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Aug 07 2012

Criteria for Selecting Hydraulic Models, Part 4

Continuing with our coverage of the new HDS 7 manual from FHWA.

Part 1, Part 2, Part 3

Countermeasure Design

The HEC-23 manual provides guidance on designing countermeasures for stream instability and scour. Many countermeasures, including spurs, guide banks, and transverse dikes, significantly alter flow paths and flow distributions. Two-dimensional models that are set up with a complete three-dimensional representation of the channel and countermeasure provide an accurate simulation of the flow field in the horizontal plane including locations of high velocity, flow separation and flow circulation. Three-dimensional models, CFD modeling, and physical hydraulic modeling may be required for analyzing extremely complex flow fields with large vertical velocity components that can occur at countermeasures.

Typical Guide Bank

Summary

No numerical model provides an exact representation of the complexities of an actual flow condition. This is especially true where roadways cross natural water courses with variable channel bathymetry, floodplain topography, land use, and vegetation. The assumptions that are required for one-dimensional models are often violated to a greater degree than is commonly thought, though in many cases experienced hydraulic engineers can compensate for some of the limitations of one-dimensional models. Because two-dimensional models avoid many assumptions required by one-dimensional models, they better represent the physics of the flow and provide more realistic hydraulic results, especially at highway encroachments. Therefore, two-dimensional models should be used for many bridge hydraulics and scour problems. Table 4.1 provides guidance on selecting one- versus two-dimensional modeling for bridge hydraulic and scour analyses. Two-dimensional models provide more accurate results for hydraulically complex conditions. Table 4.1 does not include three-dimensional numerical modeling, computational fluid dynamics (CFD), or physical hydraulic modeling because these methods are used primarily to simulate individual piers or other bridge elements and are rarely used to analyze the entire bridge reach.

Table 4.1  Bridge Hydraulic Modeling Selection

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