Between 3-5 March, a huge āwhā off the east coast of the South Island pushed cold south easterly rain onto Te Pātaka o Rākaihautū and Ōtautahi causing significant flooding and land sliding. This was a 1-in-100 year āwhā. SH75 between Wairewa and Akaroa was closed and there was significant surface flooding in the Little River and Cooptown area. The majority of rain fell in a 24-hour period from 10am on 4 March. Rainfall predictions from the MetService were for moderate rainfall up until the evening of 3 March, when it was upgraded to severe. By this time it was too late to open the lake, which was at 1.80m, due to the 5-6m swells rolling in.
The rainfall in the Ōkana catchment was extremely intense for a short period, which was the primary reason for the flooding to occur. The heaviest was at 7am on 5 March, which followed a long period of sustained high-intensity rain. Local resident, Bill Tini said this was the worst flooding he had seen in the district and it was the first time he had had to evacuate his whare due to the flooding. The water was lapping onto his doorstep. The marae escaped damage from the flood waters, however it was affected by the power outage, as power surges blew most of the circuits in the electronics at the marae. It also affected the wi-fi, to the disappointment of James Daniels.
The rainfall in the catchment was very intense, which generated considerable surface flooding. The Christchurch City Council have engaged a consultant to look at where the main flooding occurred and to what level. This will be useful information in assessing how influential the lake level was in exacerbating the floods in Little River. The lake prior to the storm was sitting at about 1.80m, which is not particularly high (about 25% of the recordings since 1994 have been above this level) and the lake would not normally be considered for opening at this level. The predicted rainfall was not particularly high until very close to the actual event (about 12 hours out and at night), so it would have been impossible to open the lake prior to the main rainfall.
It is likely that the main cause of surface flooding in the Little River area (including Cooptown) was the stream channels and drains being unable to cope with the very high flows, rather than the lake backing up and blocking drainage. Stream channels like lower Police Creek (below the state highway), Ōkana (SH 75 Bridge) and Ōpuahou (Cooptown) have a lot of old willow and poplars blocking channels, which will have increased the flooding out of the stream channels, into the domain and down the state highway, which in turn flooded the town and people’s homes and businesses.
Flooding down SH75 in Little River.