Coastal observation: monitoring erosion and managing the coastline
Between land and sea
Transcript of the video
Coastlines have evolved naturally since the dawn of time, but man has only recently become acutely aware of this. About 27% of France's mainland coasts are eroding. This is spoken about more and more because the vulnerability of these zones is increasing, and particularly so in recent years.
With more and more people in France and worldwide living on the coasts, or within 25 km, new problems are being raised. Not just erosion and the receding coastlines, but also major questions of society.
A coastline is a living space, and all that happens there - interaction between the waves and the mainland, the organisms that live there, including humans - makes it a very complex environment. The evolution of nature isn't always on the same scale as that of man, even if we can grasp the overall effect of change in some places. Understanding the mechanisms governing a coastline's evolution over the long term requires study over the long term.
The French coastline is made up of different types of coastal environments. To simplify things, let's say there are three types. There are estuaries, there are cliffs, and there are sandy beaches. And the morphology of these environments differs greatly. So the processes that cause the evolution of their erosion also differ greatly. The tools and methods that enable us to characterize and understand coastline evolutions are also different, and require complementary approaches.
What is a coastline? There's not one definition, but several. A cliff is simple. But is the coastline at the top or the bottom? For a sandy beach, how do we define it in purely geometric terms? The limit the sea reaches at high tide? Or do we mark the limit according to vegetation? So there are various ways of defining a coastline. But that's not the problem. It's defining a coastline that we'll measure over a long-term period. And when you must combine the total of these coastlines to produce a homogenous figure for the whole of France, then it's a problem.
Although coasts shift a lot, we must attempt to fix the coastline, or our territorial limit, on a map. This is the task of SHOM: the French Navy's hydrographic and oceanographic service, and IGN, the National Geographic Institute. On screen, we see the whole of Finistère. In black, the coastline. This is an example of the precise coastal mapping produced by Litto3D.
Litto3D is the project that aims to create a continuous land-sea altimetric model that will establish a geographic reference figure for the mainland and overseas France coastline. We will finally be able to define a coastline, and on top of that, it's of legal importance because these limits will enable us to define French territorial waters and an exclusive French economic zone.
The position of the coastline is different when measured over the long term or at the present time.
On this beach, we see the position of the coastline at different moments. We have the current position of the coastline, at the uprush level. That's the small sandy heap in a line along the beach. It's forming right now. For a few hours, it will mark the position of the current coastline. If you look higher up, around where my feet are and along the morphology oscillating along the coast, there's the same thing as down below, but from when the sea was higher, typical of the storm we had a few days or weeks ago. That's another coastline marked by another uprush. And even higher, along the beach, at the top of the beach is a tide mark with driftwood and vegetal debris deposited during very strong paroxysmal events. If the sea level reaches the very top of the beach, where the last coastline is, we're at risk of flooding. What's typical in this system is that the lower coastline, the heap at my feet, corresponds to a construction mechanism. Sand has accumulated. But up top is the mark of the total destruction of the beach. When the sea reached the top of the beach, all of this sand system was out at sea. It has come back during periods of fine weather.
We're on this site because there's a very fast coastal dynamic. To us, the coastline is: the line between the foot of the dune and the beach. That's the indicator we follow. It's practical, because with the break in the slope, we can find and study it easily on old photos, and we can note the evolution of this coastline over several years or even decades. The particular thing here is that we have a fairly deep channel which evolves quickly, with erosion rates that can reach 20 to 25 meters a year. That's what we had here in the winter of 2013-2014, with a series of extraordinary storms. The coastline retreated more than 20 meters here, and the beach lowered by 2 to 2.5 meters just after the winter. This resulted in big changes to the environment and serious consequences for the facilities behind the beach. We can see on the beach, which is in an advanced state of erosion today, that paleosols are appearing in outcrops. "Paleo" means old. This is the soil of a former forest. This clay, or rather peat, which we see on the beach, is several thousand years old, or several hundred years on some sites. So this is a shifting environment. And although there is erosion today, we did manage to build a dune at some time, which we're losing today. We tend to forget how much the coastline shifts, and what we see today is a good reminder of that.
