Genesis and Shapes of Delta Distributary Networks

Rudy Slingerland and Douglas A. Edmonds
Penn State University, University Park, PA

In frontier areas it would be useful to predict the geometries and properties of fluvial-deltaic sandbodies given known inputs such as water and sediment supply, sediment caliber, and accommodation space. To accomplish this task, sediment morphodynamic models must accurately describe the physics of channel extension, bifurcation, avulsion, and overall network dynamics. Here we use CFD models Fluent and Delft3D to better understand these processes and the resulting feedbacks between evolving self-formed channels, the turbulent plane jets at their downstream tips, and the evolving middle ground bars (MGB). Results of numerical experiments using fine sand and silts and typical initial basin water depths define a standard sequence of distributary progradation and abandonment: 1) initial growth of subaqueous levees until the MBG lies 12 channel widths from the subaerial channel; 2) rapid widening of the MGB, creating two bifurcate channels that diverge at an angle set by channel aspect ratio; 3) upstream growth of the MGB resulting in increased back-pressure and water surface elevations upstream; 4) flow divergence and crevasse initiation; 5) abandonment of one of the bifurcate channels due to reduction in discharge in the main channel; and 6) creation of a new stable bifurcation, crevasse healing, or avulsion, depending upon the ratio of the water surface slopes and the depth of incision of the crevasse. These results are consistent with observations on 24 modern deltas which show that the typical distributary network unit cell consists of a parent channel spitting asymmetrically into two bifurcates, one approximately two-thirds as wide and long as the other.