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scott

Q Type Pits

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Hi,

I am modelling some drainage improvements for a residential property within a larger catchment model. Grated cutoff drains are proposed to reduce runoff directed at the dwelling. I have created depth vs inflow curves and created Q type pits to model flow into these drains. Originally I tried connecting these pits to the 2D domain via SXS and SXG (conn_2D entry) but found neither selected the correct cells to match the proposed drain alignment. As can be seen in the attached plot my latest trial was using SX links to define the drain alignment and direct into the pits, but the results are not what is expected. It appears some water (very minor gets into the pipe) but the pit is bypassed and therefore the depth vs flow relationship is ignored. Has anyone else encountered this type of modelling? Or does anyone else have some additional ideas? Any help would be appreciated.

 

Thanks

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The SX lines will be connecting directly to the pipe, so the Q pit won't have any influence, but should take flow from the cell where they are located into the pipe. The SX line will take flow from the cells that it is in, and that will enter the pipe.


As you have found, a pit channel will only connect at the cell in which it is located unless you use the SXS or SXG commands, which don't let you control which cells are chosen. One solution to this is to use multiple pits. Put a pit node in each of the cells where your drains are, and connect these with a pipe to where you have the pit at the moment. That should act as if you have a continuous drain.

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I have modelled cut off drains as 1D channels before, rather than as pits, as this accounts for the limited capacity of the cutoff drain in conveying flow to the pipe or connector pit. Plus you get to specify which cells connect to the cutoff drain.

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Hi Scott

 

Did you also specify the Z flag (eg. SXSZ)?  To ensure that the invert of the pit is the same as the 2D cell's ZC level, you need to specify the Z flag and set the pit's inverts to -99999.  If the pit's invert is higher than the 2D cell then no water will enter the pit until the water level rises to the pit's invert level.   If the pit's invert is lower, then you should get an ERROR message or WARNING depedning on the setting for the "SX ZC Check ==" command.  You could also consider the L flag instead of the Z flag.  The L flag will lower the ZC level by the pit's "US_Invert" attribute value and set the pit invert level to the lowered ZC level, thereby creating a slight depression to concentrate the flow towards.

 

Also note that you don't have to have your pits snapped to the 1D network.  You can specify the "Pit Search Distance ==" value to automatically connect pits to the nearest pipes, so this may help if you take up tmashby's suggestion.

 

Cheers

Bill

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Hi Guys,

Thanks for your advice, much appreciated. I have modelled the pits as suggested by tmashby and used the flags suggested by Bill and now have the model running and producing results. Upon inspection of the 2D results it appears to be acting the way I anticipate. I have attached 3 pictures:

 

1. Shows the model setup, I am blocking the house out with a z shape and have put pits and pipes into to drain the water that collects near the house (located in a small gully), the aim being to ensure floor level is protected (pits and pipes modelled with 50% blockage factor). (pic - house_drainage)

 

2. Shows the 2D results when there is no drainage (approx. average depth of ponding is 1m) (pic - no_drainage)

 

3. Shows the 2D results with the drainage (pic - with_drainage)

 

However, upon inspection of the 1D results I was surprised to see very little/no flow through some of the pits even though there is enough depth to cause flow into the pits. The flow through the pits doesn't appear to match the depth/flow relationship I have specified for the majority of the pits. I have inspected the .eof file and found that the flow through the majority of the Q type pits is being reported as 'N' regime (at all time steps when there is flow in the pit), exercpt from manual below

 

 

N

Upstream controlled friction flow occurred in a Steep (S) channel with a Froude Number less than 0.5.  N stands for normal flow, however, in this case the upstream controlled friction flow approach was adopted.  This may occur during the transitioning of flow from downstream controlled to upstream controlled.  If it occurs repetitively, the configuration of the channel should be reviewed.

 

I have inspected the model setup and can't seem to find what would be causing this and haven't been able to identify anything. Does anyone have any suggestions? I am expecting the 2D results to look like they do but would like to see the 1D results validated in which case there should be more flow through the Q type pits. Does TUFLOW produce any results files to show the depth vs flow relationship during the run (there doesn't appear to be anything in the .eof? Below are the depth vs flow relationship , the first relationship for the 6 pits running north-south, and the second relationship for the pits running east-west at the northern end. Any advice would be appreciated.

 

 

Depth Flow   Depth Flow 0 0   0 0 0.013979 0.003131   0.01 0.002348 0.018128 0.005237   0.02 0.00664 0.021105 0.007006   0.03 0.012198 0.023509 0.008578   0.04 0.018781 0.02737 0.011354   0.05 0.026247 0.030488 0.013809   0.06 0.034502 0.033149 0.016046   0.07 0.043478 0.035494 0.018123   0.08 0.05312 0.037606 0.020075   0.09 0.063385 0.039538 0.021924   0.1 0.074237 0.041325 0.023677   0.12 0.116446 0.043002 0.025322   0.14 0.171862 0.046845 0.029061   0.16 0.234035 0.050326 0.032394   0.18 0.302297 0.053544 0.035421   0.2 0.376146 0.05656 0.038207   0.25 0.582799 0.062131 0.043224   0.3 0.817581 0.067236 0.04768   0.35 1.077453 0.07199 0.051714   0.4 1.311419 0.076465 0.055416   0.45 1.402596 0.080712 0.058849   0.5 1.488198 0.084767 0.062057   0.55 1.569136 0.092406 0.066629   0.6 1.6461 0.099536 0.069735       0.106261 0.072278       0.112651 0.074374       0.118758 0.076104       0.124621 0.077529       0.130273 0.078697       0.135737 0.079643       0.141034 0.080397       0.146179 0.080983       0.151189 0.081419       0.156073 0.081722       0.160843 0.081907       0.165508 0.081984       0.170074 0.081984       0.181106 0.081984       0.191654 0.081984      

 

 

 

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As far as I know Tuflow doesn't output anything directly as far as depth vs flow results, but you should be able to set a point to output a timeseries in a cell (i.e. where you have a pit) and then relate this to the 1D _Q.csv results for that pit to see if it is behaving as it should.

 

You may find that the pipe connecting the pits together is flowing at capacity, so only a little flow will enter. Maybe do some handcalcs (or use Colebrook-White tables) to check the flow in the pipe that you have connecting the pits. Might need to make the pipe bigger for the pits to work as you want them to.

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