For outlet control type of flow, finding the value of H is not the complete solution for the Headwater (HW) which is dependent on factors such as slope of the culvert barrel and outlet conditions.
The headwater (HW) can be defined as follows:
ho is the greater of TW and where hc<or = D
So is slope of the flow line (m/m)
Refer Figure 27.10 (MSMA,2000) or Figure 18.6 (MSMA,2011) for the determination of ho.