System Description Noord-Holland Coast

Abstract

The safety of a large part of the Netherlands is dependent on the safety level of our flood protections. The height and volume of the dunes and the near shore zone influence this safety level. Robust dunes have provided natural safety against the sea for centuries. In the last centuries, structural erosion of the coastline and the increase of the use of the hinterland have made coastal maintenance a necessity in order to provide this safety. The Noord-Holland coast is one of the most extensively maintained coastal stretches of the Netherlands. In the last twenty years Rijkswaterstaat nourished this coastal stretch with a total volume of 44 million cubic meters, in the shape of beach and foreshore nourishments. To assess the need of such extensive maintenance and to map sedimentation and erosion trends a system description is made. Over the area from IJmuiden till Den Helder, the coas is divided into seven coastal cells (van Rijn, 1997). Per cell the near shore volume evolution is analysed. To describe the Noord-Holland coast it is chosen to use the JARKUS database. Forty five year of coastal morphologic data is used, over the period 1965 to the year 2010. The error with regards to the JARKUS data is found to be limited when a large number of profiles is analysed. This can be explained by the law of large numbers and a convergence of the systematic error. Accuracy in the order of 15 – 21 m2 over the surface of a profile needs to be taken into account. In the year 1990 a new coastal maintenance policy was introduced; “Dynamic Preservation”. This policy had the strategic objective “to guarantee a sustainable safety level and sustainable preservation of values and functions in the dune area” (Min V&W 2001). To reach the objective a coastal state indicator has been implemented. In the year 1990 the position of the coastline was established through a concept called the Basal Coast Line. Combined with a benchmarking principle a method was formulated to assess when coastal maintenance, in the form of nourishmens, is needed. Since the implementation of this benchmarking principle and the “Dynamic Preservation” policy the nourishment volume increased vastly. By correcting for the artificially added volumes an autonomous volume evolution is presented for each cell. With the assumption that similar erosion rates would have been present without coastal maintenance, an indication of the state of the Noord-Holland coast is given. The near shore volume corrected for nourishments over the period 1990 – 2010 shows an autonomous degradation of similar order compared to the years 1970 – 1990. This indicates that for the whole Noord-Holland coastal stretch, the concept of the autonomous behaviour as conservative indication of erosion / sedimentation rates holds plausible values. The system description indicates that the coastal stretch of Noord-Holland received a significant larger nourishment volume than deemed necessary to reach the objective of the “Dynamic Preservation” policy. The autonomous volume changes over the period 1965 to 2010 are used to calculate yearly sedimentation and erosion rates per coastal cell. By adopting alongshore transport rates over the + 3 to – 8 m zone proposed by Van de Rest (2004) a sand budget model is made. The results indicates that for the most northern cells Van de Rest underestimates the alongshore transport gradients. By adopting the calculated transport rates from Stive and Eysink (1989), better results are obtained. Although the sand budget model holds some limitations, the results are quite acceptable.