crow (1977) conducted experiments to determine roughness factors for earthen channels of small slope planted to wheat, sorghum, lespedeza, and grasses. The roughness factors were intended for application to the design of diversion terraces. However, the data can be applied to the design of any terrace, or they can be used to estimate the roughness of flood plains planted to the type of vegetation used. 16