An unprecedented number of heavy rainfall events have hit Whakatāne recently, with three of these events described as “very extreme rainfall events”. As a result, Whakatāne’s urban stormwater system has come under severe pressure and been unable to cope with the high rain levels and intensity.

• What is the Council doing to improve the stormwater system and reduce flooding in Whakatāne?
• How does the Whakatāne stormwater system work?
• Why is flooding occurring more frequently in Whakatāne?
• Why has the system been unable to cope during recent heavy rain?
• Why can’t stormwater pumps be activated sooner?

What is the Council doing to improve the stormwater system and reduce flooding in Whakatāne?

The Council is currently completing two stormwater studies. A whole-of-catchment study for both Whakatāne and Ōhope is being conducted in conjunction with the Bay of Plenty Regional Council. In addition, the urban catchment stormwater network is being assessed. Studies are also being made on overland water flows paths in urban areas.

This information will be used to identify ways to minimise the risk of flooding to Whakatāne and Ōhope urban areas and to determine the level of affordable flood protection that can be provided to the urban areas. The Council will then develop a programme to upgrade the stormwater system based on the studies' recommendations and affordability.

At present, the Council is working on preparing a new maintenance contract for stormwater drains, floodgates and silt trap cleaning.

How does the Whakatāne stormwater system work?

There are four major stormwater catchments in the Whakatāne town area - Apanui, Hinemoa, Whakatāne South and Awatapu catchments. The pipe network covers 48 kilometres and includes 903 manholes and 14 pump stations.

The stormwater system helps mitigate flooding in urban areas by releasing water through a pipe system, open drains and using pumps to discharge stormwater into the river.

Operation and maintenance of the stormwater system costs the Council $1.3 million per year and the total asset value of the system is $61 million.

Why is flooding occurring more frequently in Whakatāne?

Whakatāne has recently experienced ten high rainfall events. These range from one-in-five year to one-in-100 year events. Every event puts extreme pressure on the stormwater system. The Town’s stormwater system does not have the capacity to cope with these high intensity rainfall events. The volume of storage that is required in high rainfall events is difficult to find in built up areas, and stormwater is stored not only in the pipes and open drains but also in open public spaces like reserves, playgrounds and especially roads.

Water and debris flow from the upper hill catchments surrounding the town has increased in volume and speed due to changes in land use. This puts pressure on the stormwater system when heavy rainfall is occurring and afterwards. In addition, the build-up of silt entering pipes from hill catchment run-off affects the system’s ability to cope during subsequent heavy rainfall and increases maintenance costs.

Located in between hill ranges and the river, excess water has few avenues to escape. Most of Whakatāne town is situated on flat terrain, again limiting drainage.

Frequent heavy rain events and higher than normal average rainfall has led to the elevation of the ground water table over time, particularly in the Apanui catchment. This reduces infiltration into the ground and increases surface water flow. Urban development and infill housing has also intensified the flood risk, due to increased water run-off from roofs and hard impervious surfaces such driveways.

Why has the system been unable to cope during recent heavy rain?

The design standard of the town’s stormwater system is set to cope with a one-in-ten-year flood event. The present system is not meeting these standards. In addition, some of the stormwater pump stations are under capacity. The Council is currently investigating work required to improve the stormwater system and increase the design standard to ensure it can cope with events bigger than a one-in-ten-year event without flooding houses.

Why can’t stormwater pumps be activated sooner?

Pump stations are activated once water reaches a certain preset safe operational levels. If pumps are set to turn on too soon, they may start and run out of water, which will cause them to seize. This can cause significant damage to pumps that could take up to a week to repair at a considerable cost.

A telemetry system monitors water levels remotely, and pump station operators have 24-hour access to control the pumps. Pumps can be activated manually in special circumstances.

For more information, please contact Manager Utilities on 07 306 0500.