59th Annual Report on Research 2014

Figure 1: Surface images of one lobe-building cycle that is composed of A) channelization, B) backfilling of channel, C) refocus of flow in a new location, and D) channelization that pushes sediment into basin at accelerated progradation rate and creates a lobe. The autogenic timescales of experimental non-cohesive prograding deltas are determined to be a function of basin depth. These timescales are a function of characteristic lobe surface areas that are necessary for the completion of each lobe-building event (Figure 1). In a deeper basin there is a larger volume to be filled in order to achieve the characteristic volume. Therefore, autogenic timescales increase with basin depth (Figure 2).

Figure 2: Laterally-averaged shoreline position against time. Autogenic sediment release events as times of increased progradation rates are indicted by arrows. We used radially averaged shoreline positions to determine times of accelerated progradation rates, which corresponded to fluvial autogenic storage and release events. By dividing the shoreline into three sections, it was possible to isolate areas of increased progradation rate and to determine the shoreline-filling timescale, or the amount of time it took for all three sections to be activated. Because a lobe-building event was necessary in each section to complete a shoreline-filling event, the shoreline-filling timescale is roughly three times longer than the lobe-building timescale in our experiments. We also observed that lobe geometry varied with basin depth. Shallower basins produced wider lobes while deeper basins produced elongate lobes (Figure 3). Lobe planform geometry may be influenced by channel cross-sectional geometry. The experimental results show that deeper, narrower channels form in deeper basins while shallow and wide channels are more common in shallow basins (Figure 4). Therefore, the deep, narrow channels are associated with the elongate lobes while the wider channels are associated with the wider, shorter lobes.

Figure 3: Lobe width to length ratio (Ω). A value of 1 would correspond to lobes of equal length in the down basin direction and width in the radial direction. Higher values correspond to wide lobes and smaller values correspond to elongate lobes.

Channel geometry in the current experiments was may have been influenced by the processes on the delta foreset. The longer foreset in the deep basin allowed a sediment bypass system to develop and lock the channel in place. Channel avulsion that releases fine sediment also produces a strong locking mechanism. This focused flow further narrowed and deepened the channel and produced a narrow and elongated lobe. Future work to capture changes in foreset slope processes under varying basin depth would greatly enhance our understanding of the control of basin depth on the fluvial autogenic timescale.

Figure 4: A) Channel depths and B) width measured at upper and lower transects. Measurements were collected from topography scan collected during channelized flow (i.e., sediment release event).