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rachel.jensen

Differences between PO and Grids

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Q: I've got substantial differences between my time series and plot output data and the max grids. Why is this?

A:

There are a few things that could be going on here, depending on the differences that you’re seeing.

BIG differences (>0.1m)

The max WSL level is a tracked maximum, which means that at every time step Tuflow checks the current water level at the ZC and saves it if it’s a maximum for the model run. This is then effectively a max output calculated for every time step.

Compare that to the TS output which is only at your time series output interval. Similarly, a  WaterRide output is an envelope of the maximum grids at every output time, not of every cell at every time step.

 If you’re seeing a big difference in the gridded results and the TS results, it could be that the temporal resolution in WaterRide and your TS layer maybe missing a point of instability.

Investigate the mass balance at that location to see if it spikes, also try seriously reducing the time series output interval (down to something close to your timestep if possible) to see if the TS can produce the same results as the grids.

 

Small differences

There are two mechanisms used to create ascii files that can influence the difference between grid and PO.

 The first one is the extrapolation from the cell centers to the cell corners.

Tuflow calculates the water surface level at the ZC then when you grid results using Tuflow_to_GIS. It extrapolates out to the ZH’s on the cell edges and doesn’t preserve the ZC values. This is why you get a mapped grid cell size half of  your modelled grid cell size

This isn’t normally an issue except where you have really steep topography or incised channels.

What you can do to help this is output the ascii files within Tuflow. When you write them from within Tuflow, you get the ZC values, IE, no extrapolation. This can help reduce the difference.

 (You could also use HIGH RES for an output option. This means that you will get output at cell centers, mid-sides and corners, but be aware that you output size will be about 4 times the size and some programs (such as TUFLOW to GIS and dat_to_dat) don’t yet recognise this format.)

The second one is the north/south orientation of the ascii grid.

Ascii grids are always north/south orientated. That is, the ascii cells do not line up with your modelled orientation. Thus, there is an interpolation to rotate the grids. This can also cause issues on steep topography where there are large changes in water level or ground level.

Unfortunately, there is nothing you can do to fix this one. You can look at the tuflow .dat files for raw results and confirm to yourself that the modelled results are hitting the mark and that only the presentation of results is different.

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