THE CLOGGING OF COARSE GRAVEL RIVER BEDS BY FINE SEDIMENT

Laboratory investigations were carried out to examine the clogging process in coarse gravel river beds. Field samples taken from the Langeten river in Switzerland show that the intruded fine particles of the flow are deposited in the top layer of the-river bed. For lower discharges the decrease of the hydraulic conductivity depends mainly on the dimensionless shear stress of the flow, the concentration of the suspended load, the hydraulic gradient between river and groundwater and the grain-size distribution of the river bed. However, when the armour layer breaks up during high discharges and the whole river bed is mobilized, the deposited fines are flushed downstream and a new layer with an initial maximum hydraulic conductivity is formed. The relationship between the dimensionless shear stress and the hydraulic conductivity is discussed. Changes in the nature of the catchment area or in the river itself which can accelerate the clogging process in the affected river are also discussed.