Case studies: Coast Oostende-Mariakerke

Monitoring shore face nourishment

Traditionally at the Belgian coast, beach nourishments are used to reinforce the upper part of the beach. This will break the storm waves in a very efficient way and will create additional space for recreation.  However, other types of nourishments like a shoreface nourishment are also possible. By feeding the shoreface (the underwater part of the beach) an additional sand buffer is created that can be transported towards the upper beach by the natural dynamics of the system. The advantage of a shoreface nourishment is the more efficient and thus cheaper execution in comparison with traditional beach nourishment.

The disadvantage of shore face nourishments at the Belgian coast is the limited knowledge of the natural behavior of such a type of nourishment. The Coastal Division within the Agency for Maritime and Coastal Services has executed a shoreface nourishment in 2014 in Ostend, in the area of Mariakerke. This area has been monitored extensively.

Method

The entire active profile (seafloor, shore face, Intertidal beach and the dry beach) were monitored by several measurement methods such as bathymetrical measurements, measurements by LIDAR technology and GPS. Although the shoreface nourishment was executed at the area of Mariakerke, the study area contained as well the adjacent zones, from Middelkerke towards to western jetty of the harbour of Ostend. The monitored data was used to make analyses of sand volumes and evaluation of the evolution of the cross shore profiles of the entire study area.

Results

The monitoring data showed that:

• There is a clear impact on erosion and sedimentation rates due to the nourishments.
• The nourishments induced bar formation. The volume of the bar Is relatively stable, the shape is dynamic. There si a dominant tendency to migrate towards the shore.
• Most of the nourished sand stays ni the morphological system of the study area. A part of the sand migrates alongshore towards the jetty of the harbour of Ostend. Another part of the sand migrates cross shore, towards the shoreface and the dry beach.

Local context

The Belgian coast has some typical features which are important for the behaviour of nourishments:

• There is a clear difference between the western part and the eastern part of the coast with regards to beach slopes, grain sizes and beach widths
• In general, the beaches are wide and very gently sloping, the shoreface is very shallow
• The tidal range is 3.5 m at neap tide and 5 m at spring tide
• There is a dune system that protects the low lying hinterland from flooding. The dune system is interrupted by 4 harbours and several coastal towns with sea walls.

Future research directions

• Further monitoring is needed because the results of the pilot show that:
  1. The morphological development of the study area is still continuing: further evaluation of the long term trends and their potential changes can provide information for long term coastal policies.
  2. There is a tendency for natural recovery of the beaches after storm surges
• Sea level rise will affect the coastal safety level of the Belgian coast and many other countries as well. Gaining knowledge of the morphological response of a sandy coast towards sea level rise will be needed.

Documents:

All documents of the BwN project can be found at publications,  below you find the documents belonging to this specific case study:

• National Analysis, Flanders, Belgium (pdf, 4.9 MB)
• Flanders Coastal Division Policy brief (pdf, 1.7 MB)