Land-Ocean Interactions:  Estuarine Circulation

Land-Ocean Interactions: Estuarine Circulation PowerPoint PPT Presentation


  • 258 Views
  • Uploaded on
  • Presentation posted in: General

Schematic of a typical Estuary. very fresh. quite salty. Density gradient along axis of estuary. and in the vertical (strongly stratified). Stratification evolves over time in response to freshwater inflow shows time scale of estuary residence time is long. Smaller estuary: salinity shows tidal variability.

Download Presentation

Land-Ocean Interactions: Estuarine Circulation

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


1. Land-Ocean Interactions: Estuarine Circulation Estuary: a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage. (Pritchard,1963) A typical estuary has most of the freshwater entering at its head, and has a transitional section between the body and the coastal ocean. It is a common practice to classify estuaries into different categories, using varying schemes (based on their geomorphology or circulation) for the purposes of enabling us to study, qualitatively, common properties across multiple geographic locations. Subtleties of the terrestrial inputs and unique circulations affected by complicated coastline and bathymetry conspires to make most estuaries more unique than alike. But we can understand common physical dynamical processes that estuaries have in common…A typical estuary has most of the freshwater entering at its head, and has a transitional section between the body and the coastal ocean. It is a common practice to classify estuaries into different categories, using varying schemes (based on their geomorphology or circulation) for the purposes of enabling us to study, qualitatively, common properties across multiple geographic locations. Subtleties of the terrestrial inputs and unique circulations affected by complicated coastline and bathymetry conspires to make most estuaries more unique than alike. But we can understand common physical dynamical processes that estuaries have in common…

3. Chesapeake – a really big estuaryChesapeake – a really big estuary

4. About 90% of the nutrients that enter the Chesapeake from river runoff are assimilated by plankton in the Chesapeake Bay and exported as inorganic nutrients into the coastal ocean. The estuary acts as a large filter, processing and modifying the inputs. The capacity of an estuary to act this way depends heavily on the residence time, which is affected by the physics of the estuarine circulation.About 90% of the nutrients that enter the Chesapeake from river runoff are assimilated by plankton in the Chesapeake Bay and exported as inorganic nutrients into the coastal ocean. The estuary acts as a large filter, processing and modifying the inputs. The capacity of an estuary to act this way depends heavily on the residence time, which is affected by the physics of the estuarine circulation.

5. The James River – a much smaller estuary feeding into the Chesapeake Bay - shows a noticeable signature of tidal variability in the surface salinity distribution. Many estuaries have a pronounced cycle in the vertical stratification associated with the tides. The James River – a much smaller estuary feeding into the Chesapeake Bay - shows a noticeable signature of tidal variability in the surface salinity distribution. Many estuaries have a pronounced cycle in the vertical stratification associated with the tides.

6. Characteristics of estuaries Most estuaries: strong tidal forcing large density difference between river and ocean complex topography Long and narrow – can often be approximated by 2-dimensional vertical/along-axis flow (relatively little across axis flow) Mathematically we have equations for salt, mass (volume) and momentum significant forces: friction (mixing), pressure, nonlinearity, acceleration (time variability) typically small: wind, Coriolis, longer that tidal period coastal sea level (tides are important) most common dynamic balance is between pressure and friction/mixing Mixing affects the salt balance … … which affects the pressure distribution and pressure gradient Can classify estuaries based on their physics (relative magnitude of different terms), or topography/geomorphology

7. Physics essentials: Fresh river water encounters salty ocean water Fresh = light; salty = heavy Freshwater flows seaward at the surface Get landward flow of more dense, salty, water estuarine or gravitational or baroclinic circulation time scales of ~1 day … so Coriolis force is usually of secondary importance circulation is evident averaged over a few tidal cycles mixing and entrainment processes are central to details of the salt and volume transport balance Long and narrow Approximated by 2-dimensional vertical/along axis flow (relatively little across axis flow) Mathematically we have balance equations for salt, mass and momentum possible forces are friction, pressure, nonlinearity, unsteady (acceleration) typically small: wind, Coriolis, coastal sea level (expect where it drives the tides) relative magnitude of these terms one classification scheme Dynamic balance is between pressure force and friction/mixing Mixing affects the salt balance – which affects the pressure distribution and pressure gradient Common features: strong tidal forcing large density difference between river and ocean complex topography Can classify estuaries baaed on their topography/geomorphology Long and narrow Approximated by 2-dimensional vertical/along axis flow (relatively little across axis flow) Mathematically we have balance equations for salt, mass and momentum possible forces are friction, pressure, nonlinearity, unsteady (acceleration) typically small: wind, Coriolis, coastal sea level (expect where it drives the tides) relative magnitude of these terms one classification scheme Dynamic balance is between pressure force and friction/mixing Mixing affects the salt balance – which affects the pressure distribution and pressure gradient Common features: strong tidal forcing large density difference between river and ocean complex topography Can classify estuaries baaed on their topography/geomorphology

