Double entry drains

Panic! · 30 Jan 2016, 1:45 pm

Sometimes, for various reasons the builders of drains decide to join two drains together. Often it seems to be they divert the upper reaches of an existing drain to a nearer exit to better cope with 1 in 100 year events. Examples of these include parts of The Maze, Anzac and The Falls, Big Ears, Burbon and Critts, Nylex, Very Tidal and Pho-Chi-Min. It is weird when you're exploring a drain side passage and find another drain heading downstream that's not the drain you started in.
Have others encountered these?

sydnavigator · 02 Feb 2016, 5:08 am

I know Fortress in Sydney was an amplification of another channel which was suffering capacity issues. So instead of fixing that drain they blasted a huge tunnel straight through a cliff to divert the upstream part of the catchment to the ocean via the new outfall. Strange thing is that the amplification follows no natural watercourse, so it's impossible to guess where it goes from above ground. I haven't been inside but apparently it runs really really really straight, enough to see the light at the end of the tunnel from 800m away.

Unfortunately the amplification and the original drain don't meet up though

Found a good chunk of info from the Sydney Water website:
https://www.sydneywater.com.au/SW/water ... rchresults

Panic! · 02 Feb 2016, 6:37 pm

sydnavigator wrote: I haven't been inside but apparently it runs really really really straight, enough to see the light at the end of the tunnel from 800m away.

Nahh, not quite. There's a ramp not far inside the entrance and a distinct curve right at the entrance so yeah, you can't really see light 800 metres away. But I've only been in it three times and all at night. Maybe it meets the other tunnel up past the submergence.

iso · 03 Feb 2016, 11:06 am

Not too sure, but I think Dwellers' Tomb may a similar set-up. There's a side drain (4850/6) that almost links up with the original outfall. Where this side drain is supposed to be I spy what looks like a 1 m RCP from above (to the west of the outfall), but I could be wrong. I assume it just leads to a small side pipe in the main drain that you otherwise wouldn't care about.

And there's the classic case of the opposite, where Anzac and Como Falls would have converged but were separated.