Physical Processes - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 74

of evaporation also depends on partial pressure of the vapor in the atmosphere, the
surface temperature, and the intensity of turbulence in the atmospheric boundary layer.
Particulate matter can also be exchanged through atmospheric deposition. This
mechanism is the main source of particulate matter for the Mediterranean Sea, but
it is negligible in coastal lagoons and estuaries when compared with the particulate
matter transported by rivers and streams, due to the small dimension of the free
surface of these water bodies.
Rain is a source of water and has implications for salinity. In the ocean, rain is
the major source of salinity reduction in upper latitudes, and it can also be a source
of heat. In the case of coastal systems, the amount of water entering the system through
rainfall is negligible compared with the water discharged by the catchment areas.

3.5.3.2 Momentum Exchange

The exchange of momentum is a major mechanism generating the flow in coastal
lagoons, except in case of tidal systems. This flux depends on the atmospheric
intensity of turbulence (proportional to the square of the wind velocity) and on a
friction coefficient.
In the case of bottom shear stress, the friction coefficient is a function of
roughness. The difference is that along the bottom, roughness is a property of the
bottom (and relative roughness, a function of that and of water column height), and
at the free surface, roughness is also a function of wind speed, through wind waves,
where amplitude increases with wind speed.

3.5.3.3 Energy Exchange

Energy is exchanged with the atmosphere in the form of latent and sensible heat,
radiation, and mechanical energy. Part of the mechanical energy is the work of the
shear stress transferring momentum, and part is exchanged in the form of turbulent
kinetic energy.
Latent heat is lost through evaporation and is a direct function of the amount of
moisture exchanged with the atmosphere. Sensible heat is a function of the temper-
ature difference between the atmosphere and the water surface and of the turbulence
intensity in the atmosphere.
Heat exchange by radiation has features of both latent and sensible heat
exchange. The heat radiated by the water surface depends on water temperature, and
the heat received from the atmosphere depends on the season, time of day, cloud
cover, and atmospheric temperature.
Turbulent kinetic energy is the fraction of the mechanical energy lost by the atmo-
sphere and not converted into work by the water velocity. This fraction of energy is
accounted for by the turbulence models for computing vertical diffusivity and viscosity.

3.5.4 C OHESIVE AND N ONCOHESIVE S EDIMENT P ROCESSES

In coastal environments, solids can be found in dissolved, colloidal, or particulate
forms. The limits defining these forms are somewhat arbitrary. A substance is
considered as dissolved if it can pass through a 2-

µ m filter. Colloidal substances do