ENVIRONMENTAL FLUID MECHANICS. Estuaries

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1 ENVIRONMENTAL FLUI MECHANICS Estaries Benoit Cshman-Roisin Thayer School of Engineering artmoth College Estaries are regions where rivers meet the coastal ocean. They are biologically niqe becase these are the only water bodies where salinity vales take all intermediate vales, from zero pstream in the river to that of the ocean. A nmber of organisms (plants and shellfish) can only live at those intermediate salinities. Hence, estaries have pecliar ecosystems.

2 A prime eample in the United States: The Chesapeake Bay ( Estarine ynamics Comple interaction of - river inflow (gravity driven) - density-driven salinity intrsion (boyancy indced) - tidal motions (water pmping from open sea) - srface wind stress (if bay wide enogh) - instabilities and entrainment between srface and bottom layers.

3 Tidal crrents and salinity distribtion in Chesapeake Bay For animation, see Traditional classification of estaries Estaries come in a diversity of sizes and shapes and are fed by rivers of varios strengths. In some, the river flow is powerfl enogh to keep the seawater literally at bay, while some other estaries have weak rivers or are deep enogh to allow pstream intrsion of seawater along the bottom of the river channel. Finally, some estaries are wide enogh to feel the effect of the rotation of the earth, skewing their circlation (tendency of flow to be concentrated to the right in the Northern Hemisphere). - Salt-wedge estaries Eample: Mississippi River - Partially stratified estaries Eamples: Chesapeake Bay - Well mied estaries Eample: elaware Bay - Fjord-type estaries Eample: Norwegian fjords (

4 To classify the varios types of estaries, one defines the Estarine Richardson Nmber as g Q R river 3 0 Wtide in which: Q river = the volmetric flowrate of the feeding river W = width of river tide = rms tidal velocity = 0.71 ma. This can be recast in terms of velocities: R river density 3 tide in which: river = Q river /Wh = incoming river velocity density = g ( / 0) h g' h tide = rms tidal velocity = 0.71 ma. - Salt-wedge estaries (0.8 < R) Strctre: Sharp salinity gradient in the vertical reaching srface toward the sea ynamics: eeper basins, large rivers, weak tides Eamples: Mississippi River, Silver Bay (Alaska), Teign River Estary (England) Many river estaries dring wet season As freshwater meets seawater, the former begins to float over the latter. The velocity shear created at the interface of the two waters generates billowing and vertical miing. The salt and water ths entrained pward gradally increases the salinity of the pper layer. The incoming salinity mst be provided by an pstream flow at depth, which is driven by negative boyancy. The salinity gradient is relatively sharp. Sailing on the srface of the water, one notices an almost abrpt change in salinity and water color.

5 River Teign estary in sothwestern England Note the color contrast in the offshore waters is de to tides ( Note: Arrows are for mariners and have no scientific meaning.

6 Partially stratified estaries (0.08 < R < 0.8) Strctre: Tidal flow comparable to river flow ynamics: Relatively vigoros miing in vertical smearing the salinity gradient The salinity now varies as mch horizontal as vertically Eample: Chesapeake Bay, Hdson River Srface salinity distribtion in Chesapeake Bay Note how gradal is the srface salinity variation. There is only one sharp gradient, in the James River (sothwestern tribtary, at bottom of map), which has a significantly higher flowrate than the other rivers.

7 - Well mied estaries (R < 0.08) Strctre: Salinity homogeneos in vertical, gradally varying in horizontal ynamics: Tidal pmping is so vigoros that waters are well mied throghot the vertical, and the salinity varies strictly in the horizontal, with or withot sharp gradient Eamples: elaware Bay, Fitzroy Estary Fitzroy Estary and Keppel Bay in Central Qeensland, Astralia ( Longitdinal dispersion in an estary ocean vale river vale ds d S d d S( ) S0 ep S 1 ln S S m 1058 m / s

8 K t t t t M c K c c c t c 4 ) ( ep 4 Trbidity in an estary Case of the Seine River, France The Seine River (France) is highly contaminated by indstrial waste. Some of it has fond its way into bottom sediments, which are lifted and re-deposited with every tidal cycle, and ths progressing very slowly downstream.