Bed surface bed profile adjustments to a series of water pulses in gravel bed rivers

Abstract:

This research aims to explore the interactions between the bed surface texture, the bed topography and the sediment transport (rate and grain size distribution) to a series of water pulses in gravel bed-rivers. We conducted a set of runs in a 18 m-long tilting flume, 1 m-wide. Low flow discharges (Q = 65 l/s) during periods of variable duration (between t = 10 h and t = 1 h) were alternated with high flow rates (Q = 90 l/s) of constant duration (t = 1.5 h). Sediment was fed at a constant rate (Qfeed = 7.5 kg/h) throughout the runs. Eight experiments were consecutively conducted: the final configuration of the previous run was the initial condition for the subsequent experiment. The initial bed texture of the experiments was obtained after a 280 h-long run at low flow, the last 40 h of which under starving conditions. The initial bed slope was S₀ = 0.022 m/m. A poorly-sorted grain size distribution (D_g = 5.65 mm and s_g = 3.05) was used as a feeding material. The same material was used as the initial condition for the antecedent experiment (280 h-long). Intensive measurements of the bed surface, bed topography and sediment transport were taken during the runs.

Provisional results of the experimental campaign demonstrate that: (i) bed topography rapidly adjusts to water pulses: bed aggrades during low flow periods to subsequently degrade during water pulses. However, the rate of change of the bed profile decreases with the number of water pulses; (ii) the surface texture maintains an approximately invariant texture during the runs with no significant changes before and after the pulses; (iii) bedload transport dramatically adjusts to water pulses (increasing its rate and getting coarser). The relative increase in the bedload transport during the pulses diminishes as the number of pulses increases. A detailed analysis of the evolution of the bed profile shows the formation of transverse ribs during low flow periods which slowly migrate upstream. These bedforms are not vanished during the water pulses. On the contrary, their amplitude increases during pulses and decreases during low flow periods. The experiments also show that the bed surface locally adjusts to allow a rapid increase in the bedload transport rate while keeping an overall constant grain size distribution.