Dynamic Drainage Bypass Shape - Road Weir vs Conduit

[Allan McManus]

Hello,

I am modelling a bypass string between two nodes, the upstream node being a kerb outlet (see VicRoads Standard Drawing 2051) and the downstream node being at the base of a swale.
The kerb outlet is at a sag point in the road.

Would it be more appropriate to use 'road weir' or 'conduit' for this bypass string?

It makes a significant difference in the results.

Many thanks

[Owen Thornton]

Hi Allan,

I just looked up the drawing you mentioned, and it is for flow rates no greater than 20 L/s. I would be inclined to model the outlet drain link as a conduit, not a weir. And I would probably model the kerb outlet node as on-grade, not a sag, as water won't really have an opportunity to pond.

What sort of difference in results are we talking about, with conduit vs weir?

[Allan McManus]

Hi Owen,

In the major storm water actually backs up almost all the way to the kerb outlet, which seems to 'choke' the water a bit.
The HGL at the kerb outlet drops by about 100mm when conduit is used, compared to road weir which is higher.

Kind regards,
Allan

[Owen Thornton]

I am fairly confident that "road weir" is not appropriate for this case. I might be inclined to investigate the "side weir" option, but I confess I don't know how appropriate it might be.

If the flow rate through the little kerb outlet is small, as one would expect, I just wouldn't expect a terribly significant loss across the kerb outlet, that drastically alters the flood extents on the road.