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Lih_Chong

Pit inlet capacity and inflow relationships

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(Hi Bill :rolleyes: )

I want to model a 1D stormwater pipe network within a 2D domain. I believe that the network capacity is constrained by the pit inlets rather than the pipes. Is there a way to model the pit inlets using the 1D pit objects, eg setting the pit object to type "R" and applying the lintel length and height (for a kerb entry pit)?

Otherwise, is there a way to apply a known pit inflow relationship to the pit (similar to a pump, except limited by the tailwater conditions)?

I have previously been modelling pit inlets using a weir structure connected to the pipe network but I think this may be overestimating the pit inlet capacity.

Thanks

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(Hi Lih :rolleyes: )

Yes, it is better to specify pits as Channel_Type R with shallow/wide dimensions. Some modellers have created test models to establish the shape of the dimensions to give a similar inflow rating as per the pit inflow relationships they use. As a rule, using the dimensions of the gully trap (eg. 2m wide by 0.15m high) generally seems to work well. The R Channel_Type will tend to work much better than a weir, for, as you say, the weir will tend to over estimate the inlet capacity.

The Q Channel_Type has been reserved for specifying a pit inflow relationship. This feature has been requested by a number of modellers and is planned for the next release.

You should also use Build 2007-07-AF onwards due to the improved performance of culverts.

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Hi Vincent

For R (or C) pit channels, the flow is modelled as a zero length culvert channel. This generally means that if the flow is upstream controlled, flow Regime A (critical at inlet) or B (orifice flow at inlet) occurs, and if drowned out (downstream controlled) Regime D or F (see Table 4.13 and Figures 4-4 and 4-5 in the July 2007 manual). An R pit channel essentially behaves similar to a fixed gate structure (note the 1d_nwk Culv_H_Contraction_Coef attribute is akin to the sluice gate coefficient - usually 0.6 to 0.65).

For W pit channels, the broad-crested weir equation or submerged flow are the two regimes that can occur.

Cheers

Bill

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