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Section 5: Scour

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Analysis

Bridge foundations for new bridges over waterways require a scour analysis. Conduct scour analyses in accordance with the following:

  • Guidelines outlined in Evaluating Scour at Bridges (HEC-18).
  • Do not calculate abutment scour because none of the equations to date yield acceptable results. Protect abutments against potential scour through use of a flexible revetment, where possible.

The table below defines the minimum scour design flood frequencies and scour design check flood frequencies for a given hydraulic design flood frequency. These values are to be used to ensure that a bridge will remain stable for a given design flood frequency.

Anchor: #i1054971Table 5-3: Hydraulic Design, Scour Design, and Scour Design Check Flood Frequencies

Hydraulic Design Flood Frequency, QD

Scour Design Flood Frequency, QS

Scour Design Check Flood Frequency, QC

Q10

Q25

Q50

Q25

Q50

Q100

Q50

Q100

Q200

Q100

Q200

Q500



Evaluate existing bridge foundations for potential scour using the guidelines outlined in either of the following:

Determine scour at bridges using the following guidelines:

  • Use the following table to determine susceptibility of competent rock to scour when it is present at moderate to shallow depths. Consider materials deemed either not susceptible or mildly susceptible to scour the limit of the maximum scour depth.
    Anchor: #i1039154Table 5-4: Material Susceptibility to Scour

    Material

    Subtype

    TCP Values

    Susceptibility

    Rock

    Hard (granite, limestone, shale)

    < 4 in./100 blows

    Not susceptible

    Soft (shale)

    < 12 in./100 blows

    Mildly susceptible but not considered over time span of one flood event

    Clays

    Hard (redbed, shaley clays, very stiff clays)

    < 12 in./100 blows

    Mildly susceptible but not considered over time span of one flood event

    Soft to medium

    > 12 in./100 blows

    Susceptible to scour at a moderate rate

    Sands

    All

    All

    Very susceptible



  • Monitor shales and stiff clays for long-term degradation. Shales and stiff clays tend to break down and disintegrate when exposed to repeated wetting and drying, a major problem in northeast Texas where head cutting in the Sulphur River basin has resulted in the channels down-cutting into the shale. The typical rate of degradation of shale in this situation is typically on the order of inches per year. As a result, most shales and stiff clays are not considered susceptible to scour during a single flood event. Consider long-term history of channel cross sections when evaluating these materials.
  • For channels in cohesionless materials, such as sand and gravel, calculate contraction and pier scour using the following methods:
    • Contraction scour: use the equations in HEC-18.
    • Pier scour: use either the equations in HEC-18, Froelich’s Equation, or Sheppard’s Equations.
  • For channels in cohesive materials, such as clay, calculate contraction and pier scour using one of the following methods:
    • Limit d50 to 4 x 10-3 in. (3.33 x 10-4 ft.). For contraction scour, use the equations in HEC-18. For pier scour, use the equations in HEC-18 with a reduction factor of 0.5 for soils with 11% or more clay.
    • Use the SRICOS Method.
    • Use Annandale’s Erodability Index Method.
  • For channels in layered soil, calculate scour using one of the following methods:
    • Conduct a scour analysis layer by layer using the equations specified above for individual layers. If the scour analysis indicates a value that is greater than the thickness of the layer, remove that layer and recalculate the hydraulic variables. Then continue the scour analysis with the next layer.
    • Use the SRICOS Method.
    • Use Annandale’s Erodability Index Method.

Because of conservatism built into equations for calculating scour and limitations and gaps in existing knowledge, apply engineering judgment when using results from scour computations.

Before using the scour analysis for bridge foundation design, check the scour predictions to ensure:

  • That the scour calculations account for layered soil/rock profiles.
  • That the scour calculations account for the soil/rock properties (that is, clay, silt, sand, gravel, rock, etc.)
  • That the predicted scour depths do not extend into competent rock.
  • That the predicted scour depths are not added onto the foundation design lengths.

Determine if the scour predictions exceed the foundation disregard depth. If so, use the following to evaluate the scour predictions:

  • Performance of the existing structure during past floods (compare historic data of cross section changes at the bridge with the scour predictions).
  • Hydrologic characteristics and flood history of the stream and similar streams.
  • Recalculation of the scour analysis using a step-wise procedure that incrementally removes material and recalculates the required hydraulic variables. This may decrease the total scour depth.

Do not allow scour predictions to control foundation design because TxDOT uses deep foundations. An exception is large rivers, especially those with sand channels.

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