Experimental simulation of meandering in clay mixed sediments

Abstract

This paper presents recent works on the simulation of river planforms in the laboratory conditions using cohesive mixed sediments on layered flood-plains. The two-layered flood-plain consists of uniform diameter sand ( d 50 = 0.28 mm) in the lower layer and a mixture with different ratios (1:9 to 1:1) of kaolinite and fine sand ( d 50 = 0.11 mm) in the top layer in a small flume of size 1.8 m × 0.9 m. The controlled condition of small flow and sediment movement on the predefined channel with decreasing level of cohesion on the flood-plain leads to different end results of planforms as straight, meandering and braiding. Straight and braiding channels were obtained with 50% and 10% cohesion, respectively; however, moderate to high sinusoidal meandering was obtained from the experiments with 20% cohesion with many similarities and differences with natural meandering channels. Some of the extreme observations of natural meandering channels as formation of scrolled bars, cutoff channel and clay plugs were observed with the present experiments. Further slump block failure and decomposition were uniquely observed as the new phenomena on the outer channel and their roles on the planforms development are defined. The geometry of the slump block including its failure and decomposition were entirely dependent on many factors as cohesion content, flow conditions, initial geometry, flood-plain consolidation characteristics etc. The meandering obtained on these experiments is found as the transition stage towards the straight channel by cutoff or towards braiding by swallowing and distributing flow over whole width. The continuous increase in the flow path by the progression towards outer bend reduces the slope of the channel which ultimately reduces its carrying capacity and started deposition; finally forced to change its planforms.