Spatial organisation of large clasts and pebble clusters in gravel-bed rivers.

Spatial organisation of large clasts and pebble clusters in gravel-bed rivers. Pebble clusters and large protuberant clasts such as boulders introduce a strong variability in the distribution of roughness in gravel-bed rivers and exert a significant influence on flow resistance and sediment transport. However, understanding the complex morphology of the bed at a reach scale is limited by the scarcity of sampling strategies to locate these roughness elements in the reach. In this paper, we present two sampling strategies to characterize the spatial organisation of boulders and pebble clusters in four river reaches. We have mapped 1) isolated boulders and pebble clusters and 2) bed topography at a high spatial resolution to compare the characteristics of the spatial organisation of these elements with those obtained from random distributions. We observed that the concentration of boulders and pebble clusters, the distance between neighbouring elements and the characteristics of transverse ribs are similar for reaches of the same width. Furthermore, some characteristics of the spatial organisation of the elements in a natural reach are similar to that of random distributions. A dynamic interpretation of the spatial organisation of bedforms that includes these observations is proposed.