CIRCULATION and TRANSPORT MODELS in the coastal area of AGIA PELAGIA - MILATOS

Evangelos Pafilis

2D Wind Coastal Circulation Model

2D Advective Pollutant Transport Model

This report describes the practice that took place in July ’99 at I.M.B.C. The main objective was to study and to predict the behavior of the coastal area near Heraklion, from Agia Pelagia to Milatos. This was achieved with the development of two computer programs. At first these computer programs calculate the sea circulation, based on the bathymetry of the area and the wind stress, and after they compute the transport of a certain pollutant in the same region. The final results were saved on files easily accessed and animated providing infos in a simple and understandable manner.

Numerical Models are predictive scientific tools that use data taken right away from the observation field. In order to make the prediction a great number of parameters is taken under consideration. These parameters are expressed through, usually complex, mathematical equations. Nowadays by the use of computers a greater size of calculations is possible, providing this way even more accurate results and setting aside the traditional methods such as laboratory models.

Coastal Domain is defined as the area that extends from the coastline to the continental slope. So it comprises of marine regions where human interference is extensive.

Coastal circulation is defined as the development of generally non-steady velocity and surface elevation field’s in a coastal zone. This means that for each point of the area being studied, a vector for the velocity and a number for the elevation of the surface are computed. This number can either be positive or negative depending on whether the surface rises or lowers.

As input it uses the data obtained from the circulation model and processes them through mathematical equations that describe the phenomena of advection and diffusion of pollutants. The output is the concentration of the pollutant at each point after a certain period of time.

The two models in total provide what is so called Coastal Zone Management (CZM), providing valuable data for scientists, engineers and decision-makers. They have been used in the past by IMBC — MDNS (Institute of Marine Biology Crete — Marine Dynamics and Numerical Simulations) in a great number of projects with the most recent:

1. Coastal Engineering studies in the construction of the new Airport in Heraklion
2. Oil spill’s environmental impact prediction in Gialova’s lagoon
3. Prediction of transport and dispersion of a sewage pipeline outflow in Ierapetra

The project took place in July ’99 at IMBC — MDNS. The hardware used was an SGI O₂ system linked to an ORIGIN 200 server. As programming language FORTRAN 77 was chosen and GrADS was the animation package.

The bathymetry data were retrieved from maps provided by the Geographical Service of Defense. The domain of Agia Pelagia — Milatos was divided in squares with grid dx = 463 m, each one with its own depth.

Creating the code was the next step. The Circulation model accepts as inputs:

1. the depth of each square
2. the wind direction (user defined)
3. the wind velocity (user defined)

The process of data occurs according to the following mathematical equation:

where: h: depth, z : surface elevation, U: horizontal component of velocity, V: vertical component of velocity, : horizontal surface friction ( stands for density), : vertical surface friction, : horizontal bottom friction, : vertical bottom friction, : Coriolis coefficient, dt: time step (set to 2,2 sec for stability reasons).

The equation is solved using the finite differences in stagger grid method. The output is the horizontal and vertical velocity components as well as the surface elevation.

The transport model as input accepts U, V and z calculated in the circulation model. The user can define the following parameters: