Sediment average grain size was generated from sediment categories.

EUSeaMap sediment map of the European waters was used to obtain sediment distribution, grain size range (mm) per sediment group and % fraction of each main sediment group (Rock, coarse sediment, coarse and mixed sediment, mixed sediment, sand, muddy sand, sandy mud, fine mud or sandy mud, fine mud). Upper and lower boundaries for rock, pebble and clay size were derived from iso 16665 description. Fraction per category were derived from Folk (1954) sediment classification modified after EUSeaMap European broad-scale seabed habitat map description of categories.

Folk, R. L., 1954. The distinction between grain size and mineral composition in sedimentary rock nomenclature, Jour. Geology 62, 344–359.

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.

Sediment categories were recoded with the computed average grain size and the value was attributed to the corresponding polygons.

The categories - grain size correspondance are:

"fine mud" = 0.031525 mm, "sandy mud" = 0.09243549 mm, "Fine mud or sandy mud or muddy sand" = 0.17008322 mm, "Sandy mud or muddy sand" = 0.23936233056056 mm, "muddy sand" = 0.386289173 mm, "sand" = 0.94424275 mm, "mixed sediment" = 1.67029356 mm, "coarse & mixed sediment" = 6.34059641 mm, "coarse sediment" = 11.01089925 mm, "Rock or other hard substrata" = 2032 mm