THEME 2: morphodynamics on time scale of days (storms) to years (lifetime of nourishments) for the two pilot areas Mariakerke and Koksijde

During four years intensive monitoring was performed at two locations along the Belgian coastline. At Mariakerke the coastal defence exists of a dike and beach, whereas in Koksijde a dune and beach protect the hinterland from storm impacts. Investigating both hard and soft coastal protection measures in situ give better insight in their interaction and efficiency.

1. Permanent static terrestrial laser scanning of the dry and intertidal beach yields vertical accuracies < 2 cm and permits very time intensive scanning.
    
2. A special purpose mobile LiDAR vehicle with RTK-GNSS and IMU was developed and gave an absolute accuracy and precision as good as 2 cm, making it a highly accurate and precise technique for topographic monitoring of the intertidal zone at a hyperspatial resolution.
    
3. A one-year series of time intensive PLS measurements were acquired at Mariakerke-Beach and during a longer time span of 3 years, incidental mobile LiDAR scans and UAV photogrammetric measurements at both Mariakerke and Koksijde were acquired, in addition to numerous ground-based survey techniques.
    
4. The intertidal zone in Belgium is not only shaped by waves, but equally by tidal currents and, to a lesser extent, by natural variations in sediment supply.
    
5. A unique data set, consisting of wind conditions and aeolian sand transport rates, has been obtained through field experiments in Belgium on the natural beach of Koksijde and the developed beach of Mariakerke. Between 2016 and 2018, 40 datasets on aeolian sand transport (20 in Koksijde and 20 in Mariakerke) were obtained during moderate to strong wind conditions. Although sand transport by wind is easily observable, reliable and accurate data sets of sand transport rates are still scarcely available due to measuring difficulties. This accurate field data set has an added value in understanding how coastal environments (managed or natural) respond to wind forces over short to long-term timescales.
    
6. On short-time scales (hours to days), saturated aeolian sediment transport rate is cubic related with wind speed by a new modified Bagnold model. This model is validated by our own field data set and other international field data sets. The modified Bagnold model performs reasonably well in predicting sand transport.
    
7. On decadal timescales, the Belgian coastal dunes grow linear in time with a constant rate. Dune growth varies between 0 and 12.3 cubic metre per metre per year with an average dune growth of 6.2 cubic metre per metre per year, featuring large variations in longshore direction. Dune growth is primarily caused by aeolian sediment input from the beach during west to southwest wind conditions.
    
8. Based on the modified Bagnold model, onshore potential aeolian sediment transport ranges to maximum 9 m³/m/year, while longshore potential aeolian sediment transport could reach up to 20 m³/m/ year towards the Netherlands.
    
9. There is strong correlation between observed and predicted dune growth on decadal timescales (long-term). Most of the predicted data are within a factor 2 of the measured value. It suggests that annual differences in forcing and transport limiting conditions (wind and moisture) only have a slight effect on the overall variability of dune volume trends.
    
10. Aeolian sand from the foreshore is deposited at the foot of the steep cliff due to a decrease in shear velocity. Large shear velocities are measured at the berm lip due to compression and acceleration of the flow field. Onshore aeolian sand transport starts at the berm lip and increases rapidly towards a maximum downwind until it decreases to a lower equilibrium. The deposition at the foot of the cliff and erosion at the berm lip causes the cliff to change to an equilibrium profile.
     
11. The steep cliff in front of the human-constructed coastal berm of Mariakerke is very sensitive to erosion. Sand being eroded from the berm lip is deposited in front of the dyke and in the trench. This specific beach topography is a general good solution to minimize sand transport to the hinterland, but only serves temporally.
     
12. Further research should focus on better quantifying aeolian sediment transport processes by more innovative monitoring techniques, especially when long-term monitoring is required. A camera-system to monitor the overall weather and wave conditions, bar welding, beach morphology, and the frequency and magnitude of erosional events could be useful. The images could be used to extract moisture maps, beach dimensions, fetch distances and vegetation cover. It would be also of interest to introduce a self-rotating vertical sand trap that measures the whole transport column from surface to a certain distance above the surface to get information of the entire flux profile. A change in decadal dune behaviour due to climate change is also very relevant to study. A changing wind field could cause the dunes to erode instead of the current growth.
     
13. Morphological features of the intertidal bars, embryonic dunes and backshore berm play an important role in beach recovery.
     
14. Developed beaches in urban areas can retain some natural ability to rebuild after the storm. In the test sections 98-104 (Mariakerke) up to one third of the eroded volume due to Storm Dieter was recovered within 5 months.