DELTA GROWTH AS A TWO-STEP PROCESS: FLOODS AGGRADE THE DELTA FRONT THEN CHANNELS INCISE IT DURING LOW FLOW (Invited Presentation)

Existing models of delta front evolution dictate that bed aggradation and channel extension happen simultaneously during floods. Repeat bathymetric surveys of a set of four sub-aqueous distributary channels on Wax Lake Delta in coastal Louisiana show that this is not the case. Instead, floods aggrade the bed and the extension of multiple distributary channels is a degradational process that occurs during the periods of low river flow. During the 2011 flood, widespread aggradation occurred (average: 0.3 m) both inside and outside of the channels and channels widths increased by up to 100%. The increased channel widths and decreased channel depths acted to reduce the degree of channelization on the delta front. During the period of low flow preceding the 2011 flood, the study area was significantly reworked. All four measured channels extended basinward (150 - 500 m) and channel beds incised up to 0.80 m or 160% of their previous depths. Measurements of fluid velocity and suspended-sediment flux show that the low flow reworking occurred during ebb-tidal flow. Shear velocities estimated from velocity profiles measured at distal channel tips exceeded 50 mm/s during these times; large enough to fully suspend the fine sand building the delta (D₅₀ = 200 μm, fall velocity = 19.5 mm/s). A Helley-Smith sampler placed 0.09 m above the bed measured a near-bed suspended-sediment volume flux of 1.4 x 10^-7 m²/s during ebb flow. During flood tide, shear velocities decreased to 10 mm/s and sand transport ceased, but river discharge remained sufficient to prevent flow reversal. We conclude that delta front evolution should be divided into two phases, one associated with river flooding and the other associated with delta-front reworking during intervals of low river flow.