cowles mar 555 fall, 2009 1 week 14: estuaries introductory physical oceanography (mar 555) - fall...

CowlesMAR 555 Fall, 2009

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Week 14: Estuaries

Introductory Physical Oceanography (MAR 555) - Fall 2009

G. Cowles

From M. Sundemeyer MAR620 Notes


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Key Concepts:1. Definition

2. Importance

3. Basic Circulation

4. Empirical Classification

5. Mixing Rates

6. Residence Time

7. Flushing Time


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Estuary - Definition

An estuary is a semi-enclosed coastal body of water which has free connection to the open sea, extending into the river as far as the limit of tidal influence, and within which sea water is measurable diluted with fresh water derived from land drainage. – Pritchard, (modified by K. Dyer).


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Estuary - Importance

• Nursery Ground (Crab, Shad, Flounder)• Habitat (Crab, Shrimp, Clams, Birds)

Biologically - Very Productive

• Provides Shelter (Harbors), Food, Place to dump Effluent, Recreation and Snacking

Humans

• Loss of Habitat• Effluent Pollution – Nitrogen and Runoff• Manmade modifications – irrigation, flood control measures, etc. modify habitat, circulation, and sediment load.

Concerns


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Types of Estuaries (by formation)

• Drowned-river valley

• Fjord

• Bar-built

• Tectonic

Source: http://www.mast.udel.edu/200/

(Chesapeake)

(Pleasant Bay)

(San Fran Bay)

(Nassau)


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Ideal Estuary Cross Section


Density Discontinuity

Seaward Flow of Fresh Water

Landward Flowof Salty Water

No Tides, No Wind, No Waves, No Mixing at the Interface

Ideal:


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More Realistic Cross Section


Mixing at the Interface leads to Entrainmentof dense salty water from bottom layer into fresh top layer leading to smoothing of the interface

Realistic:


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Estuary Schematic

Tidal Forcing

Mixing: Driven by Tidesand Turbulence alongThe Fresh/Salt Interface

From Open Ocean

- Density Driven Flow (principally salinity) - Balance of Forces: Pressure Gradient and Friction- Role of Coriolis on circulation is minor

Wind and Waves may influencemixing but typically is fetch limited


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Unsteady Circulation with Tides

U. Washington Ocean 200

Flood

Ebb


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Net Circulation


• Flood and Ebb average almost to zero• Near Surface Layer: Ebb stronger than flood• Bottom Layer: Flood Stronger than ebb (inflow needed to replace water lost to entrainment)


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Classification of Estuaries


Salinity field is a balance between advection of fresh water and diffusion of salt

This balance can be roughly described using a ratio of two params

The volume of fresh water discharged by the river over a tidal cycle

R

VThe volume of water entering the estuaryduring the flood tide – this is a measure of mixing

We will classify estuaries based on R/V


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Classification of Estuaries


Salt-wedge

Partially mixed

Well-mixed

R/V > 1

.005 < R/V < 1

R/V < .005


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Salt-Wedge Estuaries



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Salt-Wedge Estuaries



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Salt-Wedge Estuaries Special Case:Fjords


Sill Blocks Deep Water Return Flow

Isohalines (and Isopycnals) are nearly horizontal


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Partially-Mixed Estuaries



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Partially-Mixed Estuaries


• Rough Balance between Freshwater forcing and mixing• Halocline weaker than in a salt wedge• Mixing and entrainment are stronger


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Well-Mixed Estuaries


• Areas of Fast Tidal Currents away from River Mouths• Typically shallow (easier to mix vertically)• Isohalines nearly Vertical• Isohalines oscillate back and forth with tide• Net Circulation is Not Two Layers, Outflow at all Depths (averaged over the tide)


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Mixing: Internally-Generated Turbulence


€

Ri = −g

ρ

∂ρ

∂z

∂u

∂z

⎛

⎝ ⎜

⎞

⎠ ⎟2

Strength of Mixing Along the halocline depends on gradient Richardson number:

For Ri > .25 Mixing Suppressed, Principally generated throughInstabilities known as Holmhoe Waves

