Advanced Time Step Control-Precipitation

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Advanced Time Step Control-Precipitation

Luis Partida
With the new HEC-RAS release im sure everyone has been attempting to decrease their model run times with this new feature. However, if you are modeling large areas with precipitation, be weary of this. The courant numbers discussed in the supplemental manual dont fit well with flat/mild sloping terrains. Courant numbers could be extremely high by following the criteria of velocity(dt/dx) . For models in the past where i have had stability and continuity balance at a certain dt, the criteria for optimizing this model will shift the dt even lower.

For precipitation models, in areas where velocities will never be high, the Residence Time option should be utilized instead.

What i am unsure of is: how does the criteria for residence time coincide with the options given for input?

Max Courant
Min Courant
Number of steps below....etc. It was my understanding that residence time is independent of these inputs.
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Re: Advanced Time Step Control-Precipitation

cameron
Residence time should not be used for rainfall models. Yes you will get models that are stable, but they will be very very slow. This is also true for very steep areas.

Grid cells that only a small volume of water get into will cause the residence time timestep to need to be very small.  

I have found that you need to take care in which values you specify for the min and max values or else the model either does not run faster or you get bad results.
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Re: Advanced Time Step Control-Precipitation

Luis Partida
Thanks cameron, this is my issue though. The Min and Max values are essentially picked from the air.

I have models in the range of 1.6 million cells, and the selection of these variables, like you said, can prove to run a model even slower than previously. The guidance for the selection of these variables was poorly outlined imo. Any help on this topic is appreciated.
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Re: Advanced Time Step Control-Precipitation

Mohsin
In reply to this post by Luis Partida
I need some help regarding adding the precipitation in the HEC-RAS.

1- What does the actual mean of time series of precipitation? Is it mean the accumulated rainfall?
for example

Date                                 Simulation time                             Precipitation (mm)
11-01-2019 07:00                   00:00                                               30
11-01-2019 08:00                   01:00                                               30
11-01-2019 09:00                   02:00                                               30

Does it mean the total rainfall added on the 2D flow area during the whole simulation time is 90 mm? Please elaborate?

Thanks