Basin Controls on the Channel Morphodynamics of Juvenile River Deltas

Juvenile deltas represent the earliest subaerial expression of a growing delta. Due to the small delta size of very recently formed juvenile deltas, their morphodynamics are primarily controlled by fluvial processes, rather than tidal or wave effects at the delta periphery. We focus here on the concurrent modeling of delta growth and channel development as the delta advances into a receiving basin. Field data show that channel bank elevation decreases basinward in the transition from old channels (upstream) to young channels (downstream). Upstream channels resemble river channels with relatively high banks and a well-developed cross section. Channels at the delta periphery, on the contrary, tend to be shallow and wide because flow expands over the subaqueous banks. We present a quasi-2D model of delta morphodynamics that predicts spatial trends in channel width and depth in time. Our model accounts for varying bed and bank elevation and separates the delta topset into channelized and non-channelized areas. A channel maturation factor is introduced. The maturation timescale depends on the channel bank accretion rate, to a first-order approximation. We find that the delta advancement rate is a key parameter regarding the geometry of delta channels at the periphery. Delta advancement rate directly depends on the input sediment flux and basin accommodation space via mass balance. As such, we find that basin geometry directly contributes to channel morphodynamics on the delta topset. Deltas advancing into broad, wide-angle, or deep basins have lower advancement rates, which slows channel creation and allows for periphery channels that mimic mature river channels. Deltas advancing into shallow basins constantly develop new channels at the periphery and extend the region of the delta topset characterized by subaqueous channel banks.