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).
- For single soil layer, input the D50 in feet/meter.
- For two soil layers, input: (top layer thick)*(top layer D50)+(bottom layer D50). For example: 2.5*0.05+0.25
- For three soil layers, input: (top layer thick)*(top layer D50)+(2nd layer thick)*(2nd layer D50)+(3rd layer D50). For example: 2.5*0.05+5*0.25+2.5
- There shall be no blank space between input data, and the maximum number of soil layers is limited to three.
- Where rock is encountered, use a D50 value of 100 to represent the rock. For example, consider a two layer condition where the top soil layer is 3 feet
thick with a D50 value of 0.1 ft.; and the bottom layer is scour resistant rock with an (assumed) D50 value of 100. Enter this information as 3*0.1+100.
Further study may be needed to estimate the scour in rock.