Method for Quantifying Freshwater Input and Flushing Time in Estuaries

A method is developed to quantify total freshwater input and flushing time in estuaries using measured flow and salinity data at the estuary-ocean boundary. A formula is derived to calculate the volume of mixed estuarine water that leaves the estuary on the ebb tide. The calculated volume of mixed estuarine water, along with the volume of new ocean water that enters the estuary on the flood tide, is used to quantify the total volume of freshwater input. The calculated volume of mixed estuarine water is also used to quantify the flushing time. The developed method is applied to Barnegat Bay, N.J. The volume of freshwater input calculated from the new method compares well with that estimated using watershed, aquifer, and meteorological data. The flushing time calculated from the new method is between that calculated from the classical tidal prism method and that from the fraction of freshwater method. An assessment is made on the sensitivity of calculated values to the quality of measured data at the estuary-ocean boundary. The volume of freshwater input and the flushing time calculated from the developed method are highly reliable when there is a significant salinity difference between flood and ebb tides.