Estimate long-term degradation, live-bed or clear-water contraction scour, simple-pier scour, and Froehlich, HIRE, or NCHRP abutment scour with documented equations from FHWA HEC-18. Pier choices include CSU/HEC-18, FDOT, coarse-bed clear-water, and maximum-potential cohesive methods. Compare design scenarios and report governing total scour depths and elevations.
Bridge scour is a public-safety design calculation
Use hydraulic-model output for each subarea and have the analysis reviewed by a bridge hydraulics professional. This implementation intentionally stops short of pressure flow, cohesive time-rate scour, erodible rock, complex foundations, debris, and opaque empirical substitutes.
The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law. Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.
How the HEC-18 synthesis works
- Establish the design-life bed elevation by lowering the existing bed for independently estimated long-term degradation. Aggradation is documented but is not used to offset contraction or local scour.
- Compute contraction scour with the Laursen live-bed or clear-water equation for the hydraulic subarea. A live-bed analysis can also apply the HEC-18 armoring check by comparing the clear-water depth based on Dm = 1.25D50.
- Select the applicable simple-pier equation family: CSU/HEC-18, FDOT, coarse-bed clear-water, or maximum-potential cohesive scour. The CSU option can apply the documented wide-pier correction where its limits are satisfied. Cohesive time-rate and complex-foundation component methods are intentionally not inferred from incomplete inputs.
- Compute an abutment check with Froehlich, HIRE, or NCHRP. Froehlich/HIRE local scour is added to contraction. The NCHRP result already includes contraction, so it is not added twice. The calculator then converts each total depth to an elevation in the supplied project datum.
The inputs should come from a hydraulic model divided into physically meaningful channel and overbank subareas. Scour equations cannot repair an incorrect flow distribution.
Frequently asked questions
What components make up total bridge scour?
HEC-18 separates long-term bed degradation, contraction scour caused by the reduced bridge opening, and local scour at piers or abutments. This calculator reports each component and combines the applicable components into scour elevations referenced to the entered bed elevation.
When should I use live-bed versus clear-water contraction scour?
Use live-bed when bed material is transported into the bridge reach from upstream. Use clear-water when the approach reach is not transporting the bed material represented by D50. HEC-18 recommends calculating both where coarse particles may armor a live-bed scour hole and using the smaller depth.
Which abutment method does the automatic option choose?
The automatic option uses Froehlich when projected embankment length divided by approach depth is 25 or less, and HIRE when that ratio is greater than 25. Froehlich itself uses the separate active-flow obstructed length. Both equation results remain available in the calculation output.
What does the cohesive pier result represent?
HEC-18 Equation 7.35 gives maximum potential scour from a material-specific critical erosion velocity. It does not predict the smaller time-dependent depth for a finite flood. That calculation needs erosion-rate test data and the FHWA time-rate procedure.
What FHWA bridge-scour modes remain outside this calculator?
Pressure-flow scour, erodible rock, stratified-bed time sequencing, debris, and complex-pier component superposition remain unsupported. The complex-pier procedure requires detailed stem, footing, pile-cap and pile-group geometry; chart-derived factors; adjusted hydraulics; and iteration. Use the FHWA Hydraulic Toolbox or a specialist bridge-scour model for those cases.
How does the NCHRP abutment option work?
Supply the amplification factor read from HEC-18 Figure 8.9 or 8.10, or from the FHWA Hydraulic Toolbox. The calculator applies Equations 8.3 and 8.4. Because that result already includes contraction scour, the synthesis does not add contraction a second time.
Why is the computed pier scour sometimes above the HEC-18 rule of thumb?
HEC-18 Equation 7.1 is retained without silently capping it. The 2.4a or 3.0a value is reported as a round-nose aligned-pier reasonableness check. An exceedance should trigger review of the input hydraulics, skew, geometry and method applicability.
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Last verified: August 2026