Site characterisation

Site characterisation is understanding how a particular site will respond to a rainfall event and the possible effects arising from stormwater runoff. This includes:

  • hydrological parameters (e.g. site coverage, soil type, groundwater, rainfall intensity, topography etc)
  • constraints (e.g. services, space availability, proximity to buildings etc)
  • contaminant generation (e.g. site use, likely contaminants, typical concentrations, annual loading etc).

Guidance on site characterisation

Waterways, Wetlands and Drainage Guide (WWDG)

Waterways, Wetland and Drainage Guide provides the most comprehensive guidance for site characterisation, covering all of the key parameters. Part B, Chapter 6 (PDF 8.02MB) sets out the methodology for doing this.

Stormwater Management Devices in the Auckland Region (GD01)

For site characterisation, refer to Section B1.4 (PDF 15MB) (and in particular Table 6), for an outline of the characterisation process. The Auckland data sources can be substituted with the Canterbury specific ones below.

Stormwater Treatment Standard for State Highway Infrastructure (NZTA)

This treatment standard (PDF 14.55MB) includes guidance on the site characterisation process and requirements (Section 5.2).

Runoff – rational method for small sites

The New Zealand Building Code (E1 Surface Water (PDF 1.59MB)) provides a verification method for the estimation of surface water runoff (section 2). This is a simplistic method suitable for small sites.

Information sources

Canterbury Maps GIS data

The Canterbury Maps website provides data on depth to groundwater, soil characteristics, and other information needed for site characterisation. Examples of useful maps are listed, but it is easy to also prepare custom maps:

Rainfall intensity

Local guidance should always be used where available, but there are two main options for areas where local standards are not available:

  • NIWA’s high intensity rainfall design system (HIRDS)
  • NZTA’s treatment standard (PDF 14.55MB) has a nationally applicable method for determination of water quality depth (Section 6.3 – but local methods should be used where available).

Soil type

S-Map online is produced by Landcare Research and has interactive soil maps. Of particular use for stormwater are:

  • soil drainage
  • depth to hard soil/gravel/rock.