Clogging potential: orifice vs. slot

[pointsman]

Fellows,

let's consider a concrete lined drainage channel conveying stormwater collected from a railway platform. It should be relatively free of debris, but obviously twigs leaves and branches are a possibility for clogging. The flow out of the channel must be regulated to 25 m3/hr, at the maximum channel head of 0.5 m. Sketch of channel attached. I've been exploring solutions to avoid clogging, and i started with a v-notch weir, but found the notch angle to be only 5° (unsuitable).

I'm now considering an orifice or slot to choke the flow exiting the channel. Which do you think would be more advantageous from a maintenance perspective? I find that the orifice would have a diameter of 7 cm, while the slot would be 4 cm x 10 cm (width x height).

Much appreciated.

 

[swazimatt]

I would put my vote on the slot. if something blocks it, the water level will fill by the blockage amount and there will be open slot above it, but not sure how that will affect your required flow. You could also account for some blockage and increase the width of the top of the slot (stepped slot)

not sure what your grade is, but steep channel is not going to provide much storage

 

[psmart]

As swazimatt points out, you also need to consider the available storage. Assuming that you are trying to limit (regulate) the flow to a certain maximum, you will have to detain the excess volume and release it over time. You cannot do this with a flow-limiting device alone. You also need an appropriate amount of storage to hold the detained volume.

There was another thread with a similar issue, wanting to limit the flow across a boundary line by installing a flow restriction, but not making any allowance for the volume of water that it would inevitably detain. In each case you are basically building a detention pond.

 

[pointsman]

The volume is taken care of, the slope is very shallow and we are only collecting from a width of a few meters along the tracks.

Just need to sort out the maintenance on this hole.

 

[swazimatt]

this SW tank manufacturer has a novel anti-clogg system, but not sure if it can be scaled up to work for you. Possibly a smaller dwonstream weir that allows a tailwater to develop an provide backpressure on the orifice

TankGuard®️ | Promax

TankGuard®️, the innovative solution for enhanced stormwater detention system performance, ensuring controlled discharge, easy application, and blockage prevention.

www.promax.co.nz

 

[pointsman]

Having taken a look at the constructability side, I've come to the conclusion that a circular orifice will be the best fit for the project.

That's a rather interesting method for anticlogging Swazimatt. The overflow equalizes pressure across the obstruction and then the crossflow velocity sweeps it away. For this situation I think it would be overkill. However I'm curious, have you had experience seeing if it works in practice?

 

[swazimatt]

I have seen it work as a demo at a water conference, not sure what the real-life results are, but NZ is small and rainwater tanks are becoming a requirement on all new builds so imagine I would have heard if they didn't work. NZ is also highly regulated so there would definitely be first-flush diverters on all these systems

Having taken a look at the constructability side, I've come to the conclusion that a circular orifice will be the best fit for the project.

That's a rather interesting method for anticlogging Swazimatt. The overflow equalizes pressure across the obstruction and then the crossflow velocity sweeps it away. For this situation I think it would be overkill. However I'm curious, have you had experience seeing if it works in practice?