Week 5 work

Definition and Etymology of Estuaries

• Etymological Origins:

  • The word "estuary" originates from the Latin word aestus, which means heat, boiling, or tide.

  • The related adjective asetuarium translates specifically to "tidal."

• Formal Definitions:

  • Cameron & Pritchard (1963): Defined an estuary as a semi-enclosed coastal body of water with free communication to the ocean, within which ocean water is diluted by freshwater derived from land.

  • Day, Crump, Kemp & Yáñez-Arancibia (2013): Defined it as the portion of the earth's coastal zone where there is interaction of ocean water, fresh water, land, and atmosphere.

Significance and Global Importance

• Ecological Productivity: Estuaries are among the most productive ecosystems in the world (McLusky & Elliot, 2004).

• Property Exchange: The unique dynamics of estuaries control the exchange of properties between continents and the ocean, including:

  • Sediments.

  • Nutrients.

  • Pollutants.

• Biodiversity: They provide diverse habitats due to the range of salinities found within them, from freshwater to seawater levels.

• Human Intersection and Urbanization:

  • Of the 32 largest cities in the world, 22 are located beside estuaries.

  • They serve as hubs for shipping, transport, recreation, and tourism.

  • They are critical habitats and nurseries for both ecologically and commercially valuable species.

• Industrial and Human Impact:

  • Estuaries often receive intentional and unintentional discharge of effluent (sewerage), industrial waste, and storm water.

Environmental Threats and Climate Change

• Direct Human-Induced Changes:

  • Loss of shallow water habitats and marshlands.

  • Pollution from industrial and urban runoff.

  • Decline of native species and the introduction of invasive species.

• Climate Change Impacts:

  • Sea Level Change: Altering the boundaries and depths of estuarine systems.

  • Storm Regimes: Heightened intensity or frequency of storm events.

  • Runoff Timing: Changes in the timing and volume of freshwater delivery.

  • Oceanic Conditions: Changes such as hypoxia, the timing and strength of upwelling, and ocean acidification.

  • Sediment Delivery: Alterations in how sediment is delivered to coastal zones.

Geological Perspective and Formation

• Timeline: Present-day estuaries generally did not exist between 10,000 and 15,000 years ago.

• Formation Process: Current estuaries formed when sea levels rose following the last glaciation period. Since their formation, they have been progressively infilling with sediment.

• Future Evolution: The future location and existence of estuaries will be determined by the ongoing combination of sediment infill and sea level changes.

Geomorphic Classifications of Estuaries

• Coastal Plain (Drowned River Valleys):

  • Result from an increase in sea level that floods existing river valleys.

  • Characteristics: Typically wide (measured in kilometers) and shallow ($~10\,m$).

  • Example: Chesapeake Bay (USA), which has a maximum depth of approximately $30\,m$.

• Fjords:

  • Result from intense glacial activity at high latitudes.

  • Characteristics: Elongated deep channels featuring a sill or moraine at the mouth.

• Bar-Built Estuaries:

  • Embayments that become semi-enclosed by the formation of a sand bar or spit.

  • Example: "Lake" Macquarie, NSW.

• Tectonic Estuaries:

  • Formed by earthquakes or fractures in the Earth's crust.

  • Faulting causes a part of the crust to sink, forming a hollow basin that is subsequently flooded.

• Case Study: Derwent Estuary:

  • The Derwent Estuary is classified geomorphologically as a drowned river valley.

Basic Estuarine Dynamics and Density Gradients

• Vertical Stratification and Density:

  • Ocean Water: Salty and/or cool, making it "heavy" (high density).

  • River Water: Fresh and/or warm, making it "light" (low density).

  • This leads to vertical stratification and horizontal density gradients.

• Mixing Behaviors:

  • When fresh and salty water meet, a gravity current often forms.

  • In an unstable state with a strong horizontal salinity gradient (e.g., $S=35\,g/kg$ vs $S=0\,g/kg$), the fluid reorganizes itself.

  • Stable configuration: Saltiest fluid moves to the bottom, and freshest fluid stays at the top.

• Tidally-Driven Mixing:

  • Tides generate mixing through bottom friction.

  • The classification of an estuary as well-mixed, partially mixed, strongly stratified, or a salt-wedge depends on the balance between the strength of tidal mixing and the strength of river flow.

