The self armouring process under unsteady flow conditions

Abstract

AbstractLaboratory experiments were conducted to investigate the effect of the original size distribution of sediment and unsteady flow characteristics on the composition of an armour layer in a straight open flume. Bed sediments having different size distribution were subjected to flow hydrographs that varied in depth (D) and duration (T). The sensitivity of the sequence of changed bed sediment compositions to the variation in hydrograph duration was more than that in hydrograph depth. Such sensitivity increased as the geometric mean diameter of the original sediment decreased and as the geometric standard deviation of the sediment increased. The gradation of the armour coat developed under unsteady flow condition was greater than that developed under equivalent steady flow condition (i.e. having the same mean discharge and friction slope), and was found to depend on the value of the parameter P = (D/T)(1/u*) up to the limit of P = 0·01, where u* is the shear velocity. A model is proposed to predict the final composition of the armour layers developed under the effects of unsteady flow conditions.