By Foo Yun Chee AMSTERDAM, May 23 (Reuters) - After holding back the sea for 75 years, the 30 kilometre-long dike protecting much of the Netherlands from floods is due for a $1 billion upgrade against mounting risks from rising sea levels and tsunamis. "We plan to invest up to 750 million euros ($1 billion) to strengthen the Afsluitdijk," said Hans Vos, senior adviser to the Dutch Transport and Public Works Ministry. On Thursday, the Netherlands marks the 75th anniversary of the completion of the Afsluitdijk, the country's longest dike, which keeps the North Sea at bay. It protects the country's biggest freshwater lake and source of drinking water. Water is a real threat to the Netherlands, two-thirds of which lies below sea level. Fears that huge waves could flood the country increased after the 2004 Indian Ocean tsunami that left some 230,000 people dead or missing, Vos said. Rising sea levels due to global warming have also added to concerns. Memories of the 1953 flood when a massive North Sea storm breached the country's dikes and resulted in about 1,800 deaths still linger. Vos said proposals to strengthen the Afsluitdijk by 2020, which stands about 7.5 metres above sea level, include raising its height, widening the barrier which now extends to 125 metres in some places, and building more sluice gates to improve water flow control. "There is also a suggestion to plant artificial grass on the dike," he said. In the aftermath of Hurricanes Katrina and Rita in 2005, U.S. officials turned to the Dutch for their expertise on flood control and water management systems.
