User Forum of Software BASEMENT

basemind

User

Registered: 2015-10-28

Posts: 15

Approximating bridges via combination of gate/weir

Hi,

as the title states, I want to approximate a bridge via a combination of inner boundaries, specifically gate and weir.
The following graph shows my approach:
I want to simulate the bottom edge of the bridge via a gate with a fixed, non-time-dependent height set to the bridge's bottom edge and the top edge of the bridge via a weir set to a fixed, non-time-dependent height set to the bridge's top edge.
I wanted to implement this via setting TWO inner boundaries for EACH of my 2 bridge boundaries (inflow and outflow). This would make sense to me - as long as the bridge isn't overtopped, the outflow of the weir would be 0, as soon as it begins being overtopped, the outflow of the weir would be >0. The output of the gate would always be >0 (as long as there is water flowing in the river, of course), as long as the two boundaries are considered separately. So the outflow should simply be outflow(gate) + outflow(weir).

Basement doesn't let me set two inner boundaries to one stringdef, however. Does anybody know of a similar way/another approach to simulate bridges? I know I could use a h/Q relation, but as this doesn't consider downstream flow states (depth etc.) and I would have to calculate those relations first (via [3rd-party] 1-d calculation?) I would like an all-in-one solution inside basement. The tools are there, I just can't use them (yet).

The arrows next to the weir/gate height shall demonstrate the no flow areas created by them when considered separately, the green area can never be used for any throughput (that's the outcome I would like to see).

Last edited by basemind (2015-11-18 18:05:15)

sajjad

User

Registered: 2014-10-01

Posts: 101

Re: Approximating bridges via combination of gate/weir

Dear fellow member

Add the two hyd. structures in succession  i.e one after the other, separated by a small distance, rather than on the same stringdef.

Regards

basemind

User

Registered: 2015-10-28

Posts: 15

Re: Approximating bridges via combination of gate/weir

If I do like you suggest, I think there are two scenarios:

1. I put the gate first and the weir second

   • No overtopping of the bridge is possible due to the gate with infinite height

   • The weir blocks all throughput until it overtops - it acts like a wall with a non-infinite height (all water must go OVER the weir)

2. I put the weir first and the gate second

   • The weir blocks all throughput until it overtops - it acts like a wall with a non-infinite height

   • The gate doesn't make any sense anymore since the waterlevel has risen to unrealistic high values before due to the weir

Either way the results would be dramatic floods...

I need the two conditions to be combined with an OR connection (=same location) not an AND connection (=different location).

sajjad

User

Registered: 2014-10-01

Posts: 101

Re: Approximating bridges via combination of gate/weir

You want to take into account the non-variability over a section but with depth-averaged equations which basement solves, this dimension is non-existant. In any case, when the bridge is overtoppped, below the flow is through orifice (which is unavailable in basement) and overtopped flow is very very turbulent and choppy. So the degree of finesse that u are trying to achieve  by trying to exclude the bridge deck area from the flow is overly focussed on having the correct flow area. The phenomenon is too complex and you should not try to be so very faithful to its representation or else use a 3d model ( success is not guaranteed, of course). If u have to model only bridge overtopping,  use a 3d model; if its part of a large model then let go of bridge deck and use increased roughness to represent its effect.

ckatsoulas

User

Registered: 2015-12-21

Posts: 4

Re: Approximating bridges via combination of gate/weir

Hi Basemind
You could try putting the weir side by side to the gate or HQ relation for the culvert. This way both will work at the same location.