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Jul 26 2012

Criteria for Selecting Hydraulic Models, Part 1

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

Multiple Openings

Multiple openings along an embankment are often used on rivers with wide floodplains. Rather than using a single bridge, additional floodplain bridges are included. Although one-dimensional models can be configured to analyze multiple openings, the judgment and assumptions that are made by the hydraulic engineer in combination with the assumptions and limitations of the software result in an extreme degree of uncertainty in the results. The proportion of flow going through a particular bridge and the corresponding flow depth and velocity are important for structure design and scour analysis. Because multiple opening bridges represent a large investment, two-dimensional analysis is always warranted.

Multiple OpeningsFigure 4.1. Two-dimensional model velocities, US 1 crossing Pee Dee River.

Another type of multiple opening is multiple bridges in series. There are conditions when this bridge configuration should be analyzed using two-dimensional models. These include unmatched bridge openings or foundations that do not align. An upstream or downstream railroad or parallel road may significantly alter the flow conditions and warrant two-dimensional analysis.

Figure 4.1 shows two-dimensional model results (velocity magnitude) for the U.S. Route 1 crossing over the Pee Dee River in South Carolina. Flow is generally from top to bottom in this figure. This model illustrates several reasons for selecting two-dimensional modeling. The floodplain width ranges from 4,000 to 8,000 ft (1,200 to 2,400 m) and has highly variable land use and vegetation. The US 1 crossing includes a 2,000 ft (600 m) main channel bridge and two 500 ft (150 m) relief bridges. There is also a railroad crossing downstream. Although the railroad also has three bridge openings, they are shorter and not aligned with the US 1 bridges. The highest velocity, greater than 8 ft/s (2.4 m/s) occurs in the main channel. However, the center relief bridge has an average velocity of nearly 6 ft/s (1.8 m/s) and the eastern relief bridge has velocities of over 7 ft/s (2.1 m/s). The floodplain area under the main channel bridge, however, has velocities ranging from 1 to 3.5 ft/s (0.3 to 1.1 m/s). Therefore, overall conveyance would be improved and backwater would be reduced by shortening the main channel bridge and lengthening the relief bridges. If changing the bridge lengths would adversely impact the downstream railroad bridges, the two-dimensional model results would also quantify those impacts.

Wide Floodplains

Floodplains often include features that significantly impact flow conveyance and flow distribution. Historic channel alignments and changes in land use or vegetation affect floodplain flow distribution. In a one-dimensional model, two cross sections that are a short distance apart may have significantly different vegetation, such as wooded versus cleared, or may have significantly different topography due to land use activities. If the hydraulic engineer uses these cross sections exactly as they exist, the one-dimensional model will depict a sudden change in flow distribution that is not physically possible.

GCID oblique looking us

To better depict the flow conditions, the hydraulic engineer would need to adjust the cross section locations or alter the Manning n values, although this is difficult to implement. The two-dimensional model avoids these difficulties because in the simulation all the flow is interconnected. Therefore, wide and complex floodplains benefit from two-dimensional analysis.

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