Presented byMd. Tariful Islam Fuad

ID. No.: 13207059Marine Science

Session: 2012-13 (2nd Year)Institute Of Marine Sciences & Fisheries,

University Of Chittagong.

Flushing time Of Estuary

The flushing time concept is one of the tools of estuarine management. It is often used to determine how much of a potentially harmful substance an estuary can tolerate before its ecosystem is adversely affected to significant degree. The flushing time concept is used to evaluate where, how and in what quantities the particular substance can be disposed of.

Flushing Time of EstuaryThe flushing time is defined as the time needed to drain a volume V through an outlet A with current velocity v. More specifically, the flushing time tF of an estuary can be defined as the time needed to replace its freshwater volume VF at the rate of the net flow through the estuary, which is given by the river discharge rate R:

tF = VF / R

Flushing Time of Estuary

The most rigorous approach estimates the freshwater volume of the estuary from measurements of salinity. If mixing in the estuary is fully turbulent the fresh water fraction f in a water sample is given by

f = (S0 - S) / S0

Where S0 is the oceanic salinity found outside the estuary mouth & S is the salinity of fresh water from any section of estuary. The fresh water volume of the estuary or of any particular section of the estuary, is the volume integral over f:

f ’ =∫fdv

If V is the volume of the estuary or of the section under consideration and f ’ is the average fresh water fraction found by integration over (S0 - S) / S0. It follows that the flushing time is given by

tF = (f ’ V) / R ………(1)The flushing time tF changes with variations in the river discharge but not in direct

proportion to 1/R because f ’ varies with R to some degree, too.

Knudsen FormulaKnudsen formula also known as salt balance equation.The core assumption of the Knudsen formula is that all sea water that enters the estuary, leaves it in the upper layer after complete mixing with fresh water. If this is not the case and the interface between the two layers is more diffuse, the upper layer will not maintain zero salinity, downstream and some sea water from the lower layer will mix with upper layer that is already slightly saline. The resulting mixture will have a salinity larger than the upper layer’s salinity.The flushing time of estuary can be obtain from the Knudsen formula.

Flushing time according Knudsen formulaIf R denotes the river discharge rate as before and the volume transports of the water entering is Qbottom and leaving the estuary at its mouth is Qtop with respective salinities Sbottom and Stop, the continuity of volume and the continuity of mass (salt) give:

Qtop - Qbottom = R

Qtop Stop = Qbottom Sbottom

So, from the equation (1) we can write:tF = V (Sbottom - Stop) / (Sbottom R)

tF = V/R(1- Stop/Sbottom)

This determination of the flushing time requires the knowledge of the estuary volume and of the river discharge as before and in addition the measurement of Sbottom and Stop, which can be obtained from a single station at the mouth of the estuary.The Knudsen formula therefore underestimates the true flushing time. It also assumes complete mixing of fresh water and oceanic water. In practice, the Knudsen formula appears more appropriate for highly stratified and salt wedge estuaries