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RMA2 Time Step Interval: 0.25 hour or 15 minutes time step gives different results

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Dear RMA2 users:

I am trying to figure out why I am getting different results when I used 0.25 hour and 15 minutes times step in the RMA2 Model Control window. I expect the results to be the same since 0.25 hour is the same as 15 munites. Any idea why I am seeing different results? Does RMA2 allows the time step to be input in muinutes? Thank you very much for your support.

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The time step is always entered into the model control in hours.

I am also using RMA2 and 4 to run very small time step simulations and need some help. I enter the time steps in hours into RMA2, run the model, however, when I attempt to run RMA4, I get the error...

--> SUB GETVEL about to divide by zero

RMA4 TIME= 1.6666699E-02

RMA2 TIME READ= -999.0000 CORRECTED= 0.0000000E+00

RMA2 TIME PREVIOUS = 0.0000000E+00

Problem during interpolation...

Present TIME - Previous TIME are equal ....

I have entered both times as accurately as possible (1 minute = 0.01666667 hrs) and made sure the time steps are consistent between the two models, however, I still get this error. Any help is greatly appreciated.

Thanks.

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The time step is always entered into the model control in hours.

Thank you very much for your support. I have difficulty in getting the model to run to completion when I used 0.25 hour time step but I soon as I change the time step to 15 minutes, the model runs to completion. Any idea why the model is behaving this way? Thank you very much.

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Thank you very much for your support. I have difficulty in getting the model to run to completion when I used 0.25 hour time step but I soon as I change the time step to 15 minutes, the model runs to completion. Any idea why the model is behaving this way? Thank you very much.

Sounds like you are running a 0.25 hr timestep (=15 minutes) and a 15 hour timestep. I dont know of any other way to specify a 15 minute timestep aside from 0.25 hrs.

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What are your boundary conditions doing 15 hours out? If you are using a 15 hour timestep, it might run to completion because you've simplified the boundary conditions by jumping ahead in time.

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What are your boundary conditions doing 15 hours out? If you are using a 15 hour timestep, it might run to completion because you've simplified the boundary conditions by ahead in time.

The tide data was collected at 15 minutes time interval. All I did was copy the data from excel and pasted it on the XY series editor in the boundary condition drop-down menu and then give a time step of 15 in the "time step size" box. The output tide shows a smooth sinusoidal tide that is also 15 minutes times interval.

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The tide data was collected at 15 minutes time interval. All I did was copy the data from excel and pasted it on the XY series editor in the boundary condition drop-down menu and then give a time step of 15 in the "time step size" box. The output tide shows a smooth sinusoidal tide that is also 15 minutes times interval.

Remember, all of the RMA2 and RMA4 dialogs are expecting times to be specified in hours, so you will want to convert from minutes to hours before entering the information into the dialogs. See the RMA4 tutorial for an example of setting up transient boundary conditions. It is found in the SMS XX.X\tutorials\Models\RMA4 directory. The Users Guide To RMA2 Version 4.5 also provides guidance on selecting time step sizes - See "Selecting A Time Step Interval" on page 40.

  • Typically, a diurnal tide can use a 1 hour time step, while a semidiurnal tide or mixed tide requires a 0.5 hour time step.
  • For dynamic simulation runs, the computational time interval should be as small as necessary to capture the extremes of the dynamic boundary conditions and maintain numerical stability. Traditionally, this time interval varies between 0.25 hours and 1.0 hour for tidal boundary conditions at the coasts of North America, provided that that the hydrodynamics do not include complex features, such as wetting and drying, etc.
  • An easy way to review the RMA2 cards involved with time is to search on "hours" in the Users Guide To RMA2 Version 4.5. The WPLIM variable on the HW card is the only value specified in seconds I believe.

Did you change the time step size when you entered your boundary conditions as 0.25 hours rather than 15 hours? You will probably need a smaller time step since the boundary conditions will be changing much more quickly when entered as 0.25 hours as opposed to 15 hours.

- John

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Remember, all of the RMA2 and RMA4 dialogs are expecting times to be specified in hours, so you will want to convert from minutes to hours before entering the information into the dialogs. See the RMA4 tutorial for an example of setting up transient boundary conditions. It is found in the SMS XX.X\tutorials\Models\RMA4 directory. The Users Guide To RMA2 Version 4.5 also provides guidance on selecting time step sizes - See "Selecting A Time Step Interval" on page 40.

  • Typically, a diurnal tide can use a 1 hour time step, while a semidiurnal tide or mixed tide requires a 0.5 hour time step.
  • For dynamic simulation runs, the computational time interval should be as small as necessary to capture the extremes of the dynamic boundary conditions and maintain numerical stability. Traditionally, this time interval varies between 0.25 hours and 1.0 hour for tidal boundary conditions at the coasts of North America, provided that that the hydrodynamics do not include complex features, such as wetting and drying, etc.
  • An easy way to review the RMA2 cards involved with time is to search on "hours" in the Users Guide To RMA2 Version 4.5. The WPLIM variable on the HW card is the only value specified in seconds I believe.

Did you change the time step size when you entered your boundary conditions as 0.25 hours rather than 15 hours? You will probably need a smaller time step since the boundary conditions will be changing much more quickly when entered as 0.25 hours as opposed to 15 hours.

- John

No, I did not. I will try to do that now. Thanks for the information. I used 15 minutes (not 0.25 hrs) in both the XY series editor of the dynamic boundary and the time step size box. I am with the impression that since both are in the same format the result should be okay.

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It is important that you specify the time in hours - the equations used by RMA2 require this to give correct results.

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