Propagation of aggrading sediment fronts in a laboratory flume

Abstract

Channel bed aggradation due to sediment overloading was studied experimentally. A series of aggradation tests, with uniform lightweight sediment, were performed to observe the hydro-morphologic response of a laboratory flume to bed-load sediment transport and nonequilibrium upstream sediment feeding. The hydro-dynamic and the sediment feeding rates were kept constant in time. The temporal evolution of the longitudinal profiles of the stream bed and of the water surface was measured by imaging methods. The experimental data were used to (i) provide a phenomenological description of the aggradation process, recognizing different morphologic features superimposed on one another and (ii) characterize the height and velocity of propagation of aggrading sediment fronts. The front heights increased with increasing sediment supply, while the front celerity decreased. The celerity values were compared to a few predictors available in the literature. The literature formulae return celerity values scattered over almost two orders of magnitude, and present conflicting trends (a predicted celerity may either increase or decrease for increasing sediment feeding rate), thus stimulating further research on the topic.

On the Relationship between Experimental and Numerical Modelling of Gravel-Bed Channel Aggradation

“…Local erosion due to the troughs between following dunes was also observed. Analogous morphological characteristics were found in all the flume aggradation experiments of References [26][27][28][29], in agreement with prior literature descriptions (particularly those of References [24]). One interesting remark is that the fixed bed elevation at the downstream sill was not always respected.…”

“…A detailed description of the image processing algorithms was provided by References [27,29,30]. The aggradation tests (T1, T2, and T3) presented in this manuscript are part of an extensive experimental campaign carried out over the last two years ( [26][27][28][29]; specifically, T1, T2, and T3 are runs AE11, AE21, and AE17 in Reference [29]). Table 1 furnishes the main parameters of each test, including T = duration of the experiment, Ud = mean velocity at the downstream control section, and Frd = Froude number at the downstream control section.…”

“…where: A = wetted cross-section area, β = momentum coefficient, g = gravity acceleration, zB = bed elevation, h = water depth, Sf = friction slope, qB = bed-load flux per unit width, slB = source term specifying a local input or output of material (per unit width), t = time, and x = stream-wise The aggradation tests (T1, T2, and T3) presented in this manuscript are part of an extensive experimental campaign carried out over the last two years ( [26][27][28][29]; specifically, T1, T2, and T3 are runs AE11, AE21, and AE17 in Reference [29]). Table 1 furnishes the main parameters of each test, including T = duration of the experiment, U d = mean velocity at the downstream control section, and Fr d = Froude number at the downstream control section.…”

“…Furthermore, laboratory tests enable a deep insight to be obtained of the mechanisms governing the observed phenomena, since laboratory conditions are obviously better controlled than the field ones. In the context of aggradation processes due to channel overloading, numerous experimental investigations have been conducted in the past covering a wide range of flow and sediment loading conditions (see, for example, [21][22][23][24][25][26][27][28][29]), with the aim of improving the phenomenological characterization of the process.…”