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Flushing Time
Citation:
Dettmann, E. Flushing Time. Encyclopedia of Estuaries. Springer Netherlands, , Netherlands, , 329-330, (2015).
Impact/Purpose:
This is a contribution to Springer Verlag’s Encyclopedia of Estuaries. It describes a quantity (flushing time) that quantifes the residence time of fresh water in estuaries, and which may be used to calculate the residence time and concentration of dissolved in the estuary of substances that enter with fresh water, and the fraction of these substances that flow through the estuary to the sea. Flushing time is an important basis of many screening-level models.
Description:
The flushing time of an estuary is generally defined as the turnover time of fresh water in the estuary, that is, the time required to replace the fresh water contained in the estuary with freshwater inflow. Thus, the flushing time of an estuary is the ratio of the volume of freshwater that it contains to the total rate of freshwater inflow. Flushing time decreases as freshwater inflow increases.Flushing time may be determined by mass balance calculations from the estuary volume, its volume-weighted average salinity, and the salinity outside the seaward boundary. This is termed the freshwater replacement method. Flushing time may also be estimated by introducing a conservative tracer, such as dye, at a constant concentration into the freshwater inflow until the mass or average concentration of tracer in the estuary at a given tide stage attains equilibrium. After termination of tracer input, concentrations will decrease approximately exponentially as tracer is flushed from the estuary. The time required for tracer concentration to attain e-1 times its initial value, the e-folding time, is an estimator of the flushing time. Flushing time, as described above, applies to an estuary as a whole. The concept of a flushing time may also be applied to a portion of an estuary, e.g. in a box model. Flushing time is a useful indicator of the behavior of materials introduced into an estuary with freshwater. For instance, the fraction of nitrogen entering the estuary from the watershed that flows through the estuary to the sea, and the fraction lost within the estuary to processes such as denitrification and permanent burial in sediments, may be estimated using the flushing time. The turnover or mean transit times of conservative materials introduced with fresh water are equal to the flushing time, while those of nonconservative materials are shorter. Other related concepts are sometimes used in describing material movement through an estuary. Examples are estuarine residence time, i.e. the residence time in the estuary of a conservative substance introduced uniformly in concentration throughout the estuary; and pulse residence time, the residence time of a conservative substance introduced as an instantaneous pulse in a limited portion of the estuary.
