A study done by Rijkswaterstaat in 1937 showed that the sea defenses in the southwest river delta were inadequate to withstand a major storm surge. The proposed solution was to dam all the river mouths and sea inlets thereby shortening the coast. However because of the scale of this project and the intervention of the Second World War its construction was delayed and the first works were only completed in 1950. The North Sea flood of 1953 gave a major impulse to speed up the project. In the following years a number of dams were built to close off the estuary-mouths. In 1976, under pressures from environmental groups and the fishing industry, it was decided not to close off the Oosterschelde estuary by a solid dam but instead to build the Oosterscheldekering, a storm surge barrier which is only closed during storms. It is the most well-known (and most expensive) dam of the project. A second major hurdle for the works was in the Rijnmond area. A storm surge through the Nieuwe Waterweg would threaten about 1.5 million people around Rotterdam. However, closing off this river mouth would be very detrimental for the Dutch economy, as the Port of Rotterdam - one of the biggest sea ports in the world - uses this river mouth. Eventually, the Maeslantkering was built in 1997, keeping economical factors in mind: the Maeslantkering is a set of two swinging doors that can shut off the river mouth when necessary, but which are usually open. The Maeslantkering is forecast to close about once per decade. Up until now (January 2012), it has closed only 1 time, in 2007. The project was finished with the construction of the Maeslantkering in 1997.