Monitoring silt content in sediments off the Dutch coast

Silts, present in sand extraction sites, can potentially mobilize in the water column and result in increased turbidity of the North Sea. The main knowledge gap in assessing this potential risk is the capacity to buffer silts in the sandy sediments. To assess this buffering potential, Rijkswaterstaat and Stichting LaMER started a Monitoring and evaluation program (MEP Sandmining) to monitor, amongst others, the concentration and variation of fines in the sediments off the Dutch Coast. We monitored the concentration of fines in the sediment along 3 cross-shore tracks, ranging from 0-9 km off the coast, with the Medusa system and by taking sediment samples. The Medusa system results in continuous measurements of the silt content of sediments along a line, the samples have been used for validation purposes. In the period September 2009 - Oktober 2010, 6 campaigns have been conducted. Measurements show that, although the silt content is very low (<5%), the silt content can vary strongly on small spatial scales. Temporal variations show how the silt content along the profiles change. Comparing silt concentrations to the daily averaged wave height before the measurements, indicates that the silt contents increase with declining wave height.