Sediment average grain size in French Mediterranean waters was generated from sediment categories.

SHOM sediment map in the French Mediterranean waters was used to obtain sediment distribution, grain size range (mm) per sediment group and % fraction of each main sediment group (Rock, pebble, pebble and gravel, gravel,sand, sand and fine sand, fine sand, mud, silt and clay). Upper and lower boundaries for rock, pebble and clay size were derived from iso 16665 description.

Römkens et al. (1997) equation enabling the estimation of average grain size from sediment typologie and fraction was applied:

X = exp[ 0.01 * SUM( fi * ln( mi ) ) ]

where X :average grain size (mm) ; mi : estimated average grain size of ith sediment category (mm) and ; fi : corresponding fraction of mi (%).

Römkens, M. J. M., Young, R. A., Poesen, J. W. A., McCool, D. K., El-Swaify, S. A., & Bradford, J. M., 1997. Soil erodibility factor (K). Renard KG et al., Predicting soil erosion by water: a guide to conservation planning with the Revised Universal Soil Loss Equation (RUSLE). USDA Agriculture Handbook, (703), 65-99.