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

Do not use abutment scour equations because none of the equations to date yield acceptable results. Use contraction scour equations to calculate total scour at abutments. Protect abutments against potential scour through use of a flexible revetment, where possible.

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.

Table 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 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 the following methods:

Limit d
50
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.
Use the SRICOS Method. Refer to NCHRP Research Report 915 for determination of erodibility parameters.
Use Annandale’s Erodibility Index Method.

For channels in layered soil, calculate scour using the following methods:

Conduct a scour analysis layer by layer using the methods specified above for individual layers. If the calculated scour of a given layer 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. Refer to NCHRP Research Report 915 for determination of erodibility parameters.
Use Annandale’s Erodibility Index Method.

the scour calculations account for layered soil/rock profiles.

the scour calculations account for the soil/rock properties (that is, clay, silt, sand, gravel, rock, etc.)

the calculated scour depths do not extend into competent rock.

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.