Sustainable coastal management, past, present and future or how to deal with the tides

Abstract

Over the centuries communities living in the coastal areas of the North Sea and the Atlantic Ocean have made a living by using the in- and out-going tides to their benefit. They did so by letting the high tides in freely. How did this happen? Why did they do so and what lessons can be learned from these past examples of sustainable coastal management? Within the framework of rising sea level, one of the future challenges will be how long the Dutch can remain safely behind dikes and dams that have to be constantly raised. Four different coastal areas will be discussed, each with a different chronology, land use and hydrology. The first case concerns tidal landscapes in Brittany and Anjou (France). Using the incoming tides a network of channels and retention areas developed here since mediaeval times. In these basins a slow evaporation process led to high quality salt. The second example includes areas in the SW-Netherlands and the Wadden area, which have been used for grazing. How did grazing affect the development of that tidal landscape, and what happened when grazing stopped? The third example comes from former Zuiderzee polders near Kampen, where low dikes (kaden) were built. These dikes could withstand the ordinary summer tides but allowed the higher winter tides to flood the polder, resulting in sediment deposition which raised the level of the low-lying land. The article goes on to describe a fourth example which draws on the case studies presented here. The final study aims to restore the disrupted low-lying surface levels of the oldest polders to a much safer, higher level. By allowing high tides to enter the polders in a regulated way, sediment deposition can be re-introduced and the level of the polder can be raised. Rather than completely excluding the tides, the regulated flooding deposits sediment builds up the level of the polders, thus making them easier to defend.