Thursday’s Flood Event

Thursday’s rainfall in some places in the Wellington region was the heaviest seen for more than 50 years, according to the regional council. The Porirua Stream carried the heaviest flows since 1980.

Saturday, 16th May 2015

The weather front that funnelled heavy rainfall and north-westerly winds onto parts of the Wellington region on Thursday 14th of May 2015 contained numerous bands of heavy rain and pockets of thunderstorms. Initially, the worst effects were noticed on the Kapiti Coast early in the morning. However, as the morning progressed Porirua, Tawa and parts of the Hutt Valley began to experience intense rainfall.

A media release issued by the Greater Wellington Regional Council (GWRC) today notes that, “Over the course of the day, rain gauges at MacKay’s Crossing, Battle Hill and Tawa Pool recorded totals greater than the rainfall normally expected during the entire month of May.”

“The 24-hour rainfall total at Battle Hill near Pauatahanui was equivalent to a 1-in-50 year event and in Porirua, the Porirua Stream had the largest flow since 1980.” The Battle Hill rain gauge recorded 144 mm for the day, and McKay’s Crossing 145 mm.

During the most intense rainfall, the Tawa Pool rain gauge recorded 62 mm of rain over a two-hour period, accumulating a total of 115 mm for the day – a one in 20 year total. At its peak, the rainfall intensity reached a rate expected once every 40 years. This resulted in creeks that feed into the Porirua Stream overflowing, causing pockets of flooding along Tawa’s Main Road.

The report says, “The Porirua Stream peaked at a flow rate of 66 cubic metres per second which was the third largest flow seen there since the floods of 1976, and the largest since 1980. It ranks as the largest flood to have occurred since the Seton Nossiter and Stebbings flood detention dams were constructed in the upper catchment.”

Tawa Weather, a privately owned weather station, recorded 96 mm for the day, the largest daily rainfall figure in its ten years of operations.

On the Kapiti Coast, The GWRC reports, “Heavy rain started to fall in the early hours of 14 May on the Kapiti Coast with a total of 145mm recorded in 24 hours at McKay’s Crossing. A rainfall total of this magnitude is expected once in every 40 years at this location. To the north, Waikanae received 102mm of rain in 24 hours which is expected once every 6 years. An intense burst of rain was recorded at Te Hapua Rd, south of Te Horo Beach, where 27mm fell in just one hour. This is equivalent to a 10 year event.

The Waikanae River peaked at a flow 270 cubic metres per second above SH1. This was the 3rd largest flow recorded since 1975 – only January 2005 and October 1998 were larger. A flow in the river of this size is expected once every 13 years.”

Flooding hit parts of the Hutt Valley as well, most notably at Petone and on parts of State Highway 2. The GWRC reports, “In the Hutt Valley, 42mm of rain was recorded in Avalon in just one-hour – that’s estimated to be in excess of a 1-in-50 year event.”

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