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praalankar

CFW X and Y values

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Hello,

I'm looking to represent culverts using CFW factors as a simplified approach.

Should I draw a polygon such that both X and Y CFW values in the grid check file are updated? Or just the one along the minor direction of the culvert?

Thanks in advance,

Prabhanjan

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Hi Prabhanjan,

TUFLOW calculates velocities at the cell sides (ZV, ZU) so any cell width reduction factors would need to be applied in the direction of flow i.e. velocity in the Y direction is calculated at ZV so flow width is considered at this point. Often flow is not perfectly aligned with your grid so it’s necessary to change both ZV and ZU flow width. For example if your culvert was perfectly aligned with your grid and water was flowing north, you would only need to apply a cell width reduction factor at your ZV point as the ZU velocity component is equal to zero. If your culvert was not perfectly aligned with your grid, then you would need to apply a cell width reduction factor to both ZV and ZU as both would have a velocity component greater than zero (and therefore both contributing to the velocity magnitude and direction).

If using the CWF as a simplified approach for culverts, typically the 2d_cwf polygon (Section 6.11.2 of the 2017 TUFLOW Manual) should be digitised with the same length and width as the culvert(s). This will ensure all relevant ZU and ZV points are modified.

Alternatively using a 2d_fcsh (Section 6.12.2 of the 2017 TUFLOW Manual) is another approach and is generally preferred. It has only slightly more attributes to populate however allows you to consider other mechanisms such as additional energy loss, the manning’s n of the culvert wall, and the culvert obvert. Similarly the 2d_fcsh feature should be digitised as a polygon or line with the same length and width as the culvert(s).

 

U_V_Velocity.jpg

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