CMS-Flow:Boundary Conditions

Wall Boundary

The wall boundary condition is a closed boundary and is applied at any cell face between wet and dry cells. Any unassigned boundary cell at the edge of the model domain is assumed to be closed and is assigned a wall boundary. A zero normal flux to the boundary is applied at closed boundaries. Two boundary conditions are available for the tangential flow:

1. Free-slip: no tangential shear stress (wall friction)
2. Partial-slip: tangential shear stress (wall friction) calculated based on the log law.

Assuming a log law for a rough wall, the partial-slip tangential shear stress is given by

$\tau _{wall}=\rho c_{wall}U_{\parallel }^{2}$

(1)

where $U_{\parallel }$ is the magnitude of the wall parallel current velocity, and $c_{wall}$ is the wall friction coefficient equal to

$c_{wall}=\left[{\frac {\kappa }{ln(y_{P}/y_{0})}}\right]^{2}$

(2)

Here, $y_{0}$ is the roughness length of the wall and is assumed to be equal to that of the bed $(i.e,y_{0}=z_{0})$. The distance from the wall to the cell center is $y_{P}$.

Flux Boundary

The flux boundary condition is typically applied to the upstream end of a river or stream and specified as either a constant or time series of total water volume flux (Q). In a 2DH model, the total volume flux needs to be distributed across the boundary in order to estimate the depth-averaged velocities. This is done using a conveyance approach in which the current velocity is assumed to be related to the local flow depth (h) and Manning’s (n) as $U\propto h^{r}/n$. Here, r is an empirical conveyance coefficient equal to approximately 2/3 for uniform flow. The smaller the r value the more uniform the current velocities are across the flux boundary. The water volume flux $(q_{i})$ at each boundary cell (i) is calculated as

${\overrightarrow {q}}_{i}=h{\overrightarrow {U}}_{i}={\frac {f_{Ramp}Q}{{\bigg |}\sum \limits _{i}({\hat {e}}\cdot {\hat {n}}){\frac {h_{i}^{r+1}}{n_{i}}}\Delta l_{i}{\bigg |}}}{\frac {h_{i}^{r+1}}{n_{i}}}{\hat {e}}$

(3)

where:

i = subscript indicating a boundary cell

${\overrightarrow {q_{i}}}$ = volume discharge at boundary cell i per unit width [m²/s]

${\hat {e}}$ = unit vector for inflow direction = $(sin\ \varphi ,cos\ \varphi )$

$\varphi$ = inflow direction measured clockwise from North [deg]

${\hat {n}}$ = boundary face unit vector (positive outward)

Q = specified total volume flux across the boundary [m³/s]

n = Manning’s coefficient [s/m^1/3]

r = empirical constant equal to approximately 2/3

$\Delta l$ = cell width in the transverse direction normal to flow [m]

$f_{Ramp}$= ramp function [-] (described in Chapter 3).

The total volume flux is positive into the computational domain. Since it is not always possible to orient all flux boundaries to be normal to the inflow direction, the option is given to specify an inflow direction $(\varphi )$. The angle is specified in degrees clockwise from true North. If the angle is not specified, then the inflow angle is assumed to be normal to the boundary. The total volume flux is conserved independently of the inflow direction.

Water Level Boundary

The general formula for the boundary water surface elevation is given by

${\bar {n}}_{B}=f_{Ramp}({\bar {\eta }}_{E}+\Delta {\bar {\eta }}+{\bar {\eta }}_{G}+{\bar {\eta }}_{C})+(1-f_{Ramp}){\bar {\eta }}_{0}$

(4)

where:

${\bar {\eta }}_{B}$ = boundary water surface elevation [m]

${\bar {\eta }}_{E}$ = specified external boundary water surface elevation [m]

$\Delta {\bar {\eta }}$ = water surface elevation offset [m]

${\bar {\eta }}_{0}$ = initial boundary water surface elevation [m]

${\bar {\eta }}_{C}$ = correction to the boundary water surface elevation based on the wind and wave forcing [m]

${\bar {\eta }}_{G}$ = water surface elevation component derived from user specified gradients [m]

$f_{Ramp}$ = ramp function [-] (described in Chapter 3).

