BACKGROUND: To avoid disasters when water levels rise near major cities, exposing citizens to dangerous floodwaters, civil engineers are taking a closer look at new ways to build dams. This could help protect coastal cities from tragedies like that of the broken levees due to Hurricane Katrina's strong winds and rising water. Taking inspiration from the Netherlands, a country that's mostly below sea level, civil engineers are studying computer models of a series of dams and levees between water and city -- rather than just one layer.
THE DUTCH BARRIERS: The Dutch barriers are several layers of dams and levees that protect the country from rising waters during storms. The most complex one, the Oosterschelde Barrier, was the one studied. This barrier's several layers include:
- A beam under which water flows when gates are open
- A steel gate that is lowered when sea level reaches "danger" height
- A 5-million-ton stone block beam steadies giant piers, which have holes that fill with sand
- A synthetic "mattress" filled with sand and gravel to strengthen the nearby sea floor
STORM SURGE: During a storm, high winds cause a wall of water to build up in front of the storm. Strong winds push on the oceans surface, and the pressure pushes the water upward, above sea level. Storm surges are particularly dangerous when they arrive near land at the same time as high tide, when the water is naturally at its highest. If the sea floor in the path of the storm surge is deep and sloping, the wall of water can be dispersed more easily. But in areas where the sea floor slopes up gradually and is very shallow for a long distance from the shore, winds push the wall of water directly up the slope.
DAMS VS. LEVEES: A dam is a wall built across a waterway, such as a river, that controls the flow of water along its natural path. Sometimes dams can be used to create lakes or to propel water-generated electric plants. A levee is a wall or embankment built along the side of a body of water to divide the water from the land, and prevent the water from overflowing its natural boundaries.
The American Society of Civil Engineers contributed to the information contained in the TV portion of this report.