§ 10.3.4.4. Free Surface at Outlet and Full Flow at Inlet - Type AB

If the frictional slope is greater than the culvert slope and the outlet water surface H _o is less than the culvert soffit at the outlet, calculate the H _i using the following steps.

Step 1 - Start with the outlet depth H _o and

Step 2 - Use a standard step backwater to determine the point along the conduit where the water surface will intersect the soffit.

Step 3 - At this point along the culvert length, the remaining culvert length L _f is substituted for L in the Equation 10.3.4.2.b to determine h _ff in Figure 10.3.4.4.

Step 4 - With the hydraulic grade line at the culvert inlet, the headwater elevation at the entrance can be calculated. See Energy Balance at Inlet section for further calculations.

Figure 10.3.4.4 - Headwater due to Full Flow at Inlet and Free surface at Outlet
(Source TxDOT, Hydraulic Design Manual, 2011)