Measuring Subaqueous Progradation of the Wax Lake Delta with a Model of Flow Direction Divergence

Abstract

Abstract.Remotely sensed flow patterns can reveal the location of the subaqueous distal tip of a distributary channel on a prograding river delta. Morphodynamic feedbacks produce distributary channel tips that become shallower over their final reaches before becoming deeper over the unchannelized foreset. The flow direction field over this morphology tends to diverge 10 and then converge providing a diagnostic signature that can be captured in flow or remote sensing data. Twenty-one measurements from the Wax Lake Delta (WLD) in coastal Louisiana, and 317 measurements from numerically simulated deltas show that the transition from divergence to convergence occurs in a distribution that is centered just downstream of the channel tip, on average 132 m in the case of the WLD. With these data we validate the Flow Direction to Channel tips (FD2C) inverse model for remotely estimating subaqueous channel tip location. We apply this model to 33 remotely sensed images of 15 the WLD between its initiation in 1974 and 2016. We find that the distributaries grew unevenly, 6 of the primary channels grew at rates of 60-80 m/yr while one grew at 116 m/yr. We also estimate the growth rate of the total area enclosed by the subaqueous delta platform to be 1.83 km 2 /yr with no obvious rate changes over time.

Changes to Subaqueous Delta Bathymetry Following a High River Flow Event, Wax Lake Delta, USA

“…Their method is not as sensitive to tides, floods, and storm surges which can inundate the vegation used as a proxy for land in the classified satellite imagery. Still, negative land growth rates were estimated in their study, but not as rapid as those estimated by Allen et al, 2012. Shaw et al, 2018 tracked the growth of the subaqueous platform by using a method that identifies the location of subaqueous channel tips in satellite imagery between 1974 and 2016.…”

“…Finally, the third objective follows from the observation that integrating area change over short timescales (1 to 5 years) can result in growth rates drastically different than integrating over decadal timescales in previous studies of the WLD (Allen et al, 2012;Olliver & Edmonds, 2017;Shaw, Estep, Whaling, Sanks, & Edmonds, 2018). Allen et al, 2012 used time, water discharge, and tide level to determine the growth rate of the subaerially exposed delta from satellite imagery between 1983 and 2010.…”

Distributary Channel Networks as Moving Boundaries: Causes and Morphodynamic Effects

“…On one hand, a channel network could outgrow the delta front if sediment discharge was too small relative to channel progradation rates. The contrast between constant channel progradation rates and a gradually declining marsh area creation rate on the WLD (Shaw, Estep, et al, ) is potentially the result of this scenario. On the other hand, the delta front could become clogged with sediment if the sediment discharge was too large compared to the network progradation rates.…”

“…Since then, the WLD has grown to cover ∼50 km 2 of subaerially exposed land and marsh (Olliver & Edmonds, ), with an additional 80 km 2 of significant subaqueous deposits (Shaw et al, ). Decadally averaged progradation rates for primary distributary channels were remarkably consistent at each channel between 1974 and 2016 and were 69–116 m/year for an individual channel tip (Shaw, Estep, et al, ). Progradation of primary distributary channels that are 3 m deep into the delta front of recently deposited sediments that is about 1 m deep is accomplished by erosion along the tips and margins of the subaqueous distributary network (Shaw & Mohrig, ).…”

“…Then the evolution of channel boundary continues to the next time step. Numerical experiments were run for 50 years ( T Final ) to be similar to the development time of the modern WLD (Roberts et al, , Shaw, Estep, et al, ). Q was set as the median discharge 1985 and 2015 (Figure ), so no intermittency factor was necessary to relate simulation time and morphological time.…”