Lec21_Estuaries - Lecture 21 Estuaries An estuary is a...

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Lecture 21 Estuaries
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An estuary is a region where a free flowing river meets the ocean Estuarine regions are characterized by tidal reversals and brackish water (mixture of salt water and fresh waters) Mixing in estuaries is primarily caused by: •T i d a l W a v e • Wind Stresses • Internal Density Variations
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Types of Estuaries The term estuary covers a diversity of shapes and sizes. (a) Sharply stratified estuaries such as fjords and salt-wedge tuaries estuaries (b) Partially-stratified estuaries where there is a significant ertical density gradient vertical density gradient (c ) Well-mixed estuaries
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Mixing caused by wind Wind is the dominant source of energy in large lakes and the open ocean. t i i t t b In estuaries it may or may not be important. If the estuary is wide wind effects become important. On the shallow side the current flows with the wind and on the deeper side it flows against it. he wind induces approximately uniform stress The wind induces approximately uniform stress everywhere on the water, so the line of action is through the centroid of the water surface. But the center of mass is displaced towards the deeper side as there is more water there. Hence a torque is induced that causes the water mass to rotate.
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Mixing caused by Tides Tides cause mixing in two ways: (a) Friction of the tidal flow running over the ottom of the estuary generates bottom of the estuary generates turbulence and leads to turbulent mixing. Large-scale currents are produced by the interaction of tidal wave with the bathymetry . This part is like flow in a river but goes back and forth. (b) Tidal “pumping” and “trapping”. uperimposed on the back- d- rth flow Superimposed on the back and forth flow is a net, steady circulation, often called the “ residual circulation ”. This is not immediately obvious to a casual observer. The residual circulation (velocity) can be obtained by averaging the velocity at each point over a tidal cycle.
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Residual circulation gives information on preferential exchange.
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Net Estuarine Flow 1 () T n Qq t d t T = ò 0 We can use sinusoidal approximations of the flood and ebb flows and integrate them to obtain the mean flow over a tidal cycle.
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This note was uploaded on 01/20/2011 for the course ENE 804 taught by Professor Hashsham,s during the Spring '08 term at Michigan State University.

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Lec21_Estuaries - Lecture 21 Estuaries An estuary is a...

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