Free Surface Flow (Type A)

If free surface flow is occurring in the culvert, the hydraulic parameters are changing with flow depth along the length of the culvert as seen in Figure 8‑9. It is necessary to calculate the backwater profile based on the outlet depth, H
o
.
Outlet Control Headwater for Culvert with Free Surface (click in image to see full-size image)
Figure 8-9. Outlet Control Headwater for Culvert with Free Surface
By definition, a free-surface backwater from the outlet end of a culvert may only affect the headwater when subcritical flow conditions exist in the culvert. Subcritical, free-surface flow at the outlet will exist if the culvert is on a mild slope with an outlet depth (H
o
) lower than the outlet soffit or if the culvert is on a steep slope with a tailwater higher than critical depth at the culvert outlet and lower than the outlet soffit.
The
Direct Step Backwater Method
is used to determine the water surface profile (and energy losses) though the conduit. The depth, H
o
, is used as the starting depth, d
1
. For subcritical flow, the calculations begin at the outlet and proceed in an upstream direction. Use the depth, H
o
, as the starting depth, d
1
, in the Direct Step calculations.
When using the direct step method, if the inlet end of the conduit is reached without the calculated depth exceeding the barrel depth (D), it verifies that the entire length of the conduit is undergoing free surface flow. Set the calculated depth (d
2
) at the inlet as H
i
 and refer to Energy Balance at Inlet to determine the headwater.
When using the direct step method, if the calculated depth (d
2
) reaches or exceeds the barrel depth (D), the inside of the inlet is submerged. Refer to Type AB - Free surface at outlet and full flow at inlet for a description. This condition is possible if the theoretical value of uniform depth is higher than the barrel depth.