Model Coupling: Difference between revisions

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where <math> \bar{\eta} </math> is the mean water level, and <math> \nabla \eta </math> is the component due to tidal, wave, and wind generated surface gradients. <math> \bar{\eta} </math> can be estimated from water level boundary conditions and is generally much larger than <math> \nabla \eta </math> so the later may be neglected. For some cases in which the surface gradients do not vary significantly over time (open coast beach), the second term may be approximated as
where <math> \bar{\eta} </math> is the mean water level, and <math> \nabla \eta </math> is the component due to tidal, wave, and wind generated surface gradients. <math> \bar{\eta} </math> can be estimated from water level boundary conditions and is generally much larger than <math> \nabla \eta </math> so the later may be neglected. For some cases in which the surface gradients do not vary significantly over time (open coast beach), the second term may be approximated as


     <math>\nabla \eta^{n+1} = \nabla \eta^{n} = \bar{\eta}^{n} - \eta^{n} </math>.
     <math>\nabla \eta(x,y)^{n+1} = \nabla \eta(x,y)^{n} \approx \bar{\eta}(x,y)^{n} - \eta(x,y)^{n} </math>.





Revision as of 22:10, 13 May 2010

Introduction

CMS-Flow and CMS-Wave can be run separately or coupled together using a process called steering. The variables passed from CMS-Wave to CMS-Flow are the significant wave height, peak wave period, wave direction, wave breaking dissipation, and radiation stress gradients. CMS-Wave uses the update bathymetry, water levels, and currents from CMS-Flow. The time interval (constant) at which CMS-Wave is run is called the steering interval. The steering process is as follows:

1. CMS-Wave model is run for the first two time steps (time = t and time = t+Δtw) and the wave information is passed to CMS-Flow.
2. CMS-Flow runs from time = t until time = t+Δtw+Δt and interpolates the wave variables from during the simulation
3. CMS-Wave is run again for time = t+2Δtw and
4. The process is repeated over until the end of the steering simulation.

Prediction of Water Levels and Currents

Because CMS-Wave requieres the water surface elevation at times that are ahead of the hydrodynamics, the water surface elevation and currents. If the steering is relatively small (~<30 min), than the values from the previous time step may be used without significant error. This may be expressed in the following form

     η(x,y)n+1=η(x,y)n  

where η(x,y) is the water surface elevation and n indicates the CMS-Wave time step.

However, in many coastal engineering projects it is desirable and common to use relatively large steering intervals of 2-3 hours and 3 hours is especially common since many wave buoy data products are at 3 hour intervals. In cases where the relative surface gradients at any time are much smaller than the mean tidal elevation, a better approximation of water level may be obtained by decomposing the water level into

     η(x,y)n+1=η¯(x,y)n+1+η(x,y)n+1  

where η¯ is the mean water level, and η is the component due to tidal, wave, and wind generated surface gradients. η¯ can be estimated from water level boundary conditions and is generally much larger than η so the later may be neglected. For some cases in which the surface gradients do not vary significantly over time (open coast beach), the second term may be approximated as

    η(x,y)n+1=η(x,y)nη¯(x,y)nη(x,y)n.


Symbol Description Units
η Water surface elevation m



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