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

SummaryA 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 ≈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.