Breaking Leads to Entrainment we


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Mixing: Internally-Generated Turbulence Kelvin-Helmholtz Instability


€

Ri<1/4

Light Fluid

Heavy Fluid


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Mixing: Boundary-Generated Turbulence


€

u(z)

τ ρ=

1

κln

z

zo

z

Velocity Is Zero at the Wall

Turbulent Flow over the Bottom

Log Law:

The wall (bottom) injects turbulence into flow causing mixing at higher levels

Zo : roughness length (related to physical roughness (substrate grainsize)

τ : shear stress on the bottom


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Partially Mixed Estuary:Internal and Bottom Mixing Interact

Highly Stratified Estuary:Internal and Bottom Mixing Separate

K. Dyer , Estuaries, a Physical Introduction

Mixing: Combined


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Mixing Time Scales

Key question for managers: How much time is required for a pollutant or tracer introduced into an estuary to diffuse to a given level

Example: Nitrogen from septic systems introduced into the Capes estuaries through groundwater.

Key focus of Mass Estuaries Project: What is the TMDL of nitrogen that can be introduced in each estuary. This information is key at town level where huge $$$ decisions regarding wastewater treatment must be made

http://www.oceanscience.net/estuaries/reports.htm


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advection only


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diffusion only


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advection and diffusion


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Time Scale: Residence Time

1) Average amount of time a particle has spent in an estuary2) Average time a particle spends from entrance to exit (a.k.a.

“Transit Time”3) Time until a given particle leaves (most common)

Start Time + Location + Definition of Estuary Boundary

Information Required

Typically Numerical Models (including segmented boxed models) are used to estimate residence time


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Residence Time Calculations

Tejo Estuary, Portugal

Residence Times (days)(following particles with a numerical model)

Willapa Bay

Numerical Models: Track time ofNeutrally Buoyant Particles in Estuary

Note: Spatial DependencySource: unknown?

Banas and Hickey, JGR 2005

Res Time in Days


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Time Scale: Flushing Time

Time required for freshwater inflow to replace freshwater originally present in estuary (Dyer, 1973)

No Mixing(Plug Flow)

At end of flushing time, all fresh watercompletely new

Perfect Mixing At end of flushing time, 1/e original remains (66%) of water is new

Relation with to Residence Time:

- Average residence time from head to mouth of region

We will look at two ways to calculate flushing time


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Flushing Time Calculation

Time required to replace the freshwater volume VF of an estuary at the net rate of flow given by the river discharge R

€

t f =VF

R

€

f = 1−S

So

⎛

⎝ ⎜

⎞

⎠ ⎟ Freshwater Fraction with So the ocean Salinity

€

f * = fV

∫∫∫ dVTotal freshwater:

€

t f =f *V

RRequires knowing S(x,y,z), R, V


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Simplification: Perfect Mixing – The Tidal Prism Method

€

VT +VR( )S* =VTSO +VR *0

Model:

VT

S=So

VR

S=0

Volume VT of ocean water enters estuary as does R*T of fresh water where T is tidal period

VT+VR

S=S*

At Flood, Perfect Mixing of VT+VR occurs with S=S*. This flows out of the estuary during ebb.

Flood tide

Ebb tide

Salt Balance Equation:


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Simplification: Perfect Mixing

€

S* =VT

VT +VR

⎛

⎝ ⎜

⎞

⎠ ⎟SO

€

f * = 1−S*

So

⎛

⎝ ⎜

⎞

⎠ ⎟

€

f * =VR

VT +VR

€

t f =VF

R

€

R =VR

T

€

t f =TV

VT +VR

T*V

Tidal Prism (see next slide)

Salinity of Mixed Water

Freshwater VolumeIn Mixed Scenario

Residence Time Def.


