Massive Sedimentation Events at the Mouth of the Rotterdam Waterway

Abstract

Under storm conditions, often more than 0.5 million tons of mud and fine sand are deposited in less than one week in the in-land part (Maasmond) of the access channels to the Europoort harbors at the mouth of the Rotterdam Waterway. The majority of this material comes from the sea. Transport of this material has previously escaped detection, and a satisfactory physical description of these massive-sedimentation events is lacking. A mechanism is proposed that can explain these sedimentation events. In the coastal zone, combined wave-current flows winnow muds and fine sands from the bottom, generate sediment-induced stratification in a layer a few decimeters thick, and transport these wave-induced near-bottom high-concentration suspension layers (WI-HCSLs) with the near-bottom water currents. In a large area residual flows in the lower portion of the water column are directed to the Maasmond, because of water-density differences from the relatively large Rhine fresh-water outflow. In a few days, sediments from a vast area are thus transported near the bottom to the Maasmond. Once in the Maasmond, they settle due to a considerable reduction of wave action and tidal current velocities there. Due to the high deposition rates, fluid-mud layers are formed that spread and consolidate slowly. A simple data-driven particle-deposition model simulates the transport and deposition of suspended sediment that enters the sedimentation area. The description of the sedimentation in the Maasmond with WI-HCSLs and with the particle-deposition model, is supported by a wide range of observations in this case study, such as sediment composition, and the positive correlation of sedimentation amounts with wave energy and fresh-water outflow. Further support is given by numerical examples based on transport processes and field data.