Geoguide 52p Gb Web

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Understanding floods and flood risks in the Gardon river catchment Discover Collias ã Village walking trail: 1 hour ã Gorges walking trail: 4 hours Contents 2. A…
Understanding floods and flood risks in the Gardon river catchment Discover Collias • Village walking trail: 1 hour • Gorges walking trail: 4 hours Contents 2. A few facts about floods. The Inter-regional Mediterranean 3. History and floods. Arc Flood Unit 4. The Gardons river catchment, physical and human features. This geo-guide was produced with support from the 6. The Gardon d’Alès catchment. Inter-regional Mediterranean Arc Flood Unit. 9. The Gardon d’Anduze catchment. The flood unit was created in 2017 by the Prefect for the South 12. The Gardonnenque. Defence and Safety Zone. It covers continental French regions 15. The Gorges. most exposed to torrential flooding, such as Provence-Alpes-Côte 17. The Uzège. d’Azur, Occitanie, Corsica, as well as the Drôme and Ardèche Departments. 19. The Lower Gardon. 22. Discover Collias, circuit 1: The unit reports to the DREAL* PACA which is also the DREAL* the village walking trail. branch office here, in the local area. 23. Stop 1, Historic floods. The flood unit strives to improve the performance of flood risk 26.Stop 2, Les “Machines”. prevention schemes and ensure that such schemes are applied consistently from zone to zone and within the Mediterranean 27. Stop 3, Joliclerc bridge. Arc as a whole. To do this, it works on inter-Ministerial and multi- 28. Stop 4, Le Ron de Fabre. partnership initiatives that foster synergies between civil protection 30. Stop 5, The Alzon. and risk prevention stakeholders. The working method used 32. Discover Collias, circuit 2: is based on grass-roots approaches to apply the principles, The Gorges de Collias to experiment and even innovate, to learn lessons or promote La Baume walking trail. useful guidance for all stakeholders in the Mediterranean Arc. 33. ection 1, Exiting the gorges. 34. Section 2, The riparian buffer. 35. Section 3, The gravel pits. 38. Section 4, The 2017 fire. 39. Section 5, The karst temple. Ardèche Ardèche Hautes-Alpes Drôme Drôme Hautes-Alpes 42. Section 6, La Baume Lot Lot Lozère Lozère de St-Vérédème. Aveyron Aveyron Alpes- Alpes- Tarn-et- Tarn-et- Vaucluse Alpes-de- Vaucluse Alpes-de- Maritimes Maritimes Gard Gard Haute-Provence 44. Staying informed and safe Garonne Garonne Haute-Provence Tarn Tarn if a flood happens. Gers Gers Bouches- Bouches- Haute- Haute- Hérault Hérault du-Rhône du-Rhône Var Var 46. Glossary. Garonne Garonne Aude Aude 47. Maison du Castor visitor centre. Hautes- Pyrénées Hautes- Ariège Pyrénées Ariège 48. Acknowledgements, Pyrénées Orientales Pyrénées Orientales Haute- Corse Haute- Corse bibliography and websites. Corse- Corse- du-Sud du-Sud The Gardonnades caught on camera! You can view videos taken during major flood events, especially those in Area covered by the Mediterranean Arc Flood Unit September 2002. Departments belonging to the Mediterranean Arc outside the South Simply scan the QR codes with your Defence and Safety Zone. smartphone or tablet. Departments belonging the Mediterranean Arc within the South Defence and Safety Zone. Other Departments in the South Defence and Safety Zone also exposed to heavy rainfall and flash floods. - A glossary, on page 46 explains terms indicated with an asterisk*. Editorials We often fail to fully appreciate where we live or The Gardons river catchment is very special given spend our holidays. Equally, we can neglect local the sheer range of scenery, coupled with outstanding history, the processes at work around us, together natural and cultural heritage. The area oozes appeal with potential dangers among all the qualities that but you have to learn to live with the Gardon and attract us to a place. Yet, these principles are key to its floods, which are just as impressive. They can being at one with our surroundings and make the exceed 7,000 m3/s, as happened in September most of the local area and the activities on offer. 2002, which those who experienced it remember That way, you won’t be caught out by known or only too well. This geo-guide takes you on a journey predictable events in a flood-risk area. of discovery, from the Lozère and Cévennes to the Water has often shaped the landscape in the Gardon d’Anduze and Alès river catchments, via the Mediterranean Arc and influenced land-use. Floods Gardonnenque and its gorges, on to Uzès and the are part of our past and present. They are frequently lower Gardon and finally, to where the river joins the powerful and destructive yet forge the identity of Rhône. You’ll see how the local people have adapted the catchment areas around us. There’s no need to to “Gardonnades” with infrastructure (the Sainte be alarmist about floods but it is best to be realistic. Cécile d’Andorge and Saint Geniès de Malgoirès So, let’s find out more about them to be better dams, the flood barrier at Aramon, and ‘tancas’ in the prepared for when