Bridge Flow Class
The losses associated with flow through bridges depend on the hydraulic conditions of low or high flow.
Low flow describes hydraulic conditions in which the water surface between Zones 1, 2, and 3 is open to atmospheric pressure. That means the water surface does not impinge upon the superstructure. (This condition should exist for the design frequency of all new on-system bridges.) Low flow is divided into categories as described in Table 9-2 “Low Flow Classes”. Type I is the most common in Texas, although severe constrictions compared to the flow conditions could result in Types IIA and IIB. Type III is likely to be limited to steep hills and mountainous regions.
Type Designation | Description |
I | Subcritical flow through Zones |
IIA | Subcritical flow Zones 1 and 3, flow through critical depth Zone 2 |
IIB | Subcritical Zone 3, flow through critical Zone 2, hydraulic jump Zone 1 |
III | Supercritical flow through Zones 1, 2 and 3 |
High flow refers to conditions in which the water surface impinges on the bridge superstructure:
- When the tailwater does not submerge the lowchord of the bridge, the flow condition is comparable to a pressure flow sluice gate.
- At the tailwater, which submerges the lowchord but does not exceed the elevation of critical depth over the road, the flow condition is comparable to orifice flow.
- If the tailwater overtops the roadway, neither sluice gate flow nor orifice flow is reasonable, and the flow is either weir flow or open flow.