A Precipiton-Based Approach for Multi Grain-Size Transport Models

<p><span>Multi </span><span>g</span><span>rain-</span><span>s</span><span>ize </span><span>t</span><span>ransport </span><span>models </span><span>t</span><span>hat</span><span> simulate transport </span><span>of various grain sizes </span><span>along with the bed stratigraphy </span><span>consider that only the sediment present in an active layer at the top of the substratum participate</span><span>s</span><span> in sediment transport. The thickness of this </span><span>well-mixed </span><span>layer </span><span>may be</span><span> f</span><span>ixed </span><span>but also</span><span> calculated according t</span><span>o </span><span>the coarsest grain size it contains </span><span>or to </span><span>t</span><span>he shear stress </span><span>applied at the surface of the substratum</span><span>. </span><span>H</span><span>owever, </span><span>this approach puts the emphasis on the </span><span>conservation of the active layer thickness and </span><span>on </span><span>the availability of the various sizes within this layer. This means there is little consideration </span><span>i)</span> <span>for heterogeneity </span><span>in</span><span> grain size distribution when mixing </span><span>together </span><span>adjacent </span>stratigraphic layers that differ significantly in composition and ii) for grain s<span>izes that could prevent</span><span> or slow down removal of </span><span>the others. </span><span>To </span><span>cope with these limitations</span><span>, we </span><span>developed</span> <span>an</span><span> algorithm with the ability to capture the transport of </span><span>heterogeneous sediments and the related </span><span>strat</span><span>i</span><span>graphic record of erosional and depositional events based on the behavior of the various sizes within the </span><span>bed</span><span> layers. </span><span>W</span><span>e </span><span>buil</span><span>t</span><span> a mult</span><span>i </span><span>grain-</span><span>size</span><span> module based on the precipiton method: the time spent by a precipiton </span><span>(volume of water that carries sediment)</span><span> on a pixel determines the </span><span>grain-size specific </span><span>magnitude of deposition and erosion. </span><span>The newness of our work is that the magnitudes </span><span>of erosion </span><span>may be corrected according to the sizes that slow down the erosion of the others (</span><span>zero or slow erosion rate</span><span>) and stratigraphic layers with similar composition only may be merged. A</span><span> few tests </span><span>were </span><span>conducted</span><span> to </span><span>study the morphological evolution of </span><span>a </span><span>1D-river reach under various conditions (water discharge, sediment source, etc.)</span><span>. A lake was added at the end of the reach to record the various sizes existing the reach over time. </span><span>At low water discharge when only the threshold of fine grains is exceeded, an armoring layer made of coarse grains develop at the surface of the substrate. At a water discharge when all the grains are in motion, the finer the grains </span><span>are</span><span>, the further downstream they are transported. This downstream fining pattern may be associated with change</span><span>s</span><span> in the concavity of the river profile. T</span><span>his multi grain-</span><span>size</span><span> algorithm</span> not restricted to the precipiton approach <span>has the potential to </span><span>unravel the role </span><span>of heterogeneous sediments </span><span>in the formation of sorting patterns </span><span>and, therefore, it is to be implemented </span><span>in the landscape evolution model RiverLab (former Eros).&#160;</span></p>