Scour Depth Calculator

Example data table

These examples show typical input ranges and resulting design scour depths.

Formula used

1) Froude number

Computed from approach depth and velocity:

Fr = V / √(g · y1)

2) Pier local scour (HEC-18 / CSU)

Used when you select the pier method:

ds / y1 = 2.0 · K1 · K2 · K3 · K4 · (a / y1)^0.65 · Fr^0.43

K1–K4 represent pier shape, flow angle, bed condition, and armoring effects.

3) Abutment local scour (Froehlich-style screening)

Used when you select the abutment method:

ds / y1 = 2.27 · (L / y1)^0.43 · Fr^0.61 + 1.0

This is a screening estimate. Confirm with applicable design guidance for final work.

4) Contraction scour (screening width-ratio)

Quick estimate for early-stage checks:

ys = y1 · ( (B1 / B2)^m − 1 )

Default exponent m = 0.60. Set contraction to “No” if not applicable.

5) Total and design scour

Total scour = local + contraction + degradation
Design scour = Total scour × Safety factor

How to use this calculator

1. Select your units and the local scour method.
2. Enter approach depth y1 and velocity V.
3. Provide pier width a, or abutment length L when needed.
4. Set K-factors for piers to match your geometry and flow angle.
5. Enable contraction scour and enter B1, B2, and exponent m if required.
6. If long-term degradation is known, enable it and enter the depth.
7. Click “Calculate scour depth” and review the design value shown under the header.
8. Use the CSV or PDF buttons to export the saved result.

Use the notes below to interpret inputs, evaluate sensitivity, and document decisions for bridge and culvert foundation planning.

Scour depth and foundation risk

Scour is the erosion of bed material around a structure during high flows. Local scour forms holes at piers or abutments, while contraction scour lowers the bed through a narrowed opening. For foundations, design scour depth is used to set embedment and to size countermeasures. Severe scour can expose footings, reduce pile lateral resistance, and trigger rapid settlement.

Key hydraulic inputs and typical ranges

The calculator uses approach depth (y1) and velocity (V) to compute the Froude number, Fr = V/√(g·y1). In practice, y1 is often 0.5–6.0 m (or 2–20 ft) and V commonly 0.5–5.0 m/s (or 1.5–16 ft/s), depending on channel slope and flood magnitude.

Pier local scour drivers

For piers, the CSU relationship scales scour with pier width a, depth y1, and Froude number. Larger a/y1 ratios and higher Fr typically increase local scour. Flow skew and pier shape are handled through K-factors; changes in K2 can noticeably shift predicted depth under identical hydraulics.

Abutment scour behavior

For abutments, the screening relationship emphasizes the length projected into the flow (L) and Fr. Long embankments that protrude into floodplain flow can produce deep scour near the abutment toe. Use consistent geometry: L should represent the effective obstruction at the design stage.

Contraction and long‑term degradation

Contraction scour is driven by the width ratio B1/B2. Early checks often see ratios near 1.0–2.5; higher ratios indicate stronger narrowing and a greater likelihood of bed lowering. Long-term degradation is added separately when profiles, sediment budgets, or river training works indicate persistent lowering.

Selecting K-factors and safety factor

K1 reflects shape, K2 flow angle, K3 bed condition, and K4 armoring. If you lack detailed values, start with typical defaults and run sensitivity cases. Safety factors in preliminary work are often 1.05–1.30, aligned with project risk and data quality. Use higher factors when field data is sparse.

Interpreting results for embedment and protection

Use the design scour depth to check that footing or pile tip elevations remain below the predicted scour surface, with allowance for construction tolerances. When scour is large, consider riprap, guide banks, spur dikes, or pier collars, then confirm performance with hydraulics and constructability checks.

Documentation, checks, and site monitoring

Record the discharge, water level, velocity estimate method, and assumptions used for K-factors. Compare outputs against past flood observations, soundings, or inspection notes. After major events, measure bed levels at piers and abutments; repeatable monitoring data helps calibrate future estimates.

FAQs

1) Which local scour method should I choose?

Use the pier method for isolated columns in the flow. Use the abutment method for embankments or spill-through abutments that block floodplain flow. If both exist, evaluate each and design for the larger depth.

2) Do I need discharge (Q) to run the calculation?

No. This version uses depth and velocity to compute Froude number. Enter Q if you want it shown in reports, or if your workflow tracks scour estimates by design flood discharge.

3) How should I pick K-factors for piers?

Start with K1=1.0, K2=1.0, K3=1.1, and K4=1.0 when details are limited. Then update using your pier shape, measured skew angle, and bed condition notes, and run sensitivity checks.

4) What does contraction scour represent here?

It is a screening estimate based on the width ratio B1/B2. Use it to flag whether a narrowing may lower the bed. For detailed design, use a hydraulic model and sediment guidance, then input refined degradation or add separate documentation.

5) Why is the design scour depth larger than total scour?

The design value multiplies the summed scour components by the safety factor. This accounts for uncertainty in hydraulics, geometry, and bed material. Choose a safety factor that matches project risk and available site data.

6) Can I use D50 and D84 for armoring effects?

Yes, optionally. When both are provided, the calculator applies a modest reduction to local scour for coarse, well-graded beds. Treat this as a screening adjustment and confirm armoring behavior with project-specific geotechnical and hydraulic checks.

7) How can I validate the result on site?

Compare predicted scour with historical sounding data, inspection photos, and post-flood bed surveys. If possible, install reference marks or sonar monitoring near critical piers. Use consistent cross-sections so trends can be tracked over time.