Vertical exchange of coarse bedload in desert streams

The tracing of coarse particles, transported by flood events in two ephemeral streams in the Negev Desert, Israel, reveals that a size-independent, highly skewed longitudinal dispersion pattern is accompanied by vertical distribution throughout the scour layer. An analysis of the intermittent burial and resurfacing of these particles from one event to the next leads to the formulation of a concept of equilibrium in vertical exchange. This equilibrium requires that, for the 'dominant' flood event (frequency 1-2 years), the number of previously buried particles exposed by that event is balanced by an identical number of previously surficial particles being buried. Such an equilibrium can be maintained only if the scour layer reaches sufficiently deep into the 'sub-armour' layer to entrain a requisite number of particles of armour-layer size. The resulting functional relationship between (1) the size of coarse particles in relation to the matrix; (2) the thickness of the armour and 'sub-armour' layers and the distribution of coarse particles; and (3) aspects of the flow regime, enables the derivation of hydrological information from studies of the three-dimensional distribution of coarse particles in modern channel bed sediments of arid streams. It also helps to explain the vertical distribution of pebbles in ancient continental clastic sediments.