Upgrade of existing water canal/pipe

[SA07]

Hi
We are working on a project to refurbish a water canal which carries raw water from a small dam on a river to a water tank 7 km away. The water flows through gravity between the dam and the water tank. There is a height difference of 35 m. This is an old installation since 1888. Some parts are open canal, some parts are pipes. We have problem of soil erosion, soil falling inside the open canal, trees falling inside the open canal, people who break the pipes or canal to take water to irrigate their vegetable fields. The expected flow is around 1000 m3/hr. The water is used for sugar cane field irrigation and factory operation. Most of the route of the canal is inside sugar cane field and at some places it passes under the road where there is normal traffic.
We are considering to install pipes buried underground. Can you plz advise us on the following?
a) Type of pipe we can use Ductile iron, HDPE or others. What is the lifetime of recommended pipe?
b) Diameter of pipe
c) Depth to bury the pipe underground
Any other important aspect to consider?
Thks

 

[coloeng]

Would recommend that you engage the services of a civil engineer that has experience in this area. There are too many unkowns to provide answers to all your questions.

 

[SA07]

Civil engineering contractors will be requested to quote for the project. In parallel, we want to double check their proposals with up to date practices from abroad.

 

[LittleInch]

Randhir,

If you're doing the concept design or something similar then you need to think about the following things. Is the contractor a design company or installation contractor? At this point it sounds like you need someone with design experience. YOu can also try the AWWA publications

https://www.awwa.org/

You might need to buy them but they will add a lot to your understanding and background knowledge.

You need a profile all the way. However if the current design is an open channel then it would appear that there are no high points along the route and if falls evenly ( apart from road crossings) all the way from dam to water tank and you have no valves or restriction in flow at the tank. 35m over 7km is quite a decent fall, so 500mm per 100m or 5000Pa/100m.

So if you want to enclose the "canal" then you have two choices - a full pipe or a partially full pipe (like a sewer).

A full pipe is smaller, but would need some pressure control at the tank end in order to maintain the pipe in a full condition and hence your pipe would need to be capable of about 3.5 bar pressure rating.

An open channel flow design could be larger with air vents at regular intervals to maintain the open channel. If you have some long flatter sections then you may end up with a full pipe. A constant fall is recommended. But then this could be something like concrete pipe with a low pressure rating.

So type of pipe is often dependant on what is available at lowest cost in your particular location - is it Mauritius?

DI lasts for decades and is usually quite cheap. Bends etc available. Needs some sort of connecting mechanism to prevent joints pulling out as you can't weld it.
PE similarly, very flexible and forgiving. Easily fused together to create a single long pipe
Concrete segmented pipes could be used depending on design pressure. Usually push fit so needs connectors or anchor blocks at changes of direction. Last a long time and resistant to people trying to tap into it.
Concrete lined steel pipe may be a choice, but probably too small

Diameter - work it out from freely available graphs. For a full pipe design for 1000 m3/hr I think it's about 20" / 500mm for DI and PE pipes.
For an open channel flow you're probably into 30-36" 900mm pipe.

Depth of cover is usually 3 ft / 900mm min. Enough to protect the pipe without needing to dig a huge hole


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[SA07]

Thks a lot LittleInch for info & advice.

 

[LittleInch]

Last thing - you need to try and maintain a velocity of about 0.8 to 1m/second in order to keep any sediment in train and not fall out and block up the pipe. Having some manholes every say 500m is a good idea if you need to access to flush out a section with e.g. a truck load of water or insert a pig or other type of cleaning device.

PE is very smooth and if you remove the internal bead it should help to prevent any internal growth of plant / animals etc or build up of dirt. DI and concrete can get rough over time and allow more debris to attach.

Also 130 yr old system is pretty good to last that long. Any photos?

Let us know how it goes and what you decide to do - we like feedback on this site, even if it's months later,

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SA07]

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[LittleInch]

So a 20" pipe looks about right.

That's a right mixture of pipes and channels you have there....

Could you re-use some of those pipe sections and just lay more pipes inside the channel sections to tie it all together?

It doesn't look like 1000 m3/hr or is a lot getting bled off by others?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

a) Type of pipe we can use Ductile iron, HDPE or others. What is the lifetime of recommended pipe?
The type of pipe will depend on the local pipe manufacturing industry if there is one. Pipe in this application will last as long as your present system, over 50 years.

b) Diameter of pipe
The diameter of the pipe will depend on the elevation drop on the pipe route.

c) Depth to bury the pipe underground

You appear to be in a warm climate and the minimum depth for pipe burial is about 1 meter.

Do you have drawings of the existing system? Do you have easements for the route?

 

[SA07]

We have a water right of 1000 m3/hr. The flow is controlled by a manual valve near the dam. We have a way leave. The canal passes partly through the land of our company, partly near the airport.