Submerged Weir Discharge Computation
Submerged Weir Discharge Computation
(OP)
What is the best method to compute Q over a submerged weir?
My discharge is 344 cfs, weir length = 40'. Tailwater is 1.4' above the weir (H2). Assume a sharp crested weir.
The procedure in HEC-RAS results in no reduction, so H1 = 2.0'
The equation developed by Brater & King, '76 results in H1 = 2.3'
Which is the better answer?
My discharge is 344 cfs, weir length = 40'. Tailwater is 1.4' above the weir (H2). Assume a sharp crested weir.
The procedure in HEC-RAS results in no reduction, so H1 = 2.0'
The equation developed by Brater & King, '76 results in H1 = 2.3'
Which is the better answer?





RE: Submerged Weir Discharge Computation
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Submerged Weir Discharge Computation
RE: Submerged Weir Discharge Computation
RE: Submerged Weir Discharge Computation
As suggested by Lincoln. Just noticed your post, thanks.
RE: Submerged Weir Discharge Computation
I compared formulae/charts provided by Ackers (1971); King and Brater (Villemonte) (1963); Davis (1952); Bligh (1927) and Bos (1989). The comparison showed that there are significant differences in the results obtained by these different approaches. Withough going into too much detail I found that if you are using the Villemonte equation then you are probably getting results at the conservative end of the scale (ie indicating that drowning has a greater impact on discharge, with a lower modular limit). The more recent Ackers (and Bos), on the other hand, indicates that the modular limit is at around 0.9 and flow rates decrease rapidly above this. It sounds like this is more akin to what HEC-RAS is telling you.
So, it comes down to whether you need to be conservative or are struggling to get something to work. Personally I put a lot of faith in Bos in these things, but it is aimed at flow measurement structures and so may give an optimistic view of things on other less "refined" weirs. Hope this provides some useful context.
RE: Submerged Weir Discharge Computation