Cliffs make up a second main type of coastline. Unlike beaches, which advance and retreat, cliffs only retreat. This retreat becomes even faster when the materials of the scarps have low resistance. Cliff erosion is due to a combination of factors: aggressive action - waves hitting the foot of the cliff, called "subaerial erosion", along with freeze-thaw, run-off and infiltrations - and gravity, which can cause a scarp to collapse. The combination of these factors explains the retreat of the cliffs.
When a cliff collapses, because it contains flint, the chalk dissolves quickly. The flint is then rolled by the sea, turning it into pebbles. These pebbles protect the cliff-foot and the valley outlets from flooding. The coast of Haute-Normandie is made up of chalk, but from Cap d'Antifer and Étretat to the Somme estuary, it's not the same chalk. That's why we have different types of cliff and different evolution speeds. At Étretat, the cliff-foot is more resistant, a little more crystallized, so the retreat speed, on a human scale, is very slow. Whereas here, at Cap d'Ailly, near Dieppe, or at Criel, the cliffs are composed of less resistant chalk and retreat speeds reach 20, 30, 40 cm a year, which is considerable. We also have cliffs composed of granite or metamorphic rock, which are very resistant and, on a human scale, unshifting. Or, on the contrary, clay cliff, like at Vaches Noires, which experience landslides and very fast retreat speeds.
In the 1970s, man decided to fix a permanent coastline. And to fix this coastline, he constructed various structures, like sea walls, groynes, riprap banks, and so on. But these structures tend to impact the morphosedimentary dynamic of beaches, like here, with the Curnic jetty at Vougot beach in Guissény.
This jetty was built in 1974 to protect a cove where, in summer, fifty or so yachts drop anchor. It's obvious that this structure is vastly oversized for the job it's meant to do. But it does protect this cove, and for the yachtsmen, who have formed an association, removing this sea wall is out of the question.
But after construction, the wall had an immediate impact: it blocked sedimentary transport between the protected beach and Vougot beach on the other side. And so, by the end of the 1970s, Vougot was no longer fed with sand and started eroding. To limit this erosion, riprap was laid at the foot of the dune after the stormy winter of '89-'90.
When you look at this riprap, you can't help but notice that the dune is totally disconnected. It has retreated 10 to 12 meters back from it. The riprap was badly laid and badly dimensioned, because the sea very soon got around it and submerged it, and now it serves no purpose at all. In any case, the money spent at the time was pointless because it hasn't protected this dune. So its primary function hasn't been fulfilled at all.
All structures have an impact: a positive impact, which is expected of them. A longitudinal structure's job is to fix the coastline. Most of them achieve this in the short term. They fix the coastline, so they meet a need in a positive way. But each structure also has an unwished-for negative impact. The heavier and bulkier the riprap is, the more negative the impact. When waves hit it and then retreat, they take more sand out to sea than they bring in, and that generates erosion.
Groynes were constructed in the 1970s. The trouble was, they blocked the sand on the south side. They generated a form of erosion in direct proximity. They only partially met the need. So they've experimented with another form of protection for 15 years: breakwaters. The breakwaters are riprap laid in the sea to break the onslaught of waves and allow sand to deposit behind. This type of structure should form what's known as a "tombolo": a strip of sand from the beach which deposits when it hits the breakwaters. But here too, it's not all positive. Sand is deposited behind the breakwaters, but a gap is left in the spaces in between. So there's still erosion to the side. Today's solution for preserving this zone, with its homes and businesses, is different. We now bring in sand every spring before the summer season. This is longer-lasting and better for the whole dynamic, because the balance is restored. So we must switch from a philosophy of protecting a coastline for ever by erecting barriers to a philosophy of gradual management, a bit like tending your garden.
Different coastal environments have different problems. At Ault, in just a century, the town has lost a street due to a cliff collapse. To preserve the Rue de St-Valéry, another street further on along the ledge, a protection called a "cap" now consolidates the cliff-edge. In 1983, a riprap seawall, "Dyke 83" was constructed along the beach. The high cost of the work indebted the town for 30 years.
