Bed clusters in humid perennial and Mediterranean ephemeral gravel-bed streams: The effect of clast size and bed material sorting

A short review of the literature on particle clusters on gravel river beds reveals investigations of both process and form dominated by the intensive study approach, using a restricted geographical sample or evidence from flumes. An alternative, presented here, is extensive sampling – from three climatic zones, several channels in each and at multiple transects at each site. It uses insights provided by a more intensively studied ‘base station’ [Wittenberg, L., Newson, M.D., 2005. Particle clusters in gravel-bed rivers – an experimental morphological approach to bed material transport and stability concepts. Earth Surface Processes and Landforms 30(11), 1351–1368]. Transect surveys were completed in each selected reach to establish flow_depth, bed material size and bed structure. A total of more than 5000 sample points reveals the vital presence of bed material of (almost equal)100 mm D50 for all sub-types of clusters to occur; thereafter, cluster frequency relates directly to the D90, with an improving correlation at D90 > 256 mm. A better integration of data from the diverse hydrological/hydraulic regimes can be achieved by correlating cluster frequency with a sorting index for bed material. Further analysis of hydrological and hydraulic data for all sites is required to develop a dynamic explanation.