Venturi effect, stormwater drains

[Trevor Bannister]

Apologies, physics is more my thing rather than engineering so please bear with...

A river and a road run parallel to each other. There are drains from the road that run into the river. Under normal conditions, the river level is lower than the road, any surface water on the road simply runs ino the drain chamber, through the lateral pipe and into the river. The end of the pipe is flush with the vertical river wall and has a hinged flap over the opening, the pressure of the water in the pipe pushes the flap open.

When the river is in spate, the river is above the outlets and the river water pushes the flaps closed; thus preventing river water backing up through the pipe and spilling onto the road. In some locations this is fine but in other locations it prevents the roads from draining leading to regular flooding and road-closures.

Would it be beneficial to remove the flaps from the ends of the pipes and put a 45° to 90° elbow on the end of the pipe, the new outlet being alligned with the flow of the water. The thinking being that the river water rushing past the angled outlet would create a venturi type effect and draw water through the pipe such that, in theory at least, the river level could be higher than the road level and still draw off water from the road. In spate, the river flows at between 2m/s and 8m/s depending on cross sectional area, pipe diameters vary but are typically between 100mm and 300mm.

Many thanks if you are able to advise on this. If you think it's practial, we can look at developing this as a tool to help reduce the flood risk.

 

[gbam]

Maybe, first off you are introducing head loss into your pipe network by placing a 45^ or 90^ angle in the pipe. One could run through the hydraulic computation for a couple of key locations to see if more locations would benefit with the evaluation.

I will mention if there are flap-gates on a system I would definitely check the surrounding area for elevation compared to the flood stage. There is a reason the gates are on the system and it may be more in-depth than just roadway drainage.

 

[cvg]

in general rainfall runoff from roads does not always coincide very well with river flood stages, so in practice this may be only partially effective. if water is spilling onto the road than there is not enough freeboard on the levee. the typical and more robust approach is to raise the road higher.

 

[Ekenny_G]

To add to what has been mentioned already, water will flow downhill unless there is an obstruction. if the River flow is higher than the pipe outlet, water will back up the pipe unless there is added pressure from the upstream drain chamber. Angling the pipe outlet in the same flow direction as the river is expected to allow some minimal flow to continue from the pipe as the river rages downhill. you will have to compare the pipe outlet velocity to the the river flow velocity and if the river flow is laminar or turbulent.
Additional research or testing in a lab is warranted or some empirical flow data to support your proposal. This could be a cheaper (less expensive) alternative to raising the road.