"Dyke 83" has borne fruit because it has saved a street. If we hadn't built it, the sea would be inland now, 20 or 30 meters further on. So we're still fighting against the sea to preserve the coast and the homes on the seafront. Regarding the pebbles, a natural phenomenon takes them out and brings them back. It's a phenomenon that has been going on for ever. Our groynes retain some of the pebbles, but the force of the sea takes a lot away. They go off sailing! So we need to bring some in to strengthen the riprap. I don't have the money, so I "harvest" some from nearby Mers-les-Bains. So I thank my counterpart for the pebbles. It's he and the mayor of Le Tréport who pay.
During the 20th century, about 3000 m³ of pebbles were taken every year from Criel-sur-Mer beach for the construction of a new railroad. An estimated 3 million m³ of pebbles were taken all along the coast. Half of the pebbles: for industry, silica, earthenware, pottery, ceramics, etc. And despite seawalls built to break the natural transport of pebbles, there's a huge deficit in the natural protection of the cliff-feet.
Work done by the Dutch and Japanese on this subject show that once man starts changing an environment, he sets in motion a set of gears that cause these changes to continue by themselves, forcing us to protect the neighboring environment, etc. So we must be very careful with the environment, because we don't know what effect an action will have on a place dozens of kilometers away.
We're at Carry-le-Rouet, a resort to the west of Marseille. These cliffs date from the Oligocene and have layers of marl, sandstone and conglomerates. They were formed more recently than the nearby Jurassic and Cretaceous formations. These formations disintegrate heavily, mainly due to water circulation and rockfall. This site is interesting due to some recent events, notably one in late January, of which you can still see traces. Here, 60 to 80 cubic meters have gone. Another important factor with these faces is that the sea undermines the cliff-foot, creating cave-like features. This is enforced by the swell, especially since there's a prevailing swell. Vegetation can also play a role in causing rockslides. A negative role, if there are a lot of roots causing rainwater to circulate, which accelerates the dislodging of rocks along cracks and other discontinuities in the cliff. But vegetation can also - as we're trying to show in certain works - play a positive role by keeping finer formations on the ground, with a layer on top that slows down erosion. So what happened here is, the town chose to spray concrete over it to stop it from evolving. The reason for regularly recording occurrences like this is to try to forecast, with different time perspectives, the volumes that might fall in 100, 1,000, 10,000 years. For this, we have different methodologies. We have the regular recording of events along the coast, like the one in January 2015. And new technologies, such as scanning cliffs, making digital models of the terrain at different periods, and then comparing them to assess the material loss.
Coastal erosion isn't a new problem. It's been a known phenomenon for 200 years. The first works to fight erosion included the planting of the Les Landes pine forest. One aim was to drain the marshland. The other was to halt the migration of coastal dunes inland. Some towns are adopting more flexible techniques than riprap and concrete to fight erosion.
Sandy beaches are particular because they're dynamic environments. Sand offers little resistance to the waves' energy. The beaches can change with each tide. Here, the only thing elected officials care about is: "Will the dune hold?" Because the village of Biscarosse-Plage is the same level as the beach. So the only thing stopping it from being submerged by the ocean is the dune. Here we have what we call "soft" protection. The idea is to plant certain species to keep the sand in place and to stop it shifting during windy periods. Geotextiles have also been placed at the foot of the dune to try to stabilize the base. In theory, the geotextiles should be covered in sand. That was so until 2013. That winter, they were uncovered by the force of the waves.
You can only understand the coast if several experts study it together. It's a place where you must study its physics: the interaction between waves, coast and beach. You must study the biology: how organisms, including humans, live on and transform the coast. You must study its geology: understanding long-term evolution, how the coast constructs and deconstructs.
This is why, over the past decades, the CNRS has become very involved with these issues in order to better understand the evolution of our coasts. One example of this involvement is the creation of a national observation service for coastal and coastline dynamics. What's the aim of this service? To collect measurements so we can forecast coastal evolution in the coming years. What do we do exactly when studying the coastline? What must we measure? First we must measure the topography, the evolution of the relief. We must also measure the subsea topography, what we call bathymetry. We must measure what we call forcings, the elements that will cause the coast to evolve, such as the weather out at sea: storms, and even bigger meteorological events, like cyclones, in our overseas territories. To observe coastal evolution, we must work, recurrently and in the most harmonized way possible, and across the entire French maritime façade, on these key parameters.
