The overshoot and equilibration of saltation

Abstract

Downwind of the leading edge of a saltating surface, the saltation process develops toward an eventual stream wise equilibrium. The nature of this development is investigated in a portable wind tunnel by measuring the vertically integrated streamwise flux Q of saltating sand grains as a function of distance x from the leading edge of the erodible surface, at wind speeds between 8 and 13 m s −1 . It is found that Q ( x ) increases from zero at x = 0 to a maximum value at x around 7 m, before decreasing toward an eventual equilibrium value (about half the maximum value) when x is greater than about 15 m. These results are in qualitative agreement with two recent numerical simulations of the approach of saltation to equilibrium. The experiment indicates the minimum length of wind tunnel required for studying the saltation process in its equilibrium state, and the order of magnitude of the measurement errors if shorter tunnels must be used.