Local Scour Depth at a Bridge Pier
The HEC-18 local pier scour relation with the nose shape, bed condition, and angle-of-attack factors.
Example
You enter
- Pier width (ft) 6
- Pier length (ft) 36
- Approach flow depth (ft) 15
- Approach velocity (ft/s) 8
- Nose shape factor K1 1
- Bed condition factor K3 1.1
- Angle of attack (degrees) 30
- Wider pier to compare (ft) 12
- Foundation margin below scour (ft) 2
You get
- Approach Froude number 0.3640
- Local scour, aligned 11.8 ft below the ambient bed
- With the wider pier 18.4843
- Wider pier increase (%) 56.9168
- Angle-of-attack factor K2 2.408 at the entered skew
- Local scour at the skew 28.4 ft -- 2.4 times the aligned depth
- Skew ratio 2.40836
- Foundation below ambient bed 30.3697
Details, formula, and sources
The worked 6 ft pier scours 11.8 ft aligned and 28.4 ft at a 30 degree skew, because a skewed pier presents its length. Contraction scour and long-term degradation add to this.
y_s / y_1 = 2.0 K1 K2 K3 (a / y_1)^0.65 Fr_1^0.43, with Fr = V / sqrt(g y) and K2 = (cos theta + (L/a) sin theta)^0.65.
FHWA HEC-18 Evaluating Scour at Bridges, with K1 the nose shape (1.0 round, 1.1 square, 0.9 sharp) and K3 the bed condition (1.1 plane bed and antidunes).
HEC-18 is published by the Federal Highway Administration and is freely available.
Estimate. AHJ and licensed professional govern.
Field names used by the API: pier_width_ft, pier_length_ft, flow_depth_ft, velocity_fps, nose_shape_factor, bed_condition_factor, angle_of_attack_deg, wider_pier_width_ft, foundation_margin_ft, froude_number, scour_depth_ft, wider_pier_scour_ft, wider_pier_increase_pct, angle_factor, skewed_scour_ft, skew_ratio, foundation_depth_ft
- Angle of attack a geometry problem rather than a hydraulic one, and channels migrate -- a pier aligned when built is not necessarily aligned nowbridge hydraulic study
- Scour mechanism removes the soil providing both the bearing and the lateral support, often during the flood when the bed cannot be seenHEC-18