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e " . e Note: Headwater depth determined in b(3) becomes in- creasingly less accurate as the headwater com- puted by this method falls below the value V2 D + (1 + ke)2g' (See discussion under "Culvert Flowing Full with Outlet Control", p. 5-9.) c. Compare the headwaters found in Step 3a and Step 3b (In- let Control and Outlet Control). The higher headwater governs and indicates the flow control existing under the given conditions for the trial size selected. d. If outlet control governs and the HW is higher than is acceptable, select a larger trial size and find HW as in- structed under Step 3b. (Inlet control need not be checked, since the smaller size was satisfactory for this control as determined under Step 3a.) Step 4: Try a culvert of another type or shape and determine size and HWby the above procedure. Step 5: Compute outlet velocities for size and types to be considered in selection and determine need for channel protection. a. If outlet control governs in Step 3c above, outlet veloc- ity equals t, where Ao is the cross-sectional area of flow in the culvert barrel at the outlet. If dc or ~ is less than the height of the culvert barrel use Ao corres- ponding to dc or ~ depth, whichever gives the greater area of flow. Ao should not exceed the total cross- sectional area A of the culvert barrel. b. If inlet control governs in step 3c, outlet velocity can be assumed to equal mean velocity in open-channel flow in the barrel as computed by Manning's equation for the rate of flow, barrel size, roughness and slope of culvert selected. Note: Charts and tables are helpful in computing outlet velocities. (See references p. 5-14.) Step 6: Record final selection of culvert with size, type, required headwater, o~tlet velocity, and economic justification. 5-17