TUFLOW Forum

# 1d/2d connection for Field Inlet and Kerb Inlet

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Hi all,

For C,R and W Type pit, the Tuflow manual says that they are weirs (in the vertical) . Does the vertical weir mean field inlet or kerb inlet? My understanding is kerb inlet. Once the water level goes above the kerb inlet height, orifice flow equation will be used. If this is correct, how shall we input the width and height for field inlet in order for Tuflow work for both weir and orifice flow?

Regards,

Eric

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Hi Eric,

As the variables describing the orifice geometry are for a 'vertical orifice', these variables cannot be directly used for a horizontal 'field inlet' type orifice (see QUDM section 7.05.4) without some fudging.

I think the best way will be to make a 'pit database' for field inlets. QUDM gives two equations for field inlet flow, 7.04 when flow is shallow and the inlet acts as a weir, and 7.05 when flow is deep enough to induce orifice flow.

You could plot both curves in a spreadsheet for a series of depths, and take an envelope of these to form the combined inlet curve.

Cheers

Paul.

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Hi Paul,

That's a great idea for the horizontal field inlet.

An issue regarding the transition point between weir flow and orifice flow is: To "take an envelope of these to form the combined inlet curve" is purley mathimatic data processing, you will just use the graphic cross point between the weir curve and orifice curve as the flow changing point. But at this point, the water surface level may be much under or above the field inlet level, which is not the appropriate water level for flow changing from weir to orifice. QUDM says that "For shallow depths the flow will behave as for a sharp crested weir. For greater depths the inlet will become submerged and inflow will behave as for an orifice".

So Tuflow may need to add a switch for flow transition based on water level (ie when the field inlet is submerged). It is better for Tuflow to add additional fields for width and height of the field inlet, so Tuflow can solve field inlet (horizontal) just same as kerb inlet (vertical). Tuflow may need a different layer for field inlet or define a flag for field inlet in 1d_nwk layer. It's just personal suggestion to BMT_WBM.

Thank you very much.

Regards,

Eric

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Hi Eric,

When modelling as a "W" type, the vertical input is the weir coefficient, which is typically set to 1 as default and does not represent a height.

As Paul has suggested, the most accurate approach would be to develop a pit inlet database with a Qh relationship for the inlet configuration. You would calculate this for the weir flow and orifice flow components separately and take the envelope of the two curves. You would then model the pit inlet as a "Q" type, rather than a "W" type, refering it to the appropriate inlet curve.

Cheers,

Dan.

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Hi Dan,

If you 'take an envelope of these to form the combined inlet curve', you will just use the graphic cross point between the weir curve and orifice curve as the flow changing point. But at this point, the water surface level may be much under or above the field inlet level, which is not the appropriate water level for flow changing from weir to orifice.

Only the Tuflow model knows when each field inlet is submerged, and when the flow type should be changed from weir to orifice for each field inlet.

Thank you very much.

Regards,

Eric

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Hi all,

I'm by no means a regular user of inlets, but just from reading the previous posts, instead of taking the envelope of the weir and orifice curves, couldn't you apply the weir curve until the height at which the field inlet is submerged and then use the orifice equation above that? (This may need a check to make sure it's still monotonic). You know where the top of the field inlet is just as well as TUFLOW does, don't you?

Any use?

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Hi PHA,

The 'H' in QH curve for weir/orifice flow is the water depth on the 2d dells around the field inlet which is different from the water level on/above/below the field inlet. Say there may be 50mm deep water on the 2d cells around the field inlet, but the water level in the field inlet may be 1 meter below than the field inlet. In this case, the field inlet is not submerged. See figure 7.05.4 and 7.05.5 in QUDM (Queensland Urban Drainage Manual) for the 'H' in QH curve of weir/orifice flow and water level on/above/below the field inlet.

I think that we have different understanding about "WHEN". I mean we do not know "at what time" each field inlet is submerged. This time only can be caught by Tuflow during the simulation, and the flow calculation should change from weir to orifice at this time for the submerged field inlet. Of cource, we know that once the water level on the ground goes above the field inlet, the flow changes from weir to orifice.

Regards,

Eric

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Ah, I see. Not so straight forward as I'd hoped!

I think I'll be keeping an eye on this thread in case I do ever have to do field inlets myself. Good luck then, and let us know if you come to any satisfactory solution.

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Hi Eric,

Yes, with the approach of taking the envelope of the two curves (or alternatively taking a fixed depth above which the inlet becomes submerged) you are making an assumption as to when the inlet actually becomes submerged.

However, there is inherent uncertainty elsewhere in the modelling, such as the assumed blockage of an inlet. Then the level of uncertainty in the hydrology massively outweighs that of the hydraulics anyway.

I'm approaching this from a flood modelling perspective, where the uncertainty in hydraulic representations such as these are small in the big scheme of things. However, the importance of such issues will obviously change according to the intended use of the model and the required outcomes.

Many thanks for initiating such an interesting discussion.

Cheers,

Dan.

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Hi Eric,

It would be nice to use equations to set up the field inlets and also the kerb inlets. Lab testing usually assumes inlet control, so when the d/s HGL rises, flow rate would normally reduce as the HGL flattens, which is not captured in Lab testing usually.

The equations from QUDM for the Field Inlet do look ok, but as discussed, there is a transition between weir and orifice flow where the equations would not work so well. As Dan suggests, in the greater scheme of things the shape of this transition and its impact on flood levels is probably negligible. See attached spreadsheet and the Field Inlet worksheets.

Using the weir and orifice equations to match a kerb inlet inflow is quite difficult, without a lot of fudging of geometry and coefficients. See attached spreadsheet and the Kerb Inlet worksheets.

Therefore, to gain control and assure compliance with QUDM, it seems best to use the Pit Databases for Kerb Inlets (as it is hard to get a good fit using equations), and because a vertical orifice (used in TF) is different to a Field Inlet's flat orifice(not used in TF), use Pit Databases for Field Inlets also.

See also this post for reverse flow:

Cheers

Paul.

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Here is the spreadsheet...change the extension from .txt to .xlsx

Field Inlet Setup.txt

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