At the Aquitaine Coastal Observatory, we have deployed a network of markers to do our surveys of the coastline and to take measurements along the profiles perpendicular to the coastline. We have pairs of yellow markers like these. There's one here and another behind me in the forest. This one is G19: the 19th marker in the Gironde. We go north to south. We have about 50 pairs of markers across the Aquitaine sandy coast. These allow us to position a Differential GPS that measures to the nearest cm. We take readings once a year. This one's interesting because it's the 3rd generation. The previous one was 15 meters further west. That position has vanished. It's in a void. We've leapfrogged this one back as the coast evolves.
We're here to take topography measurements. We have two GPS stations and a receiver behind me positioned on a known base. We know very precisely the position of this fixed station. We have a mobile base, now on a quad bike, which comes to record, at different dates, the position of the coastline and the morphology of the beach. What you see behind me is the section north of Port Barcarès. It's suffering from chronic erosion due to the position of the port and jetty. With sand shifting along this Catalan coast from south to north, the whole section has eroded, with the sand shifting to where we are now.
The tides are weak in the Mediterranean, so GPS measurements are taken in Port Barcarès twice a year. Whereas at Biscarosse and its surroundings, GPS measuring by quad bike is done every 2 weeks at low tide. Once the beach has been covered, the overall line comes on-screen. The reworked data will help with modeling the beach's morphology.
On the coast, we can see the processes. We can see the breaking waves, the coastline itself... All observable by video imaging, which allows us to measure its evolution on varying scales: on daily, seasonal and interannual scales. Our challenge is to link up these scales so as to understand the long-term impact of a storm or other extreme event. At Biscarosse, our video system comprises four cameras, which allows us to monitor 2 kilometers of coastline. We'll monitor it every 15 minutes over several years. Then we'll reconstruct the beach's profile, which flattens during storms and rises again afterwards.
The researchers will also use complementary satellite data.
The data from SPOT-5 is used to study all the phenomena which can occur on mainland surfaces. So the coastline can be studied using SPOT-5. It allows us to observe a good stretch of coastal areas. After SPOT-5, CNES moved to another type of satellite with improved resolution, which provides much finer detail. This Pleiades satellite is very agile and can capture lateral images, so it can see what's happening on the coasts. Monitoring the coastline is also linked to the sea level, and the monitoring of that. For this, we use altimetry satellites. Sentinel satellites also provide a whole range of measurements on the dynamics of the oceans which allow us to observe the impacts on coastal areas.
The type of sediments has a lot to do with beach dynamics. Size and quantity of the sediments, along with the swell, are the main factors in defining the shape of a beach.
At SHOM, we take samples from beaches. Like these for example: one from the edge of the dune at Le Pilat, and one from the beach at Biscarosse. The samples differ a lot from one beach to the next. So the characteristics of a beach are directly linked to those of its sediments. Using our measurements of the swell and those of the sedimentology, we can run models to predict, after a storm, what a beach will look like in terms of its height and slope. When there are several storms, we observe that a beach tends to shape itself into a configuration of response to the storm, meaning the sand conforms to the swell. So at the next storm, the beach will be much more resistant to the swell caused by the storm. This knowledge, a kind of "apprenticeship" of the hydrodynamic conditions, can be done with modeling and can be compared to real-life measurements.
We're on Porsmilin beach, a few kilometers from Brest, in order to observe beach erosion. We're using a special technique: aerial very-high-resolution imagery. We're not using satellites or planes, but small camera-fitted drones which will give us a resolution of a few centimeters per pixel. We'll place a few markers on the beach. They're these red plates, which are easily identifiable in the photos.
We're setting up these markers which will allow us to readjust our on-ground digital model. We'll take measurements with our Differential GPS with a precision of 1 to 2 centimeters.
