An analysis of dune evolution on Dutch barrier islands using a 3-dimensional model

Abstract

Barrier islands are long landforms in front of coasts and are a vital part of the coastal defence. The islands protect the mainland from storms. Furthermore, they provide unique habitats, such as salt marshes. Barrier islands are densely populated and cover a large part of the ocean shoreline. Dunes are an important part of barrier islands, as they provide protection against waves from storms, and they provide and protect the unique habitat behind the dunes. In the Netherlands, a barrier island complex called the Wadden exists. The Wadden is comprised of five large barrier islands and smaller island plates in between, which are subject to frequent inundation overwash from storms. The dynamics of the dunes on these islands is important to know, considering climate change and sea level rise. Dune gaps can play a significant role in battling sea level rise, as they increase transport of sediment inland. In the Netherlands projects were started where artificial dune gaps were excavated to achieve this effect. On the Wadden Islands naturally formed gaps already exist, however it is unknown how stable they are and under what conditions they are formed. Using the 3d-model Barrier3D the conditions of the Dutch Wadden Islands were simulated. Through the addition of the Dutch storm climate, and interactions between vegetation and dune growth, the development of the islands was simulated. As a result of ratio changes between the parameters of dune growth, storm strength, and diffusion, the possible development scenarios for a similar barrier island as are present in the Dutch Wadden could be uncovered. By experimenting with the values of the parameters the barrier island develops differently. Contrary to earlier studies, the results yielded not two stable states but three for the dune complex on the barrier islands. Dunes can grow tall, and be untouched by storms. Dunes can be eroded, either quickly when embryo dunes are unable to develop, or through lateral erosion. Finally, the dunes can stabilize. This is a state where dune complexes are stable for a long time with gaps in the dune row. Under the right conditions, gaps are created between higher elevated dunes. Gaps have a lower elevation and are remain at that elevation due to the interaction between storms and vegetation. The gaps can expand, collapse, or stay stable for decades. The characteristics of the gaps change depending on the parameter settings but do coincide with the gaps found on the barrier island of Schiermonnikoog, with gap widths ranging between 30 and 250 metres. The island behind the dunes responds to the three different states. With high dunes in front of the interior of the island, little sediment is deposited, this can pose problems in the future as sea level rise puts the islands at risk. Gaps promote sediment transport behind the dunes in turn creating washover fans, when gaps are open for hundreds of years, the island interior can increase in elevation with several metres.