Designing a weir wall

[alfaromeo99]

Hi,
I need to design a weir wall. The weir is to be sized to come into operation at flows of 1300L/sec and upwards. The expected extreme flows can be up to 4000L/sec, therefore my weir wall needs to be sized to remove flows from the existing system upwards of 1300L/sec to a max of 4000L/sec (2700L/sec).
The weir (and bypass) is to be constructed in a tunnel system (sandstone) and the size of the tunnel is 1.5m width x 2.5m high and is unlined (rough as guts although the invert is conc lined). The invert is bowed in shaped. I reckon the height of the weir has to be approximately 0.45m high but am unsure how to work out the length of the weir. Can anyone show me the calculation to determine the actual height and weir length?
A

 

[BRIS]

Your post is not clear but I assume the following

You have a tunnel and you want to construct an overflow/by-pass with a fixed weir.

The weir will start to operate when the flow down the tunnel reaches 1300 l/sec and you want it to skim off up to 2700 l/sec such that the maximum flow down the tunnel is 1300 l/sec. If my understanding is correct - what you want to do is not possible with a fixed weir. The solution will depend on how accurate you want to control the flows.

The method of calculating the head over the weir can be found in any standard text book. The length of the weir will depend on how much you can allow the head to rise between the water level at the crest of the weir up to its maximum discharge of 2700 l/sec. The length can be increased by using duckbill shapes. (i.e zig zagging the plan shape)

Other non-mechanical options, instead of a weir, that can limit the rise in level include self priming air regulated siphons.

But, you will need to describe the problem in more detail.

 

[rcooper]

BRIS,

What you describe in your third paragraph can just about be achieved by having a long, long channel with a side weir height equal to the critical depth of flow at 1300l/s. Having travelled along the channel, the water flows off the end of the weir into a chamber and away. The length of weir required is a function of the amount of flow which has to go over the weir.

I have come across a weir designed like this once before. It worked really well at the flow rate it was designed to operate at The problem we had was changing the flow rate. We ended up demolishing the weir and re-building another with a flow control penstock and flow meter providing a feed back signal.

 

[vpl]

You might find this helpful:

http://www.ieindia.org/publish/mc/0404/apr04mc4.pdf

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[alfaromeo99]

Thanks for the input.
Another thought was that if a single side weir results in a really long length of weir wall then we can look at designing a double sided weir.
What are your thoughts on this?
A