Perfect, there's nobody around. Ready, and we're off! OK, 40 meters... 54... 60... 73... 100 meters and stable! It's perfect. We'll have 200 to 300 photos which we'll use for calculations that will give us our on-ground digital model. In other words: 3D mapping.
With this 3D map, from one field trip to the next, they can estimate sedimentary shifts due to storms and climate forcings. In this image which combines readings from 2009 to 2015, we see that the beach has grown 2 meters higher in places and lost a depth of 2 meters of sand in others.
Here at Le Grand-Dellec, we're studying the erosion of the cliffs. They're interesting due to their lithology, or types of rocks. There's a very wide variety. Over there is some mica-schist. Over there, in a lighter layer, is diorite. And just behind, various gneisses. These cliffs are very fractured, so our aim is to better understand how far they are retreating, depending on rock type and fracture level. That's why we have a ground laser scanner. Around the ground laser scanner, we place a number of markers, these small white reflective objects. Then we geo-reference the markers with a tacheometer. OK, Marion, you're on marker 12. How does it work? Well, a beam of light is emitted, and it sweeps around panoramically, through 360 degrees. And with successive readings, every 4 to 5 months, we'll have an image of the evolution of the cliff erosion, information on the influence of the lithology, and also information on the fracturing, which we'll be able to analyze and thus discover the role the fracturing plays in the erosion.
We monitor the evolution of this coast which, as you know, constantly changes, notably after big storms, very high tides, or both combined. To monitor this evolution closely, we have equipped our laboratory with a LIDAR, a very special tool. It's an airborne LIDAR which we take up in a plane, and it scans the coast with a laser beam. The aim is to reconstruct the topography, the relief of the coast, sometimes over vast areas, and to return regularly to compare successive states so as to calculate the evolution.
Other measurements are taken: the height of the waves, the strength of the currents, the height of the tides. Data from ground laser scanners is also used as a complement to the airborne LIDAR, which emits on the red wavelength.
We use another, more "confidential" range of lasers, the bathymetric lasers that we use at SHOM. They're different because they emit on the green wavelength. So the light signal penetrates the sea. This gives a return on the surface, in the water column and on the seabed. This return signal is what interests us. Here's a group of red dots. These are what we eliminate, what we filter out. Now we can clearly see the return zone on the sea surface. This is the final Litto3D image that we want. Here you can see the land-sea continuity between, on one side, the depths, then the sandy zone plus the beach. This continuity gives us precise mapping of the coast.
In coastal management, Litto3D data is also a good complement to field observations, notably in the Montpellier area, which is a tourism zone.
This is Les Aresquiers beach, a few kilometers east of Frontignan, a small village east of Sète. The characteristic of this zone is this: it accumulates all the aggravating effects that cause erosion. As you can see, it's fairly flat, apart from the work behind to raise the level against flooding. The erosion is so bad that the only remaining matter is that brought in by man. And in the sea, there is zero sedimentary matter. It's bare rock. When you reach that configuration, the coastal system is dead. There's no available sediment, no sand to absorb the effect of the waves. So you can consider this coast dead. This configuration is one of high-energy waves hitting the coast and permanently preventing the return of the sand. It's at a dead end, which is why we call it a dead coast. With erosion this bad, there are several ways to react. Just behind us, work is being done, because in this zone, there are high stakes, notably the road, which can't be moved for many reasons. It's a thoroughfare and much needed for safety reasons in case of an industrial accident in Sète, so it can't be closed. It can't be moved back due to the lagoon and environmental concerns. So the authorities have decided to protect the road by adding tough, solid structures.
Large-scale work was also done in recent years on the Lido beach in Sète. The Sète to Agde road, once running through the middle of this beach, was regularly damaged by winter storms. So it was moved, and the beach was totally reconstructed artificially. The dune ridge is now protected by stake fences which slow the effects of the wind and collect sand deposits. Beach restaurants were also regularly damaged, and have been replaced by buildings that are dismantled for the winter.
