Cowles MAR 555 Fall, Week 14: Estuaries Introductory Physical Oceanography (MAR 555) - Fall 2009 G. Cowles From M. Sundemeyer MAR620 Notes

Cowles MAR 555 Fall, 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, 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).

Cowles MAR 555 Fall, 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

Cowles MAR 555 Fall, Types of Estuaries (by formation) Drowned- river valley Fjord Bar-built Tectonic Source: (Chesapeake) (Pleasant Bay) (San Fran Bay) (Nassau)

Cowles MAR 555 Fall, Ideal Estuary Cross Section Source: Density Discontinuity Seaward Flow of Fresh Water Landward Flow of Salty Water No Tides, No Wind, No Waves, No Mixing at the Interface Ideal:

Cowles MAR 555 Fall, More Realistic Cross Section Source: Mixing at the Interface leads to Entrainment of dense salty water from bottom layer into fresh top layer leading to smoothing of the interface Realistic:

Cowles MAR 555 Fall, Estuary Schematic Tidal Forcing Mixing: Driven by Tides and Turbulence along The 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 influence mixing but typically is fetch limited

Cowles MAR 555 Fall, Unsteady Circulation with Tides U. Washington Ocean 200 Flood Ebb

Cowles MAR 555 Fall, Net Circulation U. Washington Ocean 200 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)

Cowles MAR 555 Fall, Classification of Estuaries Source: 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 V The volume of water entering the estuary during the flood tide – this is a measure of mixing We will classify estuaries based on R/V

Cowles MAR 555 Fall, Classification of Estuaries Source: Salt-wedge Partially mixed Well-mixed R/V > < R/V < 1 R/V <.005

Cowles MAR 555 Fall, Salt-Wedge Estuaries Source:

Cowles MAR 555 Fall, Salt-Wedge Estuaries U. Washington Ocean 200

Cowles MAR 555 Fall, Salt-Wedge Estuaries Special Case: Fjords U. Washington Ocean 200 Sill Blocks Deep Water Return Flow Isohalines (and Isopycnals) are nearly horizontal

Cowles MAR 555 Fall, Partially-Mixed Estuaries Source:

Cowles MAR 555 Fall, Partially-Mixed Estuaries U. Washington Ocean 200 Rough Balance between Freshwater forcing and mixing Halocline weaker than in a salt wedge Mixing and entrainment are stronger

Cowles MAR 555 Fall, Well-Mixed Estuaries U. Washington Ocean 200 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)

Cowles MAR 555 Fall, Mixing: Internally-Generated Turbulence U. Washington Ocean 200 Strength of Mixing Along the halocline depends on gradient Richardson number: For Ri >.25 Mixing Suppressed, Principally generated through Instabilities known as Holmhoe Waves Breaking Leads to Entrainment w e

Cowles MAR 555 Fall, Mixing: Internally-Generated Turbulence Kelvin-Helmholtz Instability U. Washington Ocean 200 Light Fluid Heavy Fluid

Cowles MAR 555 Fall, Mixing: Boundary-Generated Turbulence U. Washington Ocean 200 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 Z o : roughness length (related to physical roughness (substrate grainsize) τ : shear stress on the bottom

Cowles MAR 555 Fall, Partially Mixed Estuary: Internal and Bottom Mixing Interact Highly Stratified Estuary: Internal and Bottom Mixing Separate K. Dyer, Estuaries, a Physical Introduction Mixing: Combined

Cowles MAR 555 Fall, 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

Cowles MAR 555 Fall, advection only

Cowles MAR 555 Fall, diffusion only

Cowles MAR 555 Fall, advection and diffusion

Cowles MAR 555 Fall, Time Scale: Residence Time 1)Average amount of time a particle has spent in an estuary 2)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

Cowles MAR 555 Fall, Residence Time Calculations Tejo Estuary, Portugal Residence Times (days) (following particles with a numerical model) Willapa Bay Numerical Models: Track time of Neutrally Buoyant Particles in Estuary Note: Spatial Dependency Source: unknown? Banas and Hickey, JGR 2005 Res Time in Days

Cowles MAR 555 Fall, 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 water completely 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

Cowles MAR 555 Fall, Flushing Time Calculation Time required to replace the freshwater volume V F of an estuary at the net rate of flow given by the river discharge R Freshwater Fraction with S o the ocean Salinity Total freshwater: Requires knowing S(x,y,z), R, V

