2022
Explaining the formation of sedimentary structures under antidunes using a 2D widthwidth-averaged numerical model
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A new numerical model for computing the formation of sedimentary structures in a fluvialA new numerical model for computing the formation of sedimentary structures in a fluvialenvironment is presented. The mode model has been used to explain the strata -forming processprocesses underantidunes in detail, including water velocities, turbulence, free surface location, erosion,sedimentation and formation of each sediment layer . The Reynolds Reynolds-Averaged Navier Navier-Stokesequations wer e solved together with the k k-epsilon turbulence model to determine the water flowfield and the bed shear stress on a widthwidth-averaged two two-dimensional grid. The sediment transportwa s computed by solving convection convection-diffusion equations for multiple sediment sizes. An empirical formula for r esuspension at the bed wa s used , enabling the computation of erosion and depositionfor each sediment fraction. Five sediment size s were used, from 0.5 to 2.5 mm. The bed sedimentstratification wa s recorded using a very fine grid, resolving the bedding with up to 1000 cells in thevertical direction. A novel algorithm wa s used to vertically adapt the bed grid, enabling thecomputation of very large g radients of the grain size distribution, including thin laminae. The modelwas tested by computing sediment deposition in a channel with a Froude number of 1.21. Thehydraulic parameters and the average sediment size were selected similar to an earlier lab oratoryAccepted manuscriptflume studyflume study where downstreamwhere downstream-migrating antidunes were observed. The antidunes started as small migrating antidunes were observed. The antidunes started as small bed irregularities and increased in size bed irregularities and increased in size as they moved as they moved in the downstream direction. Recirculation in the downstream direction. Recirculation zones were observed zones were observed behind behind bedformsbedforms with large slopeswith large slopes on the lee side. Here, n the lee side. Here, sediments deposited sediments deposited in slides. Smaller sediment waves moving on top of the in slides. Smaller sediment waves moving on top of the bedformsbedforms depositdepositing layers of a fine and a layers of a fine and a coarse material simultaneously. coarse material simultaneously. After the initial phase, sAfter the initial phase, stationary antidunes tationary antidunes formed in the upstream formed in the upstream and middle partand middle part of the flume, producingof the flume, producing wavy sedimentary structures. wavy sedimentary structures. Sinusoidal crossinusoidal cross-lamination formed formed in d eposits under eposits under bedforms bedforms of different downstream velocitiesof different downstream velocities. The The antidunes antidunes in the in the downstream part of the flume moved in the streamwise direction. downstream part of the flume moved in the streamwise direction. They were affected They were affected by occasional by occasional upstreamupstream-moving surface waves, causing complex sedimentary structures to form. Figures and moving surface waves, causing complex sedimentary structures to form. Figures and videos show the stratavideos show the strata-forming process in detail, including velocity vectors, turbulence and layers forming process in detail, including velocity vectors, turbulence and layers of different particle diameters. of different particle diameters.

Nils.R.Olsen[at]ntnu.no