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TUFLOW HR Output: Difference between revisions

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=Water Level Interpolation Method=
Similar to the standard output, the HR output also needneeds to interpolate cell centre water levellevels to cell corners. However, the interpolation methods for the standard output (<font color="blue"><tt>Map Output Corner Interpolation </tt></font> <font color="red"><tt>== </tt></font> <font color="black"><tt>Method C</tt></font>) can produce "bumpy" HR water level output in direct rainfall modelmodels alongwith narrowsteep streamsterrain, as the water level is linearly interpolated from the cell centres/corners, while the change of sub-grid elevations may not be linear. This often happens between fully wet cells and sheet flow cells, as the water level is linearly interpolatedillustrated below.
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applies a minimum corner water level of “corner elevation + cell wet/dry depth” if all the surrounding 4 cells are wet. However, this may produce “bumpy” water level outputs along narrow streams in a direct rainfall SGS model between sheet flow cells and fully wet cells when using the new high resolution output. This is caused by applying a linear water level interpolation between SGS cells that have a non-linear minimum SGS elevation as illustrated below.
 
[[File:corner_h_intp_output.png|480px]]
interpolated from the surrounding SGS sampled elevations.
 
*''under construction''
In the HR water level output, these locations open have high water level with triangular shape.
 
[[File:HR_Intp_A.png|480px]]<br>
'''HR Interpolation Approach == Method A'''
 
Two additional methods have been added for the HR corner water level interpolation:
 
<font color="blue"><tt>HR Interpolation Approach </tt></font> <font color="red"><tt>== </tt></font> <font color="black"><tt>{Method A} | Method B | Method C</tt></font>
 
* Method A is the default option that applies the same water level interpolation method used for the standard output.
* Method B performs sheet flow checks at cell faces and ignores the water level from the upstream cell.
* Method C applies the same sheet flow checks as the Method B. In addition, it also uses the number of wet SGS sampled points as a weighting that biases non-sheet flow cells that further improves the mapping to in-stream water levels.
 
The two images below present the high-resolution water level output at the same location, but with HR Interpolation Approach == Method B and Method C. As can be seen, the water level along the narrow stream is “smoother”.
 
[[File:HR_Intp_B.png|480px]] [[File:HR_Intp_C.png|480px]]<br>
'''HR Interpolation Approach == Method B (left) and Method C (right)'''
 
Note that when these two methods are applied, the interpolated corner water level is biased to the non-sheet flow cells, and consequently, sheet flow cells may appear as “dry” cells. The improved approach that takes into the account of the sheet flow water level is currently under development.
 
Whilst Methods B and C can substantially improve the water surface mapping of SGS models using direct rainfall (rain-on-grid), there will always be inaccuracies with mapping at a higher resolution than the 2D cell resolution due to interpolation and extrapolation. Regardless of the software, the greater the ratio of 2D cell size to the high-resolution DEM cell size, the greater the potential for mapping inaccuracies. Should better mapping accuracy be required, reducing the 2D cell size to compute the spatial variation in water surface and velocities more accurately is, by far, the best course of action.
 
Please also note that it is not necessary to use these options for non-rainfall on grid models.
 
=Interpolation near Thin Breakline =
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