8. Fjords Glacial valleys flooded by rising sea level Found poleward of 43o latitude Narrow, deep inlets Shallow sill connect fjord with ocean Freshwater flows out in a thin surface layer Deep water is near oceanic salinity and relatively motionless Alaska, British Columbia, Norway, Scotland Chile, New Zealand River valleys deepened by glaciers Very deep due to glacial scouring 800 m deep Shallow sill at mouth (terminal moraine of glacier)Alaska, British Columbia, Norway, Scotland Chile, New Zealand River valleys deepened by glaciers Very deep due to glacial scouring 800 m deep Shallow sill at mouth (terminal moraine of glacier)

9. Coastal Plain Estuaries River valleys flooded by sea level rise following glacial period (sometimes sediment-filled fjords) Little sedimentation Ancient river valleys determine the topography Shallower than fjords and more uniform in depth Extent of salt influence depends on forcing more than bathymetry Tides are often the most important source of mixing Sketch partially mixed or salt wedge type sectionSketch partially mixed or salt wedge type section

11. Bar-built and Lagoon Estuaries Drowned river valleys with high sedimentation rates Very shallow Often branch toward mouth into a system of shallow waterways (lagoons) Narrow connections to the ocean Sediment accumulates at mouth contributing to bar formation Shallow lagoons can be well-mixed by tides and winds Complex topography: channels, island and shoals Multiple sources of freshwater

13. The incoming water is at ocean salinity, but the outgoing water is partially ocean water that just entered and partly water that has been well mixed with the freshwater river inflow. Asymmetry of the inflow and outflow geometry favors increased salt exchange through the mouthThe incoming water is at ocean salinity, but the outgoing water is partially ocean water that just entered and partly water that has been well mixed with the freshwater river inflow. Asymmetry of the inflow and outflow geometry favors increased salt exchange through the mouth

14. The asymmetry can also be effective outside the mouth to the estuary.The asymmetry can also be effective outside the mouth to the estuary.

15. Fjord: high relief in bathymetry, shallow sill Coastal plain estuaries (ria = drowned coastline): Drowned river valleys with high sedimentation rates Very shallow Can branch toward mouth into a system of shallow waterways (lagoons) Bar built estuaries sediment accumulates at mouth contributing to bar formation Shallow lagoons can be well-mixed by tides and winds Complicated factors are Complex topography: channels, island and shoals Multiple sources of freshwater All these can modify the salt balance and residence time Estuaries can be a significant mixture of all these features depending on their geological history. Fjord: high relief in bathymetry, shallow sill Coastal plain estuaries (ria = drowned coastline): Drowned river valleys with high sedimentation rates Very shallow Can branch toward mouth into a system of shallow waterways (lagoons) Bar built estuaries sediment accumulates at mouth contributing to bar formation Shallow lagoons can be well-mixed by tides and winds Complicated factors are Complex topography: channels, island and shoals Multiple sources of freshwater All these can modify the salt balance and residence time Estuaries can be a significant mixture of all these features depending on their geological history.

16. Morphological classification Wave dominated: waves are significant at the mouth where sediment is eroded from the coastline and transported alongshore to form a spit. This constricts the mouth and reduces the role of tides. Minimum of energy in mid estuary – often extensive mudflats and marshes Morphological classification Wave dominated: waves are significant at the mouth where sediment is eroded from the coastline and transported alongshore to form a spit. This constricts the mouth and reduces the role of tides. Minimum of energy in mid estuary – often extensive mudflats and marshes

17. Macrotidal: 4 to 6 m ranges Hypersynchronous: convergence exceeds friction and tidal currents increase toward the head; generally funnel shaped – this fosters mobility of sediments throughout the estuaryMacrotidal: 4 to 6 m ranges Hypersynchronous: convergence exceeds friction and tidal currents increase toward the head; generally funnel shaped – this fosters mobility of sediments throughout the estuary

18. Classification based on salinity structure (= physics yay!) The majority of estuaries in populated coastal regions are in the coastal plain category (locally: Chesapeake, Delaware, Hudson) Within this group there are large differences in circulation patterns, density, residence time, and mixing A better classification is one based on salinity and flow characteristics It’s Physics!It’s Physics!

19. Physics essentials: Fresh river water encounters salty ocean water Fresh = light; salty = heavy Freshwater flows seaward at the surface Get landward flow of more dense, salty, water estuarine or gravitational or baroclinic circulation time scales of ~1 day … so Coriolis force is usually of secondary importance circulation is evident averaged over a few tidal cycles mixing and entrainment processes are central to details of the salt and volume transport balance Long and narrow Approximated by 2-dimensional vertical/along axis flow (relatively little across axis flow) Mathematically we have balance equations for salt, mass and momentum possible forces are friction, pressure, nonlinearity, unsteady (acceleration) typically small: wind, Coriolis, coastal sea level (expect where it drives the tides) relative magnitude of these terms one classification scheme Dynamic balance is between pressure force and friction/mixing Mixing affects the salt balance – which affects the pressure distribution and pressure gradient Common features: strong tidal forcing large density difference between river and ocean complex topography Can classify estuaries baaed on their topography/geomorphology Long and narrow Approximated by 2-dimensional vertical/along axis flow (relatively little across axis flow) Mathematically we have balance equations for salt, mass and momentum possible forces are friction, pressure, nonlinearity, unsteady (acceleration) typically small: wind, Coriolis, coastal sea level (expect where it drives the tides) relative magnitude of these terms one classification scheme Dynamic balance is between pressure force and friction/mixing Mixing affects the salt balance – which affects the pressure distribution and pressure gradient Common features: strong tidal forcing large density difference between river and ocean complex topography Can classify estuaries baaed on their topography/geomorphology