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tidal prismhigh tide

low tide

Tidal Prism

This is something we can reasonably measure


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Tidal Prism (cont’d)

Note: the above assumes perfect replacement – i.e., none of the water removed from the estuary during ebb returns during the next flood, and vice versa

Source: www.soc.soton.ac.uk/soes/teaching/courses/ oa217/€

⇒ t f = V

estuary

Vprism

×T

hAVestuaryprism


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Flushing Time Method 2: Knudsen Formula Estimate

• Low mean salinity => long freshwater residence time• High mean salinity => short freshwater residence time

Vtop, Stop VR

S=0

Model

Vbot, Sbot

€

t f =f *V

R=

1−Stop

Sbot

⎛

⎝ ⎜

⎞

⎠ ⎟

RV


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Flushing Time Summary

Choice depends on available data and estuary type

Matthias Tomczak, Shelf and Coastal Zone Lec. Notes

A 4th option: Numerical Modeling with FVCOM!!


37Source:

0 100 200 300 400 Residence Time (days)

Corpus Christi BayAransas BaySan Antonio BayMalagorda BayBrazos RiverGalveston BaySabine LakeCalcasieu LakeAtchafalaya-Vermillion BaysTerrebonne/Timbalier BaysBarataria BayMississippi RiverBreton-Chandeleur SoundsLake PontchartrainLake BoerneMississippi SoundMobile BayPerdido BayPensacola BayChoctawhatchee BaySt. Andrew BayApalachicola BaySuwannee RiverTampa BaySarasota BayCaloosahatchee RiverCharlotte Harbor

Flushing times for Gulf of Mexico estuaries, NOAA data, calculated using the freshwater fraction method

Flushing Time Estimates


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Flushing TimeExample: Boston Harbor

Source: http://data.ecology.su.se/MNODE/North%20America/bhbud.htm


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Flushing Time (cont’d)Example: Boston Harbor (cont’d)

Source: http://data.ecology.su.se/MNODE/North%20America/bhbud.htm

Area of Boston Harbor: 100 km2

Average Depth: 5.5 mAverage Tidal Range: 2.7 mTotal Freshwater Input: 40 m3s-1

Average Salinity: 29.5-31.5 PSU

Tidal Prism =Tidal Exchange =

108 m2 x 2.7 m = 2.7 x 108 m3

2.7 x 108 m3 / 12 hrs = 6250 m3s-1


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Boston Harbor FlushingTime: Tidal Prism Method




€

⇒ T = V

estuary

Vprism

×Ttidal

= 108m × 5.5 m

108m × 2.7 m×12 hrs ≈1 Day


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Assume Boston Harbor Salinity = 31.0 PSU Assume Mass. Bay Salinity = 31.5 PSU

€

⇒ T =

1-S

estuary

Ssea

⎛

⎝ ⎜

⎞

⎠ ⎟ Vestuary

Qriver

= 1-

31.0

31.5

⎛

⎝ ⎜

⎞

⎠ ⎟ 5.5x108m3

40 m3s-1≈ 2 days

Boston Harbor Flushing Time: Freshwater Fraction Method


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New Bedford HarborExample: Effects of Hurricane Barrier

Source: Abdelrhman, M. A., 2002. Estuaries V25(2) pp177-196

Flow patterns for speeds </= 0.1 m s-1 during peak spring currents: (a) flood tide (hour 96) without barrier, (b) flood tide (hour 96) with barrier, (c) ebb tide (hour 90) without barrier, and (d) ebb tide (hour 90) with barrier)


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Normalized average concentration of tracer versus time after beginning of flushing for: (a) freshwater distribution and (b) uniformly distributed tracer. The time required for normalized concentration to reach 1/e times its initial value give the average residence time.


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Average residence times (h) with and w/o hurricane barrier.


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Natural Changes to Flushing:Pleasant Bay

Patriots Day Storm: 2007

Nor’Easter1987


46U. Washington Ocean 200


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Materials and Other Courses

K. Dyer, Estuaries: A Physical Introduction (Wiley)

MAR615: Dynamics of Estuarine Circulation – Dan MacDonald

Books

SMAST Courses

MAR620: Case Studies in Estuarine Dynamics – Sundermeyer and Howes


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Key Concepts:1. Definition

2. Importance

3. Basic Circulation

4. Empirical Classification

5. Mixing Rates

6. Residence Time

7. Flushing Time

cowles mar 555 fall, 2009 1 week 14: estuaries introductory physical oceanography (mar 555) - fall...

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