Effect of dredging on SPM concentrations in the North Sea : set-up, validation and results of Delft3D model simulations

•A 3D model was set-up to simulate the suspended sediment concentrations of a silt fraction and a sand fraction in part of the Wadden Sea and part of the North Sea and simulate the effects of overflow from dredging activities on these concentrations offshore Den Helder. •The model includes effects of a temporally and spatially varying wind field, waves and salinity. •The model represents natural background suspended sediment concentrations offshore Den Helder and the effects of a sediment plume from the Marsdiep tidal inlet reasonably well. •The model results show encouraging agreement with observations of suspended sediment concentrations made on 17 September 2007. •The model underestimates the observed suspended sediment concentrations on 1 and 2 October by about a factor 2. This means roughly that the model predicts 2 mg/l near the surface where 4 mg/l was observed. •Sediment release rate due to overflow from a dredger have been schematized based on observed flow velocities and concentrations in the suction pipes on a dredger and the amount of sediment (dry weight) in the dredger. Data were made available by RWS Directie Noordzee. •This resulted in a realistic dredging scenario 1 in which the release rate from the dynamic overflow plume was 130 kg/s and from the passive flume 20 kg/s. Simulation were made also for scenario 2 in which the release rates were a factor 2 higher, i.e. 260 kg/s from the dynamic plume and 40 kg/s from the passive plume. •The added near-surface concentrations in the plume from overflow by dredging amount generally to 0.5-1 mg/l. The added concentrations may locally (up to 2 km from the dredging location) and temporally (1 or 2 hours) reach values of 2-4 mg/l. •The added concentrations due to overflow from dredging are generally smaller than the natural background concentrations.