Gerwin Stam

Because of the on-going erosion along the Dutch coast, sand is nourished regularly to maintain the coastline position and safety levels. Since the year 2000, a volume of 12 million m³ of sand is supplied artificially every year. Current plans are to increase this depositing volume in relation with the amount of sea level rise. In the past, these nourishments have been carried out with relatively small volumes and as a consequence of this, the effects are mainly local. This automatically implies that multiple nourishments on different locations along the Dutch coast are needed to counteract this on-going erosion.

However, recently large beach extensions have been carried out near Ter Heijde (Sand Motor), Noordwijk and Petten that besides safety also serve natural and recreational functions. The main advantage of these large beach extensions is that large stretches of coastline are protected for a long timescale (e.g. 20 years). In this way, the frequency of nourishing decreases significantly. This is not only cost effective, but it is also positive for preserving local ecology. Besides, this method is considered future-proof; additional sand can always be added to maintain safety levels, while this is much more difficult for a hard sea defence. These large scale nourishments are also proposed by the ‘Deltacommissie 2008’ because of their ‘great social benefit’ for the Netherlands.

It is expected that these two types of large scale nourishments will be applied more often in the near future, both in the Netherlands and worldwide. However, one of the most challenging issues in the design of large scale beach extensions is estimating the erosion rates in time and consequently the lifespan of a beach extension. A more exact prediction of the erosion rates and lifespan is relevant because it can lead to a more efficient design of beach extensions or can give more control over maintenance. It can also lead to a more reliable long term planning with respect to safety, nature and recreation.

The focus of this research will be on the relation between the erosion rate and the characteristics of large scale nourishments. These characteristics are for instance size, shape and grain size. The primary research goal of this study is to develop design graphs for the erosion rates of beach extensions at the Dutch coast.

Furthermore, it is expected that there will be a critical seaward extent for which the nourishment itself starts to influence the external forcing. This external forcing is in turn responsible for the morphological changes of the nourishment. If the nourishment becomes big enough, then:

• Constriction of tidal forcing is likely to occur which results in increased flow velocities and hence larger sediment transport capacities.
• The depth contours of the nourishment will influence the propagating waves from offshore to nearshore. Due to this refraction, different incoming wave angles are to be found in close proximity of the nourishment.

It is likely that the effects described above have an influence on the erosion rates of nourishments. Secondary goal of this research is to find out at what seaward extent (or L/W ratio) the erosion rates are influenced by the dimensions of the nourishment itself.