The most important consideration in culvert hydraulic performance is whether the flow is subject to inlet or outlet control. Figure 3.5 shows eight typical conditions under which culverts may operate (i.e. the range of flow types commonly encountered in culverts). For inlet control two distinct regimes exist, depending on whether the inlet is submerged or not submerged. Outlet control occurs in long culverts, culverts laid on flat grades and culverts with high tailwater depths.
A study of Figure 3.5 will reveal that the prediction of inlet or outlet control is not precise and the standard procedure for calculation is to assume each control situation in turn, the control situation with the highest calculated headwater being adopted for design.
Checks should be made to ensure a submerged inlet type (HW > 1.2D). Submerged inlets usually have more driving head which helps improve culvert capacity/performance, which is preferable.
The desirable control condition is outlet control as shown in Type 7 in Figure 3.5.
Source: Herr and Bossy, cited in DTMR (2010).