Overflow management is deciding where a full cistern’s extra water goes: out through an overflow pipe set at the same level as the inlet, screened against insects and animals, and led away to a soakaway or drain well clear of the tank and its foundations. Without it the tank is damaged or the water backs up into the gutters, and in a wet season the tank will certainly be full more than once.
Part of Cisterns and Rainwater Storage
Overflow management is the planned path for water that arrives after a tank is full. It protects the tank, the ground around it and the water inside from the one event every rainwater system meets: more rain than storage.
A Full Tank With No Exit Damages Itself
When the tank is full and rain keeps falling, the water has to go somewhere. Without a planned route it rises up the inlet and backs into the gutters, spills over the rim and erodes the soil at the foundations, or pushes on the lid and walls. The cisterns page lists “no overflow pipe” as a cause of a damaged tank or water backing up into gutters, and its fix is an overflow pipe at the inlet level routed to a soakaway.
In the worked example the roof yields 108,000 L in the wet half-year (Rainfall Data) against a tank of 20,000–30,000 L. The tank fills several times over, so overflow is the normal wet-season condition and not a rare accident.
The Two Parts Of The Answer
- The pipe that takes the water out of the tank: Overflow Pipe
- The place the water goes: Soakaway, or a drain or ditch
What You Need
- An overflow pipe at least as big as the inlet, set at the inlet level
- A fine mesh or flap on the outlet end, against insects and small animals
- A disposal route — a soakaway or a lined ditch — downhill of the tank and of any well
- A walk-through at the start of every wet season
Method: Plan The Exit Before The First Rain
Step 1: Decide where the water goes while you are still designing the site.
Step 2: Set the pipe at the inlet level so a full tank spills out and not back.
Step 3: Screen the outlet. An open pipe is a path for mosquitoes, rodents and frogs into the tank.
Step 4: Check the route during the first heavy rain and watch where the water goes.
Common Mistakes
- Sending overflow toward the tank wall. It undermines the foundation.
- Connecting overflow to a drain or sewer that can back up. Dirty water can flow back into the tank. Leave an air gap and keep the tank’s overflow separate from any waste line.
- No screen on the outlet. Mosquitoes breed in the tank.
- A pipe smaller than the inlet. The tank cannot pass what arrives.
What’s Next
- Learn next: Overflow Pipe — sizing and placing the pipe
- Combine with: Flood Control and Drainage — where the water goes next