So for starters, what is a retention or a detention tank?
Here is a clear definition from Auckland city council.
Detention tank: Detention tanks work by temporarily storing stormwater runoff during a rainfall event and then slowly releases the water into the public stormwater system. These can be located either above or below ground.
Retention tank (Water Harvesting) : A retention tank is used to permanently hold rainwater, which can then be reused for hose taps, toilet use and laundry purposes. Also known as a single purpose rain tank.
Dual purpose tank: Dual purpose tank both retain and detain rainwater. The bottom third of the tank is used to store rainwater permanently (retains) for non-drinking water supply and the upper two thirds of the tank temporarily holds (detains) rainwater and slowly releases it until it reaches the retaining level.
Water supply rain tank: These tanks store rainwater collected from roof areas and provide the main water supply to a household, this includes drinking water. This practice note does not address potable water supply requirements; for further information refer to Clause G12 of the NZ Building Code, which can be found at www.mbie.govt.nz.
Why are detention tanks desired by Auckland city council?
Auckland is growing! As such, its infrastructure must also grow to keep up with the demand, this includes its public sewer and storm water systems into which all properties discharge. Keeping up with this growth could cost tax payers Billions of dollars.
1000’s of new homes are being built with even more homes being extended adding hundreds of thousands of square meters of roof surface collecting additional rain water.
All this water needs to somehow be managed , if not, our public storm water systems will get overwhelmed affecting our public sewer lines and that is something no one wants for obvious reasons. So Auckland city council have come up with a solution that will slow down this infrastructure growth.
The solution was to simply make the managing of this additional rainwater the homeowners problem by introducing detention tanks. The surface area of the roof will determine the size of the detention tank. Rule of thumb is that 1 square meter or roof surface area will equal to 100 liters of detained water (this can change depending on council or conditions).
Detention tanks are designed to take the first load of rain, then with a controlled outlet slowly release the water back into the public storm water system. The size of the controlled outlet will be specified in your consent application.
The controlled discharge will allow the public storm water system to cope with the initial downpour, at some point the tank may completely fill up because the controlled outlet may not keep up with ongoing downpour. At this point in time the tanks will automatically overflow directly into public system. In theory, this should only happen every 100 years :).
In the images below you can see underground detention tanks have been installed (no pump). Having them underground is a great way to hide them. These specific models are very handy because they are only 1 meter deep allowing us in most cases to find a gravity solution rather than having to use a pump.
Drainage NZ can advise on many variations of retention and detention setups. This includes commercial and residential solutions. Simply give us a call if you have any questions.
My next article will discuss a maintenance schedule council urges home owners should maintain.