Shift of estuarine type in altered estuaries

To better understand the alteration of the estuarine circulation caused by estuarine dams, four major Korean estuaries were classified by using the Hansen and Rattray stratification–circulation classification scheme. The stratification and circulation parameters were calculated for both discharge and no-discharge periods from the tidally averaged salinity and velocity data obtained from these four Korean estuaries. The estuarine types of the altered Korean estuaries were compared with the previous results for natural estuaries in other countries of similar magnitude in tidal range, water depth, and discharge. This comparison revealed that the estuarine types of the altered Korean estuaries have been shifted from a partially mixed to a well-mixed type (Nakdong River Estuary), from a partially mixed to a coastal bay or a fjord type (Yeongsan River Estuary), and from a well-mixed to a well-mixed type with less tidal modulation (Geum River Estuary). The controlling factors that determined the type in natural estuaries were tide, discharge, and water depth, whereas for altered estuaries, they were the controlled river discharge and water depth. The different estuarine dam gate types with their different modes of operation (surface or bottom discharge) played an important role in the mixing and circulation of the altered estuaries.