Wind Forcing¶
Wind friction on the water surface can be accounted for by inclusion of the wind shear stress in the momentum equation.
The wind shear stress \(\tau_w\) is expressed as:
Equation: Wind shear stress at water surface from atmospheric conditions and wind speed.
where,
- \(T_{fac}\) — topographical factor
- \(C_{w}\) — wind friction factor, default value 0.0025
- \(\rho_{a}\) — density of the air, using value of 1.3
- \(V_{10}\) — wind velocity 10 m above the surface water
Taking the wind direction into account, the wind force is projected into the direction of the channel and included as a forcing in the momentum equation (Q-points) on reaches with open cross sections.
The wind direction and velocity can be specified as constant or time series. It is assumed that the wind direction is specified in the same coordinate system as the network.
The topographical factor is used to reduce the impact of the wind at locations that are sheltered from the wind, having a value of 1.0 for open areas, and 0.0 where the reach is completely sheltered from the wind. It can be defined globally or locally for each reach in the network.
The surface area to which the wind stress applies excludes any additional flooded areas and is equal to the width of the reach multiplied by the distance between h-points on either side of the Q-point
The topographical factor is specified in the Hydrodynamic editor under the “Wind” tab. The wind friction factor can not be specified, but are always given as the default value of 0.0025. The friction factor in the wind tab in the Hydrodynamic editor is the friction factor between stratified layers in the water column, and not for the air-water interface. The wind direction and speed are specified in the boundary editor as global and/or distributed boundaries, as either constants or time series.