Case studies: Krogen Denmark
Increase safety level and stop profile retreat with sand nourishment
Krogen is a very narrow sand barrier in Denmark. The natural active profile retreat here is 1 m/year. The barrier is part of a common agreement in which a 110 km long stretch of coast is protected by the state and the 4 municipalities. The aim is to have a safety level corresponding to a 1/100 year event, and to stop the profile retreat. This is achieved by sand nourishment, both on the beach and the outer bar.
The Krogen area has the most narrow dune on the 110 km stretch. The area consists of natural dunes and areas with holiday houses in the dunes. The beach is wide and the sediment is mainly sand and includes gravel. The amount of holiday houses close to the beach puts retraints on how much aeolian transport can be accepted, hence brushwood fences and marron grass is used to trap sediment at the beach and foredune.
The stretch is and has been frequently nourished with sand.
Method
The method was to design a shoreface nourishment to provide safety, and to analyse the effects of reducing aeolian transport into the hinterland.
The effect of the nourishment perpendicular to the coast was assessed by using surveys. The aeolian transport was assessed by using airborne Lidar and small drone footage.
Results
The main result is that shoreface nourishment stabilised the beach. The shoreface nourishment diffused downstream and strengthened the bar.
Another main result is that aeolian transport of sand from the beach to the dunes can be reduced by installing brushwood fences and planting marron grass.
The analysis also shows that building with nature measures, such as sand nourishment, wave and wind sediment transport, also preserves habitats and a natural coast which is a foundation for the local coastal tourist economy.
The analysis shows that it is essential to know the natural variability in order to assess the effect. At least 4 yearly surveys for at least 5 years must be done before human impact is made.
Because of the many nourishments it is difficult to extract the net effect of one nourishment.
Local context
The dominant wind direction is shorewards and the climate is windy. This provides an opportunity to use aeolian transport as an approach.
Future research directions
Further research is needed on quantifying the natural variability and the impact of shoreface nourishment on that. It requires a lot of surveys that are difficult to conduct because of the severe wave climate prompting for new survey techniques, e.g. satellites.
Documents
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