doi: 10.3319/TAO.2011.08.22.01(Hy) * Corresponding author E-mail: choyk@snu.ac.kr Terr. Atmos. Ocean. Sci., Vol. 23, No. 1, 119-130, February 2012 Spatio-Temporal Variation of Flushing Time in the Sumjin River Estuary Dinesh Chandra Shaha 1 , Yang-Ki Cho 1, * , Tae-Wan Kim 2 , and Arnoldo Valle-Levinson 3 1 School of Earth & Environmental Sciences, Research Institute of Oceanography, Seoul National University, Seoul, Korea 2 Korea Polar Research Institute (KOPRI), Korea Ocean Research and Development Institute, Incheon, Korea 3 Department of Civil and Coastal Engineering, University of Florida, Gainesville, Florida, USA Received 21 March 2011, accepted 22 August 2011 AbSTRAcT Flushing is a very complicated process in estuarine environments. In order to examine the effects of tidal amplitude, river discharge, and stratification on the spatially varying flushing time of the Sumjin River Estuary (SRE), 24 longitudinal salinity transects were obtained during spring and neap tides from August 2004 to April 2007. The widely accepted freshwater frac- tion method has been used to calculate the flushing time for multiple estuarine segments using a spatially varying freshwater fraction. The effects of tidal amplitude, river discharge, and stratification on estuarine flushing were identified reasonably well by the spatially varying time scale. The flushing time appears to be close to the semidiurnal (M 2 ) tidal period during spring tide, but it is twice as long during a neap tide near the mouth. The flushing time increases in the central regions with a decrease in the tidal amplitudes and reduces in the inner-most regions owing to the strong influence of gravitational circulation. A linear function negatively relates estuarine flushing to the tidal amplitudes near the mouth of the estuary, whereas a power-law func- tion relates estuarine flushing to the freshwater inflow near the head. In addition, strong stratification induced by freshwater discharge and small tidal amplitude exerts dominant control to reduce the estuarine flushing in the central and upper regions of the estuary during a neap tide. Key words: Freshwater fraction method, Potential energy anomaly, Multiple segments, Spatio-temporal variations Citation: Shaha, D. C., Y. K. Cho, T. W. Kim, and A. Valle-Levinson, 2012: Spatio-temporal variation of flushing time in the Sumjin River Estuary. Terr. Atmos. Ocean. Sci., 23, 119-130, doi: 10.3319/TAO.2011.08.22.01(Hy) 1. InTRoducTIon The flushing time is an important indicator for the assessment of the estuarine water quality (Thomann and Muller 1987; Huang and Spaulding 2002; Huang 2007). It is commonly defined as the time required to replace the fresh- water then present in the estuary (freshwater volume) at the rate of freshwater inflow (Officer 1976; Dyer 1997). In gen- eral, the flushing time of a given system depends on the tidal exchange between the system and the adjacent ocean which is a complicated function of the freshwater runoff, tidal range, density stratification, bathymetry, and wind (Pilson 1985; Choi and Lee 2004; Ji et al. 2007). The flushing time describes an overall feature of the estuary and is often used to estimate the removal rate of a pollutant carried by fresh water. A long flushing time implies that it would take a long time to flush pollutants from an estuary, which in turn may involve outbreaks of harmful algal blooms (Garcon et al. 1986; Bricelj and Lonsdale 1997). Several methods are used to calculate the flushing time, such as the freshwater fraction method (FFM), Knudsen’s hydrographic theorem, tidal prism method, modified tidal prism method, and mixing length theory (Dyer 1997). In particular, the FFM has frequently been used to estimate the estuarine flushing time in response to freshwater inputs (Pil- son 1985; Williams 1986; Dyer 1997; Huang 2007; Kumari and Rao 2009). This method determines the flushing time as the ratio between the freshwater volume of the estuary and the freshwater inflow rate (Officer 1976; Officer and Kester 1991; Dyer 1997; Ji 2008). This method is applicable only when detailed measurements of freshwater flow and salinity are available. Freshwater fraction models are most appropri- ate for estuaries where there is a measurable difference in the salinity between the estuary and the adjacent water body (Choi and Lee 2004; Valle-Levinson 2010). In general, the appropriateness of the estimation methods for flushing time