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Quantified bed dynamics of the Netherlands Continental Shelf and the river Waal

Thaïenne van Dijk, Sytze Van Heteren, Marco H.P. Kleuskens, Pieter J. Doornenbal,  C. (Kees) van der Tak, P.J. Menninga and C. F. van der Mark

Quantified bed dynamics of the Netherlands Continental Shelf and the river Waal.Sedimentary beds of rivers and shallow seas are dynamic and may therewith form a risk to sea and river traffic and offshore engineering projects, such as offshore wind farms, pipelines and cables. Knowledge of bed dynamics is also relevant to archaeological investigations and geological modelling. For the Netherlands Continental Shelf (NCS), quantitative studies of seabed dynamics are local in scope. Therefore the large-scale spatial variation in seabed dynamics is still largely unknown. During the past years, Deltares has developed an objective method to quantify seabed dynamics from time series of high-resolution, digital echo soundings. These digital soundings cover much of the NCS and thus form a dataset that is highly suitable for NCS-wide quantification of vertical dynamic trends. The time series reveal that the coastal zone and offshore regions with rhythmic bedforms, primarily sand waves, are particularly dynamic. Detailed analyses of these dynamic zones show that sand-wave migration rates vary spatially between 0 and 20 m/year. This spatial knowledge of natural morphodynamics is applied to improve monitoring strategies. A recent project comprised the validation and optimisation of the re-survey policy of the Netherlands Hydrographic Office for the NCS. In a GIS-overlay method, predicted bed levels based on the quantified seabed morphodynamics were combined with  grounding dangers of ships as modelled from maritime (AIS-) data in order to assign re-survey frequencies to certain areas of the NCS. In another project, time-series of higher temporal resolution are used to study what environmental parameters (both natural and human intervention) control seabed and river-bed dynamics in order to optimise the monitoring strategies of Rijkswaterstaat in the approach channels to IJmuiden and Rotterdam as well as in the river Waal. The inclusion and consideration of environmental parameters and the effect of dredging may lead to more flexible and more efficient hydrographic monitoring.