Information supporting the design principles for stormwater assets

Our stormwater design requirements

Adequate stormwater soakage may not be available at depths less than 5m in our district. Developers should reference the Engineering Code of Practice to find details of stormwater design requirements.

Other guidance to refer to:

Design Rainfall

We have developed a series of design rainfalls for those sites having high resolution temporal data. To calculate design rainfall depths for a given township within the Selwyn district follow our documented procedure.

Water Quality assessment

  • The water quality rainfall depth is generally accepted to be the first 15-25mm of rainfall (first flush) within the Canterbury Region. There are numerous means for calculating this depth with no specific requirements from the Regional Council.
  • Within the Selwyn District, where the water quality depth is calculated to be greater than 25mm then 25mm shall be adopted. Conversely, where the water quality depth is calculated to be less than 15mm the water quality depth shall be taken as 15mm.
  • Higher risk developments, e.g. industrial development, shall adopt 25mm as the water quality depth regardless of the calculated water quality depth.
  • For stormwater management devices designed based on flow rate, the design rainfall intensity shall be no less than 10mm/hr.
  • Three memos commissioned by Selwyn District Council and prepared by Pattle Delamore Partners (PDP) are provided for background information:

    Disclaimer: the memos shall only be used as a demonstration of the method and each site owner will need to provide site specific data for their particular development. SDC and PDP accept no responsibility for the use of the contents of these memos by any other party. Any use of the memos or reliance on decisions that are made by other parties based on these memos is their responsibility.

Water Quantity assessment

Control of peak storm discharge is required for systems discharging to surface water. Post development flows shall be no greater than pre-development flows for the following events:

  • During 50% AEP - Critical duration storm (2-year event)
  • During 10% AEP - Critical duration storm (10-year event)
  • During 2% AEP - Critical duration storm (50-year event)

The District Council or Environment Canterbury may also specify additional attenuation requirements specifically aimed at proving flood, erosion and scour protection.

Where the discharge of stormwater is to ground , the system should be designed for 'total detention' (combination of storage and overflow soakage) of the 2% AEP critical duration storm event. The suitability of the natural ground to receive and dispose of the stormwater without causing damage or nuisance to neighbouring property, shall be determined by a suitably qualified person/engineer.

This requirement may be relaxed, at the discretion of the Council, provided that a well defined secondary overland flow path is available.