Median Soil Grain Size D50

Median diameter of the bed material, the diameter of which 50% of the sizes are smaller, in feet or meters. Refer to Chapter 14, Appendix B for guidance on selecting samples of bed material for analysis. Input the D50 in feet/meters.

The HEC-18 pier scour equation computes the same scour depth regardless of the size of the bed material; it is unaffected by the D50 grain size.

The Florida pier scour equation includes a factor for grain size but its use should be limited to soils with a D50 between 0.1 mm (0.0003 feet) and 100 mm (0.3 feet). The equation may provide unrealistic answers for bed materials with a D50 outside of these limits.

The following discussion applies to the Florida pier scour equation for soil layers containing D50 materials between 0.1 mm (0.0003 feet) and 100 mm (0.3 feet).