User Forum of Software BASEMENT

Matteo Facchini

Developer

From: Trento

Registered: 2014-09-05

Posts: 281

bridge simulation with hq-relation

Hi,

I am trying to simulate the flow of a stream under a bridge.

1. I set the upstream boundary (red line)
2. I calculate the discharge (uniform flow) corresponding to increasing wse, until the wse value reaches the ceiling of the bridge; from that point on the wse increases, while the Q stays constant;
3. I set an hq linked boundary condition using the discharge calculated at 2

The problem I am facing is that if I do not give the exact value of the discharge two thins are occurring:

a. Q > Qflow --> mass balance problem in the cells upstream of the boundary condition
b. Q < Qflow --> the ∆Q flows to the downstream elements (black triangles) which are very steep thus I get crazy high velocities (more than 2000 m/s) in those cells and that slows the simulation down.

All I want to achieve is to allow all the water to flow under the bridge until a maximum value of wse is reached. From that point on the Q remains constant. Thus, I would like to set my hq file as follows:

h,Q
0,0
wse_min,0
wse_max,Qmax
1000,Qmax

Any thoughts/suggestions?

I think making a hole in the mesh where I have a bridge would bring something, but I would like to simulate also the bridge overtopping.

Last edited by Matteo Facchini (2020-06-12 08:56:23)

Lukas Vonwiller

Developer

Registered: 2014-09-04

Posts: 127

Re: bridge simulation with hq-relation

hi Matteo!

We used h-q relations also at the Sihl river, see page 10:
http://people.ee.ethz.ch/~basement/base … hlmann.pdf

We defined holes for the bridges and applied 'linked' boundaries.

1) h-q relation for open channel flow using a stepwise increasing hydrograph or analytical approach using uniform flow
2) Gate
3) + weir in case of overflow

But instead of 2 and 3 you can also define a max. discharge using a 'steep' h-q relation!

Maybe try in your h-q file without (0.0, 0.0):

mean bed elevation on the string (see h5 container), 0.0
wse_max, Qmax
1000, Qmax

hope this helps!
cheers, Luke

Matteo Facchini

Developer

From: Trento

Registered: 2014-09-05

Posts: 281

Re: bridge simulation with hq-relation

Ciao Lukas,

that's great!

I think I will remove the bridges from the mesh and play with the hq relation to simulate my scenarios.

However, it would be great to have a version of the hq relation that uses the fluxes to have a kind of transparent hq relation!

Cheers
Matteo

PS: just because I was not finding the averaged wse: to have the stringdef results in the h5 container one must have ns_hyd_discharge as output variable in his/her results.json file!

Last edited by Matteo Facchini (2020-06-12 13:37:49)

isabo

User

Registered: 2020-11-03

Posts: 6

Re: bridge simulation with hq-relation

Hi,

I've been trying to simulate a culvert in Basement v3. I set the linked boundary condition at the up - and the downstream, each with two attached nodes (so just one String). But I don't seem to set the hqrelation_file right. The following numbers are my .txt file which i connected to the boundary.

0.0,0.0
477.62,0.0
477.72,0.025
477.82,0.05
477.92,0.1
478.02,0.15
478.12,0.2

It starts to simulate and plotting massive ammounts of "mass balance problem" until the programm isnt responding anymore.

Is the h-q-relation the right type or do I have to use anotherone?
And how do I have to set up my Text file, that the culvert works the right way?

Cheers, Stefan

Lukas Vonwiller

Developer

Registered: 2014-09-04

Posts: 127

Re: bridge simulation with hq-relation

Hi Stefen,

I am not sure if I understand you correctly, but for a linked boundary condition you will need two separate strings.

Furthermore, I recommend to check the mean bed elevation on your upstream string in the .h5 container. Try to start from this elevation (or maybe some centimeters above) to make sure you have water at your string before trying to extract water with your hqrelation.

I hope this helps!
cheers, Lukas

isabo

User

Registered: 2020-11-03

Posts: 6

Re: bridge simulation with hq-relation

Hi Lukas,

Yes there are two separate strings.
Thanks for the tip. But the Results arent plausible. Whenever the water reaches the culvert (Linked Boundary Inflow) it seems like its pushing the water back like a floodwave. The nodestrings should be ok so far and the elevation was set on de mean bed elevation (also testet with 0.0 to 0.5, same results). Its like there's an invisible wall instead of a culvert.

Maybe theres another type which can Simulate a culvert? or do you got any more Ideas.

Cheers, Stefan

Lukas Vonwiller

Developer

Registered: 2014-09-04

Posts: 127

Re: bridge simulation with hq-relation

Hi Stefan,

It's hard so say from far away, but just some random thoughts:

• Maybe the first line (0.0, 0.0) is not needed, because your start from zero discharge at the second line (477.62, 0.0)

• check if upstream_direction (left/right) is correct

• check if you can improve your mesh, especially near the culvert

• make sure that your h-q relation makes sense from a hydraulic point of view

Hope this helps!

cheers, Lukas