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The most common barriers used on dams and major waterways are radial or fuse gates. The radial gates are controlled both mechanically and manually. The flood gates open and close in response to the level of water in a dam. When the sensors rise to a certain level, the valve opens and releases water. Some radial gates are connected to hydro-energy plants and water is released to the plant then returned to the dam from the plant. Fuse gates are stationery pilings that release water when it reaches the top of a dam. These barriers do not allow an operator to control the flow of water from the dam and are not used in areas where there is a need to control the release of water. Mobile flood gates are being used for a unique purpose. As the water level in oceans continues to rise, cities throughout the world are threatened by the influx of the rising water. These barriers have been designed to prevent flooding of major cities in the world. Towers located near the installation area activate the flood gates. The gates lower to a specific water level. When water reaches above that level, the barrier stops it. These barriers protect the city from being flooded as ocean water rises. Since 1982 there have been mobile barriers designed and built in some major cities. The construction of barriers continues on cities that have been encroached upon for centuries and are threatened with disastrous flooding. There are some concerns by marine biologists regarding the continuous use of these types of mobile barriers to protect cities. One argument against mobile barriers is that the exchange of water providing important nutrients to sea life is reduced or stopped when the gates are activated. When the barriers are used continuously, there is little opportunity to take advantage of hydro-energy. This has created some concern as the use of fossil fuel has created more demand for more efficient energy sources in the cities where the barriers are used. Some of the barriers have been installed on feeder rivers at the mouth of the sea. These barriers require a significant amount of dock space that cannot be used by ships. Therefore, the generation of revenue for major shipping docks is reduced. The barriers have also been anchored in the sea. These barriers cover several miles of ocean and are controlled via radio transmission. The gates are activated each day when the tide rises. Scientists are studying the cities where daily use of the mobile barriers is used to determine the amount of damage that is occurring to the eco-system in that area. As the oceans continue to rise the use of flood gates will continue to rise. The flood control barriers take, on average, between ten and twenty years to be completed. The planning for the size of the gates is based on predictions of future needs so that there is no need to replace the gates once they have been installed. For more discussion of this topic, check the links below: flood east auckland
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