20. Tides are the principal source of mixing energy Velocity shear and turbulence are generated in the bottom boundary layer from friction and bottom drag in the velocity shear across the halocline but density difference works against mixingTides are the principal source of mixing energy Velocity shear and turbulence are generated in the bottom boundary layer from friction and bottom drag in the velocity shear across the halocline but density difference works against mixing

22. Variations of current and salinity over a tidal period Mean velocity profile shows seaward flow at the surface, and landward flow at the bottom The salinity is not uniform – it can’t be because the up-river flow at the bottom implies a slat flux into the estuary, which must be balanced over several tidal cycles. Mean velocity profile shows seaward flow at the surface, and landward flow at the bottom The salinity is not uniform – it can’t be because the up-river flow at the bottom implies a slat flux into the estuary, which must be balanced over several tidal cycles.

23. Salinity in a salt wedge estuary If the tidal excursion, or tidal volume, is small compared to the freshwater input (R times T_tide) freshwater floats on saltier water w/o much mixing wedge of salt water extends into estuary with little mixing most of the water exchange is at the frontIf the tidal excursion, or tidal volume, is small compared to the freshwater input (R times T_tide) freshwater floats on saltier water w/o much mixing wedge of salt water extends into estuary with little mixing most of the water exchange is at the front

24. Salinity in a highly stratified estuary

26. Mass transport in a highly stratified estuary Entrainment adds salt to the surface layer, so salinity increases seaward Vigorous circulation aids flushing Can enable upstream biological transport (retention of larvae in estuaries) Entrainment adds salt to the surface layer, so salinity increases seaward Vigorous circulation aids flushing Can enable upstream biological transport (retention of larvae in estuaries)

27. Download, install and run the CHIMP viewer Examples: No wind, high Susquehanna flow see surface and ocean floor velocity and salinity Add strong southerly wind Holds water in estuary Switch to strong northerly wind Improves flushing of bay but wind-driven vertical mixing can supress salt wedge and estuarine circulation Turn off river See salt wedge advance up estuaryDownload, install and run the CHIMP viewer Examples: No wind, high Susquehanna flow see surface and ocean floor velocity and salinity Add strong southerly wind Holds water in estuary Switch to strong northerly wind Improves flushing of bay but wind-driven vertical mixing can supress salt wedge and estuarine circulation Turn off river See salt wedge advance up estuary

28. Salinity in a slightly stratified estuary If the tidal volume is increased we get more mixing gives a slightly stratified or “partially mixed” estuary significant vertical mixing everywhere – dilution of the lower layer salinity If the tidal volume is increased we get more mixing gives a slightly stratified or “partially mixed” estuary significant vertical mixing everywhere – dilution of the lower layer salinity

29. Salinity in a vertically mixed estuary If tidal volume and/or tidal mixing increases still further, locally the salinity stratification is nearly eliminated. Well mixed estuary Salinity increases toward the sea but does not vary with depth Penetration of salt upstream is by horizontal mixing only – no longer any overturning estuarine circulation An estuary can transition between the well mixed and partially mixed regimes on the spring-neap tidal cycle This can actually mean the estuary is less flushed on the more energetic spring tide the river outflow transport is distributed over the entire water depth, so the outflow current is weaker, whereas when the estuarine circulation is active there is faster outflow at the surface balanced by inflow at the bottom. If tidal volume and/or tidal mixing increases still further, locally the salinity stratification is nearly eliminated. Well mixed estuary Salinity increases toward the sea but does not vary with depth Penetration of salt upstream is by horizontal mixing only – no longer any overturning estuarine circulation An estuary can transition between the well mixed and partially mixed regimes on the spring-neap tidal cycle This can actually mean the estuary is less flushed on the more energetic spring tide the river outflow transport is distributed over the entire water depth, so the outflow current is weaker, whereas when the estuarine circulation is active there is faster outflow at the surface balanced by inflow at the bottom.

30. Salinity in an inverse estuary

31. 3-dimensional circulation Coriolis force concentrates flow on the right bank both ebb and flood Get weaker mean flow on left (looking seaward) than right If the effect is strong, the mean flow on the left can be into the estuary producing a strong secondary circulationCoriolis force concentrates flow on the right bank both ebb and flood Get weaker mean flow on left (looking seaward) than right If the effect is strong, the mean flow on the left can be into the estuary producing a strong secondary circulation

32. Ebb and flood channel asymmetry.Ebb and flood channel asymmetry.

33. Secondary flows Density driven lateral tidal cells – axial convergence Asymmetry is because of friction acting on different water depthsSecondary flows Density driven lateral tidal cells – axial convergence Asymmetry is because of friction acting on different water depths

  • Login