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Summary of previous lessons. Estuarine Dynamics

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1 Summary of previous lessons. Estuarine Dynamics

2 Equations format for Env. Modelling
Isolated System: Material System Moving with the flow (lagrangian formulation): Fixe point (Eulerian formulation):

3 Lotka Volterra
This model describes the dynamics, but it is quantitatively incorrect.

4 Results: no interannual variability

5 Source and Sink terms. The Phyto Eq.



8 Why to study an estuary in the course?
Estuaries are among the richest ecosystems, Estuarine ecosystems require knowledge of a large number of disciplines, They are a adequate to introduce a general ecosystem approach that can be simplified for particular ecosystems.

9 Estuaries vs Coastal Lagoons
Fine (organic rich sediments settle in the upper estuary due to flocculation induced by salinity

10 Estuary Definition
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea.[1] In the Water Framework Directive: An estuary is a “Transition Water” ) from a fresh water body into a coastal water body.

11 Estuarine Life Estuaries are home to an astonishing variety of plants and animals and important in the lifecycles of many more. Salt Marshes importance Salt marshes form in shallow, quiet water, where the water is salty and still enough for the suspended particles to settle to the bottom. These marshes are some of the most productive lands in the world and produce so many organic nutrients that their influence can be seen far out into the coastal ocean waters. Marshes are based on one plant, the Saltmarsh Cordgrass, which helps stabilize the environment so that many other plants and animals to move in.

12 Why are Estuaries so much demanded by people?
Because they generate opportunities: Goods, Navigation, Recreation, …….. The estuarine ecosystem is biologically rich because estuaries are rich in nutrients/primary producers. They provide food and protection to many species of juveniles (they are nursery zones). What about eutrophication?

13 Classification of Estuaries
Coastal Plain Estuaries or Drowned river valleys, Tectonic Estuaries, Bar-built Estuaries or Lagoon type, Fjords.

14 Coastal Plain Estuaries
1. Coastal Plain Estuaries are formed by the sea level rising and filling an existing river valley. Examples of this are the Chesapeake Bay in Maryland and the harbor in Charleston, South Carolina

15 Tectonic Estuaries 2. Tectonic Estuaries are caused by the folding or faulting of land surfaces. These estuaries are found along major fault lines, like the San Francisco Bay area in California.

16 Bar-built Estuaries (Rias/Lagoas em Português)
Bar-built Estuaries form when a shallow lagoon or bay is protected from the ocean by a sand bar or barrier island. Examples of these are found along the Eastern Seaboard and the Gulf Coast of North America.

17 Fjords Fjords are U-shaped valleys formed by glacial action. Fjords are found in areas with long histories of glacier activity, like northern Europe, Alaska and Canada.

18 Are this the only types of estuaries?
Most of today's estuaries formed because the sea level has slowly risen during the last 18,000 years, drowning river valleys and filling in glacial troughs. This classification is relevant yes because it helps us describing estuaries. It is not because many estuaries do not fit in any of these types….

19 Estuarine Hydrodynamics: Tides
Tides are the periodic rise and fall of the ocean waters. They are caused by the gravitational pulls of the Moon and (to a lesser extent) Sun, as well as the rotation of the Earth.


21 Classification based on water circulation

22 Salt wedge In this type of estuary, river output greatly exceeds marine input and tidal effects have a minor importance. Fresh water floats on top of the seawater in a layer that gradually thins as it moves seaward. The denser seawater moves landward along the bottom of the estuary, forming a wedge-shaped layer that is thinner as it approaches land. As a velocity difference develops between the two layers, shear forces generate internal waves at the interface, mixing the seawater upward with the freshwater. An example of a salt wedge estuary is the Mississippi River.[5]

23 Partially mixed estuary
As tidal forcing increases, river output becomes less than the marine input. Here, current induced turbulence causes mixing of the whole water column such that salinity varies more longitudinally rather than vertically, leading to a moderately stratified condition. Examples include the Chesapeake Bay and Narragansett Bay.[5]

