3.8.3 Outlet Control Conditions
With outlet control, the culvert flow is restricted to the discharge which can pass through the conduit for a given level of water in the outlet channel (tailwater level). The slope, cross-sectional area, roughness and length of the culvert barrel have to be considered as these losses exceed the inlet losses. However, inlet size, shape and edge geometry can still affect the capacity.
In general the control will be at the outlet if the culvert slope is less than critical. A tailwater depth equal to 80% or more of the height of the culvert barrel/cell will usually indicate outlet control, except in rolling or mountainous country with the culvert on natural surface slopes. However, a check of the design assuming inlet control is such an easy process that it forms part of standard design procedure.
Culverts flowing with outlet control can flow with the culvert barrel full or with the barrel part-full for all of the culvert length. With outlet control, having both the inlet and the outlet submerged, the culvert flows full and under pressure. The culvert also can flow full over part of its length with part‑full flow at the outlet. The point at which the water surface breaks away from the barrel obvert depends on the tailwater depth and culvert grade, and can be determined by using flow profile calculations.
If the culvert is laid at a flat grade, outlet control can occur with both inlet and outlet not submerged, and the part-full flow throughout the culvert length will be flowing under sub-critical conditions.
Variations of these main types of flow conditions can occur, depending on the relative value of critical slope, normal depth, culvert height and tailwater depth. While the potential flow conditions shown in Figure 3.5 are the most common for simple culverts, different flow conditions are possible where complex culvert structures are required. Advice may be required from an expert in such cases.
In Figure 3.5, Types 1, 2, 6 and 7 have outlet control.