Flooding at Tawa

Intense rain across the Wellington region caused serious flooding in a number of areas today. The Tawa weather station recorded its wettest day in ten years.

Thursday, 14th May 2015

Intense rain across the southern part of the North Island brought localised flooding to several parts of the Wellington region today. By early afternoon, Wellington had been almost cut-off from the rest of the North Island by road and rail, and several areas were experiencing local flooding.

The rain event brought significant rain to the Kapiti Coast area early in the morning, causing flooding to Waikanae, Paraparaumu and Raumati and disrupting the flow of commuters into Wellington city. Around 8 o’clock a stream of water pouring down the hillside above Centennial Highway between Pukerua Bay and Paekakariki brought a slip of mud and rocks onto the highway, closing State Highway 1 in both directions at a narrow two-lane stretch of the road that runs along the coast below steep scree banks.

Until this time, the weather had been largely westerly, perhaps sheltering areas further south from the intense rain bands coming in from the west. As the morning progressed, the pattern tended more north-west bringing thunderstorms and intense bursts of rain further south. Until that time, Tawa had largely escaped the deluge, recording 23 mm of rain in short intense bursts.

This changed around 10 o’clock with a series of thunderstorms and downpours that dropped another 27 mm of rain on Tawa over the next hour. A further 21 mm fell to midday, causing surface flooding of roads, and several of the creeks that drain the western hills of Tawa into the Porirua Stream overflowed the culverts that they have been directed into as Tawa Flat has been developed.

The rain paused around midday, but by then streams at several locations along Main Road Tawa had overflowed onto roads and into buildings in low-lying areas. Collins Avenue had flooded and, further north, Porirua Railway Station’s subway had flooded. Flooding occurred at a number of locations in Porirua, and Kenepuru Hospital was isolated for a time by flooding on local roads.

Thunderstorms again rolled into Tawa around 1 o’clock, but they only delivered another 9 mm of rain by 2 p.m.

Those commuters who had managed to get in to the city from the Kapiti Coast faced a difficult choice for getting home. State Highway 1 was closed on the coast at Centennial Highway. State Highway 2 (to the Hutt Valley) was also closed northbound by flooding north of Ngauranga.

Those who left the office early, hoping to get home, faced lengthy delays with State Highway 1 carrying the northbound traffic for both highways. Hutt Valley commuters were hoping to travel the long route by taking SH1 to Mana and then head east via Haywards Hill in a hope to get home. Some western coast locals chose to try and travel north via back roads, but the Ngaio to Johnsonville link road was almost at walking pace for a number of hours.

Civil Defence recommended that commuters for points north of Pukerua Bay opt to stay in the city overnight, much to the delight of city hotels which are reported to be fully booked. The Wellington City Council has opened a couple of relief centres, one of which is the city library. Tired out-of-towners will be allowed to hunker down on the library’s mezzanine floor to catch a few Z’s if they can’t find alternative accommodation.

For some this might be a wise idea. A fellow Tawa-ite who was working at the bottom of the Ngauranga Gorge decided to bail out of the office at 2:30 p.m. when the traffic through the interchange was obviously approaching mayhem. After vainly watching full buses crawling up the gorge heading north, he decided to head south into the city to catch a north-bound bus at Courtenay Place before it became filled with commuters.

This worked, but the bus trip from Courtenay Place to his first interchange at Johnsonville took a remarkable two hours, a trip that would normally take 20-25 minutes at that time of day. The connection to Tawa was only another half-hour despite the dense traffic. All-up a 4-hour commute.

Tawa is home to two fire stations – a large rural fire depot which houses vehicles for the Rural Fire Service and can be busy in a hot summer fire season, and the operational Tawa Fire Station which provides fire, rescue and paramedic services for the local area and motorway incidents. Tawa Fire Station has good volunteer support and the siren has been well-sounded today! The Tawa Fire Station attended 24 callouts in Tawa and the Porirua basin today (as at 7:45 p.m.) and is probably continuing with mop-up work. They do a fine job, which is much appreciated.

Tawa Weather has only been operating since January 2005, but today’s rainfall has already broken its record for daily rainfall during that time. At the time of writing (9:30 p.m.), 95 mm of rain has been recorded in Tawa today. The previous wettest day for Tawa Weather was recorded on 25th May 2010 when 80 mm of rain was recorded.

Despite the intense bursts of rain today, the peak rain rate calculated by Weather Display software, which Tawa Weather uses, was 1.5 mm per minute (92.3 mm/hr) at 13:47 today. This is below the deluge of 10 mm/minute which Tawa Weather recorded on 15 May 2006.

Forecasts for the next 24-hours are mixed and change regularly as this weather event passes through.

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