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paul_ollett

'Pit Inlet Database' - reverse flow

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Hi there,

Just wanting to ask how TF detects when a Pit Inlet should be under reverse flow (ie, pipe HGL d/s of pit is greater than pit HGL). If reverse flow does occur, what equation is used? Eg, an expansion loss equation?

Pit Inlet charts are normally based on a free inlet under lab testing. However in reality (consider an orifice equation), as the HGL in the pipe rises, the head differential will reduce, and the amount of flow from pit to pipe will reduce. Therefore, do classical pit databases artificially inject too much flow into the pipe, up until the point where reverse flow is triggered? Hope this makes sense.

Cheers

Paul.

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Hi Paul

Good question. For Q pits (which I assume you're referring to), the depth value used to interpolate the flow is dependent on whether the water level (or pressure head) in the pit is above the pit inlet. The logic is as follows:

(i) When the water level in the pit is below the pit inlet level, the depth used to interpolate the pit flow is the 2D water level minus the pit inlet level.

(ii) When the water level or pressure head in the pit is above the pit inlet, the depth is taken as the 2D water level minus the pit water level or pressure head, ie. the depth is the head difference between 1D and 2D, not the actual depth of water. This means that as 1D pit water level (or pressure head) rises or approaches the 2D water level, the "depth" value approaches zero, and as TUFLOW requires the depth discharge curve to pass through 0,0 the flow will also approach zero.

When reverse flow starts due to the pit water level or pressure head being above the 2D water level, the same curve is used but in reverse and the "depth" value is the 1D water level minus the 2D water level (if the 2D cell is dry, the 2D ZC level is used). On the list is to be able to specify a different curve for reverse flow, but this isn't available as yet.

Below is the extract from the manual which should essentially say the same as above.

Cheers

Bill

Extract from Section 4.5.1.2 of the 2010-10 manual:

The depth value used to derive the discharge through the pit channel is taken as the water level in the 2D domain less the pit invert (usually the ground/2D level). If the downstream water level of the pit channel rises above the pit invert, the depth value used for deriving the pit channel flow is the difference in water level. This provides a smooth transition from upstream controlled flow to drowned flow. If the pit channel starts to be surcharged (ie. flow is reversed), the flow is extracted from the depth-discharge curve using the downstream water level less the upstream water level (pit invert if dry) as the depth value, and is applied as a negative discharge so that flow surcharges onto the 2D domain.

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Hi Bill,

Further to your comment above regarding the specification of a different Q pit curve for a reverse flow; is this option available for the current TUFLOW build?

Cheers

Nick

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