During last year's flood, more than 200mm of rain fell over a six-hour period overwhelming Napier's stormwater system which performed "as it was designed to" more than 50 years ago. Photo / NZME
During last year's flood, more than 200mm of rain fell over a six-hour period overwhelming Napier's stormwater system which performed "as it was designed to" more than 50 years ago. Photo / NZME
More than $341 million would need to be spent to protect against future flood events and overhaul Napier City Council's stormwater assets, a new report has found.
The report, released today on the one-year anniversary of the Napier floods, found that while it appeared parts of the system weren't running,it was "working as it was designed to".
However, the rainfall event, originally described as a "one in 250-year event" with over 200mm falling in the six hours between 2pm and 8pm on November 9, also highlighted "deficiencies in the Napier system".
Napier's stormwater assets have a replacement value of $341 million. Photo / NZME
By some counts, the event was closer to a "one in 50-year storm". The report noted with climate changes these return periods would become less relevant as such events return more often than the rating suggests.
While the stormwater system worked as it was designed to do, the specifications of the system was rated below the rating of this storm, with 40 per cent of the stormwater having been designed over 50 years ago
"The system was designed for the population, climate and standards of the time," the report stated.
"Since then, in Napier, there has been significant population growth and changes in weather patterns that have eroded the ability of the system to perform."
In addition to pumps and pipes, stormwater assets include roadways, waterways and open spaces which help minimise flood damage. Photo / NZME
Not limited to just pipes and pumps, stormwater systems are also made up of roadways, waterways and open spaces, which help minimise flood damage.
While Napier's parks and roadways performed as expected, further damage was caused by cars driving on roads that created water wakes.
Pipes and pumps are designed to convey stormwater to the sea. However, if the downstream is already flooded they are "not as effective" in conveying the water downstream because there is no capacity downstream to receive the water.
When this happens, areas upstream will back up and flood.
While roads and parks performed well and helped reduce flood damage, vehicles driving through floodwaters created wakes which wrecked properties. Photo / NZME
The pipes were not designed to convey the volume of water that fell during this event. They were also potentially impacted by a build-up of silt and rubble in the pipes.
The report found that pumps that did not appear to work at maximum capacity were actually pumping at full capacity when the depth of the downstream water is taken into account.
"The deeper the water is downstream, the less a pump can transfer to that water body.
"In some cases, the water from upstream had yet to arrive at a pump, so the pump was operating at the maximum level it could with the water at its inlet."
Napier's stormwater assets comprise of 241kms of pipe network, 10 pump stations and two detention dams. Photo / NZME
Napier's stormwater assets - comprising of 241kms of pipe network, 10 pump stations and two detention dams - have a replacement value of $341 million, as of June 30, 2020.
Through the Long Term Plan, NCC has committed $116m to key stormwater projects to assist with damage mitigation and floodzone remediation work, Napier mayor Kirsten Wise said.
This includes $11.6m to be spent in the next six years on CBD upgrades, $15m in seven years in Maraenui - one of the worst-hit suburbs - $14.5 million in six years in Te Awa, and $1m in three years for pump station upgrades including increased electrical and control cabinets.
A significant clean-up programme to clear pipes of rubble and silt is still ongoing.
The 241km of the stormwater pipe network is also part of a comprehensive programme of pipe jetting and cleaning, with an investigation for debris and damage through CCTV already having taken place, and part of an ongoing rota programme.
