Learning by doing

29 June 2020 - Published by Eric Boessenkool
In the Netherlands, large-scale research is accelerating the pace of understanding the interplay of water, waves and wind on the country’s sandy coastline. There is a strong sense of urgency to acquire knowledge about the morphology, hydrology and ecology of the entire coastal foundation, to enable properly informed decision making about defending the coast from sea level rises due to climate change. At the same time, there’s a need to learn quickly what works, what doesn’t work, and what other factors need to be considered. Pilot studies have proven that the use of sand nourishment is highly cost-effective for maintaining large stretches of the coast, and this now forms the core of coastal defence policy. But can it also be applied in a coastal archipelago such as the islands of the Wadden Sea? And what impact does nourishment have on the vulnerable ecosystem of such an important marine habitat?


Need for nourishment
As a low-lying coastal nation, the Netherlands recognises that failure to act in time against rising sea levels will ultimately lead to disaster. The country’s dykes, dams and other traditional “hard” barriers are unlikely to be scalable everywhere to the extent necessary to cope with climate change. Large stretches of the coast are already protected instead by a natural “soft” barrier: rows of dunes that also form an important habitat and freshwater catchment. Provided the foreshore at high tide is wide enough, the dunes are safe from wave erosion during storm surges, which could lead to a breach and subsequent inundation.

Fortunately, intensive research and pilot studies have proven that sand nourishment of the shoreface is highly effective at keeping the breaker line from creeping landwards. This entails “borrowing” sand from deeper water offshore, and spraying it into the shallower breaker zone, where it replenishes losses from waves, currents and wind. Moreover, the higher and wider beach provides sand for the wind to blow into the dunes, which in turn also become higher and stronger. The protection provided by this Nature Based Solution can last up to 5 years. The annual 12 million m³ sand nourishment campaign now forms the core of the country’s sandy coast maintenance strategy. Similar results are being experienced and shared by other countries participating in the Building with Nature project.

But in the face of rising sea level, the reliance on sand nourishment raises new questions. We talk to a person who is leading the search for answers.


Sand Motor
Carola van Gelder, project manager at Rijkswaterstaat (the Dutch Ministry of Infrastructure and Water Management), has been involved in research into coastal defence since 1998. “Our first really big pilot project was the Sand Motor,” says Carola, ‘a sand nourishment of 21.5 million m³ on the North Sea coast just south of Scheveningen in 2011. This multidisciplinary and multi-party study has proven the efficacy of sand nourishments as a protection strategy for what we call the ‘coastal foundation’: the zone from 20 meters below sea level to the inner boundary of the dune system. By sustaining that foundation, we keep the sea at arm’s length.”

But what about the impact of nourishments on the coastal ecosystem, and on the quality of the shore for recreation, for example? Carola continues: “The research is ongoing, and we are continuously monitoring aspects like currents, sand distribution, flora and fauna, and recreation value. After nearly 10 years we can say that large-scale sand nourishment is a proven concept. But we definitely need to know more.”


Carola van Gelder: “The projected sea level rise poses new questions. We will need more sand to protect our country from flooding. The question we are now trying to answer is: where, how, and how much sand do we need to maintain a sustainable equilibrium? Where can we extract and apply that sand? What functions of the coast will be affected by sea level rise, and by adding more sand?”

Carola continues: “The northwest part of the Dutch coast is defined by a string of low-lying islands in the Wadden Sea – an EU-designated Natura 2000 area with exceptionally high ecological value as a habitat for sea mammals, migratory birds, fisheries and shellfish farming, and a major destination for recreation and tourism. Can sand nourishment be used to protect the Wadden Islands and the extensive tidal mud flats? Where and how would you apply the sand, and what would that do to the marine habitat? We don’t want the cure to kill the patient.”

It was apparent to Carola and her research associates from other disciplines that too little was known about the dynamics of the underwater ebb-tidal deltas between the islands that connect the tidal mud flats or ‘Wad’ of the Wadden Sea to the North Sea. More solid, scientific evidence had to be acquired. And not just for the Netherlands –after all, low-lying sandy coastlines exist all over the world.


Coastal Genesis
In 2015, a government decision was taken to seek the answers, and extensive funding was allocated. A new research programme, ‘Coastal Genesis 2.0’ was launched with the goal of providing policy recommendations to the Ministry for the period up to 2035, and to provide a forecast for up to a century ahead. A new large-scale experiment was initiated in the Ameland inlet of the Wadden Sea, between the islands of Ameland and Terschelling.

Carola oversaw the project. “We brought a team of researchers together from every possible discipline to study the effects of the experimental sand nourishment on the entire system, both inside and outside the inlet, on the flora and fauna, on the currents, and on the beaches.”

The first challenge was to establish the baseline – the state of the tidal inlet system before the sand nourishment was applied. “We set aside two weeks in the autumn of 2017 to measure every imaginable parameter in an intense 24x7 morphological and ecological monitoring campaign of the existing situation. Below the waterline we dropped steel frames packed with instrumentation to continuously record details of currents, water turbidity, temperatures and much more at 5 locations across the ebb-flow delta. Many more observations were made from research vessels. It was exhausting for everyone, but an enormous volume of baseline data was compiled.”


Lessons learnt
Then it was time for the nourishment: 5 million m³ of sand was sprayed onto the western flank of the inlet ebb-flow channel from the start of 2018 until early 2019. It was then up to nature to do the rest. Carola van Gelder: “We learnt two important early lessons during this phase: the first was, know your seabed ecology before you start covering it with sand – the unexpected presence of a vital sand eel habitat meant we had to select a new nourishment site. The second was that you need to consider bottom clearance when choosing your sand barge. Our fully loaded barge floated too deep for the nourishment site. That meant having to lighten the payload, and the nourishment therefore took many more transport journeys than planned. These are lessons you have to learn the hard way.”

And…what else have we learnt from the project? “It has been encouraging to discover that siting the nourishment in the underwater ebb-flow delta was the right decision. This has established that the Wadden Sea captures a lot of sediment from the nourishment and gently spreads it across the tidal flats without causing permanent harm to the ecosystem. That is highly promising for the future protection of these and other tidal inlets.”


Sharing data and insights
The effect of the sand nourishment in the Ameland Inlet is still being monitored, and a colossal amount of scientific evidence has already been collected. Carola: ‘All data is shared publicly. I believe passionately in sharing. Our database, which runs into terabytes, is accessible worldwide via the internet, and we share and discuss our results via meetings and other channels within Interreg BWN. There is no better way to enable public and private research parties to compare our data with that from other projects in the North Sea region and elsewhere in the world, and to draw conclusions and make recommendations. It allows us all to benefit from the insights and ideas from the best and brightest minds in the field worldwide.”

Does Carola van Gelder have any further thoughts to share with us? “We are extremely fortunate in the Netherlands to be able to do research on this scale, and with so much support from local and national stakeholders of every kind. Although the threat from climate change is real and imminent, with serious research funding and by pooling our resources, we can develop sustainable strategies to defend our coastlines worldwide by means of Nature Based Solutions. Everybody benefits.”