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Since tides are caused by the earth- moon-sun positions which are known accurately, the tides can be predicted well in advance. This helps the navigators and fishermen plan their activities. Tidal flows are of great importance in navigation. Tidal heights are very important, especially harbours near rivers and within estuaries having shallow тАШbarsтАЩ at the entrance, which prevent ships and boats from entering into the harbour. Kolkata port on Hugli river is an example for it.