Geotechnical Manual: Scour

Section 5: Scour

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).
Abutment scour does not need to be calculated because none of the equations to date yield acceptable results. Protect abutments against potential scour through use of a flexible revetment, where possible.

Design bridge foundations to withstand the scour depths for either the 100-year flood or a smaller flood if it will cause scour depths deeper than the 100-year flood.

Check the bridge foundations against the scour depth associated with the 500-year flood. This flood event is treated as an extreme event and the factor of safety on the bridge foundations should be greater than or equal to 1 (bridge foundation factor of safety is required to be FS>1).

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

“Evaluating Scour at Bridges” (HEC-18)
“Texas Secondary Evaluation and Analysis for Scour” (TSEAS, 1993)

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: #i1039154Material 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 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 or Froelich’s Equation
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^-4ft.). 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 SCRICOS 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 SCRICOS 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 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.

In general, scour predictions should not control foundation design because TxDOT uses deep foundations. An exception is large rivers, especially those with sand channels.