Calibration for extremely low flows

[Erica Koppes]

I am trying to calibrate a model using very low flows <0.05 cfs. I have WSE data from a field survey and am trying to calibrate my 1D model using roughness values. It seems that the flows are so small the WSE’s are not sensitive to varying roughness values. Is a flow this low even worth attempting to calibrate?

[Luis Partida]

I believe it is stated in RAS documentation that calculated WSE values as low as 0.01 are considered negligeable. That being said, if a target calibration of 0.05 cfs is desired, I would imagine RAS not being sensitive to roughness coefficients. Instead, what I would try is increasing or decreasing the # of XS’s in the model because the averaging between XS data will alter results but to the hundredth of a decimal place…I am not sure. Good luck

[Chris Hauser]

Erica-

To what flow depths are you trying to calibrate these very low flows? What are your typical cross-section dimensions and spacing? How accurate are you trying to get?

Without knowing a bit more, it seems to me that you may be asking HEC-RAS to do a bit too much in this situation.

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