Napier received almost twice its average September rainfall yesterday. The city's wastewater network came under intense pressure.
Rain data taken from McLean Park recorded more than 90 mm of water falling on the city in the 24 hours to midnight last night. The average total rainfall for the whole month of September, according to NIWA, is 58mm.
Overnight the rainfall intensity was significant at times and Council placed a message on Facebook last night asking the community to help by reducing their input into the wastewater network to reduce the load. Napier City Council Chief Executive Wayne Jack thanked the public for their efforts last night to reduce their water use and strain on the network.
Staff were managing pump stations and also brought in vacuum sucker trucks to help ease the pressure.
The wastewater and stormwater networks are meant to be completely separate. Wastewater covers the sewage system, and interior building drainage such as from showers and dishwashers, while exterior downpipes on buildings feed into the stormwater network.
Problems arise, as in this case, with infiltration and inflow into the wastewater network, much of this caused by the illegal connections of stormwater downpipes and incorrectly installed gully traps, says Mr Jack. “We’re aware our network has issues and that there are multiple points of infiltration.
“To address this takes time, people and money. Our LTP allows for that, with $20.6 million allocated to address wastewater issues.”
NCC's Wayne Jack thanked the public for their efforts last night to reduce water use. “We’re aware our network has issues and there are multiple points of infiltration. To address this takes time, people and money. Our LTP allows for that, with $20.6 million allocated to address wastewater issues.”
This work includes $7.29m on pump station upgrades, $1.4m on industrial main improvements, $1.1m on improvements to the network in Taradale, $6.2m on new pipes, $2.5m on pump station improvements and a $2.15m programme targeting infiltration on top of the usual maintenance.
Next month smoke detection testing within the wastewater network will begin, to show where there are stormwater connections infiltrating the wastewater network. “The smoke is fed into the wastewater system and if we see it coming from any downpipes or other areas, it will be quite clear that these have been connected to the wrong network,” says Mr Jack.
Last night staff were aware manhole covers were beginning to lift, and the escaping water would have caused a public health risk. So the decision was taken to release pressure from the wastewater network and discharge wastewater, at 1am today, into the Purimu stormwater stream, which flows out to sea through the Ahuriri Estuary. At this time of the night wastewater flows are usually at their lowest, so it would be safe to say that almost all of the flow in the network at that time would have been stormwater.
Hawke’s Bay Regional Council, the Hawke’s Bay District Health Board and Mana Ahuriri were notified of the discharge.
Warning signs have been placed around the Estuary advising the public not to swim or collect shellfish from the area.
Water testing was conducted yesterday in the stormwater network as well as upstream in the Estuary to provide baseline data. Testing has been ongoing since the release.
“This type of rainfall – and the weather forecast is predicting more to come – means our wastewater network is still likely to be under pressure for hours and days after the rain has stopped falling,” says Mr Jack.
Rain falling in the ranges and hills outside the city are also likely to have an impact on groundwater levels - if the rivers are full, this causes groundwater levels to rise as it nowhere to drain away to, he adds.
Stormwater and high groundwater can enter the network at weak points – this is a common issue for most councils in New Zealand.
6 September 2018
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