Case studies: Sylt beach and shore face nourishment between Rantum and Sansibar
Finding solutions to protect nature and infrastructure
The sandy coastline of Sylt, between Rantum and Sansibar,is subject to coastal retreat due to storm surges, tidal currents and waves. Even though the permanently ongoing relocation of the beach- and shoreface sediments make a protection of the coastline difficult, solutions have to be found and permanently improved to protect nature and infrastructure in an area that is both heavily touristic and of sensible nature at the edge of a national park.
Excursion to Husum, 2017
Method
Three adjacent (but not directly connected) shoreface nourishments of different design have been performed to answer questions about the effects of these designs and of shoreface nourishments in particular. These measures have been accompanied by measurement campaigns to monitor the development of the beach and the shoreface.
Results
It is common knowledge that nourishments are the most economic and ecologicmeasure to protect the coast of Sylt island, if not the only one. Besides that, the assumption that shoreface nourishments are valuable to protect the beaches turned into factual knowledge.
Most morphological changes occurredin the bar-trough-system. The additional sediment supply in front of the bar temporarily leads to an enlargement of the trough. In the medium term, sediments were relocated from the bar to the trough and the beach. Additionally, the sediment is distributed longshore. Longshore and cross-shore transports are dominated by the wave energy flux.
Intermediate areas between nourished spots benefitted from the measures.
Shoreface nourishments perform a rebuild of the beach volume. When acute action needs to be taken at the beach, combined shoreface and beach nourishments were observed to be efficient.
Nourishments performed at exposed sites of the coastline should not be expected to have a long-term effect at this specific location, but more at adjacent areas downstream the longshore transport.
It is important to have a suitable morphologic and hydrodynamic monitoring concept to gain knowledge about the processes.
Local context
The proximity of the national park and the importance of tourism on the island makes it easier to find favour with BwN solutions.Sand is the best way to dissipate the wave energywiththe least negative side effects on neighbouring areas.
The orientation of the coastline is almost perpendicular to the major driving force (wave energy flux). Nevertheless, the rather small component of longshore transport is responsible for the continuous loss of sediments towards the ends of the island. Strong tidal currents at the northern and southern ends of the island work as sinks for the longshore transported sediments. Therefore, a permanent loss of sediments occurs. The national park (WaddenSea) benefits from the sediments.
Future research directions
- The lifetime of nourishments depends highly on the weather, i.e. strength and duration of storms and the resulting energy impact. Therefore, it is difficult to compare the effectiveness of nourishments of different years. A method to account for different wave energy impacts in the assessment is needed.
- How to deal with foreshore erosion at the ends of the island?
- How will climate change (sea level rise, wind) affect erosion in the future? How can we prepare the sandy coast for that? What amount of nourishment is needed to keep the coastal volume? Which is the most efficient way to place it?
Documents
All documents of the BwN project can be found at publications, below you find the documents belonging to this specific case study:
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