Land-Ocean Interactions: Estuarine Circulation

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) Coastal Ocean Estuary mouth Estuary Estuary head River 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…

Schematic of a typical Estuary

very fresh Chesapeake – a really big estuary 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 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.

Smaller estuary: salinity shows tidal variability 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. Smaller estuary: salinity shows tidal variability

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 and momentum typically small: wind, Coriolis, long time scale coastal sea level significant forces: friction (mixing), pressure, nonlinearity, acceleration (time variability) tides are important most common dynamic balance is 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

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

Fjords Glacial valleys flooded by rising sea level Topography classification: 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)

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 section

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

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!

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 Mixing across the strong vertical salinity gradient is significant Turbulence driven by velocity shear affects mixing rates Density stratification works against mixing but does not prevent it. 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

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 mixing

to ocean river

to ocean river flood tide ebb tide to ocean river Geyer, W.R. And P. MacCready, Annu. Rev. Fluid Mech. 2014. 46:175–97, doi: 10.1146/annurev-fluid-010313-141302 to ocean river Profiles of velocity, density anomaly, and eddy viscosity as they evolve over a tidal period for the strain-induced periodic stratification regime: There is complete destratification during the flood tide, leading to strong mixing and almost no vertical shear. On the ebb tide, substantial shear may develop due to suppression of turbulence by stratification, which originates from the straining of the horizontal density gradient. The stratification at the end of the flood tide results from lateral straining, whereas the increasing stratification during the ebb tide results from along-estuary straining. Geyer, W.R. And P. MacCready, Annu. Rev. Fluid Mech. 2014. 46:175–97, doi: 10.1146/annurev-fluid-010313-141302

ocean river ocean river ocean river

(averaged over several tidal cycles) V1 = V2 S2/S1 Volume balance: Salt balance: Salt in = V2S2 + R So Salt out = V1S1 V1S1 = V2S2 (averaged over several tidal cycles) V1 = V2 S2/S1 Volume balance: R + V2 = V1 R = V1 – V2 = V2(S2/S1) – V2 = V2(S2/S1 – 1) V2 = R / (S2/S1– 1) or = S1 R / (S2 – S1) V1 = S2 R / (S2 – S1) R V1 , S1 V2 , S2 Vertical flux of salt through entrainment R V1 , S1 V3 , S3 V4 , S4 V2 , S2 Difference between upper and lower transport is always R This is the same logic we used in determining the exchange flow through the Strait of Gibralter in the Nov 17, 2010, lecture

Mass transport in a highly stratified estuary River volume flow is R. Outflow from the estuary in the upper layer is 10R. This is balanced by oceanic inflow of 9R. The net outflow at the ocean end is, of course, still only 1R. 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) © 1996 M. Tomczak

Salinity in a salt wedge estuary Top: As a function of depth and distance along estuary Bottom: Vertical salinity profiles for stations 1-4 Surface salinity is close to zero at all stations. Bottom salinity is close to oceanic. 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 front © 1996 M. Tomczak

Salinity in a slightly stratified (partially-mixed) estuary Top: As a function of depth and distance along estuary Mixing is indicated by the circles. Bottom: Vertical salinity profiles for stations 1-4 Surface and bottom salinity increase from station 1 to 4, but surface salinity is always slightly fresher. 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 © 1996 M. Tomczak

Salinity in a vertically well-mixed estuary Top: As a function of depth and distance along estuary Bottom: Vertical salinity profiles for stations 1-4 Surface and bottom salinity increase from station 1 to 4, but surface and bottom salinity are always nearly identical 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. © 1996 M. Tomczak

Secondary flows Density driven lateral tidal cells - axial convergence Asymmetry is because of friction acting on different water depths

3-dimensional circulation Slightly stratified estuary with weak Coriolis effect (northern hemisphere). Slightly stratified with strong Coriolis effect Vertically mixed estuary with Coriolis effect 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 circulation Blue (dark) arrows indicate upper layer flow, and red (light) arrows bottom flow © 1996 M. Tomczak