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Publication Number: FHWA-RD-02-078
Date: November 2003

Bottomless Culvert Scour Study: Phase I Laboratory Report

5. CONCLUSIONS

The abutment scour concept of using the flow distribution at the culvert entrance to compute the primary scour depth component and adjusting that with an empirical factor based on laboratory data appears to be valid for bottomless culverts. The culvert shapes tested in these experiments did not significantly influence the scour; however, the entrance conditions did influence the scour.

Equations are presented to estimate the maximum expected flow depths at the upstream corners of bottomless culverts under clear-water conditions. Equations are also presented to estimate the riprap sizes needed to protect bottomless culvert footings from scour.

Two methods for approximating the initial representative velocity and the critical incipient motion velocity, one proposed by GKY and one proposed by Maryland DOT (Chang), were tested. Either method seemed to work reasonably well for representative velocity. The GKY method for critical velocity seemed to work better for the laboratory data; however, critical velocity is independent of the flow depth by the GKY method and is expected to give unreasonably conservative scour estimates for typical field conditions. Maryland DOT's (Chang's) equations for critical velocity were derived for field conditions with depths of 1.5 m (5 ft) or greater and they had to be extrapolated considerably below that depth to be applied to the laboratory conditions.

The limitations of the experimental setup are much more important than the details of which methods should be used for computing velocities. These results are based on laboratory flume experiments with a flat approach cross section with uniform flow conveyance, which is not typical of field conditions. The experiments were also conducted under clear-water approach flow conditions with no sediment being transported into the culvert. The study should be considered a preliminary investigation of a problem that had not been adequately addressed previously. The authors attempted to present the results in terms of overbank flow rather than geometric variables because it can account for the reduced conveyance that is typical of overbank flow for natural streams. These results have not been tested for field conditions; however, they are offered as initial guidance for field applications. An anticipated next step is that the Maryland SHA will adopt the results as preliminary design guidelines and test them for field sites using engineering judgment to decide if the applications are reasonable.

Additional research could extend and/or improve upon the study results, including:

  • Conceptual sediment balance relationships to extend the analysis to live-bed conditions. The authors propose that Laursen's "sediment-in equals sediment-out" logic (that the amount of sediment entering a stream segment must equal the amount of sediment exiting) should apply with reasonable assumptions about flow distributions. An inherent assumption is that the empirical adjustment factors from the clear-water experiments can be applied to live-bed conditions. Live-bed flume experiments with sediment transport in the main channel and clear water (no sediment) in overbank flow are needed to test these assumptions.
  • Additional riprap tests to improve the riprap analysis. More data are needed, including experiments with wingwalls.
  • Derivation of a safety factor to envelop the experimental riprap data. Engineers often find that they end up using the same class of riprap for a wide range of requirements. A safety factor provides a level of confidence in applying engineering judgment in these situations.
  • Fixed-bed experiments to accurately measure initial flow distributions and flow redistribution as it flows through the culvert. One of the problems with moveable-bed experiments is that conditions change as soon as the experiments begin. This information will help validate approximations and determine how the scour depths might diminish and redistribute beyond the culvert entrance. Fixed-bed velocity measurements need to be compared to the 1D approximations and 2D numerical model results to determine if the numerical model flow distribution would be a better platform for developing the regression equations.

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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). The hydraulics and hydrology research program at the TFHRC Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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