The importance of proper subdivision as well as the effects of improper subdivision can be illustrated dramatically. Figure 6‑6 shows a trapezoidal cross section having heavy brush and trees on the banks and subdivided near the bottom of each bank because of the abrupt change of roughness.

The conveyance for each subarea is calculated as follows:

A 1 = A 3 = 50 ft 2	A 1 = A 3 = 4.5 m 2
P 1 = P 3 = 14.14 ft	P 1 = P 3 = 4.24 m
R 1 = R 3 = A 1 /P 1 = 3.54 ft	R 1 = R 3 = A 1 /P 1 = 1.06 m
K 1 = K 3 = 1.486A 1 R 1 2/3 /n = 1724.4 cfs	K 1 = K 3 = A 1 R 1 2/3 /n = 46.8 m 3 /s
A 2 = 500 ft 2	A 2 = 45 m 2
P 2 = 50 ft	P 2 = 15 m
R 2 = A 2 /P 2 = 10 ft	R 2 = A 2 /P 2 = 3 m
K 2 = 1.486A 2 R 2 2/3 /n = 98534.3 cfs	K 2 = A 2 R 2 2/3 /n = 2674.4 m 3 /s

When the subareas are combined, the effective n-value for the total area can be calculated.

A c = A 1 + A 2 + A 3 = 600 ft 2	A c = A 1 + A 2 + A 3 = 54 m 2
P c = P 1 + P 2 + P 3 = 78.28 ft	P c = P 1 + P 2 + P 3 = 23.5 m
R c = A c /P c = 7.66 ft	R c = A c /P c = 2.3 m
K T = K 1 + K 2 + K 3 = 101983 cfs	K T = K 1 + K 2 + K 3 = 2768 m 3 /s
n = 1.486A c R c 2/3 /K T = 0.034	n = A c R c 2/3 /K T = 0.034

A smaller wetted perimeter in respect to area abnormally increases the hydraulic radius (R = A / P), and this results in a computed conveyance different from that determined for a section with a complete wetted perimeter. As shown above, a conveyance (K

T

) for the total area would require a composite

n-value of 0.034. This is less than the

n-values of 0.035 and 0.10 that describe the roughness for the various parts of the basic trapezoidal shape. Do not subdivide the basic shape. Assign an effective value of

n somewhat higher than 0.035 to this cross section, to account for the additional drag imposed by the larger roughness of the banks.

At the other extreme, you must subdivide the panhandle section in Figure 6‑7, consisting of a main channel and an overflow plain, into two parts. The roughness coefficient is 0.040 throughout the total cross section. The conveyance for each subarea is calculated as follows:

A 1 = 195 ft 2	A 1 = 20 m 2
P 1 = 68 ft	P 1 = 21 m
R 1 = A 1 /P 1 = 2.87 ft	R 1 = A 1 /P 1 = 0.95 m
K 1 = 1.486A 1 R 1 2/3 /n = 14622.1 cfs	K 1 = A 1 R 1 2/3 /n = 484.0 m 3 /s
A 2 = 814.5 ft 2	A 2 = 75.5 m 2
P 2 = 82.5 ft	P 2 = 24.9 m
R 2 = A 2 /P 2 = 9.87 ft	R 2 = A 2 /P 2 = 3.03 m
K 2 = 1.486A 2 R 2 2/3 /n = 139226.2 cfs	K 2 = A 2 R 2 2/3 /n = 3954.2 m 3 /s

The effective n-value calculations for the combined subareas are as follows:

A c = A 1 + A 2 = 1009.5 ft 2	A c = A 1 + A 2 = 95.5 m 2
P c = P 1 + P 2 = 150.5 ft	P c = P 1 + P 2 = 45.9 m
R c = A c /P c = 6.71 ft	R c = A c /P c = 2.08 m
K T = K 1 + K 2 = 153848.3 cfs	K T = K 1 + K 2 = 4438.2 m 3 /s
n = 1.486A c R c 2/3 /K T = 0.035	n = A c R c 2/3 /K T = 0.035

If you do not subdivide the section, the increase in wetted perimeter of the floodplain is relatively large with respect to the increase in area. The hydraulic radius is abnormally reduced, and the calculated conveyance of the entire section (K

c

) is lower than the conveyance of the main channel, K

2

. You should subdivide irregular cross sections such as that in Figure 6‑7 to create individual basic shapes.

The cross section shapes in Figure 6‑6 through Figure 6‑9 represent extremes of the problems associated with improper subdivision. A bench panhandle, or terrace, is a shape that falls between these two extremes (see Figure 6‑8). Subdivide bench panhandles if the ratio L/d is equal to five or greater.

The following guidelines apply to the subdivision of triangular sections (see Figure 6‑9):

Figure 6‑10 shows another shape that commonly causes problems in subdivision. In this case, subdivide the cross section if the main-channel depth (d

max

) is more than twice the depth at the stream edge of the overbank area (d

b

).

In some cases the decision to subdivide is difficult. Subdivisions in adjacent sections along the stream reach should be similar to avoid large differences in the kinetic energy coefficient (α). Therefore, if a borderline case is between sections not requiring subdivision, do not subdivide the borderline section. If it is between sections that must be subdivided, subdivide this section as well.