Why discharges happen - what's the problem
Read about what causes our city's wastewater network to overload in heavy rain and why emergency discharges happen.
What causes emergency discharges in heavy rain?
Gisborne city's wastewater network and stormwater network are separate systems. During intense or heavy rainfall, some parts of the city's wastewater network gets inundated with stormwater and the network can't cope with the volume of water going through it.
It then causes wastewater to overflow onto private properties and out of manholes onto roads, causing dire health risks to people.
To prevent this from happening, Council has to let the excess water out of the wastewater network. The only way to do this is to open valves and discharge wastewater into the river. The discharge is highly diluted with rainwater, but there's still a risk to health.
We seriously consider all options before we open the valves and discharge wastewater into our city's rivers. It's only done when it's absolutely necessary and only in the areas with issues in the network. The decision to open the valves is left until the absolute last minute when there's no other option.
We need to work together
The city wastewater network has been designed and built to manage the wastewater needs of households and businesses for growth over the next 30+ years.
It's important to note that Council only owns and manages 50% of the wastewater network. The remaining 50% is owned and managed by individual property owners.
Areas of responsibility
Council’s responsibility is from outside the property boundary.
This means you're responsible for the pipes from inside your house out to your property boundary.
At your property boundary your pipe (lateral) joins into the city's main wastewater network, which Council is responsible for.
Council’s half of the wastewater network includes 223km of mains pipes (that's roughly the distance from Gisborne to Napier), 2806 manholes, 40 pump stations, and the wastewater treatment plant.
For several years, Council has focused on fixing and improving our part of the wastewater network by:
• Increasing the size of Council’s wastewater pipes where they're under capacity.
• Upgrading the Council stormwater network to manage larger flood events.
• Replacing Council’s leaking wastewater pipes and access points.
Here's the problem...
Stormwater gets into the wastewater network and floods it. We have to get that excess stormwater out of the wastewater network by discharging it to the river.
If we don’t discharge it, wastewater overflows into people’s homes and out through manholes on the street, creating dire health risks to people.
Stormwater gets into the wastewater network from:
The diagram below ranks these issues by the impact they have on the wastewater network. The medium and high impact issues are responsible for 90% of the problems and are happening on private property.
The Council-owned and managed wastewater network is designed to accepted NZ standards, and under normal conditions would be able to cope with higher wastewater volumes in heavy rainfall events. The problem in Gisborne is that the amount of rainwater getting into the wastewater network, mostly from private properties, is extraordinarily high.
For example, it only takes 4 houses with their down pipes going directly into the gully trap, to overwhelm the sewer in a catchment area.
Just 2 properties with surface water flowing into their gully trap can do the same. Depending on the age of the house, we’re finding 10-50% of gully traps in any one street are not working correctly.
Opening the valves to discharge the overloaded pipes
We prepare before a heavy rain event. When MetService issue a heavy rain warning for the city, we go out and check sewer manholes, pump stations and valves. This is to make sure they’re working as they should. We monitor the network's hot spots, to help prevent overflows and discharges.
Our pump stations have alarms that tell us when there’s a problem, like too much rainwater in wastewater pipes.
Where possible we take measures like using big tanker trucks to pump this extra water out of overloaded areas, in an effort to avoid opening the valves into the river.
We get to a critical stage when there's just too much rainwater in the wastewater system, the above measures don’t do enough, and we unfortunately have to open the valves.
We therefore need the cooperation of homeowners in the city in order to reduce how often this happens.
We will have to keep opening the valves and discharging into rivers if rainwater keeps getting in from private property. We need the cooperation of city homeowners to reduce how often this happens.
Notification when we open valves and discharge into rivers
Our Water team notifies our Pollution Hotline when the valves have been opened.
The pollution officer contacts our Communications and Environmental Health teams. We advise residents via our Facebook page, our website and other media channels, we also send out an email to a distribution list.
The Medical Officer of Health is notified and we put temporary warning signs up at affected swimming and recreation sites.
Water quality testing is carried out within 48 hours.
Our water team notifies the pollution hotline when the valves are closed. We update the information on the Facebook post and website, The signs are removed 5 days after closing valves.
We recommend that people do not swim or gather shellfish for 5 days after any heavy rain, regardless of whether there's been a sewage discharge or not.
Number of discharges to waterways since 2017
7 September 2018 - Wainui Rd
6 August 2018 - Wainui Rd
11 June 2018 - rainfall 113mm over 7 days - Wainui Rd, Seymour/Turenne, Fitzherbert, Russell, Peel/Palmerston, Oak St sluice valve
4 June 2018 - intense rainfall in 8hrs - 59mm - Wainui Rd, Seymour/Turenne
3 September 2017 - rainfall over 48hrs, 76.6mm – Wainui Rd
28 May 2017 - rainfall over 48hrs, 75.4mm - Russell St, Wainui Rd
12 May 2017 - Cyclone Donna rainfall over 48hrs, 52.4mm - Peel St/Palmerston Rd, Russell St, Wainui Rd
13 April 2017 - Cyclone Cook rainfall over 48hrs, 64mm - Oak St sluice valve, Derby St, Portside, Russell St, Wainui Rd
4 April 2017 - Cyclone Debbie rainfall over 48hrs, 82.2mm - Oak St sluice valve, Oak St pump station, Owen /Seymour, Parau, Peel St/Palmerston RD, Portside, Russell St, Wainui Rd.