they do occur. Cévennes). You’ll see how communities have made themselves less vulnerable by adding cofferdams in This geo-guide, produced by Sudaléa, the EPTB doors and gateways or even relocating (the story of Gardons and the Syndicat Mixte des Gorges du Massillan, the village that vanished). Flood markers Gardon, invites you to explore the Gardon rivers have been put up to commemorate events and basin and discover this wonderful area with two special measures have been introduced to local great walking trails. You’ll find out about the powers plans. There are also local emergency action plans of water, past floods and flood risks. We hope you’ll and information documents for the general public enjoy using this geo-guide with your friends and (DICRIM). Vegetation is now managed and rivers are family and that it gives you a new insight into the monitored (Vigicrue system from the Grand Delta local area. Flood Forecasting Service). Activities are organised in We also hope that other stakeholders in the local schools, together with exhibitions on different Mediterranean might like a river catchment geo- topics for the very young and the general public. guide to showcase their own areas too. There’s so much going on and the EPTB Gardons actively contributes to many of these activities. Max ROUSTAN, President Ghislaine Verrhiest-Leblanc of the EPTB Gardons and Inter-regional Mediterranean Bérangère NOGUIER, Arc Flood Project Officer, DREAL* PACA. President of the Gorges du Gardon Joint Association. 1 A few facts about floods DDTM* 30 A flood is when water invades land that is normally dry. Surface runoff at Sagriès in the Uzège district on 10 October 2014. Are Cévenol episodes really Cévenol? Our Mediterranean regions regularly experience partly to relief. The steeper the slopes, the greater so-called “Cévenol episodes”. These are very heavy the runoff speeds. downpours based on the amount of water falling in Soil permeability is an important factor too. Indeed, a given time. They commonly occur in late summer the geology and types of surfaces in urban areas and early autumn, resulting in large amounts of rain are crucial. Some rock, like limestone, which forms falling in a short time. For example, a famous record a large part of the Gardons catchment* can absorb dates back to 29 September 1900 when 950 mm a lot of water, while others, like clays and artificial of rain fell in 10 hours on Valleraugue, at the foot surfaces (concrete, tarmac and tiles) are sometimes of Mont-Aigoual, i.e. the equivalent of 10 bathfuls virtually impermeable. of water for every square metre! Runoff can spread out to form large pools of surface By comparison, normal annual rainfall in Nîmes water or collect in thalwegs*. The water then travels is 762mm. That said, the Mediterranean climate along in permanent streambeds creating a surge means that this falls over just 64 days, while the in flow*. When this happens, we say the river is on monthly average combined from September to stormflow and above a certain level, the river can January is 428 mm. overflow from its streambed* into its floodplain*. Cévenol episodes can be highly localised, as was Just how far the water spreads depends on the the case on 8 to 9 September 2002, when a large surrounding relief. part of the Gard Department was hit by torrential In both cases (surface runoff or overbank flooding), rain, especially the Gardons catchment area. there’s going to be flooding! Although frequent in the Cévennes, these ‘episodes’ In a given area, the probability of a flood occurring are especially typical of Mediterranean coastal areas, that reaches a specific height and flow rate is called a squeezed between the sea and mountains. Recent climate hazard. This hazard can be high, moderate or floods in the Aude on 15 October 2018 and near residual. Cannes on 3 October 2015 are examples of this, From flood hazard to risk! but similar events have also recently hit areas in Greece, Spain, Italy and North Africa. If this hazard threatens assets like property and human life, economic activities, roads, public From runoff to a flood, how a climate event services and property, etc. we then refer to a flood develops risk. The more vulnerable the assets, the greater the risk. Surface runoff is partly due to rainfall intensity and 2 History and  Gardon , Nyme, eaux si hault desborderont, floods Qu’on cuidera Deucalion renaistre Dans le Colosse la plupart fuiront Vesta, sepulcre, feu esteint à paroistre Nostradamus* Main historical floods 03 51 57 05 97 41 768 795 846 861 890 891 907933 958 002 ? 