The Coastline Evolution of an Abandoned Deltaic Lobe and the Fate of its Relict Distributary Channel: A Case Study from the Huanghe (Yellow River) Delta, China.

A high sediment load and frequent flooding events drive rapid modification to the coastline of the Huanghe (Yellow River) delta, China. Distributary channel avulsions occur every 7-10 years, and each event results in the shifting of fluvial sediment supply over hundreds of square kilometers across the deltaic coastline. Upon lobe abandonment, the shoreline erodes at rates that reach kilometers per year, and low-lying regions of the delta are routinely inundated by tides. These processes rework the sediment deposit, and while much of this material is advected basinward, some is transported landward via tidal channels that occupy the abandoned distributary channel. Over a yearly timescale, the relict channel fills with sediment, the delta lobe converts to a tidal flat, and the rate of coastline retreat decreases. The focus of this study is to validate a physical model of the time evolution of the morphodynamics using data collected from field studies, as well as time-series satellite imagery. Sedimentological analysis of seventeen 6-m cores extracted from a lobe abandoned in 1996 documents the abrupt transition from the relict channel bed (comprised of sand) to the ongoing tidal flat sedimentation (comprised of mud). The thickness of the tidally-influenced mud deposit varies across the old channel, and is based on the inherited bed morphology and proximity to the active tidal channel. For example, sedimentation rates, as estimated using a numerical model, are higher near the tidal channel and decrease with lateral distance from this source, and are also a function of the local elevation of the tidal flat surface relative to the tidal amplitude. Overall, predicted sedimentation rates on the tidal flat - reaching several centimeters per year - are in agreement with field observations. Our results indicate that after 20 years of morphological adjustment following abandonment, this particular Yellow River delta lobe remains highly dynamic as result of active reworking of the shoreline.