Stormwater Management

Kapiti District's stormwater network is made up of nearly 7000 stormwater pipelines, with a total length of more than 180km. Each year, the Council carries out 300 open-drain inspections and 360 culvert inspections.

Open Waterways

All open waterways maintained by the Council are managed with an electronic database that automatically generates a maintenance inspection schedule. The frequency of inspection and clearing is based on the level of risk that flooding could cause in the area, as well as past experience and the weather.

In general, urban drains have more potential to cause damage in flood events, so they are inspected more frequently than rural ones. The impact of weed growth in and around waterways is affected by seasonal weather conditions, so the frequency of inspection varies between monthly, three monthly, six monthly and yearly.

The Council inspects and maintains 45km of open waterways, which includes 118 sections of open drains. It also inspects and clears stormwater flapgates throughout the District, which allow water out but not back in again.

Culverts

A culvert is a pipe crossing beneath a road. There are 104 culverts in the Kapiti District.

The condition of our piped stormwater network can be assessed by sending a small closed circuit television (CCTV) camera through stormwater pipelines.

Controlling Vegetation

The main aim in clearing vegetation from drains and stream beds is to reduce the risk of flooding. The two methods used for vegetation control are hand-cleaning and mechanical excavation. Weed-spraying is not carried out because of concerns about the environmental impact on waterways.

Mowing around waterways is carried out on a regular basis and to a higher standard in high-profile public areas to keep them tidy. These are coastal strips, parks and reserves and the Wharemauku Stream walkways.

Maintenance

All stormwater pipe inlets and outlets are inspected and cleaned quarterly, as well as after heavy rainfall. Areas with high levels of sediment, such as the Waikanae hill catchment inlets, are inspected and cleared monthly.

Beach outlets are inspected and cleared monthly with the exception of Paraparaumu Beach between the Waikanae River and the Wharemauku Stream. These are inspected and cleared on a weekly basis to prevent sand build-up which blocks the pipes and could cause flooding to residential properties.

Many public stormwater enquiries are received every day, including information requests on floodzones, land contours and house flood risk. Many of these need further investigation including site visits or other research, and often result in physical works being carried out.

Flooding

In case of flooding, a stock of 8000 empty sandbags is kept on hand, with 500 filled sandbags always ready for immediate despatch. Council staff members have been trained in their use by the New Zealand Army, and in the event of a flood they will be used primarily to protect property.

During and after floods, the Council also pumps out flooded properties and sections, with houses taking priority. It also assesses flood damage, responding in person on a case by case basis.

In major flood events, checks are made to see if any stormwater network problems contributed to the flooding, and the necessary maintenance is taken out.

Strategy

Storms and elevated groundwater levels in 2003/04 increased community interest and raised important issues about overall links between urban growth management and stormwater.

The Council responded by:

• bringing forward the stormwater review with its focus on high level issues including:
• analysis of long-term climate change impacts and implications for levels of service choices;
• assessment of implications for existing stormwater infrastructure capacity standards;
• assessment of the implications for, and relationship with, development management;
• reviewing the prioritisation methodology to give greater emphasis to groundwater issues;
• reviewing site specific concerns.

Climate change implications for stormwater reticulation and stopbanks are potentially very significant and include an increased frequency and/or volume of system flooding; increased peak flows in streams and related erosion; groundwater level changes; saltwater intrusion in coastal zones; changing floodplains and greater likelihood of damage to properties and infrastructure.

The total implications of upgrading the system to maintain existing service levels and cope with climate change effects are unknown at this point.

For any enquiries about stormwater, please contact the Council.