The external water surface elevation $({\bar {\eta }}_{E})$ may be specified as a time series, both spatially constant and varying or calculated from tidal/harmonic constituents. When a time series is specified, the values are interpolated using piecewise Lagrangian polynomials. By default, second order interpolation is used but can be changed by the user. If tidal constituents are specified, then ${\bar {\eta }}_{E}$ is calculated as

${\bar {\eta }}_{E}(t)=\sum f_{i}A_{i}\cos(\omega _{i}t+V_{i}^{0}+{\hat {u}}_{i}-\kappa _{i})$

(5)

where:

i = subscript indicating a tidal constituent

$A_{i}$ = mean amplitude [m]

$f_{i}$ = node (nodal) factor [-]

$\omega _{i}$ = frequency [deg/hr]

t = elapsed time from midnight of the starting year [hrs]

$V_{i}^{0}+{\hat {u}}_{i}$ = equilibrium phase [deg]

$\kappa _{i}$ = phase lag [deg].

The mean amplitude and phase may be specified by the user or interpolated from a tidal constituent database. The nodal factor is a time-varying correc-tion to the mean amplitude. The equilibrium phase has a uniform com-ponent $(V_{i}^{0})$ and a relatively smaller periodic component. The zero-superscript of $V_{i}^{0}$ indicates that the constituent phase is at time zero. Table 1 provides a list of tidal constituents currently supported in CMS. More information on US tidal constituent values can be obtained from the US National Oceanographic and Atmospheric Administration (NOAA) https://www.noaa.gov/ and National Ocean Service (NOS) https://oceanservice.noaa.gov/.

Table 1. Tidal constituents implemented in CMS. Constituent speeds are in degrees per mean solar hour.

Constituent	Speed	Constituent	Speed	Constituent	Speed	Constituent	Speed
SA	0.041067	SSA	0.082137	MM	0.54438	MSF	1.0159
MF	1.098	2Q1*	12.8543	Q1	13.3987	RHO1	13.4715
O1	13.943	M1*	14.4967	P1	14.9589	S1	15.0
K1	15.0411	J1	15.5854	OO1	16.1391	2N2	27.8954
MU2	27.9682	N2	28.4397	NU2	28.5126	M2	28.9841
LDA2	29.4556	L2	29.5285	T2	29.9589	S2	30
R2	30.0411	K2	30.0821	2SM2	31.0159	2MK3	42.9271
M3	43.4762	MK3	44.0252	MN4	57.4238	M4	57.9682
MS4	58.9841	S4	60.0	M6	86.9523	S6	90.0
M8	115.9364

If a harmonic boundary condition is applied, then the node factors are set to one and the equilibrium arguments are set to zero. The harmonic boundary condition is provided as an option for simulating idealized or hypothetical conditions.

The water surface elevation offset $(\Delta {\bar {\eta }})$ is assumed spatially and temporally constant and may be used to correct the boundary water surface elevation for vertical datums and sea level rise. The component ${\bar {\eta }}_{G}$ is intended to represent regional gradients in the water surface elevation, is assumed to be constant in time, and is only applicable when ${\bar {\eta }}_{E}$ is spatially constant. When applying a water level boundary condition to the nearshore, local flow reversals and boundary problems may result if the wave- and wind-induced setup is not included. This problem is avoided by adding a correction $({\bar {\eta }}_{C})$ to the local water level to account for the wind and wave setup as

$\rho gh_{B}{\frac {\partial }{\partial x}}({\bar {\eta }}_{E}+\Delta {\bar {\eta }}+{\bar {\eta }}_{G}+{\bar {\eta }}_{C})=\tau _{sx}+\tau _{wx}-\tau _{bx}$

(6)

where $h_{B}$ is the boundary total water depth, and $\tau _{sx},\tau _{wx}\ and\ \tau _{bx}$ are the wind, wave, and bottom stresses in the boundary direction (x). The wave forcing term is equal to

$\tau _{wi}=-{\frac {\partial }{\partial x_{j}}}(S_{ij}+R_{ij}-\rho hU_{wi}U_{wj})$

(7)

The correction ${\bar {\eta }}_{C}$ is only applicable when ${\bar {\eta }}_{E}$ is spatially constant as in the case of a single water surface elevation time series.

