Zuid West Ameland, historic data, full scale tests and goals to achieve
Marco Pluijm
Morfluencer / Sand Whisperer / Shaping Morphology / Seasoned Professional / Publicist / Writer / Researcher / "Resilient by Nature"
Rijkswaterstaat recently announced the start of two sea bed nourishment tests in a pair of main tidal channels in estuarine parts of the Netherlands.
One of these tests is set at the South West Coast of Ameland, an island in the North of the Netherlands [ref. 1, in Dutch].
Purpose of that test is 1) to research the potential of protecting an existing rock revetment by means of seabed nourishment and 2) to see how the added sand mass finds its way to the adjacent coast and estuarine backwater. For which assessments new track and trace methods are said to be deployed. In order to do so, a sand mass of 2.5 million m3 is currently being dumped in the channel. Unspecified is whether this is net (after dumping) or gross (including sand losses during dumping).
One of the characteristics of tidal inlets as such, is their so-called equilibrium profile as part of the overall system with its ebb- and flood channels. Which profile dimensions are directly related to the characteristics of that inlet system and its ambient conditions such as tidal prism, associated volumes, transport capacities, delta configuration and estuarine backwater system. [ref. 2]. None of which actually change due to this nourishment. Also because the dumped sand mass is very small in relation to the overall sediment balance.
So what’s going to happen is actually more the opposite. As soon as the sand is being dumped in the channel, the erosion begins and the profile will restore its original dimensions. And even when coarser sand would be used, he outcome is still the same. The only difference with fine(r) sand is the time it takes to get washed away. In terms of protecting a rock revetment by means of an inlet/sea bed nourishment, it is by all means a very predictable result.
Also because the Ameland South West Coast project location is one of most studied morphological features in the Netherlands. As a result of which basically all that matters is known.
Pictured above the headline of this post is the final report of an extremely well documented study issued by Rijkswaterstaat in 1960 [ref. 3]. It contains amongst a lot of data, processed time series of the Amelander Gat channel system since 1798 as well as hydrographic surveys of the full tidal inlet and adjacent coast over the period 1946 till1959.. Further in depth studies and analyses were done and presented in 2005 [ref. 4]. All references are in the Dutch language due to the nature of the matter, however the many colourful maps and data sheets read for themselves for those who don’t master the language.
This all being so, it is interesting to see where this test now comes in as an innovation.
Protecting a rock revetment in a tidal channel by means of an added sand mass might be seen as kind of unusual. With regard to the second goal of this test, studying the redistribution of the dumped mass along the adjacent coastline and into the backwater, system seems rather ambitious with regard to the relative small volume of 2,5 million m3 in relation to the daily hydraulic and sediment transport capacities. Unless the sand is tagged, it will be very hard to trace it. In the early days we used radioactive tracers, but that’s a method which for obvious reasons has been forbidden already a long time ago. Instead fluorescents might help out. New survey techniques are said to be deployed, so lets wait and see.
Still, this whole research project as such might have served a much broader goal when instead of dumping sand straight in the channel, as an alternative, it could have been stacked on the outer delta, outside the direct transport zones, thus enhancing the natural shallow water features, in order to improve the overall storm impact safety and resilience all together. Still able to feed the rock protection if that still would be seen as one of the goals.
Also because we seem to know basically everything about the major transport phenomena and related gradients on the spot, the planning and execution of such a test would have been and is very well doable. And for sure would provide much more valuable data than the test in its current form.
In general, based on what has been learned form the initial Resilient by Nature assessments [ref. 5], enhancing the outer breakerzone and nearby natural coastal features, such as sand banks or ridges, is an extremely effective way to improve the overall coastal stability, protection and resilience.
In order to stimulate coastal research projects to become more effective, an overarching framework could be a valuable tool to identify (most) relevant coastal process and system interactions and values. And so to improve on integrated research output and thus, benefits.
Which in four dimensional conditions like this would be highly recommended, also when it comes down to (re)building experience with and thorough understanding of coastal process and dynamics.
Such framework can be the recently launched “Room for the Coast” concept program [ref. 6]
Full scale tests are complex, time consuming and expensive and therefore shall be drafted and kitted out to the max in such a way that the outcome is as broad, reliable and valuable as possible.
The Ameland South West Coast program is now underway and can’t get adjusted any more.
Referring to the above, it is recommended that all future full scale tests and investigations are part of an overall framework or program, preferable cross border and even cross continent. Climate change is affecting us all, almost regardless of location, reason why all experience and knowledge gained shall be shared with equal partners around the globe. Combined test sites would be ideal to do so.
References
2. Stability of Coastal Inlets; P. Bruun & F. Gerritsen; North Holland Publishing Company, 1960
3. Onderzoek Kustverdediging ZWKust Ameland, 1960, Nota 60.02 ; Rijkswaterstaat, Sudiedienst Arr. Hoorn
4. De Westkust van Ameland, Resultaten van 10 jaar kustmorfologisch onderzoek in the kader van de Rijkswaterstaat programma’s KUST2000 en KUST2005, Rapport RIKZ/2005.029
5. https://www.dhirubhai.net/pulse/old-school-skills-world-virtual-reality-marco-pluijm
6. https://www.dhirubhai.net/pulse/towards-room-coast-program-marco-pluijm
Metocean | HPC | Machine Learning | Coastal & Maritime Engineering
7 年This whole area is a brilliant study area for us humble coastal engineers :-)