environment

The cost of saving a flood-prone cricket ground

How much is a community cricket ground worth? A new report estimates the bill for defending a popular playing field from flooding and worsening sea level rise, reports Eloise Gibson.

A report on a flood-prone Auckland cricket ground offers a glimpse of the kinds of decisions that will face people living in other coastal places.

Luckily no homes are in the path of rising seas and floods at Little Shoal Bay, on Auckland’s North Shore, but there is a popular cricket field, native plants, boatyards, a road and a bowling club.

The low-lying bay began flooding regularly from king tides and storm surges in 2009. In 2017, extreme tides caused at least five floods of the boat stand, cricket pitch, bowling club grounds and the road, which is called Maritime Terrace.

While today’s troubles are mostly caused by topography, mud and drainage problems, they are expected to get worse as the sea level rises, eventually turning today’s rare big tides into common occurrences.

The bay's prognosis is in a report written for Auckland Council by consultants Morphum Environmental Ltd, which roughly costs the options from doing nothing to raising the whole field by 1m. It was finished in September and released to Newsroom this month in response to an official information request.

The report traces the strange history of the bay, which was originally a natural estuary and wetland but was filled and extended about 70 years ago with rubble sourced from an old gas works, the construction of the nearby Auckland Harbour Bridge, and other big building projects. Despite its artificial origins, it has regenerated into a prized freshwater wetland, native coastal forest and, higher up, kauri forest “right in the heart of Auckland City”, says the Morphum report. The reclaimed land has become a popular sports area, walkway, and cricket-playing destination for residents of nearby Northcote and Birkenhead. But more frequent floods from the harbour are turning parts of the field back into a salty estuary.

Auckland Council decided to review the bay’s future after a seemingly sudden increase in flooding alarmed the local board and community. One immediate problem is that a culvert draining water from the wetland to a bigger culvert under Maritime Terrace is broken and eroding, which may be making flooding worse, says the report.

Morphum lists (and includes photos of) all the ways the area is getting soggy and harder to use: the Dudding Reserve cricket pitch has been turning muddy and brown from harbour water. It is growing difficult to host cricket games, because the field’s outer boundary is being inundated during king tides, even during fine, summer weather.

The road is regularly flooded and sometimes closed during extreme tides. Colonies of crabs are making the most of increased saltwater and burrowing into the muddy banks around Watercare’s water pipes “resulting in a honey comb effect reducing the … strength of the surrounding bed and bank … This will be communicated to Watercare,” says the Morphum report.

The frequent flooding is also creating an apparent risk of electrical explosions. “Inundation effects on infrastructure have included the electricity transformer units in the road reserve arcing and exploding in January 2018,” says the report.

Meanwhile, next door to the playing fields, the greens of the privately-owned Northcote bowling club have also been flooded, along with the cars of people unlucky enough to be caught out while attending parties, locals and the club told Newsroom.

Flooding at Northcote Bowling Club on Auckland's North Shore. Photo: Kyle Aitken

To fix the issue, the council, local board and community need to decide how much they want to spend to keep their recreation facilities – if anything. If the council doesn’t intervene, the area will get saltier and more estuary-like and self-seeding mangroves will keep replacing native freshwater plants that were hand-planted there by community groups. At some point the area won’t be suitable for dry-ground activities, like cricket and bowls.

But the decision is complicated by sea level rise, which threatens to overwhelm some of the cheaper or short-term solutions to the flooding.

The report traverses how sea levels have risen and fallen through the ages as Earth varied its orbit around the sun and moved in and out of ice ages. About 120,000 years ago the sea in the Auckland region may have been four to five metres higher than it is now, according to one study. At the other extreme, during the most recent ice age, a land bridge connected the North and South Islands and the Auckland coastline extended past Great Barrier Island, says another study cited by Morphum. But these changes, while enormous, were very gradual. For most of the last 2000 years, global sea levels crept up on average at only about 0.2mm a year, says the report, citing the IPCC. More recently, the long-running Auckland sea level record shows the average rate of sea level rise between 1993-2003 was more like 3.1 mm a year. While the Auckland Unitary Plan incorporates a total rise of at least 1m over the next 100 years, the long-term prognosis for sea level rise suggests levels could end up rising by as much as 5m over coming centuries, says the report.

A few centuries is probably beyond the life of a cricket ground, but even over the next 100 years, modelling using the Unitary Plan’s mapping tools and done by Morphum shows that much of the existing reserve and a large portion of the wetland will be at higher risk of flooding.

The cheapest option detailed by Morphum is building a $5000 fence to stop people falling into holes made by the broken culvert, which would fix an immediate safety issue but wouldn’t reduce the flood risk. That is an all-but do-nothing approach - cricketers would take their chances and eventually stop using the area.

The next level up would be fixing the broken culvert, which the report estimates would cost between $15,000 and $25,000, depending whether it was merely repaired or replaced with new pipes.

Then there’s the possibility of trying to reduce flooding problems by building a weir and a bridge ($50,000) that could control and slow down the flow of water during big tides. There could also be a tide gate, like a one-way valve, to control the flow of excess water ($80,000). But there would still be occasional flooding when a storm surge overtopped the road, becoming more frequent as sea levels rose.

Within 20 years these stop gaps would fail and floods would overwhelm the structures unless the council did further work, says Morphum. “Because of predicted sea level rise, these structures could become less effective over time. It is anticipated that within a 10-20 year horizon other measures would need to be taken to raise the level of Maritime Terrace, or construct bunds or dykes on the surrounding land,” says the report.

One of the more enduring options is a seawall to help defend the cricket field ($35,000) and/or upgrading the main culvert under the road with a “modified tide-gate with a fish friendly flap” to prevent floods yet still let fish move up the stream ($55,000). Again, though, the fish flap would stop working as sea levels rose, staying shut for longer periods to prevent flooding and keeping fish out.

Another option, raising the entire playing field by 1m, would keep the area dry for 50-100 years. The report didn’t estimate a cost for that.

The final option, cost unknown, is full retreat. That involves relocating the sports field, park facilities, carpark, bowling club and dry dock to new locations and raising or removing Maritime Terrace. Raised boardwalks might be built for people who wanted to walk from the nearby bush and around the area’s new-look salt-marsh. New plants more suitable for an estuary could be planted in the area – though it wouldn’t be a good place to enjoy a game of cricket.

The council and local board are considering the report.

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