A Medium Sized Rain Water Collection System -- Overflow System

This page covers building the overflow for the rain water collection system.

overflow pipe on rain water collection system

The tank needs to have an overflow system of some type to handle the situations where the tank fills up with harvested rain water and needs to have a way to get rid of the excess.

We drilled a hole in the side of the tank up toward the top and attached a drain pipe to this that pipes the water out away from the tank. 


Overflow System

We used a plastic bulkhead fitting to make a tight connection of the overflow pipe through the tank wall.  Since our tank did not already have penetration for a drain pipe, we drilled a hole to fit the bulkhead fitting through.    Once the bulkhead fitting was in, a threaded adaptor was used screwed into the bulkhead fitting, and then 1 inch PVC pipe was solvent welded into the bulkhead fitting.  The PVC drain pipe was extended down the tank and then out away from the tank a ways.  The pictures below show the construction.

The plan is to extend the overflow pipe out to the nearby trees to provide them with some additional water.

drilling overvlow pipe hole
Drilling the hole for the bulkhead fitting -- this is
a wood boring drill in a brace.
Be careful not to let the plug drop into the tank.
bulkhead fitting for overflow pipe
The bulkhead fitting. Note that since I could not
access the back side of the fitting, I just relied on
screwing the front half of the bulkhead fitting into
the tight fitting hole and sealing it with silicone -- this was
fine for a connection that is above the normal water line.


Overflow pipe.
overflow stake
Ground support stake for overflow pipe.

It may be that we skimped a little on the overflow pipe diameter.  With just gravity flow and the water level building up to (say) 6 inches above the overflow pipe, the flow rate will only be about 2 gpm.  With a heavy rain (say 1 inch per hour), the flow rate would need to be about 10 gallons per minute.  In order to push 10 gpm through the 30 ft of 1 inch overflow pipe, the head would have to be about 2 ft of water.  So, the tank would have to fill and slightly pressurize itself to get the flow out the overflow up to what a heavy rain would bring.   This is probably OK as 2 ft of head should not bother the tank, but perhaps going up to a 1.5 inch overflow line would have been better.

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Gary July 3, 2011