Mobile-bed aggradation and degradation in a narrow flume: Laboratory experiments and numerical simulations

Aggradation and degradation phenomena in fluvial beds having uniform sediments are explored experimentally and simulated numerically. Laboratory experiments were conducted in a narrow mobile-bed flume for different sediment sizes, flow discharges and bed slopes. The flume had a cross-section of curved wall and flat bed, where the shear stress distribution is not same on the bed and the walls. Tests were carried out in two conditions: (1) equilibrium condition, where, once the steady and uniform flow conditions were achieved for a given slope and discharge, the channel was fed with variable sediment discharges until the bed-load sediment transport achieved an equilibrium condition; and (2) nonequilibrium condition, where the sediment feeding was instantaneously stopped (degradation tests) or increased (aggradation tests) and the bed levels were measured over time. Experimental results enabled assessing the deposition and erosion rates and determining the empirical power law for the bed-load discharge. Numerical simulations were then performed with the Double Order Approximation (DORA) model, which solves the one-dimensional shallow water equations governing a free surface gradually varied unsteady flow on mobile beds. As closure relationship, three bed-load formulae were implemented in the numerical code: the Ackers, P. [1984. Sediment transport in sewers and the design implications. In: Proceedings of the International Conference on Planning, Construction, Maintenance and Operation of Sewerage Systems, Reading, England, BHRA (Cranfield), pp. 215-230.] and Meyer-Peter, E., Muller, R. [1948. Formulas for Bed-Load Transport. Report on the Second IAHR Meeting, Stockholm, Sweden, pp. 39-64.] ones and a formula empirically derived at equilibrium in the laboratory tests. The results show that the DORA model can reproduce the bed profiles over time except the local scouring at the transition between fixed and mobile bed. In particular, in the best simulations obtained by using the empirical bed-load power law, the time evolution of bed profiles is in good agreement with the measured data of aggradation and degradation tests, having small average relative errors.