After the redevelopment of the coast at the Lido beach, we decided to deploy several technical solutions to fight erosion. The first was to reload the beach with sand. As you can see behind me, we have two colors of sand: the natural sand of the beach and the sand brought in recently from the L'Espiguette spit, which is finer and a little grayer. A clear contrast. And to keep this load of sand on the beach long-term, we have chosen maritime solutions: a swell attenuator and a beach-drainage system. The attenuator is a geotextile that causes waves to break at sea rather than on the shore, thus attenuating erosion. We have installed, along a stretch of 850 meters, geotextile rolls, positioned about 400 meters offshore, with only 50 cm to 1 m of water above them. These break the waves during storms. The second system, a bit further along, is the beach-drainage system. The principle is this: when a wave climbs the beach, sand is suspended in the water. We filter this water to collect the sand and the wave returns to the sea without it. The drains are installed beneath the beach, and they filter the water so the sand deposits there. Pumps also evacuate water through the groynes further along. This attenuator system is scientifically very interesting. We've been monitoring it for 4 years by video. Other towns are interested because our systems are very non-invasive. There's no riprap on the beach, the systems are long-lasting and can be easily dismantled if there's a problem, unlike the riprap and concrete you can see along certain coastlines.
To the east in the Golfe du Lion lies the L'Espiguette spit. It's a particular place due to its beach, which is in overall accretion, meaning a huge natural accumulation of sand. The beach comprises 50% of sediments brought by the Rhône and 50% produced by bioclasts, the fossilized shells of organisms which transform gradually into sand.
The coast was a few kilometers inland a couple of hundred years ago. Now it's here. In a few decades, it has moved several hundred meters, and we've had to build an 800-meter sea wall to halt it and stop it from silting up Port-Camargue behind us. But the sea wall is in such a position that the sand passes over it naturally from one side to the other, totally covering the wall. So the authorities decided to use the sand that crosses over and forms a large strip advancing westwards. The sand is taken and dispersed in nearby areas suffering from erosion. This has several consequences. The first is that it removes the sand from this sector. But that's necessary as it blocks the entrance to Port-Camargue. So its removal is a good thing. But two, the sand is taken away from this system's natural dynamic.
Not all of mainland France's coasts are eroding. About 10% of them are moving into the sea by accumulating sand. Occasional events, such as big storms, erode the dunes and set the coastline back up to 10 meters, like at Guissény beach during Storm Christine on 3 March 2014. But there are also periods of regeneration. Take the average between periods of erosion and regeneration over the long term, and one observes a low retreat of the coastline of only 70 centimeters a year.
People were inclined to take long-term measurements rather than short-term measurements since the last storm. But one area of study which is developing rapidly is resilience, the capacity of coastal environments to adapt to climate change. Here's a very simple example. Over the past 50 years, the prevailing storm direction in Brittany has changed. This has been measured thanks to observations over the long term. If the direction of storms changes, the impact on beaches changes too. And the groynes built several decades ago as protection from storms coming from a certain direction are less effective, due to this change in direction of storms.
Today, towns have a new problem linked to coastal erosion: the relocation of inhabitants as homes disappear. At Ault, there's a mansion that will soon need to be evacuated as it's less than 10 meters from the cliff edge. In nearby Criel-sur-Mer, after significant collapses, the relocations have already begun.
These past few years, we've started performing expropriations due to the imminent danger of inhabitants living on the cliff-tops. So far, we've expropriated about 20 people. You can no longer see the homes, because they were demolished. The people were moved to equivalent housing with their costs covered by the Barnier risk-prevention fund.
Like Ault, the town of Lacanau has become a pilot site for a nationwide study on relocating businesses and homes launched by the Ecology Ministry. During the winter of 2013-2014, a series of storms shifted the coastline back more than 25m.
Our problem lies within our urbanized area. We have 1.5 km of property. So we needed to protect it. The protection system is based solely on restraining the dune. The total cost of the work is 3.3 million euros. And we had planned, simply for maintenance, stopping the beach from retreating and cleaning the rocks, for about 80,000 euros.
It's possible that by 2040, the sea will creep up to 60 meters inland at Lacanau.
