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On July 24, 1988, the Mangaone and Kawau streams flooded Palmerston North, triggering both a civil defence emergency and a 30-year project of significant and prolonged investment in stormwater infrastructure to better protect the city against future flooding events.
On March 8, 1988, New Zealand awoke to images of unprecedented devastation in the East Coast caused by Cyclone Bola.
With rainfall peaking at 917mm, Bola caused over $100 million in damages and seven fatalities.
Palmerston North escaped relatively unscathed comparative to the terrible destruction suffered elsewhere, and in any assessment of major weather events of 1988, Bola will always dominate.
However, in July of that year a major but less-remembered storm flooded Palmerston North, triggering both a civil defence emergency and a 30-year project of significant and prolonged investment in stormwater infrastructure to better protect the city against future flooding events.
On July 24 heavy rain overloaded the Mangaone and Kawau streams, inundating the drain network and causing flooding to various parts of the city.
It had come off the back of at least a week of steady rain in the wider region, which had filled the catchments and saturated ground surfaces to an extent that the deluge beginning on July 24 tipped the system beyond capacity.
Compounding the issue, water and silt from upstream and high-country areas to the north of the Manawatū catchment made its way downstream over several hours and added to the already bulging banks of the river.
Manawatu Heritage/Stuff
The area around Awapuni Racecourse floods as the stormwater lagoon exceeds its storage capacity.
By 11.30am the Manawatū River had risen to 4.7m, and rain was falling at a rate of 100mm per hour. In the Napier Road catchment significant residential and commercial flooding occurred around Sutton Place, as well as inundation of extensive farmland in the area between State Highway 3 and the river, severing access to the city via Napier Road.
Across town, the Mangaone and Kawau streams were posing problems for residents. With the streams rising rapidly, waters overloaded the Pioneer Highway drain pipeline and exceeded the capacity of the Racecourse stormwater lagoon, giving the floodwaters nowhere to go but into the nearby Kentucky Way, Racecourse Road, and Totara Road.
The rain kept coming, and the water backed up, until the Managone Stream overtopped its stopbanks and flooded a 20-hectare residential area around Amberley Ave, Cedar Grove, Pencarrow Street and Chadwick Place.
Attempts by the council to pump floodwaters out of Cedar Grove were quickly recognised to be futile as water swirled over the mailboxes, and residents fled.
The Mangaone hit 4.25m and tripped the spillway at Flygers Line at around 6.30pm. Floodwaters spilt across Kairanga farmland, and across Rangitīkei, Flygers and Gillespie’s Lines. The stream continued to rise, eventually peaking at 4.5m at 9.30pm.
At 10.12pm, after wide-spread power outages, a Civil Defence Emergency was declared for Palmerston North, and more than 1000 evacuees were transported to the Convention Centre to spend the night.
Over in Milson the Kawau Stream rose so rapidly it forced stormwater trapdoors shut ,and spilt over into areas from Fairs Rd to Roy St. Mixing of stormwater and sewage became an immediate concern to the council as the rain kept falling.
The Manawatū River peaked at 5m on the morning of the 25th but did not breach its banks. The Civil Defence Emergency remained in place for four days, before being lifted at 9am on July 27 as floodwaters began to recede and the clean-up effort began.
Manawatu Heritage/Stuff
Main routes into the city are severed by flooding in commercial and residential areas.
In the aftermath of the flood, the Palmerston North City Council commissioned the Comprehensive Stormwater Management Plan, undertaken by Beca, Carter Hollings & Ferner Ltd.
Completed in 1991, the report’s purpose was to identify and model flooding risks to the city, and to determine appropriate solutions to manage the risk.
In addition to modern modelling methods, rainfall data from 1949-1989, and engineering expertise, the report was informed by more than 4000 responses to a residents’ survey initiated by the council in 1990, which gathered anecdotal data about flooding impacts on properties throughout the city.
The survey results demonstrated strong public appetite for a variety of engineering solutions to mitigate flooding risk to a 100-year level, and an overwhelming preference for limiting development and intensification in flood-prone areas.
A Stormwater Management Plan was also initiated for Ashhurst, which similarly identified a risk to the public and a strong desire for the council to undertake mitigation solutions to a 100-year level.
As a result of this work, a number of solutions were ranked into first and second priorities. The solutions included a mix of improvements to discharge capacity, increasing stormwater culverts at strategic locations, and significant investment in stormwater infrastructure such as pumping stations, flumes and stopbanks.
Manawatu Heritage/Stuff
The car may be underwater, but the washing is staying dry.
The sum of the 10 top-ranked priorities was costed at just under $4 million, while the second-ranked works were not initially costed.
All proposed solutions were designed to achieve 100-year flood protection to existing habitable floors.
Planning solutions were also considered, such as raising the minimum-required floor level for new dwellings in flood prone areas to be above the 100-year flood level, protecting stopbanks, the designation of flood ponding areas, and limiting the density of development in the residential zone.
The council held a series of public meetings in school halls throughout May 1991 to seek feedback and input on the proposed solutions, and a programme of works was then adopted and approved in the 1993 Annual Plan.
Work began with $941,000 budgeted to improve the Birmingham Street pipeline, the Kelvin Grove outfall channel from SH3 to the river, and the Clausen St pump station. Work continues to this day, with mains upgrades currently occurring around Victoria Ave.
The public willingness to fund major and sustained investment in the greatest level of flood protection infrastructure available at the time showed a widespread motivation to improve the city’s resilience against flood events in the future.
These defences have been tested a number of times since 1988, notably in 2004 and 2015, and we’ve learnt more each time about different ways our infrastructure can be breached or undermined.
And after a series of wet winters, two cyclones and with national planning standards requiring medium density housing and intensification for Palmerston North, learning from past flood events is critical to ensuring that we are ready to face the next one.
Kaydee Zabelin is a historian, self-confessed stormwater geek, and a Palmerston North city councillor.
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