Sediment mapping of sand extraction pit Maasvlakte 2, using bed classification from multibeam backscatter data

The distribution of sea bed sediments is relevant in understanding both physical processes and ecology in the marine environment. Modern and innovative techniques allow for high-resolution measurements for sediment mapping. In this project, we collected multibeam bathymetry and backscatter data, as well as box core samples for ground truthing, in and around the two sand extraction pits for Maasvlakte 2, offshore Rotterdam, Netherlands. We applied a Bayesian bed classification technique, as developed by Delft University of Technology, in order to create a high-resolution acoustic sediment classification map. This project is meant to be a pilot study, also aimed at knowledge exchange. Therefore, bed classification results are preliminary. The acoustic bed classification results in 4 acoustic classes. Assigning sediment characteristics, as derived from grain-size analyses of the box core samples, to the acoustic classes results in an acoustic sediment map. Differences in sediment characteristics in the study area are subtle, and yet the acoustic techniques are able to differentiate between these characteristics. The correlation of sediment characteristics to acoustic classes revealed that median grain size and mud content alone were not fully discriminative and that sediment sorting and gravel content correlated well to the acoustic classes. The influence of gravel content and other sediment and/or bed characteristics, such as thin mud drapes or benthic fauna, are discussed in this report. The absence of mud layers at the bed in 19 of 21 box core descriptions, grain-size analyses and 4 water samples suggest that the pit is not being filled in with mud at a significant rate. However, two fine-grained samples in the eastern part of the pit fall within an acoustic class that occurs in the eastern area that was first abandoned for dredging activities, which could imply that fine sediments are slowly accumulated. More rigorous interpretation of the method, resulting bed classification map and assignment of samples is required to fully understand the sedimentological conditions of these sand pits.