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What is Stormwater Detention and Why Detain Stormwater?

When building a new home, local regulations might require you plan for and detain stormwater on your property. In a related article, we explored whether you need to detain stormwater and common requirements. This article just explains stormwater detention, what it is and why it is necessary for some properties to have.

What is Stormwater?

Before understanding “stormwater detention” it is best to first know what “stormwater” is. Isn’t stormwater created in storms from rain, so technically “rainwater”? Yes, but rainwater is commonly defined as stormwater once it hits the ground.

Unlike “rainwater”, which often refers to rain falling on rooftops and is of a higher water quality, “stormwater” is rain that meets ground surfaces. For example, what starts out as “rainwater” harvested from rooftops into a rainwater tank, becomes “stormwater” should it overflow from that tank onto the ground or into a drain.

What is Stormwater Detention?

Buildings in residential and commercial districts often need to plan for and meet local stormwater detention requirements. What is “stormwater detention” though?

Stormwater detention is where rain water flowing across ground surface areas such as driveways, paths, pebble rock, and the like, or water otherwise headed for the stormwater drain, is captured and stored before being released. Generally, stormwater is detained in a special water tank fitted with a slow-release valve that allows the water to slowly drain.

There are two tank options with stormwater detention:

  1. Two water tanks, one to harvest rainwater and the other for stormwater detention,
  2. Dual compartment tank, one for storing water, the other with a slow release value for emptying detained stormwater.

Why Detain Stormwater?

It might seem wasteful allowing otherwise high-quality rainwater to drain slowly away. Why not just buy a larger rainwater tank to store ALL the rainwater?

Some councils will allow higher tank capacities or swimming pools to replace “stormwater detention” methods of tanks. Other councils will require a stormwater detention system to be implemented. It really depends upon the council and rainfall patterns in your area.

The logic comes with understanding that during heavy periods of rain, the rain after pouring down will often pause, and then return an hour or so later. When it pours, it really pours, but then the rain subsides, before pouring down again. Understanding this, rainwater tanks will quickly fill up, perhaps even in the first downpour. This renders them useless in consecutive downpours.

Since a stormwater detention tank is intended to always empty it will be prepared to store more water in consecutive downpours. So, if a storm stops pouring down rain for an hour or two, it should be ready to detain another 2,000 litres of water by the time the storm returns. This allows the stormwater drainage network in your area to better manage water runoff caused during heavy storms.


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