User Forum of Software BASEMENT

chri_and

User

Registered: 2017-07-27

Posts: 34

Flow through a structure

Hi There

I'm trying to simulate the effect of a structure in my 2D simulation. This structure lays on the banks of the river and deviates the flow at soon as the level of the water is higher that the banks. To better explain it, the structure is meant to deviate the flow laterally during a flood, so that downstrean only a fix dicharge is achieved. To simulate the behavior of structure I tried to used the an INNER_BOUNDARY with a hqrelation. I was expecting that the flow gets through the structure according to my hqrelation untill the max discharge (100 m3/2) is reached. At this point the flow is deviated laterally. What happes though, is that the structure acts like a damm, i.e., the water accumulate behind the structure and at some point overflows it. Surely I'm doing something worng with the INNER_BOUNDARY , can anyone help with this problem?

Does the gate work better for 1D than 2D simulation? In that case I could do a 1D-2D couling.

Many Thanks
Devis

Stephan Kammerer

Developer

Registered: 2015-03-31

Posts: 67

Re: Flow through a structure

For this purpose, I would use an Inner Boundary type weir. You can also use a controller in order to define your weir level (creast of your outflow structure). For this, you define your fix downstream discharge as  MONITORED_VARIABLE and the weir level as MANIPULATED_VARIABLE like in the example below:

CONTROLLER {
        name             = weir_level_structure
        type             = PID
        output           = yes
        output_time_step = 20
        MONITORED_VARIABLE {
            name            = Q_downstream
            type            = Q
            position        = region_name.STRINGDEF name // define a Stringdef that "measures" the downstream discharge
            root_value      = 790
            root_value_type = fixed_value
            coefficients    = weir_level_Plattform:(-0.01 -0.0005 -1.0)
        }
        MANIPULATED_VARIABLE {
            name            = weir_level_structure
            position        = region_name.STRINGDEF name // string_name1 of Inner Boundary
            start_value     = 657.8 // absolute elevation [m asl]
            range           = (657.6 658.6)
            max_change_rate = 0.01
        }
    }

chri_and

User

Registered: 2017-07-27

Posts: 34

Re: Flow through a structure

Hi Stephan

Thanks for the reply, it helped me. I do have a couple of questions regarding the setup of the weir tough.

1) Is root_value my discharge in m3/2?

2) Are the start_value and the range the actual lower and upper edges of the weir?

3) Is the weir one possible solution and the controller a separate one or do I need to define an Inner Boundary + Controller? I have tried only Controller without Wier but I get a fatal error. What I don't understand for the solution Inner Boundary + Controller it is why I need to define a table for the weir with time and weir_level and again the start_value and range, which is again the weir level, if question 2) is correct.

Cheers

Stephan Kammerer

Developer

Registered: 2015-03-31

Posts: 67

Re: Flow through a structure

1) Yes, the root_value is discharge [m3/s] for type "Q" or water sursface elevation [m asl] for type "wse"

2) No, if you use an Inner Boundary type weir the start_value should be a first reasonable guess for your weir level (crest of weir overfall). The range is the upper and lower limit in which the actual solution shall be found by the controller.

3) The Inner Boundary is independent of the controller, you can also use it without. Thats why you have to define a file for the weir level in any case. For the controller of course you need a boundary string for a weir or gate. This can be either an Inner Boundary or a normal outflow boundary

chri_and

User

Registered: 2017-07-27

Posts: 34

Re: Flow through a structure

Hi Stephan

Thanks for the replay. My structure is above the rived bed, so for my case I used gate at the end, so that I have some flow through the structure at the beginnig of the flood. However, how can I consider sediment transport at a structure? Mine is a complicated case. Water and sediment should go through the structure (essentialy a wall a couple of meters above the river bed, meant to deviate the flow laterally during a flood when the river has reached a certain water level ) until the lower edge of the structure is reached. At this point the water and some sediment are deviated laterally. 

My trial till now:
use external_source, type dredge, defining the elem_ids upstream the structure and dredge_elem_ids_redist dowstream. The problem is that the sediment transport does not go through a gate/weir by default in Basement. Result is, that I have no sediment transport through the structure at the beginnig of the flood when the structure has no effect. This is caused by the definition of the dredge_trigger_height, i.e sediment transport takes place only when this height is reached.

What I could try:
1) Run a simulation without structure, monitor the water level and stop the simulation when this is as high as the structure lower edge. Use the new terrain with sediment deposition in a new simulation with structure and fix bed. Can I export the terrain (the mesh) after the simulation?
2) set additional outflow and inflow before and after the structure and controll the amount of sediment and water. Run I into problems with the inflow downstream the structure?

Any thought/idea on the problem and on how I could simulate this at best?

Regards
Devis