Vertical Salinity Structure and Hydrodynamics

• Classification Categories:

  • Salt-Wedge: Dominated by river flow; minimal tidal mixing.

  • Strongly Stratified: Strong river flow with moderate tidal influence.

  • Partially Mixed: Balanced river flow and tidal mixing.

  • Well-Mixed: Dominated by tidal mixing; river flow is relatively weak.

• Hydrodynamic Parameters (Geyer & MacCready 2014):

  • Froude Number ($Fr_f$): Represents the influence of river flow, defined as $Fr_f = \frac{U_r}{(\beta g S_o H)^{1/2}}$.

  • Mixing Parameter ($M^2$): Represents the effectiveness of tidal mixing, defined as $M^2 = \frac{C_D U_t^2}{\omega N_0 H^2}$.

• Examples of Systems by Hydrodynamic Classification:

  • Salt Wedge: Ebro River, Mississippi River, Amazon River, Fraser River.

  • Strongly Stratified: Chang Jiang River, Merrimack River, Columbia River, Hudson River.

  • Partially Mixed: Conwy River, San Francisco Bay, Tamar River, Chesapeake Bay, James River.

  • Fjord/Deep Systems: Baltic Sea, Puget Sound, Long Island Sound.

  • Well-Mixed: Narragansett Bay, Willapa Bay, SIPS (South Indian River Lagoon).

• Derwent Estuary Specifics:

  • Classified as "micro-tidal" (tidal sea surface height variation is less than $1\,m$).

  • Operates as a salt-wedge estuary.

  • It is highly stratified in its narrow upper reaches and becomes well-mixed in its lower reaches.

Estuarine Currents and Residual Circulation

• Current Components: Estuarine currents consist of Tidal currents + Residual circulation.

• Velocity Differences: Tidal currents are typically much larger than mean/residual currents (e.g., in the Derwent, tidal currents are $\sim 0.5\,m/s$ vs residual currents of $\sim 0.1\,m/s$).

• Residual Circulation: This refers to the flow remaining after tides have been averaged out.

• The Navier-Stokes Equation: Describes fluid motion through several terms:

  • Acceleration.

  • Rotation.

  • Pressure gradient.

  • Buoyancy.

  • Other forces (wind, friction, tides, etc.).

• Simplified Flow Model: Assuming steady state and hydrostatic conditions while ignoring rotation and spatial changes in y- and z-directions.

• Pressure Gradient Decomposition:

  • Barotropic: Derived from the sea surface set-up (the river end is higher because water is being added there).

  • Baroclinic: Derived from the horizontal density (salinity) gradient.

• Balance: The pressure gradient is balanced by the frictional term (drag on the bottom). Turbulent mixing driven by tidal currents rubbing against the bottom boundary smooths the exchange flow curves.

Salt Balance and the Knudsen Relationship

• Principle: Using volume conservation and salt balance, we can determine the magnitude of the exchange flow.

• Flow Mechanics:

  • The estuary is generally saltier from land (Left) to ocean (Right).

  • For salinity to remain constant, the outflow ($Q_{out}$) must be saltier than the river input ($Q_R$).

  • Consequently, Q_{out} > Q_R.

  • To maintain water volume conservation, an inflow ($Q_{in}$) is required.

• Inference: The net circulation of an estuary is significantly larger than the volume of the river input alone.

• Derwent Data (Herzfeld et al., 2005a):

  • Mapping indicates surface and bottom residual circulation over a 14-day mean flow.

  • Locations include Bridgewater Bridge.

  • Coordinates: $42^\circ 45'S$ to $43^\circ S$ and $147^\circ 10'E$ to $147^\circ 25'E$.

Estuarine Ecology and Productivity Drivers

• Variable Habitat Factors: Organisms must adapt to varying waves, currents, turbulence, turbidity, oxygen levels, sediment types, temperature, and salinity.

• Organic Matter Sources: Productivity is supported by detritus and primary producers, including:

  • Marsh grass.

  • Sea grass.

  • Mangroves.

  • Benthic algae.

  • Macroalgae.

  • Phytoplankton.

• Nutrient Efficiency: Estuaries feature abundant nutrient supplies and rapid regeneration/conservation of nutrients due to the activity of microorganisms, filter feeders, and shallow water depths.

• Light Usage: The diversity of producers ensures maximum use of light across all seasons.