At Lacanau, it was decided, from the outset, to involve a committee of local people, elected officials, and technical partners of the town and the Aquitaine coast. These two years of study resulted in four scenarios: three on relocation and one on long-term protection. They have their good points and bad points, notably regarding the present context of regulations and financing, both for relocations and protection. The media have leaped at this - the relocation study - but they often misquote it, saying that it has become the law in Lacanau. So we spend time with the media, explaining that it's just a feasibility study, and not something that will happen in 6 months. In fact, it may never see the light of day at all.
Through a project called COCORISCO, Knowledge and Understanding of Coastal Risks, in large part financed by the National Research Agency, researchers are trying to widen our vision of risks by considering hazards, problems, and the management and portrayal of these risks.
COCORISCO is an interdisciplinary project on all levels. It includes people like geologists, geographers, lawyers, economists - for financial issues - and environmental psychologists. All together, we take into account not only what we risk losing with coastal retreat or flooding, but also the measures we need to take to counter this erosion and defend ourselves against flooding. This involves both seafront structures and legislation, like new regulations and laws, or decrees to determine the type of coastal management to deploy. The other thing is representation, meaning what people think about coastal risks. This is notably covered by environmental psychology, which is a human and social science.
All the results of the COCORISCO project are grouped together in a methodology guide for the coastal managers.
Social perception is quite particular. People who frequent the coast feel like they know it. And some do, very well. But we often note a lack of overall knowledge, meaning putting the events they see in the right context. For example, the effects of the storms of winter 2013-2014. What caused them? Is it climate change? What exactly happened? There are lots of questions. People pay more attention to what's happening, but still lack the knowledge to put the elements into their context. Our task isn't to be alarmists, but to give the facts and put these evolutions into the right context over longer periods of time.
Coastline management on the sandy coast of the Occitanie region
Transcript of the video
The shoreline is a frontier, the place where earth, sea and sky meet. It's a mix of elements with individual characteristics whose complex interaction enables the coastline to be mapped.
The coastline or shoreline is difficult to define: it's the interface between sand and sea.
The coastline, by its nature, changes and fluctuates, shifting over time, according to the seasons and weather.
We often hear about "coastal erosion". This is not a recent phenomenon, but it has accelerated in recent decades.
It's due to the action of the waves dragging sediment along the beaches. That's coastal erosion.
It can be seen in various forms, and is present along most of France's coastline. The causes are natural phenomena linked with human factors. Its consequences are barely perceptible day to day, but storms, by spectacularly accelerating the erosion, regularly remind us of the fragility of coastal areas.
Everyone likes the seaside, but its apparent simplicity hides a complex structure and operation. Here comes Yann Baloin from the BRGM, who will explain. Hello, Yann.
Can you explain how the beach-dune system works?
The beach-dune system consists of 3 interdependent compartments between which sand is carried by the wind, the swells and the waves. The first compartment is the underwater beach, on which are found pre-coastal sand banks. Then comes the exposed beach where you put your towel. Lastly, the dune bank containing large amounts of sand.
Right. Mediterranean tides are minimal, so what causes the sand to move?
The main evolutionary factors are the swell created by the wind, and the wind itself that moves the sand. There are also the extreme levels caused by storms. The waves are generated by the wind. At sea and on the ocean, they build up freely: that's a swell. As the waves arrive at the shore, they break up, resulting in currents of various types. First we have currents that go from the sea to the land, which allow sand to accumulate: this is called accretion. Then there are return currents, which carry material towards the sea. It's mainly these currents that cause seasonal variation. In winter, sand is dragged towards the pre-coastal bars. In spring, we have more favourable currents that replenish the beach with sand. The breaking of the waves as they approach the coast creates a current parallel to the coastline, the coastal drift. This phenomenon plays the main role in how beaches evolve in terms of accretion, erosion and coastal stability.
Does installing protective structures along the coast change the currents?
Locally they can disturb the sand movements, and paradoxically accentuate erosion on adjacent coastlines.
To quantify the accretion or erosion of a coast, we need to count the volume of sand involved. When you look at a sketch of the coast, you can have an estuary, a port system or a rocky coastline. We can divide the coastal system into several compartments, which are relatively independent from each other, and within which we can calculate the level of sedimentation. To do this, we make use of 3 parameters. The first is the sand entering this compartment.
