6.13 Chapter Summary

Proper modeling of bridges for floodplain hydraulic analysis requires significant time, effort, and judgment on the part of the modeler. Bridge modeling requires the engineer to simulate flood flows as they contract into the bridge, with the velocity accelerating through the bridge opening, and then to model the expansion of flow on the downstream side of the bridge, as the velocity decelerates. Effects on flow from obstructions within the bridge must also be evaluated.

Flood flows through the bridge can be analyzed with the energy, momentum, Yarnell equation, WSPRO, pressure flow, weir flow method, or by a combination of methods. Flows through the bridge opening may be defined as either low flow (water surface elevation less than the low chord elevation) or high flow (elevation greater than the low chord value). The water surface elevations through the bridge reach may be subcritical, critical, or supercritical, requiring different computation methods to best define the resulting flood profile.

Bridge modeling requires specifying the beginning of the contraction and the end of the expansion for the bridge, determining the proper lengths of contraction and expansion, making adjustments to the expansion and contraction coefficients and to the n value, as well as specifying the ineffective flow area elevations and locations on both the upstream and downstream sides of the bridge opening. The estimation and refinement of ineffective flow elevations and locations is a particularly vexing problem for many modelers, often requiring an iterative analysis before an acceptable solution is reached. Modeling the actual bridge requires the incorporation of bridge and approach geometry (roadway surface and low chord elevations), pier and abutment geometry, bridge width, various coefficients for pier shape and weir flow, and the specification of modeling computation procedures for both high and low flow scenarios. Special procedures may be necessary to model dual bridges, perched bridges, low-water crossings, a bridge and embankment that act as a dam, bridge crossings that have multiple openings and/or bridges on skews.

The FHWA's WSPRO method can be used within HEC-RAS to compute profiles through bridges or to analyze bridge openings using FHWA methods. Procedures for WSPRO vary from those of the other bridge analysis techniques, especially in cross-section location and in better evaluating the effects of bridge opening features, such as spur dikes and abutments.