14 15 15 16 16 17 1 1 1 1 1 1 1 1 1 2 You don’t have to be a ‘seer’ like Nostradamus* their intensity and their consequences distinguish to warn people that the waters will rise again one flood from another. Also, only the more recent sometime. When he lived, in the 16th century, this floods leave visible marks on the landscape or can illustrious Provencal astrologist may have predicted be described by witnesses. catastrophic floods in Nîmes and along the Gardon You’ll often find the relics of floods, From the River. Perhaps, he was thinking about the flood of Cévennes to the Rhône, such as debris lines* in 9 September 1557 when he wrote these lines of the riparian* buffer zones, where small branches verse between 1558 and 1563. Maybe it was the are trapped among the trees. flood of September 1551 that swept away an arch Flood markers, painted or inscribed on buildings, of the Pont du Marché bridge in Alès. Who knows?! bear witness to the biggest floods that have left an What we do know is that the Gardon has burst impression on our friends and families.  its banks on many occasions with serious These markers were promoted in 2006 by the consequences. Records point to dozens major EPTB* Gardons, (ex-SMAGE* des Gardons), with flood events since the Middle Ages. the addition of 200 new flood markers in line with Although we have few reliable written a new nationwide design. records prior to the 13th century, a study of As a result, the ones that stand out most sediments deposited by major floods in a along the Gardon and its tributaries are karstic cavity* 15 metres above the stream the floods of 1907, 1958 and especially bed* in the Gorges du Gardon, unearthed the 8 and 9 September 2002, which is some surprising findings. It actually found never far from people’s thoughts due to that many extreme flood events occurred its scale and devastating effects. at the start of the “Little Ice Age”*, in the 15th century. The study also confirmed This geo-guide focuses mainly on these that the 2002 flood was one of the most three floods but also features more powerful that occurred since this period. recent events in 2014 and 2015, as well as the oldest-known flood, dating back What relics are left in the to 1403 which wiped out the village of landscape now? Massillan in the Gardonnenque district Seules leurs intensités et leurs (see page 13). conséquences distinguent les inondations les unes des autres et seules les plus Only Flood markers installed 3 by the EPTB* Gardons, in Rue Pélico, in Anduze. THE GARDON The Gardons river D’ALES RIVER CATCHMENT catchment: Alès > Physical and human features The catchment area* A catchment area contains a 2 complex drainage network and the Gardons river catchment 1 3 covers more than 2000 km² with 150 towns and villages. About 1 11 180,000 people live in the area, La Grand-Combe more than half in the town of 10 Alès. The catchment is very varied. There are several areas and 9 6 sub-catchments. These are 5 the Cévennes containing the 8 12 Gardons d’Alès and Anduze rivers, the Gardonnenque, the Alès gorges du Gardon, the Uzège and Saint-Jean-du-Gard the Lower Gardon. 14 13 Anduze Upstream from Anduze > 15 THE GARDON D’ANDUZE RIVER CATCHMENT Gard, Gardon or Gardons? The original term for river comes from the early Latin word, Vardo. Although the meaning of Vardo remains unclear, it gave rise to the word, Gard, which was the name chosen in 1791 when the S Departments of France were created. That said, people have long referred to the “Gardon” and “Gardons”, as featured on a 17th century map and in Nostradamus’ quatrains*. Indeed, those living along the river’s lower reaches call its upper tributaries the “Gardon”, which means “Small Gard”. If you break down the tributaries into their geographical origins, you get the Gardons d’Alès, d’Anduze, de Mialet, de Ste-Croix, de St-Martin and de St Germain rivers. The name “Gardon” subsequently became commonly used to refer to all the water courses along the river until it joins the Rhône. 4 LA GARDONNENQUE 01 The Gardon d’Alès Moussac 02 The Gardon (official source) 03 The Dourdon 04 The Grabieux 05 The Galeizon 06 The Salandre 07 The Avène 08 The Gardon de St-Jean 9 The Gardon de Ste-Croix 10 The Gardon de St-Martin 11 The Gardon de St-Germain 12 The Gardon de Mialet THE UZEGE 13 The Salindrenque The River Alzon, in the Eure Valley, at Uzès 14 The Amous 15 The Gardon d’Anduze 16 The Droude 17 The Esquielle 18 The Braune 19 The Ruisseau de Vallongue 4 20 The Goutajon 21 The Bourdic 7 22 The Seynes 23 The Alzon 24 The Valliguiere 25 The Briançon 1 16 THE GORGES 22 The gorges at Sanilhac 21 23 Uzès 24 Saint-Chaptes Collias 17 Remoulins 18 Photos: EPTB Gardons 20 25 19 Saint-Mamert-du-Gard Nîmes 10 Km N THE LOWER GARDON The Seuil de Remoulins watershed 5 The Gardon River, at Alès. The Gardon d’Alès River starts in the southern foothills of the Montagne du Bougès, in Lozère. It then descends through thalwegs* and streams (valats*) which join to form the river. This collection of small mountain streams, the highest of which begins at 1,350 m, below the Signal de Ventalon peak, rush down the short but very steep slopes. After only 15 km, the Gardon d’Alès is just 285 m above sea level, at Collet de Dèze. This steeply sloping land causes sudden torrential, sometimes extremely rapid, stormflows. This torrential character quickly disappears when it reaches the piedmont* of the Cévennes that the river encounters just upstream of Alès. Before it flows into the Gardon d’Anduze, between Vézénobres and Cassagnoles, the river is joined by several large tributaries on its right bank, such as the Galeizon (certified as a “river in good condition”* and “wild river”*), the Alzon (not to be confused with the other River Alzon, in Uzège) and the Carriol. On its left bank, tributaries like the Grabieux join it at Alès, followed by the Avène.   