This comes mainly from the riverbanks via the estuary. Sand is also brought in by the sea. The second parameter is sediment leaks, i.e. losses from this compartment, either towards the lagoon during major storms, or towards the sea with the return current. Lastly, we need to know the amount of sediment already present in the cell, and in particular the depth of the sand inside that cell. Now we know the sediment level in the compartment, and we can assess the shoreline's erosion or accretion levels.
It's better not to react immediately after a storm, but to take time to analyse it.
Normally the beach repairs itself naturally, regaining most of its sand after a few weeks. This is known as resilience.
The Languedoc shoreline consists of low sandy beaches and coastal bars, strips of land separating a lagoon from the sea.
On this type of coastline, particular conditions apply. Firstly, tides are minimal: a maximum of 30cm on average. Secondly, drifting often occurs, caused by atmospheric pressure and the wind and waves during storms.
High winds, low-lying land and narrow bars: the Languedoc coast is a vulnerable space, and the sediment levels of the cells can be impacted by natural phenomena and human activity.
The natural phenomena which affect the coast are storms, which can cause submersion and coastal erosion.
Human activity includes sand extraction, sand pits, mechanical beach cleaning, dune levelling and urbanisation in general. But the biggest impact on coastal evolution is the development of riverbanks and in particular riverbeds. In their natural state, rivers convey to the coast sediment from the erosion of the riverbanks, contributing to the sediment levels on the beaches. But the construction of dams and embankments, as well as the practice of dredging the rivers, have progressively blocked the sediment, depriving the beaches of sand. In former times, man adapted to coastal fluctuations by not permanently inhabiting land close to the sea. But industrial development and tourism during the 1950s and 60s altered the coastal landscape.
In 1963, the Racine initiative, named after the person in charge, instigated the development of tourism on the Languedoc-Roussillon coast, involving large-scale building works.
In its wake, many sites devoted to seaside tourism or industry were built. These areas played a major role in the dynamic balance of the coastal system. For example, dune banks disappeared, and others appeared at sea level.
Thereafter, coastal fluctuation was no longer deemed possible, as large swathes of the economy depended on its permanence. To ensure this, many structures were built: longitudinal defences high up the beach, such as walls, boulders and rip-rap, and breakwaters or groynes close to the shore to protect the dunes from the sea and stop the coastline moving.
But their presence increases the action of the swell, accentuating the phenomenon of erosion in front of the structure, known as undermining. They also reduce the profile of the beach, making it narrower, and increase the erosion in unprotected zones on either side of the structure. Transversal structures such as groynes are intended partly to prevent sedimentation due to coastal drift. However, they block not only the coastal drift, but the natural transit of sediment. They create an accretion zone upstream, and an erosion zone downstream due to lack of sediment. When arranged in banks, they can heavily modify the currents from coast to sea, causing increased erosion between each groyne and a lowering of the seashore. While these measures are often said to be necessary given the economic issues, it is now vital that we fine-tune our actions on the coast to protect it from erosion. Individual interventions done in a rush have now been replaced by a deeper, more global approach to the situation.
This new way of managing the coast is required by the national strategy for integrated coastal management, adopted by the government in 2012.
There now exists a raft of new techniques which vary in scale and affordability. But none is a one-off, miracle solution. As each context is different, projects must be appropriate and take into account the lifetime of the structures.
The coastline is a dynamic, living space. It is sometimes spectacularly changed after a storm. Faced with the phenomenon of coastal erosion and submersion, there is no miracle solution. We must continually strengthen our knowledge of how the coast works. Observing phenomena will help us build, on a local scale, appropriate solutions which take into account human and economic issues as well as our natural heritage. For a long time, we thought that building sea defences was a permanent solution. Today, we understand it was just a temporary one. From now on, we must think differently about the land and how we use it, reconstructing coastal areas at risk of erosion and submersion. We must act today to preserve the attractiveness and the way of life of tomorrow's coastline.