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Section 6: Scour

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Analysis

All bridges over waterways require a scour analysis. The results of the scour analysis will determine the Bridge Inspection Coding for Item 113 – Scour. 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



* Use the values listed or the overtopping event, whichever is the lower event.

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New Bridges with Known Foundations

Evaluate new bridges with known foundations for potential scour in accordance with the following:

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  • Guidelines outlined in Evaluating Scour at Bridges (HEC-18).
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  • 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.

Determine scour at bridges using the following guidelines:

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  • 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



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  • 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.
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  • For channels in cohesionless materials, such as sand and gravel, calculate contraction and pier scour using the following methods:
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    • Contraction scour: use the equations in HEC-18.
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    • Pier scour: use either the equations in HEC-18, Froelich’s Equation, or Sheppard’s Equations.
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  • For channels in cohesive materials, such as clay, calculate contraction and pier scour using one of the following methods:
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    • Limit d50 to 0.2 mm (6.56 x 10-4 ft or 7.87 x 10-3 in). 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.
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    • Use the SRICOS Method.
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    • Use Annandale’s Erodibility Index Method.
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  • For channels in layered soil, calculate scour using one of the following methods:
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    • 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.
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    • Use the SRICOS Method.
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    • Use Annandale’s Erodibility 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:

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

Determine if the scour predictions exceed the typical disregard depth of 10 feet from the channel flow line. If so, use the following to evaluate the scour predictions:

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  • Performance of the existing structure during past floods (compare historic data of cross section changes at the bridge with the scour predictions).
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  • Hydrologic characteristics and flood history of the stream and similar streams.
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  • 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.

Upon completion of a scour analysis for a new bridge the Scour Summary Sheet for Known Foundations is to be completed and placed into InspectTech. In addition, the results of the scour analysis are to be used to determine the Bridge Inspection Coding for Item 113 – Scour. The results of the coding need to be placed into InspectTech.

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Existing Bridges with Known Foundations

Evaluate existing bridges with known foundations for potential scour using the following:

The TSEAS Manual includes two parts: a) Secondary Screening Method; and b) Concise Analysis Method. The Secondary Screening Method is only applicable to low volume Off-System bridges with known foundations. The Concise Analysis Method is applicable to both On-System and Off-System bridges that are not on the interstate system or are high priority bridges. Bridges that would be considered high priority are:

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  • Bridges on principal arterials;Bridges on evacuation routes;
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  • Bridges that provide access to local emergency services such as hospitals; and
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  • Bridges that are defined as critical in a local emergency plan (i.e., bridges that enable immediate emergency response to disasters).

Upon completion of a scour analysis for an existing bridge the Scour Summary Sheet for Known Foundations is required to be completed and placed into InspectTech. In addition, the results of the scour analysis are to be used to determine the Bridge Inspection Coding for Item 113 – Scour. The results of the coding need to be placed into InspectTech.

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Existing Bridges with Unknown Foundations

Contact the Geotechnical Branch for the evaluation of existing bridges with unknown foundations.

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Bridge Class Culverts

Evaluate new and existing bridge class culverts for potential scour using the following:

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Scour Critical Bridges

Existing bridges that require a coding in Item 113 of a 3, 2, 1, or 0 are considered scour critical. For each scour critical bridge a Plan of Action (POA) needs to be developed and implemented, as well as placed into InspectTech.

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Stone Protection at Bridges

Protecting abutments and piers at bridges is beneficial in limiting the effects of scour. The use of stone protection is recommended over concrete riprap due to its flexible nature. Concrete riprap, due to its rigidity, masks problems. Consequently, voids can form under them and eventually undermine the pavement or approach slab.

Stone protection needs to be designed for the conditions that exist at the bridge. The recommend methodology for the design of stone protection is to use HEC – 23 Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Guidance (see Volume 2 for guidance on design). Upon completing the design, the appropriate D50 will be determined. This value should then be compared to the tables in Item 432 – Riprap to determine the appropriate size of the stone protection. Once the appropriate size of the stone protection has been identified then the appropriate thickness of the stone protection needs to be determined. The thickness is a function of the conditions where the stone protection is being used. However, a general rule of thumb is that the thickness needs to be equal to or larger than 1.5 times the size of the stone protection listed in Item 432. This is consistent with the procedures outlined in HEC-23 for the thickness of the stone protection, where it is typically taken as the larger of either the 100% size (i.e. maximum size) or 2 times the D50 size. If one compares the range of the D50 values for the various sizes listed in table 2 of Item 432 and then multiplies by these by 2 one will obtain approximately 1.5 times the size of the stone protection listed. In the plans Stone Protection should be listed as follows:

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  • Riprap (Stone Protection) XX in. (where XX is the size in inches)

    Thickness = YY in. (where YY is the appropriate thickness)

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