On the influence of varying stream discharge on bed topography and tracer migration in gravel bedded channels

A series of controlled experiments were conducted in a large experimental flume at the St. Anthony Falls Laboratory to study the effect of varying discharge on bed topography and tracer dispersal. Instantaneous, high-resolution bed elevations and sediment transport rates along with travel distances of tracer particles of size representative of the grain size distribution of bed material were measured, for a range of discharges. It is shown that bedform geometry directly depends on discharge with increasing height, decreasing length, and decreasing variability in bedform aspect ratio as the discharge increases. For the case of higher discharges where the bed topography is more pronounced, it is demonstrated that the length of the bed forms acts as a first order control on tracer travel distances. Based on a multi-scale analysis of bed elevation increment series, we demonstrate that the spectral slope and the degree of non-linear dependence of higher order structure functions on moment order control the distribution of travel distances, with larger particles traveling further at low discharge and smaller particles not significantly affected by the discharge rate. Results also show that the mean travel distance of smaller particles does not get much affected by the bed topography as the dynamics of smaller particles are mainly dominated by the particle hiding effect. Our results also confirm, for the first time, the heavy-tailed distribution (truncated power-law tail) in the statistics of tracer travel distances for a mixture of grain sizes and discharges as recently hypothesized in theoretical studies and as expected in natural rivers characterized by a wide grain size distribution and extreme flood events. The implications of these results for predictive modeling of sediment transport are discussed.