Cross-shore Boundary

In the implicit flow solver, a cross-shore boundary condition is applied in the nearshore by solving the 1D cross-shore momentum equation including wave and wind forcing (Wu et al. 2010). Along a cross-shore boundary, it is assumed that a well-developed longshore current exists (quasi-steady conditions with longshore gradients in advection, diffusion, and water levels equal to zero). These assumptions are valid for relatively long coasts with shore-parallel contours and simplify the alongshore (y-direction) momentum equation to

${\frac {\partial }{\partial x}}\left(\rho v_{t}h{\frac {\partial V_{y}}{\partial x}}\right)=\tau _{sy}+\tau _{Wy}-\tau _{by}$

(8)

where $\tau _{sy},\tau _{Wy},\ and\ \tau _{by}$ are the surface, wave, and bottom stresses in the longshore direction, respectively. Equation (8) is solved iteratively to determine the longshore current velocity. The cross-shore (x) component of the velocity is assigned a zero-gradient boundary condition. The longshore current velocity is applied when the flow is directed inwards. When the flow is directed outwards, a zero-gradient boundary condition is applied to the longshore current velocity.

The water level due to waves and wind at the cross-shore boundary can be determined by assuming a zero alongshore gradient of flow velocity and negligible cross-shore current velocity. For this case, the cross-shore momentum equation reduces to

$\rho gh{\frac {\partial {\bar {\eta }}}{\partial x}}=\tau _{sx}+\tau _{Wx}-\tau _{bx}$

(9)

where $\tau _{sx},\tau _{Wx},\ and\ \tau _{bx}$ are the surface, wave, and bottom stresses in the cross-shore direction. The water level boundary condition is applied for the case when the flow is directed outwards. When the flow is directed inwards, a zero-gradient boundary condition is applied to the water level.

Tidal Constituent

The water level predictions are based on a harmonic equation with several arguments

$\eta (t)=\sum _{i=1}^{N}f_{i}A_{i}\cos(\omega _{i}t+V_{i}^{0}+u_{i}-\kappa _{i})$

(1)

where $A$ is the constituent mean amplitude, $f$ is a factor that reduces the mean amplitude and varies in time, $V_{0}+u$ are the constituents equilibrium phase and $\kappa$ is the constituent phase lag or epoch. Table 1 shows a list of the currently supported tidal constituents in CMS.

Constituent	Speed	Constituent	Speed	Constituent	Speed	Constituent	Speed
SA	0.041067	SSA	0.082137	MM	0.54438	MSF	1.0159
MF	1.098	2Q1	12.8543	Q1	13.3987	RHO1	13.4715
O1	13.943	M1	14.4967	P1	14.9589	S1	15
K1	15.0411	J1	15.5854	OO1	16.1391	2N2	27.8954
MU2	27.9682	N2	28.4397	NU2	28.5126	M2	28.9841
LDA2	29.4556	L2	29.5285	T2	29.9589	S2	30
R2	30.0411	K2	30.0821	2SM2	31.0159	2MK3	42.9271
M3	43.4762	MK3	44.0252	MN4	57.4238	M4	57.9682
MS4	58.9841	S4	60	M6	86.9523	S6	90
M8	115.9364

Flux

The water flux is specified as m^3/sec per cell along the cell string. This value is multiplied by the number of cells in the cell string to obtain the total flux. The total flux is then redistributed along the boundary according to

$q_{i}={\frac {Q_{b}}{\sum \Delta s_{i}h_{i}^{1+r}/n_{i}}}{\frac {h_{i}^{1+r}}{n_{i}}}$

(2)

Cross-shore

Along a cross-shore boundary, it is assumed that a well-developed longshore current exists. Thus, the y (alongshore) momentum equation can be reduced as follows

$0={\frac {\partial }{\partial x}}{\biggl (}\nu _{t}h{\frac {\partial V}{\partial x}}{\biggr )}+\tau _{sy}+\tau _{wy}-\tau _{by}$

(3)

The water level setup due to waves and winds at the cross-shore boundary can be determined by assuming a zero alongshore gradient of water level, or using the following equation reduced from the x (cross-shore) momentum equation

$0=\rho g{\frac {\partial \eta }{\partial x}}+\tau _{sx}+\tau _{wx}$

(4)

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Symbol	Description
$t$	Time
$A$	Constituent mean amplitude
$f$	Constituent nodal factor
$V^{0}$	Constituent equilibrium argument
$u$	Constituent equilibrium argument
$\kappa$	Constituent phase or epoch

References

• Wu, W., A. Sánchez, and M. Zhang. 2010. An implicit 2-D depth-averaged finite-volume model of flow and sediment transport in coastal waters. In Proceedings of the International Conference on Coastal Engineering, No. 32. Paper Number: Sediment 23. Shanghai, China.

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