What this computes
The CSU / HEC-18 pier scour equation: ys = 2.0 · K₁ · K₂ · K₃ · a⁰·⁶⁵ · y₁⁰·³⁵ · Fr₁⁰·⁴³ — the FHWA standard for local scour at a single pier, applicable to both live-bed and clear-water conditions. K₂ is computed from your skew angle and L/a as K₂ = (cos θ + (L/a)·sin θ)⁰·⁶⁵.
Limits: HEC-18 recommends ys ≤ 2.4·a for Fr ≤ 0.8 and ys ≤ 3.0·a for Fr > 0.8 (round-nose piers). The calculator flags when your result exceeds these — values beyond the limit indicate the equation is being extrapolated and the estimate should not be trusted without further analysis.
What it deliberately does not do
- Contraction scour — the bridge opening squeezing the flow lowers the whole bed, often more than local scour. Needs the live-bed/clear-water contraction equations with upstream transport.
- Long-term degradation and lateral migration — channel history, sediment supply, upstream interventions.
- Complex pier geometry — pile groups, caps in flow, debris rafts each need HEC-18's component method.
- Wadi flash-flood hydraulics — supercritical approach flows and hyper-concentrated sediment, common in KSA crossings, sit outside the equation's calibration. That's where 2D modelling earns its place setting y₁ and V₁ honestly.
Approach hydraulics (y₁, V₁) usually come from a HEC-RAS model at the design event — see choosing the modelling tool and setting the design rainfall. Sanity-check channel capacity with the Manning calculator.