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U.S. Geological Survey Sea Floor Stress and Sediment Mobility Database

Appendix: Critical Stress Calculation

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Surficial sediment texture data are used to estimate the critical shear stress at those points where sediment texture data are available (Poppe et al, 2005). The texture data includes the distribution of sediment over grain size classes ranging from -5 to 11 phi, ranging from gravel through sand and silt to clay. Texture observations are first classified as cohesive or non-cohesive based on the fraction of clay: if the clay fraction exceeds 7.5%, the sample is deemed cohesive, if less than or equal to 7.5% the sample is non-cohesive. Critical stress thresholds for non-cohesive sediment mixtures are calculated from the median grain size following Soulsby (1997). Because a variety of unavailable parameters influence the critical shear stress for cohesive sediments, a value of 0.1 Pa is used for all samples identified as cohesive. Additional information may be found in Dalyander et al. (2012).

Table 5. Critical stress thresholds for sand mixtures of select median grain sizes following Soulsby (1997):

Median Grain Size (d50, mm) Median Grain Size (d50, Phi) Critical Stress (Pa)
2.00 -1.0 1.17
1.00 0.0 0.48
0.50 1.0 0.26
0.25 2.0 0.19
0.13 3.0 0.15
0.06 4.0 0.12
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