September 2009

The bay of Mont-Saint-Michel, in the coastal corner of France between
Normandy and Brittany, boasts the fourth-largest tidal range in the world – up
to 14 meters (46 feet) and the sedimentary drama to go along with it. It is an incredibly complex system. The tides
carry sand and silt into the bay and leave much of it behind, augmented by the
sedimentary loads brought down to the bay by the three rivers that drain into
it. Roughly 700,000 cubic meters of sediment are added to the bay every year –
it is, quite naturally, silting up (or, more attractively in French,
experiencing ensablement – sanding up). As a result, the iconic World
Heritage Site of the monastery towering above its isolated rock is increasingly
losing its magnificent isolation and becoming united with the mainland. But this
natural process has been greatly exaggerated by human meddling over the
centuries and the rhythms of the bay profoundly changed and subdued. The natural
processes operate on a scale far beyond that of any human endeavour, but it is
possible to mitigate the effects of our meddling and restore the bay to at least
close to its unhindered condition and allow natural ensablement to take
its course. And that is exactly what is going on today in the bay, after years
of preparatory work and research by a syndicate of authorities and
institutions – a grand experiment so that Mont-Saint-Michel “ recovers pride of
place as the central feature of the Bay, amid a seascape of sands constantly
being reshaped by tidal and river water.”Mont-Saint-Michel began as an isolated rock remaining emergent from the bay
as rising sea levels inundated the land following the end of the last ice age;
it was periodically connected to the mainland by a natural spit of sand, a
tombolo. But then, in 708 so the story goes, St. Michael approached the
bishop of nearby Avranches, Aubert (later also a saint) and demanded that a
monastery be built on the rock. Foolishly, Aubert ignored this request, finally
setting about the task only after the irritated angel had burned a hole in the
bishop’s head with his finger (now there’s a warning about
procrastination and disobeying orders). The monastery and then abbey grew up over the centuries
and then declined; after the French revolution, it became a prison that
welcomed, among many anti-republicans, Victor Hugo – he described the tide as
coming into the bay “with the speed of a galloping horse.” Along with this tide
came the human one – pilgrims both religious and touristic. Today, it is the
most-visited site in France outside Paris – three million people visit each
year. It was teeming even in mid-September when I arrived, the massive parking
lot and causeway (built where the tombolo used to be) filled with cars,
camper-vans and buses. The causeway completely cuts off the natural currents and
sediment movement of the inner bay, but it is by no means the only manmade
disruption. In the 1850s, the channels of the Rivers Sée and Sélune were eroding
the agricultural area south of where they entered the bay, and a dike
(“digue”) was constructed to force their flow further north; over the following
decade, the channel of the River Couesnon was completely controlled and
“canalised” all the way out beyond Mont-Saint-Michel itself (the artificial
banks for much of this were eroded or overwhelmed by sand by the 1960s).
Critically, in 1969, a dam was built across the Cuesnon that effectively ended
the power of the river to flush sediment out into the bay. The result of all
these interventions was increased natural and manmade growth of the “polders,”
the salt marshes that invade the bay and provide valuable agricultural land,
together with accelerated sedimentation around the Mount itself. In modern
times, the foundations of Mont-Saint-Michel were buried under two meters of sand
that had built up since the nineteenth century - the entire sedimentological
system of the inner bay was changed – dramatically illustrated in the two maps
below showing the locations and changes to the river channels before and after
these “management” initiatives.

The project to restore as far as possible the nature of the bay has been
comprehensive, evaluating hydrology and sedimentology, environmental factors,
and the influence on tourism. It is is superbly described at http://www.projetmontsaintmichel.fr/en/,
much of the site available in English. The sedimentology modelling alone was a
four-year study, a key part of which was the construction of a huge physical
model capable of reproducing and predicting sand dynamics (see photos below).
The model covers 900 square meters (almost 10,000 square feet) and was used to
model 45 years of annual cycles in the bay without further intervention and with
a range of restoration options (see my earlier
post for a natural version of such a laboratory and references to other such
modelling projects).Over a recent period of forty years, 27 million cubic meters of sediment have
been added to the bay and a thousand hectares (four square miles) of new
vegetated land encroached into it. The model demonstrated that, by 2042, the
marine environment of the inner bay around the Mount would vanish completely.
Nothing can stop this happening eventually, but it can be slowed down. There are
two keys to delaying the inevitable: remove the causeway and restore the
hydraulic power of the Couesnon river to flush sediment out of the inner bay.
The projects to accomplish both of these, together with all the associated
works, were commenced in 2006 and are due to be completed by 2015. The causeway
will be replaced by a simple access structure raised above the sea floor to
permit natural flow and sediment movement, and well away from the mouth of the
Couesnon. The gigantic parking lot will be demolished, to be naturally replaced
by 15 hectares of sand. The access to the Mount will be severed under the
highest spring tides of the year, restoring the symbolic isolation of
Mont-Saint-Michel.

On the river itself, a barrage has been constructed that will
allow the waters of each high tide to be stored behind it and then released,
flushing sediment out into the bay – the hydraulic power of the river will be
restored, even enhanced. The barrage, shown under construction below, has just
been commissioned. The river below the barrage will also be dredged; the product
of the dredging, 1.25 million cubic meters of salty sand full of shell fragments
that is locally known as “tangue” will be used, as it has been for centuries,
for local agriculture and polder maintenance. Importantly, the modelling
demonstrated the importance of the river draining in two channels out
into the bay, so a structure to split the flow downstream from the barrage
opening will be constructed.

The effects of the project will be gradual, but in
thirty years or so it is anticipated that the mean level of the sea bed around
Mont-Saint-Michel will be 70 centimeters lower than today, millions of cubic
meters of sediment will have been removed (much of it quite quickly), and
the sea will have recaptured 50 hectares of today’s land. The scene today, below
left, will have evolved into something like the view on the right. This is an
extraordinary and ambitious project, a fascinating experiment in large-scale
restoration of the natural landscape.

The site linked above is the source of much of the information and
illustrations for this post, and includes a thorough documentation of the
sedimentological work, downloadable as a pdf file here;
it’s in French, but superbly illustrated. I have copied below a map of the main
components of the whole Mont-Saint-Michel project for reference and further
detail. SIGNATURE

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/2009/09/index.html