In Western Australia you are required to dispose of storm water on your block, rather than to a public storm water drain.
There are also many areas that don’t have public stormwater drains including many beach side areas.
In either case if this applies to your block then you will probably need soakwells, so here is some information.
How Soakwells Work
Basically the soakwell is an underground storage tank with holes in it so it ‘leaks’ into the soil.
The soakwell is surrounded by gravel which helps distribute the water ‘leaking’ out of the tank into the soil. It also prevents sand running into the tank.
Traditionally soakwells have been constructed out of precast concrete like this round example on the right.
Now there are a range of options which include:
Plastic Tanks Made of Polyethylene, or Glass reinforced Plastic, these are really just a version of the concrete soakwells
Drainage Cells These are plastic mesh boxes that look something like plastic milk crates. They are stacked in the excavated whole wrapped in a geotextile. Make sure you have a Silt Trap before the drainage cells.
In general soakwells should be further away from all buildings and boundaries than the depth of the soakwell (measured from ground level).
On restricted blocks I have heard of the soakwell being installed under the house. . . . perhaps OK on sand but not so good if there is clay soil (see Building on Clay)
The size and number of soakwells required depend on the following:
- Infiltration Rate The rate at which the water can be absorbed into the soil.
- Maximum Rainfall The storage volume should be able to take the flow from the maximum rainfall less the amount of water that can ‘infiltrate’ into the soil.
- Groundwater Level To be effective the entire volume of the tank needs to be above the groundwater level.
You should contact your local council to find out their requirements with regard to soakwells. A typical requirement is 1 cubic m of volume for every 80 sqm of roof.
For sites that have rocky or clay, the volume required will be much larger.
See Drainage for more posts