CMS-Flow:Hydro Eqs

Continuity and Momentum Equations

On the basis of the definitions Variable Definitions, and assuming depth-uniform cur-rents, the general depth-integrated and wave-averaged continuity and momentum equations may be written as (Phillips 1977; Mei 1983; Svendsen 2006)

${\frac {\partial h}{\partial t}}+{\frac {\partial (hV_{j})}{\partial x_{j}}}=S_{M}$

(1)

${\frac {\partial (hV_{i})}{\partial t}}+{\frac {\partial (hV_{j}V_{i})}{\partial x_{j}}}-\varepsilon _{ij}f_{c}hV_{j}=-gh{\frac {\partial {\bar {\eta }}}{\partial x_{i}}}-{\frac {h}{\rho }}{\frac {\partial p_{atm}}{\partial x_{i}}}+{\frac {\partial }{\partial x_{j}}}{\left(v_{t}h{\frac {\partial V_{i}}{\partial x_{j}}}\right)}-{\frac {1}{\rho }}{\frac {\partial }{\partial x_{j}}}\left(S_{ij}+R_{ij}-\rho hU_{wi}U_{wj}\right)+{\frac {\tau _{si}}{\rho }}-m_{b}{\frac {\tau _{bi}}{\rho }}$

(2)

where

$t$ = time[s]

$x_{j}$ = Cartesian coordinate in the $j^{tj}$ direction [m]

math>f_c</math> = Coriolis parameter [rad/s] $=2\Omega sin\phi$ where $\Omega =7.29x10^{-5}$ rad/s is the earth’s angular velocity of rotation and $\phi$ is the latitude in degrees

$h$ = wave-averaged total water depth $h=\zeta +{\bar {\eta }}$ [m]

${\bar {\eta }}=$ wave-averaged water surface elevation with respect to reference datum [m]

$S_{M}=$ water source/sink term due to precipitation, evaporation and structures (e.g. culverts) [m/s]

$V_{i}=$ total flux velocity defined as $V_{i}=U_{i}+U_{wi}$ [m/s]

$U_{i}=$ wave- and depth-averaged current velocity [m/s]

$U_{wi}=$ mean wave mass flux velocity or wave flux velocity for short [m/s]

$g=$ gravitational constant (~9.81 m/s²)

$p_{atm}=$ atmospheric pressure [Pa]

$\rho =$ water density (~1025 kg/m³)

$v_{t}=$ turbulent eddy viscosity [m²/s]

$\tau _{si}=$ wind surface stress [Pa]

$S_{ij}=$ wave radiation stress [Pa]

$R_{ij}=$ surface roller stress [Pa]

$m_{b}=$ bed slope coefficient [-]

$\tau _{bi}=$ combined wave and current mean bed shear stress [Pa]

The above 2DH equations are similar to those derived by Svendsen (2006), except for the inclusion of the water source/sink term in the continuity equation and the atmospheric pressure and surface roller terms and the bed slope coefficient in the momentum equation. It’s also noted that the horizontal mixing term is formulated slightly differently as a function of the total flux velocity, similar to the Generalized Lagrangian Mean (GLM) approach (Andrews and McIntyre 1978; Walstra et al. 2000). This approach is arguably more physically meaningful and also simplifies the discretization in the case where the total flux velocity is used as the model prognostic variable.

References

• Andrews, D. G., and M. E. McIntyre. 1978. An exact theory of nonlinear waves on a Lagrangian mean flow. Journal of Fluid Mechanics (89):609–646.
• Phillips, O. M. 1977. Dynamics of the upper ocean, Cambridge University Press.
• Svendsen, I. A. 2006. Introduction to nearshore hydrodynamics, Advanced Series on Ocean Engineering, 124, World Scientific Publishing, 722 p.
• Walstra, D. J. R., J. A. Roelvink, and J. Groeneweg. 2000. Calculation of wave-driven currents in a 3D mean flow model. In Proceedings, 27th International Conference on Coastal Engineering, 1050-1063. Sydney, Australia.

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