In all cases the stormwater:

• needs to be collected and controlled.
• may need to be treated to remove pollutants, either large gross rubbish pollutants, or hydrocarbons and oils, or just normal sediments entrained in the water flow.
• needs to be disposed of by a piped system or another method, to a suitable area, or stored.

Stormwater not properly controlled can cause extensive damage

The treatment of stormwater is becoming a common requirement these days and may require the use of a (normally buried) proprietary system, the size of which is dependent on the expected water flow. These systems treat the water in low flow events and treat the "first flush" water in larger events, as this is when the predominant content of gross pollutants and/or oils are entrained. The systems remove and hold the large rubbish, collect the oils and the sediments, and ensure that the remaining runoff water is as clean as possible.

Stormwater systems are usually designed to cater for the worst storm event in an average 20 year period, but should also have an overflow capacity in the case of a larger storm. The overflow capacity should allow overland flow and the prevention of any localised building flooding.