Cowles MAR 555 Fall, Simplification: Perfect Mixing – The Tidal Prism Method Model: VTVT S=S o VRVR S=0 Volume V T of ocean water enters estuary as does R*T of fresh water where T is tidal period V T+ V R S=S * At Flood, Perfect Mixing of V T+ V R occurs with S=S *. This flows out of the estuary during ebb. Flood tide Ebb tide Salt Balance Equation:

Cowles MAR 555 Fall, Simplification: Perfect Mixing T*V Tidal Prism (see next slide) Salinity of Mixed Water Freshwater Volume In Mixed Scenario Residence Time Def.

Cowles MAR 555 Fall, tidal prism high tide low tide Tidal Prism This is something we can reasonably measure

Cowles MAR 555 Fall, 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: oa217/

Cowles MAR 555 Fall, Flushing Time Method 2: Knudsen Formula Estimate Low mean salinity => long freshwater residence time High mean salinity => short freshwater residence time V top, S top VRVR S=0 Model V bot, S bot

Cowles MAR 555 Fall, Flushing Time Summary Choice depends on available data and estuary type Matthias Tomczak, Shelf and Coastal Zone Lec. Notes A 4 th option: Numerical Modeling with FVCOM!!

Cowles MAR 555 Fall, Source: Residence Time (days) Corpus Christi Bay Aransas Bay San Antonio Bay Malagorda Bay Brazos River Galveston Bay Sabine Lake Calcasieu Lake Atchafalaya-Vermillion Bays Terrebonne/Timbalier Bays Barataria Bay Mississippi River Breton-Chandeleur Sounds Lake Pontchartrain Lake Boerne Mississippi Sound Mobile Bay Perdido Bay Pensacola Bay Choctawhatchee Bay St. Andrew Bay Apalachicola Bay Suwannee River Tampa Bay Sarasota Bay Caloosahatchee River Charlotte Harbor Flushing times for Gulf of Mexico estuaries, NOAA data, calculated using the freshwater fraction method Flushing Time Estimates

Cowles MAR 555 Fall, Flushing Time Example: Boston Harbor Source:

Cowles MAR 555 Fall, Flushing Time (cont’d) Example: Boston Harbor (cont’d) Source: Area of Boston Harbor:100 km 2 Average Depth:5.5 m Average Tidal Range:2.7 m Total Freshwater Input: 40 m 3 s -1 Average Salinity: PSU Tidal Prism = Tidal Exchange = 10 8 m 2 x 2.7 m = 2.7 x 10 8 m x 10 8 m 3 / 12 hrs = 6250 m 3 s -1

Cowles MAR 555 Fall, Boston Harbor FlushingTime: Tidal Prism Method Area of Boston Harbor:100 km 2 Average Depth:5.5 m Average Tidal Range:2.7 m Total Freshwater Input: 40 m 3 s -1 Average Salinity: PSU

Cowles MAR 555 Fall, Area of Boston Harbor:100 km 2 Average Depth:5.5 m Average Tidal Range:2.7 m Total Freshwater Input: 40 m 3 s -1 Average Salinity: PSU Assume Boston Harbor Salinity = 31.0 PSU Assume Mass. Bay Salinity = 31.5 PSU Boston Harbor Flushing Time: Freshwater Fraction Method

Cowles MAR 555 Fall, New Bedford Harbor Example: Effects of Hurricane Barrier Source: Abdelrhman, M. A., Estuaries V25(2) pp 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)

Cowles MAR 555 Fall, New Bedford Harbor Example: Effects of Hurricane Barrier Source: Abdelrhman, M. A., Estuaries V25(2) pp 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.

Cowles MAR 555 Fall, New Bedford Harbor Example: Effects of Hurricane Barrier Source: Abdelrhman, M. A., Estuaries V25(2) pp Average residence times (h) with and w/o hurricane barrier.

Cowles MAR 555 Fall, Natural Changes to Flushing: Pleasant Bay Patriots Day Storm: 2007 Nor’Easter 1987

Cowles MAR 555 Fall, U. Washington Ocean 200

Cowles MAR 555 Fall, 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

Cowles MAR 555 Fall, Key Concepts: 1.Definition 2.Importance 3.Basic Circulation 4.Empirical Classification 5.Mixing Rates 6.Residence Time 7.Flushing Time