Riverbed fluctuation and erosion property of cohesive sediment based on long-term topographic surveys in a macrotidal estuary

Abstract

The feature of large scouring process of cohesive sediment and the long term seasonal riverbed fluctuation at the estuarine turbidity maximum (ETM) zone of the macrotidal Chikugo River estuary, Japan were studied using the intensive periodical transverse surveys of riverbed topography and sediment sampling along with continuous monitoring of water level, salinity and turbidity during 2005–2017. The results revealed that substantial erosion occurs (increase in channel capacity) when the river discharge exceeds 2,000 m3 s−1 while deposition occurs (decrease in channel capacity) when the flow falls under 100 m3 s−1. During the normal flows, the bed shear stress (values ranging from 0.2–4.0 N m−2) showed a harmonious behavior driven by the fortnightly tidal variations. Conversely, rapid variations in bed shear stress values (2.0–22.7 N m−2) were observed during the flood events. Further, a critical shear stress value of 1.8 N m−2 was estimated as the erosion threshold of mud deposit in the estuary. Finally, an erodibility coefficient of 0.0015 s2 m−2 was determined based on the reasonable relationship (R2=0.767) between erosion depth and accumulated friction velocity from the actual field data. This value is several times lower than those reported in previous studies due to the differences in deposition and consolidation properties of sediments between actual and experimental channels. This indicates that the erosion of cohesive sediments in estuaries is not only influenced by the magnitude of shear velocity, but also by its consolidation characteristics.