Víctor Chavarrías

The graded alluvial river: profile concavity and downstream fining

There has been quite some debate on the relative importance of particle abrasion and grain size selective transport regarding the river profile form and the associated grain size trends in a graded alluvial stream. Here we present new theoretical equations for the graded alluvial river profile that account for the effects of particle abrasion and grain size selective transport in the absence of subsidence, uplift, and sea level change. Under graded conditions we find that abrasion results in a mild profile concavity and downstream fining, whereas under aggradational conditions grain size selective transport can lead to large spatial changes in channel slope and bed surface mean grain size.

The equilibrium alluvial river under variable flow and its channel-forming discharge

When the water discharge, sediment supply, and base level vary around stable values, an alluvial river evolves toward a mean equilibrium or graded state with small fluctuations around this mean state (i.e., a dynamic or statistical equilibrium state). Here we present analytical relations describing the mean equilibrium geometry of an alluvial river under variable flow by linking channel slope, width, and bed surface texture. The solution holds in river normal flow zones (or outside both the hydrograph boundary layer and the backwater zone) and accounts for grain size selective transport and particle abrasion. We consider the variable flow rate as a series of continuously changing yet steady water discharges (here termed an alternating steady discharge). The analysis also provides a solution to the channel-forming water discharge, which is here defined as the steady water discharge that, given the mean sediment supply, provides the same equilibrium channel slope as the natural long-term hydrograph. The channel-forming water discharge for the gravel load is larger than the one associated with the sand load. The analysis illustrates how the load is distributed over the range of water discharge in the river normal flow zone, which we term the “normal flow load distribution”. The fact that the distribution of the (imposed) sediment supply spatially adapts to this normal flow load distribution is the origin of the hydrograph boundary layer. The results quantify the findings by Wolman and Miller (1960) regarding the relevance of both magnitude and frequency of the flow rate with respect to channel geometry.

A new technique for measuring the bed surface texture during flow and application to a degradational sand‐gravel laboratory experiment

We present a new image analysis technique for measuring the grain size distribution (texture) of the bed surface during flow in a laboratory experiment. A camera and a floating device are connected to a carriage used to take images of the bed surface over the entire flume length. The image analysis technique, which is based on color segmentation, provides detailed data on spatial and temporal changes of the areal fraction content of each grain size at the bed surface. The technique was applied in a laboratory experiment conducted to examine a degradational reach composed of a well sorted two-fraction mixture of sand and gravel. The initial bed consisted of an upstream reach that was characterized by an imposed stepwise fining pattern (the bimodal reach) and a downstream sand reach. A lack of sediment supply and partial transport conditions led to the formation of a static armor in the bimodal reach, which resulted in a more abrupt spatial transition in the bed surface mean grain size. The associated spatial transition in slope led to a backwater effect over the bimodal reach, a streamwise reduction in sand mobility, and so a static armor that was governed by a downstream fining pattern. Although a morphodynamic equilibrium state under steady flow is generally characterized by normal flow, here the partial transport regime prevented the bed from adjusting toward normal flow conditions and the morphodynamic steady state was governed by a backwater. We applied a numerical morphodynamic sand-gravel model to reproduce the laboratory experiment. The numerical model captured the hydrodynamic and morphodynamic adjustment and the static armor well, yet the armoring occurred too slowly. Although the final configuration of the experiment shows features of a gravel-sand transition (i.e., a sudden transition in slope and mean grain size), we are hesitant to claim similarities between our results and the physical mechanisms governing a gravel-sand transition in the field.

A Sand-Gravel Gilbert Delta Subject to Base Level Change: GILBERT DELTA UNDER BASE LEVEL CHANGE

Laboratory experiments were conducted on a sand-gravel Gilbert delta to gain insight on its dynamics under varying base level. Base level rise results in intensified aggradation over the topset, as well as a decrease in topset slope and topset surface coarsening, the signals of which migrate in an upstream direction. Preferential deposition of coarse sediment in the topset results in a finer load at the topset-foreset break, which creates a fine signature in the foreset deposit. Base level fall has the opposite effects. Entrainment of the topset mobile armor causes a coarsening of the load at the topset-foreset break and so a coarse signature in the foreset deposit. The entrainment of the topset substrate and fine top part of the foreset may follow, which causes a fining of the load and a fine signature in the foreset deposit. The fact that the upstream sediment supply requires a certain slope and bed surface texture to be transported downstream under quasi-equilibrium conditions counteracts the effects of base level change. This information travels in the downstream direction. In nature base level change is likely so slow that the upstream sediment load maintains the topset slope and bed surface texture and so keeps the topset in a quasi-equilibrium state. Base level change is therefore not expected to leave a clear signal in a mixed-sediment Gilbert delta other than a change in elevation of the topset-foreset interface.