02/07/2025
MetService has updated their warning from yesterday. Their advice stays much the same with a few small changes. Differences that are notable are a shorter duration of the event northwest of Motueka - now 15hrs from Thursday 3am. There is still the possibility of localised heavier rain of up to 30mm/hr, most likely in the ranges.
The other Warning is for the remainder of Tasman, including Richmond and Bryant Ranges. The peak intensities are now a little higher at 15 to 25mm/hr. Otherwise, timing is still from Thursday 6am to 9pm, expecting 80 to 110mm in the ranges and 60 to 80mm.
There is also mention of further rain and showers for the remainder of Thursday and Friday.
Rainfall peak intensities appear to be occurring mid-dayish and then petering off in the evening. This event then, is approximately 15hours, and is a short event in comparison to last week. These shorter events have much more effect on the smaller catchments, or catchments that are already sodden. The timing of the rain will give some time to allow decisions to be made in daylight, before rivers peak later in the day.
In terms of rivers, the main stem of the Tākaka River and its main tributaries, i.e., Anatoki and Waingaro Rivers, expect flows up to annual flood. The usual surface flooding should occur and there is the chance of further landslips, especially around the Wainui and Ligar Bay areas. Tākaka Hill may also suffer from further slips.
Currently, of most concern to us, is the Riwaka and Brooklyn rivers due to this area being the most likely to receive higher intensity rainfall bursts. While we are anticipating only annual flood flows in the lower Riwaka, we would take a cautious approach and plan up to 5 year flood flow impacts, given the saturation and possible elevation changes in bed level. For context, last Fridays flood peak in the lower Riwaka was in a 10 year flood flow range.
Because of the wind direction and the shorter duration, we are not expecting too much happening in the rivers of the upper Motueka, Motupiko and Wai-iti Rivers. However, if the rain affects a wider area (duration and intensity), these could react quickly. We understand that the lower Motupiko River can currently only hold an annual flood flow due to damage.
We expect flows in the main Motueka River to be less than annual, thanks to the short duration of this event.
We are noticing in the Waimea Plains conditions similar to August 2022, when the groundwater table reached the surface and flow paths formed over the lower-lying areas. This occurrence lasted for weeks, with periods of small rain holding the water levels up. So any rain we are going to receive from now will exacerbate this effect. The Pitfure area will most likely be experiencing this as well.
It is always prudent to plan for higher flows than predicted, but this is more so at this time. Many rivers have had major bed changes, with less ability to cope with floods. All of our monitoring equipment is recording correctly again (some stations were damaged).
The high tide for Thursday is at 3.27pm and Friday 3.57am, being 3.4m, which is relatively low. Barometric pressure is not particularly low, and the wind being north/north-easterly and of moderate strength, we’re not expecting issues from storm surge or wave run-up.
Overall, we can expect further surface flooding, landslipping and road closures to already compromised roads and associated structures, i.e., stop banks.
Links for more information:
• MetService warnings: https://www.metservice.com/warnings/home -south.
• Updates on local road conditions: https://roadinfo.ramm.com/nz/roadclosuremap.html?region=tasman
• Updates on state highway conditions: https://www.journeys.nzta.govt.nz/regions/tasman/
• River flow reports: https://www.tasman.govt.nz/flow-report/
• Rainfall reports: https://www.tasman.govt.nz/report/
