Corjan Nolet is a PhD candidate at the Soil Physics and Land Management group of Wageningen University. He has extensively studied wind-driven coastal processes, using remote sensing techniques and geospatial analysis.
(Tentative) dissertation title:
"How biogeomorphic feedback drives dune development along nourished coastlines." PhD supervisors:
Coen Ritsema (Wageningen University) Michel Riksen (Wageningen University) Figure 1. Overview of how plant-sand feedback dynamics drives coastal dune development. Figure 2.
The state of dune development on and along the Sand Motor in the summer of 2017. (Data derived from satellite imagery, photos courtesy of Rijkswaterstaat and Jurriaan Brobbel.) Dune cover (%) 10-35 35-65 65-90 90-100 sand transport Spreading of beach grass to new places
98 99 FO U R – D U N E D E V E L O P M E N T
we identified changes in dune height and changes in marram grass greenness. Plotting the data (Figure 5) revealed an interesting pattern, which points towards feedback dynamics in dune building. In particular, the graph clearly shows that marram grass grows better when it traps more sand, albeit up to an optimal amount of sand per growing season. When too much sand is transported towards the dunes, the graph suggests that marram grass may get overwhelmed by sand, impeding its growth. At the same time, when marram grass traps less sand than the optimal amount, it may not grow as vigorously as it could potentially, meaning it also would not trap as much sand as it potentially could.
This pattern of optimal marram grass growth has an important implication for coastal management: optimizing the potential of marram grass to grow and develop dunes means we can maximize the potential of coastal dunes to provide coastal safety. Sand nourishment strategies such as the Sand Motor should therefore aim to ensure that supply of wind-blown sand towards the dunes occurs at optimal rates for marram grass to thrive. Luckily, the Sand Motor appears to do just that and in many places on and along the Sand Motor, the dunes are developing at optimal rates. Using high-resolution data obtained by a drone has proven to be invaluable for gaining insight into how marram grass contributes to dune development and coastal safety. The continuing application of such remote sensing techniques holds great promise to better understand the coastal terrestrial ecosystem with its interactions between the dry beach, dunes, vegetation and morphology.
leisure, ecology, drinking water supply) can sometimes make striking a good balance challenging.
Comparing the maps in Figure 3 made one thing clear: almost all the dunes on the Sand Motor, both the foredunes and embryo dunes, only developed in areas where considerable amounts of sand were deposited by wind. Although no clear relationship could be observed using the satellite data, this provided a first indication that reinforcing plant-sand feedbacks drives the development of coastal dunes. To truly measure this feedback, we had to study the dunes in much more detail. One way we did this was by aerial mapping with an octocoptor drone equipped with a high- resolution camera that, once in the air, would take a photo every second. With a special software technique (photogrammetry) all these photos could be stitched together to create an accurate 3D model of the dunes. Figure 4 shows two such models of the same dune, from a flight day in April 2015 and one in September 2016. The differences between these models clearly illustrates how much the foredunes have been growing, especially at the dune foot, and that quite a large number of embryo dunes have been developing on the beach.
Another striking finding is that the marram grass appears to be much greener on the parts of the dunes that have been actively growing. In other words, it seems that marram grass is much more vital in areas that receive the most wind-blown sand. We hypothesized that this was probably the result of more vigorous growth due to sand trapping, and set out to measure these differences in marram grass leaf greenness. Luckily, we were able to do this because the sensor of the camera that we used was modified to be sensitive in the near-infrared light spectrum. Because a plant absorbs visible light for photosynthesis but strongly reflects near-infrared light, there is a large contrast in reflectance which can be used to accurately determine a plant’s vitality. During one growing season (April – August 2016), we mapped the dunes every month with our drone and using the 3D models Figure 3. (right)
Mapping changes in Sand Motor morphology and dune cover density of marram grass.
Figure 4. (left) Aerial mapping with a drone to make 3D models of the foredunes. The foredunes along the Sand Motor have been actively growing and a large number of embryo dunes have developed on the beach. Figure 5. (right) Positive feedback relationship between sand trapping of wind-blown sand and growth response of marram grass. Marram grass grows better as it traps more sand, up to an optimal amount of sand per growing season. Alongshore (km) C o rs s s h o re ( k m ) 1.5 1.0 0.5 0.0 0 1 2 3 4 5 6 1.5 1.0 0.5 0.0 0 1 2 3 4 5 6
Sand Motor morphology 2013 - 2017 no change / new coast
no change
Dune cover Marram grass 2016 - 2017 Change in cover (%) Change in height (m/y)
-0.75 to -0.25 -0.25 to 0 0 to 0,25 0.25 to 0,5 <0 0 to 33 33 to 65 C o rs s s h o re ( k m ) 0.40 0.35 0.30 0.25 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Sand trapping (m) Optimal response Measurement Response model R2 ~ 0,92 L e a f g re e n n e s s ( 0 -1 )
Growing season Marram grass Positive feedback
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aeolian activity at the Sand Motor and the project’s morphological development. The different constituents of the Sand Motor’s sand were reasonably well mixed shortly after construction. A large part of the Sand Motor is located well above storm surge level (> 3 m above MSL), and is therefore never inundated and only exposed to wind. The wind exerts a drag force on the exposed sandy material, which can lift particles up in the air and initiate aeolian transport of sand to the dunes (and beyond). The wind drag is small, especially compared to the forces exerted by currents and waves. The size and mass of individual sand grains, shells and shell fragments determines the effectiveness of the wind to move the sandy material. In practice, only silt and fine sand grains are regularly lifted. Coarse sand and shells are only moved during very strong winds. Consequently, fine material disappears rapidly from the beach surface and only coarse material remains, sheltering the underlying fine material from the wind. Over time, the beach surface is covered by a thick layer of coarse elements: the armor layer (Figure 1).
The development of an armor layer explains why the sand clouds disappeared within a half year after the Sand Motor was constructed. On more regular beaches, storm surges frequently disturb the beach, breaking the armor layer, bringing fine material to the surface, and reactivating the aeolian activity. At the Sand Motor, there was no such counteracting force due to the fact that a large part of the Sand Motor surface is well above storm surge level (> 3 m above MSL). This restricts the aeolian activity at the Sand Motor more permanently.