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It is important that the sampling area be representative of the roughness coefficient of the cross section. By examining the cross section closely in the field, a representative-sampling area can be chosen. Another way to better determine the roughness coefficient is to select several representative areas and compare the results. It should be pointed out again that cross sections should be divided into subsec- tions when changes in roughness properties dictate. All of the trees, including vines, in the sampling area must be counted and the diameter measured to the nearest 0.1 ft. Each tree diameter is measured at a height that will give an average diameter for the expected flow depth of the sample area. Determining the area of the trees in the sampling area is not very difficult. A sampling area 100 ft along the cross section by 50 ft in the flow direction is generally adequate to determine the vegetation density of an area when the sample area is representative of the flood plain. A 100-ft tape is stretched out perpendicular to the flow direction in the sample area. Every tree within 25 ft along either side of the 100-ft tape is counted. 'Ihe position of the tree is plotted on a grid system by measuring the distance to each tree from the centerline along the 100-ft tape and the diameter of the tree is recorded on the grid system. (See fig. 4.) The area, L Ai, occupied by trees in the sampling area can be computed from the number of trees, their diameter, and the depth of flow in the flood plain. Once the vegetation area, LAi' is determined, the vegetation density can be computed using equation 8 and the n value for the subsection can be determined using equation 7 with appropriate values for no, R, and C*. where L nidi = h = w = and 1 = Equation 8 can be simplified to LA. h Ln.d. 1 1 1 vegd = AL = hwl the summation of number of trees multiplied by tree diameter, in feet; height of water on flood plain, in feet; the sample area width, in feet; the sample area length, in feet. (11) Example of use of direct technique.--An example of how to compute n for a flood plain using the direct method for vegetation density is shown in figures 4 and 5. A representative sample area along the cross section is chosen. The Vegd of the sample area is determined by measuring the number and diameter of trees in the 100 ft by 50 ft area. 'Ihis is easily done by plotting the location and diameter of the trees, in the sample area on the grid shown in figure 4. The numbers by the dots in figure 4 are the diameters of the trees, in tenths of a foot, except for those numbers underlined. The numbers underlined are the diameters of the trees in feet. 20