In-Transport Modification of Alluvial Sediment: Field Evidence and Laboratory Experiments

Downstream trends in bed sediment characteristics are analysed for two mid-Wales rivers, the Severn and Dyfi. Mean grain size and size distributions are compared for different sampling procedures on the Severn, and then for surface samples on both rivers. Size and skewness both decrease, but less clearly on the Dyfi, where there are pronounced 'jumps' below tributary inputs. Roundness values show little systematic change, and this is attributed especially to continuous input of more angular material along river reaches. Shape changes show some trends, more for the Dyfi than the Severn, and these are attributed to hydraulic sorting. Clast volume changes reflect those of shape. The fact that field results indicate greater importance of hydraulic sorting than abrasion (though continued angular supply by bank erosion and tributary input are also important) is confirmed by laboratory experiments using a Kuenen-type abrasion tank and tumbling barrel. These do give different results, but neither reproduces the nature or extent of downstream changes in the field.