Hydrodynamic and Thermal Dispersion Modelling of the Effluent in a Coastal Channel

Ahmad Sana, sana@squ.edu.om Sultan Qaboos University, Oman

Contents

• Motivation • Study area • Numerical model • Results and discussion • Conclusions

Motivation • Various types of industrial plants discharge effluents to the sea. Generally an outfall structure is designed to keep minimum possible impact on the ambient water quality. • In the present situation the industrial plant is located in the proximity of a natural wadi channel connected to the sea forming a lagoon-like water body. • The high discharge of effluent with higher than the ambient temperature may cause thermal pollution along-with erosion in the channel (Q=13,500m3/hr =3.75m3/s, T = 40oC).

Study Area

Numerical model

Delft3D-FLOW is the module of Delft3D suite that provides the hydrodynamic basis for water quality computations. The main purpose is the two-dimensional (2D, depth averaged) and three-dimensional (3D) simulation of tidal and wind-driven flow by solving the unsteady hydrodynamic equations.

Numerical model The main physical phenomena, which are accounted for in Delft3D-FLOW, are: • Coriolis force. • Turbulence model to account for the vertical

turbulent viscosity and diffusivity • Shear stress exerted by the turbulent flow on the

bottom based on a quadratic Chézy or Manning formula.

• Simulation of the thermal discharge intake (advective-diffusion module).

• Simulation of drying and flooding of inter-tidal flats (moving boundaries)

Computational Parameters ITEM

CONDITION

Simulation period August 7 to 21, 2009 Initial condition

Constant water temperature (25.6 oC)

Computational domain

Horizontal orthogonal curvilinear 206x21 with one layer (depth-averaged condition)

Boundary condition Predicted water level at open boundary (eastern boundary)

Meteorological data Uniform wind, air temperature, solar radiation and humidity from historical weather data

Bottom stress Colebrooke Equation

Model domain

Bathymetry

Tidal elevation at sea boundary

Tidal elevation close to outlet

Thermal dispersion

Bottom shear stress

Possibility of sediment movement According to Shields Criterion for uniform bed material sizes, the sediment movement will be initiated if

In the present case, τb<0.03 N/m2, ρs-ρ=1650kg/m3, g = 9.81m/s2, Dmin = 0.075mm

No sediment movement will occur

Conclusions • Detailed information regarding water level, temperature,

depth averaged velocity and bed shear stress is available within the model domain.

• During the period of prediction, the effect of effluent discharge on the seawater temperature remains confined within the lagoon area.

• Seawater quality by virtue of temperature meets the MECA requirements since the zone of influence of the effluent is confined within the lagoon area where the discharge point is located.

• Extremely low values of bed shear stress in the model domain suggest that the possibility of erosion and therefore accretion of eroded material is remote.

Thank You