GRAIN SIZE CONTROL ON THE MORPHOLOGY OF DELTAS AND THEIR DISTRIBUTARY CHANNELS

Delta morphology is traditionally explained by differences in fluvial energy and wave and tidal energy. However, deltas influenced by similar ratios of river to marine energy can display strikingly different morphologies, suggesting other variables, such as grain size, may control delta morphology. Here we use numerical modeling to quantify how changes in the median and standard deviation of the incoming grain-size distribution affect delta morphology. The model setup includes a river carrying constant discharge entering a standing body of water devoid of tides, waves, and sea-level change. We conduct 23 runs only varying the distribution’s median grain size from 0.01 - 1 mm and standard deviation from 0.1 - 3. A morphological transition occurs as median grain size and standard deviation increase. At low median grain size and standard deviation, deltas have shallow topset slopes ranging from 1 x 10^-4 to 3 x 10^-4, 1 - 8 stable active channels, and elongate planform morphologies with sinuous shorelines. At high median grain size and standard deviation, deltas have steeper topset slopes ranging from 1 x 10^-3 to 2 x 10^-3, 14 - 16 mobile channels, and fan-like planform morphologies with smooth shorelines. The change in channel number occurs because an increase in grain size results in steeper topset slopes and larger equilibrium width to depth ratios of the distributary channels, which in turn leads to an instability that creates braiding. The number of distributary channels on a delta topset can be predicted using standard bar mode predictors for braided rivers. The transition from elongate to fan-like morphologies occurs because larger grain-size increases topset slope and channel number. Steeper topset slopes result in more mobile channels due to higher excess shear stresses. More channels reduce the average water and sediment discharge at a given channel mouth. The combination of these two effects suppresses bifurcation around river mouth bars in coarse-grained deltas creating fan-like morphologies as channels sweep across the topset. On the other hand, finer-grained deltas have low topset slopes and fewer channels. This allows mouth bars to create bifurcations, which in turn creates more elongate deltas as stable channels prograde basinward.