Tragic floods in 1605! Alès experienced one of the deadliest floods in the Potamologist*, Maurice Pardé* noted: history of the Gardons river catchment. On 10 August “On 10 August, many people died in Alès when it was 1605, the Gardon burst its banks and devastated the flooded by the Gardon. 40 residents perished when town. It hadn’t rained in Alès, so there must have the ramparts collapsed, weakened by the strong been a very violent storm upstream. currents. Five or six others clung to willow trees for 6 The Sainte Cécile d’Andorge dam The impressive Sainte Cécile d’Andorge dam, between Le Collet de Dèze and La Grand-Combe, is flood retention dam* built in 1967 after devastating floods in 1958. It retains all or part of the water flowing from the upper catchment to protect life and property downstream, especially the town of Alès. Its efficiency depends on how rainfall is distributed over the Gardons river catchments and affect nearby areas downstream. It has a reservoir to replenish river baseflow*. The dam is managed by the Gard Departmental Council, which operates and maintains it. At the foot of the dam is a second one, the Barrage des Cambous, built in 1955 to supply the mining industry with electricity. Nowadays, it is mainly used for leisure purposes. EPTB des Gardons Département du Gard two to three hours, calling for help, separated from the townsfolk who yelled to them sorrowfully from the walls. Seeing that no help was at hand, they prayed to god and prepared for a quick death as the trees fell and were washed away by the powerful current. Just one person miraculously escaped.” Sainte Cécile d’Andorge Dam. 7 Debris dams on a footbridge and The Grabieux, a high-risk tributary! damage following a flood by the Grabieux on 20 September 2014. Fed by its tributaries, the Bruèges, the Ruisseau Blanc and the Ruisseau Rouge, the Grabieux joins the Gardon right in the centre of Alès, at the Prés St-Jean. This small river has a torrential flow regime* and quickly descends towards the town from the hills rising above St Julien-les-Rosiers. It flows through densely built-up urban areas (housing estates, flats, shops and businesses, public services and roads, etc.). As a result, you are in an area with an exceptionally high flood risk. What’s more, numerous crossings (footbridges, road bridges and a railway viaduct) complicate the situation by generating log-jams*. Dorian Décombe On 8 and 9 September 2002, and again, on 19 and 20 September 2014, some districts, like Les Tamaris, as well as Le Moulinet and Les Prés St-Jean downstream, where the Grabieux joins the Gardon, suffered greatly from these floods, though fortunately without loss of life. To deal with these events, the local residents Check valves formed their own association, “Sinistrés du Grabieux 2002, 2014” in spring 2015 with more Water can flood part of the floodplain than 170 members. without overflowing by simply flowing However, in September 2015, the wayward back up the rainwater drainage pipes. Grabieux once again suddenly flooded certain To prevent this, anti-flood check houses. valves are fitted to pipe outlets, like the one you can see at the foot of the As a result, the residents’ association is currently Pont-Vieux bridge, on the right bank involved in considering its options, together with of the Gardon, at Alès. Alès Agglomération and l’EPTB* des Gardons. A similar association was also created following the 1933 flood in Alès. Look at the pictures of the 2002 flood that hit the districts of Les Tamaris, Le Moulinet and Les Prés St-Jean. The ground floors of the flats in Les Prés St Jean have been condemned since the flood. 8 The Gardon de St-Jean, at Peyrolles. A gathering of Gardons! Just like the Gardon d’Alès river catchment, the de St-André (de Valborgne) in its upper reaches. landscape in the Gardon d’Anduze catchment is The Gardon de St Jean has carved out spectacular very steep, both up and downstream. gorges at Peyrolles before being joined by the There are a great many gullies, or valats*, upstream Salindrenque after flowing through St-Jean-du-Gard. of Anduze, as well as streams that rush down the As for the Gardon de Mialet, it starts life where steeply sloping hills from the Cévennes. They start the Gardon de Ste Croix (a certified “river in good at the foot of a natural amphitheatre of mountain condition”*) which drains the Vallée Française, and peaks, 1,000 m high (1,225 metres on the foot the Gardon de St Martin (de Lansuscle), meet. By slopes of Mont Aigoual). Most of these temporary now, it has also been supplemented by water from and torrential regime* water courses flow from the the Gardon de
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