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Presqu'île de Giens-How do you get a double tombolo?

 

Giens is connected to the mainland by the famous double tombolo, a land formation that French geography teachers love to use as an illustration. However, residents have every reason to be proud, as it is one of only five doubles tombolos in the world. How does a tombolo work?

 

Three of the five double tombolos worldwide are found in the western Mediterranean area, of which two and a half are on French territory. In addition to Giens, the others in the Mediterranean area are Orbetello in Italy and Ifach-Calpe in Spain. Another French example is to be found in the Northern Atlantic (the only one in that area). It is a 12 km (7 ½ mile) strip joining Grande Miquelon to the Petite Miquelon (Langlade) in the French territory of Saint Pierre and Miquelon, off the coast of Canada. The other is found in the Caribbean, on Saint Martin island. One of these tombolos in France, another in Holland… The Bretons claim two other miniscule examples along the sandy Brest coastline. However, these are not internationally recognised.

What is a tombolo? It is an isthmus. So what is an isthmus then? A narrow land bridge connecting two landmasses between two stretches of water. The most famous - Suez - links Africa to Asia, but no longer actually connects the continents, now that the canal has been dug. Another is found at Panama, with an identical outcome as Suez.  

The present day definition suggests "a tombolo is a sedimentary bar connecting two land zones. It is often just a coastal bar between an island or islet and the coastal mainland or another island. The sedimentary deposit, generally formed of sand is often created by wave refraction caused by an island."

So what is wave refraction? It is the change of direction to the sea swell brought about by an obstacle. For instance, a westerly wind and swell on the "windy Mediterranean" would drive you from Toulon towards Porquerolles. Upon arrival at Petite Passe you have to change your heading from 90 to 55. If you look back, you will still have a perpendicular wave behind your boat, having gone from wind astern to broad reach. The wave pattern slows on your left as you pass the Giens peninsula, then fans out, like rows of demonstrators turning at a roundabout.

There, with the wind no longer blowing in the same direction, the momentum of the swell is lost in the turning motion, and the sea drops the sediment it is transporting on to the seabed. As the winds continue to blow the sediment builds up, forming a slope which gradually slows up the flood tide… and ends up creating a shoreline.

Based on these observations, engineers invented groynes parallel to the coastline in order to create eddies, retaining the passing sand, to extend the beaches and increase tourism (see photo of Carnon).

Let's get back to Giens and its double tombolo. The oblique waves created lateral currents linked to the winds of Provence: the "Mistral" north-westerly wind, the "Ponant" (easterly), the "Traversière" (west-north-westerly), the "Lombarde" (east-north-easterly ), the "Grec" (north-north-easterly) wind or the "Marin" (south-easterly).

The Giens peninsula is actually located at the boundary of two distinct hydraulic zones: the Gulf of Lion to the west and the Gulf of Genoa to the east. However, the influences are not equal. This results in a much larger 400 m (1,300 ft) sandy strip on the eastern side. This is where the road runs, alongside houses, camp sites, a racecourse, and pine forest, etc. The western sand bank measures 25 to 50 m (80 to 160 ft) in width, and has no vegetation, but lots of tourists and vehicles… and breaches regularly opening up in the bank. There would only be one tombolo, and no salt marshes in the centre of the peninsula, if the earth moving machines did not regularly fill in the gaps.

Why? Lack of protection: The waves are no longer slowed up by seagrass on the dunes, but crash with full force onto the beach. Lack of material: There is too much tarmac and concrete, and the rivers have been channelled, allowing too little alluvial material to flow out into the sea. The little River Gapeau which in the past flowed westward now has its mouth facing the east. Meteorological inequality: The easterly winds bring the rain, swelling the rivers that carry the sedimentary material. The westerly wind is dry. Too much activity, in spite of the efforts of the authorities. It would be politically difficult to close access to the site, even though this may be what is needed.

Urgent action is required, or else nature will take its course. The sea level will rise and it will be too late to intervene.

 

Christophe Naigeon

 

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