HEC-RAS performs for steady or unsteady, uniform or nonuniform flow. UNET is for unsteady,
uniform, and nonuniform flow. Although one-dimensional, they can give an approximate
distribution of the velocity in a cross-section. Embankment overtopping flows, in conjunction with
either free surface or pressure flow through the bridge, can be computed. The programs are
capable of computing profiles at stream crossings with multiple openings (including culverts), at
river confluences, and mixed flow regimes. They also, incorporate the effect of wide, wooded,
floodplains into the bridge backwater calculations. In addition, there are two one-dimensional
computer models which include sediment transport. These are FHWA's BRI-STARS (Molinas
2000) and U.S. Army Corps of Engineers HEC-6 (1993) computer programs.
2.11.2 Two-Dimensional Computer Models
Two-dimensional computer models give the water surface profile, and the depth and velocity
along and across the stream. Of the many models, two have the most utility for highway bridge
analysis. They are the FHWA's FESWMS (Froehlich 1996) and the U.S. Army Corps of
Engineer's (1997) RMA-2V (Thomas and McAnally 1985) models. The models are for steady or
unsteady and nonuniform open channel flow. They have all the capabilities of the one-
dimensional programs and have the utility of giving the velocity and depth distribution along and
across the channel as a function of time and distance. They can and have been used to analyze
tidal flows (Zevenbergen et al. 1997, Ayres Associates 1994 and 1997, and Richardson and
Lagasse 1999 pages 701 to 824) Both models require that a grid system be created for the river
system. However, constructing the grid is greatly aided by the use BYU's (2000) SMS modeling
A culvert is a conduit which conveys stream flow through a roadway embankment. Most culverts
are constructed of concrete, corrugated aluminum, corrugated steel, and sometimes corrugated
plastics. Culvert shapes vary from circular to rectangular, and elliptical, pipe arch, arch and metal
box sections are commonly used.
Two basic types of flow control are recognized depending on the location of the control section:
inlet control or outlet control. The characterization of pressure, as well as subcritical and
supercritical flow regimes play an important role in determining the location of the control section.
Inlet control occurs when the culvert barrel is capable of carrying more flow than the inlet will
accept. Critical flow depth is located at the inlet and the flow is supercritical in the barrel.
Outlet control flow occurs when the culvert barrel is not capable of conveying as much flow as the
inlet opening will accept. Under outlet control conditions, either subcritical or pressure flow exists
in the culvert barrel.
The hydraulic design of culverts is given in HDS 5 (FHWA 1985) and HY8 (FHWA 1998) is a
computer program for the design and analysis of culvert flows.