Traditionally made of wood or metal, sluice gates make life possible in the world’s most populous places: along rivers or coastal regions, which are often vulnerable to flooding. The basic technology behind sluice gates is ancient – Sri Lankan society used them over 4000 years ago – but their refinement continues today. Some modern sluice gates use duplex stainless steel, which offers substantial benefits over other construction materials. The projects detailed below showcase how these gates conserve material, capital, and emissions throughout their service lives.
Mont St-Michel, Avranches
Mont St-Michel is a tidal island with an abbey that appears to float on the ocean. The granite rock lies about 1km off the north-western coast of France, near Avranches. Since the 8th century, religious pilgrims crossed the muddy sands, revealed only at low tides, to reach the abbey. But the everchanging display of water and light around the monastery was under threat. A steady flow of silt from the Couesnon, Sée, and Selune rivers and sand, brought into the bay by high tides, accumulated between the rock and the coast, continuously joining Mont St-Michel with the land. The reasons for this build-up of silt and sand go back to the 19th century, when the rivers were diverted to create pasture land, preventing them from properly carrying the deposits into the sea. To make things worse, a dike built in 1869 to provide visitors with a permanent connection to the island, further impeded free drainage. Now the sands at the foot of Mont St-Michel are 3m higher than they were 200 years ago. To reduce this deposit build-up, the French government had a dam built on the Couesnon river. Eight sets of sluice gates, clad with 2205 duplex stainless steel, retain the river water at high tide and release it at low tide, allowing it to flush sediment out to sea. Additionally, the old dike that blocked water flow was replaced by a
light bridge on stilts. The bridge allows the free flow of water around the island, improving the efficiency of deposit removal from the bay.
Some 36 tonnes of duplex stainless steel plate, supplied by Industeel, clad the gates’ coated carbon steel frame. 2205 duplex stainless steel, containing 3% molybdenum, was chosen for its superior corrosion and abrasion resistance. The molybdenum content helps fend off both corrosion from seawater and abrasion from sediments. With lots of abrasive particles and high flow rates, ‘erosion corrosion’ is a significant risk for corrosion-prone, softer materials like aluminium or carbon steel. Wet-dry interfaces, inevitable in a water management system, can also exacerbate corrosion. A carefully welded duplex stainless steel surface, with its high hardness and corrosion resistance, will resist these perils far better than other metals or lesser alloyed grades.
The Couesnon dam, which faces the abbey and the open bay, has become its own tourist attraction. Project architects designed a public space atop the dam, which offers unobstructed views of Mont St-Michel and attracts millions of people to visit this UNESCO World Heritage Site every year.