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Q Pits

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Hi All,

I'm using a Q pit to control flow into a gully, and I'm wondering if there is any way to specify two different ratings for one gully, depending on downstream water level. I'm using the equations for weir and orifice flow from USFHA Hec-12. This is fine until the water level in the pipe network rises above the ground level, and the depth used for calculations is the difference between 2d and 1d water levels.

In my case this difference is small (0.1-0.2m), but I would assume the orifice formula would still apply. The standard rating uses weir flow at these lower depths - is there some way to override this when head in the pit exceeds ground level?

Cheers

Richard

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Further to this question, what exactly is the "width" attribute calculated from? I see that it is used to select the number of 2D link cells, which would suggest it's the flow width in 2D approaching the gully - if so does this still apply in sump conditions? The orifice equation uses area - is this not extracted from the table to extend the depth discharge curve?

Thanks

Richard

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Hi Richard

At some point as the 1D pit level rises above the 2D ground level the flow will become downstream controlled tending to zero as the water level difference tends to zero, but it would be possible for orifice (or other) flow to apply before downstream control kicks in. If your water level differences are only 0.1 to 0.2m, orifice flow would only occur is the total depth is shallow.

Either way, at present you can't override the current arrangement of using the difference in water levels once the pit is surcharged, but we could look into this as a new feature. There would have to be a smooth transition with the y-Q curve and it would have to tend to zero flow as the water level difference approaches zero. Any suggestions (ie. industry standard equations or an alternative curve) would be welcome.

The width value in the pit dbase is used, like you say, to control the number of 2D cells automatically linked, and is also used to calculate the area for estimating the velocity (the area used to calculate the velocity is taken as the minimum of the area in the pit dbase and the width times the depth), but is otherwise not used in the hydraulic calculations. Another width (output as the "Orifice W:" value in the .eof file) is automatically calculated that gives the equivalent flow at the top of the y-Q curve based on the orifice equation. This width times the upstream depth is then used to extrapolate the y-Q curve based on the orifice equation.

I hope this makes sense!

Cheers

Bill

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Thanks Bill, that makes sense.

I guess this is the transition between regimes L and F - in this case it sounds like it would be downstream controlled (F) so the inlet database would be ignored?

Cheers

Richard

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Yes, it would be (for a very short culvert), but note that for Q pits the flow is purely based on the depth-discharge curve which would be approximating this. We could look at overriding the y-Q curve under specified conditions, but this can be problematic, especially if the curve and the flow regime overriding it don't transition nicely (ie. a sudden change in flow occurs).

Cheers

Bill

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