24 Vertically homogenous
Tidal mixing forces exceed river output, resulting in a well mixed water column and the disappearance of the vertical salinity gradient. The freshwater-seawater boundary is eliminated due to the intense turbulent mixing and eddy effects. The lower reaches of the Delaware Bay and the Raritan River in New Jersey are examples of vertically homogenous estuaries.[5]

25 Fjord

26 Inverse estuary Inverse estuaries occur in dry climates where evaporation greatly exceeds the inflow of fresh water. A salinity maximum zone is formed, and both riverine and oceanic water flow close to the surface towards this zone.[6] This water is pushed downward and spreads along the bottom in both the seaward and landward direction.[3] An example of an inverse estuary is Spencer Gulf, South Australia

27 Estuary Flow properties
Residual velocity Residence Time Tidal prism Cohesive sediment dynamics

28 Intermittent Estuary Estuary type varies dramatically depending on freshwater input, and is capable of changing from a wholly marine embayment to any of the other estuary types.[7][8] (See also Estuarine water circulation)

29 Estuarine hydrodynamics
Shallow water equations: Hydrostatic pressure, Vertical velocity is much smaller than horizontal velocity (coriolis), Boussinesq approximation (density)

30 2D Depth Integrated Hydrodynamic Model

31 Shallow Water Equations

32 Pressure forces Hydrostatic pressure: Pressure force
Free surface elevation

33 MOHID Bathymetry Module
Defines the horizontal grid The grid can be uniform or variable The number of grid points is typically 200x200


35 Vertical grid (3D models)
Sigma coordinate Cartesian coordinate

36 Mixed coordinate system

37 The 2D model Is equivalent to a 3D model using just one layer and sigma coordinate!

38 Boundary Conditions Open boundary Closed boundary Moving boundary

39 MOHID main Modules Hydrodynamic Waterproperties (Eulerian) Lagrangian
Propriedades de escoamento (niveís, velocidades, fluxos) Waterproperties (Eulerian) Propriedades de águas (temperaturua, salinidade, phytoplankton, etc) Abordagem euleriana Lagrangian Propriedades de águas (temperatura, salinidade, phytoplankton, etc) Abordagem lagrangiana WaterQuality (Sources – Sinks) Processos de qualidade de água (fontes e poços) Funciona como um modelo 0D

40 Mohid – Input Data File Ficheiros ASCII Palavras Chaves
Organização por blocos

41 2D Hydrodynamic model parameters
Physical parameters: Bottom and surface friction coefficients, Horizontal diffusivity (viscosity).

42 Bottom shear stress α

43 Velocity Definition Cx Cx+∆x The figure represents molecules of two fluids. The velocity across the dashed line measures the volume of molecules crossing the surface per unit of area. If the velocity is null the volume crossing in one sense is equal to the volume crossing on the opposite sense.

44 Diffusion However even if there is no velocity, there is molecules exchange and consequently there is mixing. This is called diffusion. If the molecules on one side of the surface are statistically different from the molecules on the other side (different concentration) there will be a mass fluxes of individual constituents. If molecules move due to Brownian movement diffusion is called molecular diffusion. If they move due to turbulence (eddies) it will be called turbulent diffusion. Turbulence is defined as the movements with space and time scales to short to be described directly.

45 Difusive flux per unit of area
Cx Cx+∆x Along direction “x”:

46 Horizontal diffusivity in a hydrodynamic model
Hydrodynamic models consider velocity as being uniform along the all grid cell. In fact it is not and consequently there is a velocity part not resolved by the advective transport. That part has to be accounted by diffusion. Diffusivity is proportional to the grid size and to the non-resolved velocity.

47 Initial and Boundary conditions
The system exchanges material with the neighbouring areas and consequently after some time interior values do not depend on the initial conditions. Initial conditions can be any. The warming time decreases with its realism. In case of momentum there is dissipation and thus the warming time is even shorter. Boundary conditions have to be known, unless transformation processes have the time to modify the properties between the boundary and the region we want to study.

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