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Washout and over flow pipe

Washout and over flow pipe

Washout and over flow pipe

Hello their
I'm working on a ground reservoir with a volume 4000m3 , In let pipe is 300mm ( 2 pipes ) , is there any design criteria regarding the sizing of the pipe size of the washout and the over flow pipe? or rule of thump?


RE: Washout and over flow pipe

Figuring that out is what you get paid the big bucks for.
Consider this though; the washout/overflow pipe(s) needs to be sized to flow all inlet flows to the reservoir. This includes, but is not limited to:
  • flow from the inlet pipes
  • rainfall
  • runoff from the surrounding terrain

RE: Washout and over flow pipe

Also the length of pipe required to get the runoff far enough away to prevent washing out the wall, plus accounting for possible blockage by debris.

RE: Washout and over flow pipe

Maybe it's me but what do you mean by"ground reservoir"??

Is it hole, a bunded area, a tank or what

Washout?? Of what?

Why is this in the mechanical forum?

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

RE: Washout and over flow pipe

Not sure what you have, either.
For a water storage tank (ie, cylindrical container)-
Typically, the overflow is sized for the largest possible inlet flow. It will consist of a weir box, with length of weir adjusted to match flow rate, depending on 6" or 12" liquid level above the weir. And an outlet pipe. The outlet pipe is sized based on orifice flow from the liquid surface to the entrance into the pipe, as though the drop pipe beyond that point were not there. For most cases, a vertical pipe beyond that point will calculate out a larger flow rate, but with a partial vacuum in the pipe, which may or may not exist in reality. A long run of horizontal pipe may reduce the flow rate, and should be considered.

I don't know what a "washout" is. A drain nozzle? A flush type shell opening? A sump?
Anyway, in all three cases, they are sized based on convenience of the people using them.

RE: Washout and over flow pipe

Thank you all,
I mean by Ground reservoir , Concrete reservoir constructed on ground so it is not elevated tank
The Tank is square tank 4000 m3 , 30*35*hight 4-5 m , with 2 inlets and 1 suction pipe ( outlet ) to pump station.
The inlet pipes are 200mm
Outlet pipe is 200 mm - header- as a suction pipe to pump station , 2 pumps in duty + a stand by 160 m3/hr each
The inflow is around 300 m3/hr as I think

Attached is a schematic for the reservoir

RE: Washout and over flow pipe


Thanks for explanation. Still not sure why its in the mech eng forum but hey.

Dbill 74 has it right.

The system needs to be designed so that it doesn't fail and any event can be safely contained.

The outflow sizing is up to you depending on what the reservoir is being used for, how inlet flow is controlled and what max inflow could be.

Overflow needs to allow for worst case inflow with no normal outflow and maximum height allowable above the bottom of the inlet pipe. As the only driving force you have is the height of the water the overflow is often a lot bigger than the inlet pipes, especially if the inlet pipes are a pumped / forced main supply. There is also the issue that overflows often work as part filled pipes / culverts and the calculations are a bit different to normal fully filled pipes.

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

RE: Washout and over flow pipe

Thank you for your reply

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