Variability of grain sizes in a beach nourishment programme for the Danish West Caost

The coastal landscape in Denmark is characterized by multiple areas of geologic, biologic and recreational interests both national and international. In the later years several guidelines have been set up for the coastal protection area. The aims for the future are to ensure the presence of naturally shaped beaches while at the same time the risk for erosion is reduced. For this reason beach nourishment is used widely along the Danish North Sea coast (called West Coast) and this method is preferred in preference to put up solid constructions.

Beach nourishment is the primary method used by the Danish Coastal Authority. However problems have occurred in describing the spatial distribution of medium grain size d₅₀ and the gradation U of the sediment used for beach nourishment. At the same time the sedimentary composition determines how fast the nourished beach is eroded and how the beach is shaped.

The aim of the present investigation was to examine how to extract sand samples of an actual beach nourishment to obtain a satisfactory description of the sedimentary composition. In specific the goal was a description of the variation of the sand characteristics in space. The reason for this is that the compositions of sand affect the erosion and the shaping of the beach.

The results show that by collecting 13-15 sand samples it is possible to obtain a description of d₅₀ with a precision of 0.1 mm within a confidence interval of 95-99 %. If a precision of 0.05 mm is wanted, it is necessary to collect 67-94 sand samples. It is recommended to collect 13-15 samples, spaced evenly along a line throughout the nourished beach. Furthermore it is recommended that the samples shall be collected half way between the waterline and the dune, at a depth of approx. 0.50 m. The resulting precision of the grading U is then between 0.82-1.09.