A methodology to obtain the block size distribution of fragmental rockfall deposits

Rock masses detached as rockfalls usually disintegrate upon impact on the ground surface. The knowledge of the rockfall block size distribution (RBSD) generated in the rockfall deposit is useful for the analysis of the trajectories of the rock blocks, run-out distances, impact energies and for the quantitative assessment of the rockfall hazard. Obtaining the RBSD of a large rockfall deposit may become a challenge due to the high number of blocks to be measured. In this paper, we present a methodology developed for mid-size fragmental rockfalls (10(3) up to 10(5) m(3)) and its application to the Cadi massif, Eastern Pyrenees. The methodology consists of counting and measuring block fragments in selected sampling plots within homogeneous zones in the young debris cover generated by the rockfall along with all the large scattered rock blocks. The size distribution of blocks obtained in the sampling plots is extrapolated to the whole young debris cover and summed to the inventoried large scattered blocks to derive the RBSD of the whole rockfall event. The obtained distributions from the fragments can be well fitted by a power law distribution, indicating the scale invariant character of the fragmentation process (Hartmann (Icarus 2(2):201-203, 1969); Turcotte (J Geophys Res 91(NO B2):1921-1926, 1986). The total volume of the rockfall fragments has been checked against the volume at the rockfall source. The latter has been calculated comparing 3D digital surface models before and after the rockfall event.