HYDRAULIC COMPUTATIONS (1989)
a. Following the balanced design approach, establish the minimum opening required to pass the 25-year flood
with no ponding.
b. From the hydrology data the 25-year discharge has been established. The pipe size to carry this flow with no
head at the entrance, HW/D = 1.0, is determined from the charts. Using the 25-year discharge, determine
the pipe size required for both inlet and outlet control and use whichever is greater. This is the minimum
size opening required in the culvert.
c. Inlet Control. From Figure 1-4-6 through Figure 1-4-12 the headwater for a given pipe can be determined.
Using the minimum size selected for the 25-year flood, determine the headwater (for the entrance condition
desired) for the 100-year flood discharge. If this amount of headwater is acceptable for the culvert in
question, the minimum size is satisfactory for the full 100-year design discharged in inlet control. If the
headwater is too high, a larger size must be selected corresponding to the maximum permissible headwater.
Now check for possible outlet control, as follows