Dynamics of dune development: The effect of plants on sand deposition

Most researchers have left the Zandmotor and are busy analysing their data by now. Some people might wonder that there is still some evidence of our activities present at the Zandmotor. Especially near the ARGUS tower there are still some vegetation plots in place. Wageningen university aims study the processes that underlie dune formation and changes in dune morphology. The focus is on the relationship between sand transport, sand deposition and plant density of two different plant species: Ammophila arenaria and Elytrigia juncea of high and low density. 12 Plots of vegetation with different species composition and density were created, in which sand trapping efficiency and sand transport measurements are performed. Sand deposition within the plots is calculated using erosion pins as well as behind the plots using photogrammetry. In addition, 6 sand traps are placed in front the plots and 12 sand traps are placed behind the plots in order to trap the sand that is transported. The amount sand that is trapped is collected and afterwards is calculated. (Master thesis research from Angela Anastasiou, student of Wageningen University )

Vegetation plots in the wind

 

Zandmotor is under attack !

Today we are experiencing the most intense conditions so far during our MEGAPEX campaign. The cliff at the tip of the zandmotor is eroding due to the large waves.

Surfzone dye and drones

Waves, wind and tides all influence the water motion near the beach. Although we know a lot about what drives coastal currents, we can still be amazed by how complex the flow can be in a real life situation.

This week we did experiments with green dye (don't worry, it's environmental friendly!) to get a top view of the flow pattern. The green dye is released in small patches, which is then transported and mixed by the flow.

To follow the dye movement we use small drones (UAVs) to take aerial pictures. We have to specialists at the experiment right now that can make great pictures of the beach as their drones fly about 100 m above the Sand Engine and are equipped with small GoPro Camera's. 

A few snapshots of one dye deployment are shown below:

Two small patches of dye at the start of the experiment. One offshore of the bar, and one inshore of the bar. 

after 6 minutes

after 12 minutes

After 18 minutes

And after 28 minutes

These pictured give an impression on how fluid can mix material floating in the water (swimmers, sand, oil etc.) and redistribute this matter along the coast. 

Images from Ronald Brouwer (Delft University of Technology) and Patrick Rynne (University of Miami/Waterlust)

NB.. Note the small white line in the offshore region. That's probably the edge of the fresh water from the river and the salt sea water. Pretty cool!

Tilt Current Meters, take 2

In an earlier blog, the Tilt Current Meters were introduced as an innovative way of measuring currents in an extensive spatial grid. Deployment and recovery of the first batch of TCM's from a RIB boat with assistance by Shore Monitoring & Research went successfully. Since as many TCM's as possible were placed in the vicinity of ADCP stations, a valuable validation dataset has been obtained to check upon the TCM's working principle. A very promising validation plot is appended below:

5-minute averaged tilt magnitude of the TCM is shown in blue, 1 hour averaged velocity magnitude of the ADCP is shown in red. There is a dominant tidal signal visible in the data.

Yesterday the TCM's have been deployed for the second time, in a slightly different array. We hope to capture some more aspects of the tidal currents on the northern flank of the Sand Motor, as recent drifter deployments showed hints of tidal eddy formation in that region.

This week, all available resources are used to study the currents around the Sand Motor. The TCM's have been deployed, the two cross-shore and one alongshore measurement arrays are still in place, drifter deployments will be carried out and the drone teams from Delft and Miami will capture dye injections from above. Exciting data guaranteed!

Aeolian transport update - nailing it !!

Last week we were very pleased by the strong winds. Sediment was racing over the sand surface leading to some interesting measurements.

 

Underneath is an example of one of our nail experiments. We are using nails in the sand bed to measure erosion of sediment from the beach surface at short timescales. A surprising amount of sediment seems to be eroding from the intertidal zone at low tide. That was somewhat unfortunate for us since we 'want' to do these experiments with the nails at every low tide now (including the night low tides !!).

We are catching up some sleep now since the wind has been a bit less the last couple of days. BUT, we are preparing for next week where winds seem to pick up again. Ready to nail the beach again !!

Using sonars, a laser and a camera to observe change in RIPPLES!

As a part of the larger TUDelft hydrodynamic array, the University of New Hampshire (United States) is working to measure the temporal variability in the small scale roughness patterns (i.e. ripples) at the tip of the Sand Engine. With the dynamic nature of the Sand Engine, we are working to provide insight into the forcing mechanisms of rapid small scale bathymetric change.

To quantitatively measure the ripple change, we have two rotary pencil beam sonars with centimeter scale resolution, and to complement these acoustic sonar measurements, we have developed a laser grid and camera system to optically measure millimeter scale change in roughness. While, the sonar works very well in any level of water column turbidity, it has inaccuracies in resolution when the ripples get very small. At the same time, the optical measurements of the camera do not work well in very turbid water, but when bedforms are very small (generally corresponding to smaller wave forcing conditions and lower turbidity), the laster-camera system can make detailed observations. These complementary tools provide us with information about small scale morphologic change. Additionally, we are using a set of acoustic velocimeters and current profilers to attribute hydrodynamic forcing conditions to observed morphologic change.

The first image below is from a sector scan of the pencil beam sonar, the color scale is of the intensity of signal return, where red indicates a high reflector (i.e. the sediment bed). At sweep distance of 0 m and Z distance of 0 m is the center of the sonar head. The sonar rotates by means of an azimuth drive in a circle to create a local 2D bathymetric map. This image was collected on October 6th, and clearly demonstrates the presence of peaky ripples.

And, the following image is an example of the laser grid projected over a flat bed and a fish. In the presence of ripples, the grid bends over the ripple crests and through the ripple troughs (like it bends over the fish) so that ripple height, length, and direction can be determined.

So far, at the Sand Engine, we have observed a very dynamic temporal variability in small scale roughness that, at first look, changes with the wave conditions. In the next few days we are looking forward to some big waves and wind, so we can see how these ripples change as the waves get larger!!

We are very excited to be here in the Netherlands for the MegaPEX experiment! A great deal of thanks goes to the PADI Foundation for funding our investigation!

Mapping the seabed of the Sand Motor with a Sidescan Sonar and Multibeam echosounder

A combined side scan sonar and multibeam device was deployed at the Sand Motor by Delft University and Deltares technicians on monday 29th and tuesday 30th of September. The aim of the device is to investigate (1) the bed forms and (2) spatial variability in bed sediment composition. Both alongshore and cross-shore transects were sailed. Basically a high-detail map of the bed at the Sand Motor was created. The area considered extended in alongshore direction from the southern side of the Sand Motor (Monster) up till Kijkduin (The Hague). It is expected that a combination of multibeam data, backscatter from the sonar and physical sediment samples will provide a wealth of information to researchers within the NEMO and NatureCoast projects.