ABSTRACT

Experiments have been carried out in a circular flume in which silt-sized sediments were allowed to be deposited from a decelerating suspension current of density between 1.044 to 1.013. The sediment was first thrown into complete suspension at a high flow speed. An instantaneous deceleration caused rapid deposition of a graded bed. Depending on the rate of subsequent deceleration, various kinds of incomplete Bouma-type sequences were produced. Slow deceleration produced the following sequence (bottom upwards): graded bed-parallel laminae-ripple laminae--sinusoidal ripple laminae-suspension blanket. Moderate deceleration produced a similar sequence but without sinusoidal ripple laminae. Rapid deceleration caused deposition of a graded bed followed by parallel lami ae and topped by a suspension blanket. Very rapid deceleration produced a single graded bed with a suspension blanket on top.

When the speed was decreased gradually, no graded bed was formed. Instead, the sequence produced was either, (1) upstream-dipping laminae (antidune cross-stratification)--ripple laminae-suspension blanket or, (2) parallel laminae-ripple laminae-suspension blanket.

It was noted that the celerity of ripple and the upward building rate of the deposited layer were functions of the rate of deceleration. Average grain size and sorting of the deposit were also found to be affected by varying deceleration.