Turbidity is recognised as one of the major environmental problems of dredging. To get a handle on this problem the Dutch dredging industry (VBKO) commissioned a research project to develop software capable of predicting how much sediment is released into suspension during dredging operations. Major field measurement exercises are carried out in order to provide calibration data for the software. One exercise has been carried out using a trailing suction hopper dredger (Cornelia) working in the port of Rotterdam in June 2002. During dredging by a trailing suction hopper dredger one or two suction pipes are lowered on the seabed. A mixture of sediment and water is sucked up and passes into the hopper of the dredger. When the hopper is loaded beyond capacity, excess mixture leaves the hopper by means of the overflow and is released back into the water. This overflow of sediment is the main contribution to the sediment plumes near the dredging vessel. The main objective of this thesis is to find out whether the flow rate and the solids concentrations leaving the dredging vessel can be calculated sufficiently from the measured overflow samples. Special techniques were developed to sample the hopper overflow. Three novel flowthrough samplers are constructed to sample the overflow mixture. These have been used, in combination with a bottle sampler to obtain about 500 samples taken at four different locations in the overflow during 14 trips. All samples have been analysed on particle size distribution and dry matter content, some samples also on particle specific gravity. Soil samples taken in the dredging area provide the data needed for mapping the soil conditions coming up by the suction pipes. It appears that at some locations quite homogeneous soil conditions are present. Nevertheless also significant variations in the Particle Size Distributions have been found. During all dredging operations, the vessel's TDS (Tonnes Dry Solids) system has logged the characteristics like draughts and the concentrations plus velocities of the upcoming mixture in the suction pipes. These data have been reviewed to assess their reliability. After recalibration of the datasets, the concentration leaving the vessel by the overflow is computed by using sediment balances. The computed sediment releases have been compared with the measured overflow releases in order the attempt to validate the sampling method. It appeared that validation of the samples by this comparison was not possible.