Backwater computational models

Computer programs are used to calculate water surface profiles, lateral velocity distributions, flow regimes, and scour potential. For projects that are likely to involve revisions to Federal Emergency Management Agency’s (FEMA) Flood Insurance Rate Maps, selection of the hydraulic model should be coordinated carefully with FEMA. Following are some standard hydraulic models.

HEC–RAS
HEC–RAS (USACE 2001b) is the recommended computer program for performing hydraulic calculations for steady and unsteady, gradually varied (over distance), one-dimensional, open channel flow. HEC–RAS includes a culvert module that is consistent with HDS– 5 and HY–8. The bridge hydraulics algorithms now include the WSPRO models. HEC–RAS applies conservation of momentum, as well as energy and mass, in its hydraulic analysis. HEC–RAS includes all the features inherent to HEC–2 and WSPRO, plus several friction slope methods, mixed flow regime support, automatic n value calibration, ice cover, quasi 2–D velocity distribution, and super-elevation around bends.

HEC–2
HEC–2 (USACE 1990b) performs hydraulic calculations for steady, gradually varied (over distance), one-dimensional, open channel flow. One of HEC–2’s technical limitations is that the normal bridge routines and standard-step backwater computations use energy conservation only. Conservation of momentum is used only in the special bridge routines when bridge piers
are involved.

WSPRO
The WSPRO computer program was developed by the USGS and is comparable to HEC–2, except for the fact that WSPRO had special subroutines for analysis of water surface profiles at bridge locations. All of these WSPRO subroutines have been incorporated into HEC–RAS. The current version of WSPRO is no longer being supported by USGS.

HY–22
HY–22 is a small tool kit of relatively simple computer programs for performing the hydraulic analyses described in the Urban Drainage Design Manual, Hydraulic Engineering Circular No. 22, U.S. Department of Transportation Federal Highway Administration (FHWA) (1996). HY–22 includes pavement drainage, open channel hydraulics, critical depth computation, computation of storage volume, and simple reservoir routing.