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Alyssa

User

From: Aix-en-Provence, France

Registered: 2019-10-28

Posts: 41

Instability Q over weir

Hi

I have a channel 40 km long, rectangular 150m width and a low flow Q of 85m3/s. When I implement a first weir (4m high) I see some instabilities of the discharge (ranging between 60 and 120) which impacts at least 6km upstream. When I put another second weir more downstream, 10m high, the instabilities become very large (Q changes between -700 to 1000 impacting at least 10 km upstream). The water level upstream of the second weir is not flat, probably due to this instabilities. I ran the model 1000 years to see if instabilities would decrease over time but it seems it doesn't. Why is the Q changing, volumes should be constant?

I changed the calculation_type from critical to standard because when critical it seems even more unstable.
So far I'm only running with fixed Q (85m3/s) which is a low flow, eventually I would run a hydrograph, but as initial condition I need a stable condition first?

As a note, the second weir should actually my downstream boundary and in combination with a QH relation, but this didn't work out well either so instead I added 10km downstream my reach and a second weir as inner boundary (it seemed easier to investigate).

Any recommendations?
Thanks!

Alyssa

Last edited by Alyssa (2020-06-03 15:04:03)

Alyssa

User

From: Aix-en-Provence, France

Registered: 2019-10-28

Posts: 41

Re: Instability Q over weir

I solved this problem. I turns out the cross sections in between I defined the weir were too far apart and this created instability in the discharge..