The Nelson City Council has been responsible for stormwater disposal in the city since the first piped disposal system was installed in approximately 1907. Council's stormwater and flood protection system includes urban rivers and streams and the urban stormwater reticulation system. The NCC stormwater system can be categorized into two parts:
- A natural component consisting of 27km of rivers and streams
- Constructed stormwater system consisting of 175km of mains, two pump stations and 420 outfalls to rivers, streams or to salt water
The natural component consists of rivers and streams that play an important role in the support of aquatic ecosystems, areas for recreation and public use and the channeling of stormwater flows in rainfall events. During high rainfall events the rivers and streams transport large volumes of water and sediment with impressive levels of energy that are capable of causing significant damage to property adjoining these areas and within the flood path, should the flow escape the channel.
The constructed stormwater network provides water channels to drain stormwater and flows from roads and properties in the built up areas. The network includes pipes, channels, and overland flow paths such as streets that bring stormwater to the sea. The stormwater system also incorporates pump stations, and some debris detainment devices. In a large part of the city a fully reticulated system is not provided. Individual properties discharge stormwater to onsite soakage or to the road channel as part of the primary drainage system.
Rationale for Council's stormwater programme
Health and safety / property protection
Large parts of Nelson City occupy a narrow band of land wedged between the sea and low foothills, with the majority of development taking place in areas close to or influenced by rivers and streams. The provision of a robust stormwater system allows people to carry out their day to day lives with minimal disruption or adverse health effects from rainfall events or ponding water.
Protection of environment and community well-being
The natural stormwater system comprises vegetation and ground soakage with overflow to rivers and streams in high rainfall events. For the majority of time, rivers and streams sustain aquatic ecosystems and are increasingly linked to private properties and recreation activities, particularly for the larger rivers, by the residents of the city. This system is also where the bulk of materials washed from roads and private properties in the city first contact the aquatic ecosystem.
As the development of the city encroaches onto historical flood plains the multiple demands on the available river and stream areas needs to be balanced to ensure the long term best possible outcome for the city.
Council has statutory obligations to continue to provide stormwater services recognising the requirement to take a sustainable development approach that takes into account:
- the social, economic and cultural wellbeing of people and communities; and
- the need to maintain and enhance the quality to the environment; and
- the reasonably foreseeable needs of future generations