Barrier island management; lessons from the past and directions for the future

Abstract

The article focuses on the morphological development of the Wadden Sea barrier island system, with emphasis on West and East Frisian islands on several temporal and spatial scales. In addition, it integrates the insights for management purposes. Barrier island management is addressed with respect to morphology, sediment budgets, safety and natural values. We show that each of these issues is determined to some extent to various spatio-temporal scales and that the management of a barrier island has to be considered in terms of interactions on various spatial and temporal scales. Morphology of some of the barrier islands is determined by the pre-existing Pleistocene relief to a fair extent, either directly due to erosion-resistant outcrops on or near the islands, or indirectly by determining the locations where inlet systems or estuaries could develop. Where this is not the case, the larger part of the sediments are locally reworked Pleistocene or Holocene deposits eroded at the North Sea coasts of the barrier chain and deposited in the back-barrier area and on the islands as a response to sea-level rise. Hardly any sand is coming in from outside the area. In order to keep up with sea-level rise sand has thus to be nourished if coastal retreat is not allowed. During the long Holocene evolution islands and ebb-tidal deltas have been lined up during their coastward migration, forming a more or less uninterrupted barrier chain along the Frisian coasts. The present-day approach of mainly focusing on the fixation of the inhabited parts of the chain will most likely result in a de-alignment of the various parts of the chain, resulting in increasing erosion of the promontories. An inlet system is a sediment-sharing system with a tidal inlet, the ebb-tidal delta, adjacent barrier islands and the tidal basin with channels, shoals, tidal flats and salt marshes. The sand balance of a barrier island is thus directly linked to tidal inlet system development. A natural change or an intervention in the sediment-sharing system by man may thus have repercussions for the island’s development. Sediment redistribution in the coastal zone may also depend on climate, as is illustrated by the rapid growth of the islands after the demise of the Little Ice Age. On the barrier islands themselves many measures were taken during the past two centuries to ensure coastal safety. The successful attempts to stabilize the coasts and dunes of the barrier islands resulted in a reduction of sand transport from and along the shoreface to the beach and onto the islands. To some extent this has been restored by applying sand nourishments. However, vertical accretion of the islands is still largely impossible due to all the older coastal protection measures still present. On the long run sedimentary dynamics are essential if the island is to accrete vertically with sea-level rise, which forms a robust and sustainable strategy to guarantee safety during the next centuries. Massive stabilization also reduced the opportunities for pioneer vegetation. Dune belts and tidal marshes have experienced a fast succession resulting in a climax vegetation and the loss of the characteristic open landscape.