Seasonal Infilling of an Artificially Abandoned Distributary Channel on the Huanghe (Yellow River) Delta, China

Though avulsions on fluvially-dominated deltas typically leave behind abandoned distributaries near the surrounding delta elevation, engineered avulsions leave topographic lows that may or may not fill with sediment. An engineered avulsion on the Huanghe (Yellow River) Delta in 1996 left behind a tidal channel-mudflat complex with no fluvial connection. Modern mudflat accretion indicates that coastal sediment is imported to the channel, but the delta lobe has simultaneously retreated 13 km in the 23 years since abandonment. To predict how the abandoned channel will evolve, sediment import processes were examined by making in situ measurements within the tidal channel, on the mudflat, and offshore, in both winter and summer conditions. The modern abandoned distributary channel is 10 km long and drops 2 m in elevation along its length. Water column timeseries measurements indicate that the tidal channel-mudflat complex experiences a highly-mixed semidiurnal tidal signal, a strong summer spring-neap cycle, and a tidal range of 1.5 m, causing variability of mudflat inundation both temporally and spatially. Velocities measured in the tidal channel were over 1 m/s during peak ebb currents, exceeding maximum flood currents during both seasonal timeseries. While summer conditions were dominated by tidal conditions, winter timeseries were dominated by storm events corresponding with offshore wave heights up to 4 m. During storms the entire mudflat was inundated and suspended-sediment concentrations exceeded 1.2 g/L, providing ample opportunity for sediment to settle over the mudflats and contribute to infill. Measurements of inundation duration, velocity over the inundated mudflat, and sediment properties indicate that most suspended sediment transported to the mudflat settles during slack tide and does not get resuspended on the subsequent ebb, with an approximate vertical accretion rate of 2 cm/yr. These results indicate that infilling mainly occurs during winter storms, and high ebb-directed flows maintain an active tidal channel. Assuming coastal erosion and infill continue at the same rates, it is unlikely that the abandoned channel will completely infill over time, but instead will maintain its topographic low with an active tidal channel-mudflat complex as the shoreline continues to retreat.