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Hec-ras weir coefficients

Hec-ras weir coefficients

Hec-ras weir coefficients

I have received results of hec-ras river modelling showing impacts of adding 1.0m high weirs designed to improve aeration in a river. I am surprised to find that the provision of a 1.0m high weir with a length equal to the full width of the river has created a 1.0m rise in water level.

The weirs are broad crested and should have a Cd of about 1.7 (I understand the weir equation used is Q = Cd.L.H^3/2 in metric units). However the operator tells me that he cannot input a coefficient of greater than 1.4 otherwise the run crashes.

Are we missing something ? Why can we not have a weir coefficient greater than 1.4 ?

RE: Hec-ras weir coefficients

The operator is missing something.

RE: Hec-ras weir coefficients

If your flow is subcritical, then the weir might even lower the water level due to choking effects.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

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