Seawater flooding – coastal North Island
What happens to farmland flooded by seawater?
The impact will depend on how long the crop or pasture is covered by seawater. If it's only for a short time – up to 48 hours – and gets good rainfall, grasses should bounce back.
Rain is needed to wash the salt away. It's likely the salt water was diluted by the rains during the January 2018 storm which measured up to 70mm.
The last similar event in the areas was 15 July 1995. This was during winter. There was plenty of rain to wash out the salt and pasture recovered well over a 3-month period.
Effects on pasture and soil
- Salt should wash away fairly quickly with rainfall. The more porous the soils the quicker it washes out.
- Pasture dies after 3 days without oxygen. Overseas studies on inundation suggest:
- between 24 hours and 48 hours under water most pasture will recover with some discolouration
- between 48 hours and 96 hours, most of the grass will die but some stems survive, clover survives
- after 96 hours all pasture dies and requires renewal.
- Your soil texture profile affects the impact of seawater on it. Clay soils have low penetration (1cm to 2cm). Sandy soils have higher penetration (up to 15cm). This is good news for many farmers whose soil is marine clay. Near Kaiaua, there is more sand-based soil.
Animal health guidance
- Take the precautionary approach of deferring grazing until after the next heavy rainfall.
- Ensure stock have access to fresh, clean water.
- Talk to your vet if there are any concerns.
Actions to take
- If possible, pump the seawater off.
- Make a feed budget. Consider there may already be feed shortages in the area and take this into account.
- You may need to offload stock – make a plan and make decisions early.
- Talk to your accountant about possible tax flexibility or other standard income assistance for you and your workers.
- Talk to your farm consultant if your pasture has been covered for several days or longer – you may need to regrass.
- Talk to your farm consultant about soil testing for sodium or conductivity. Some consultants may be able to offer this as a rapid field test.
- Talk to your local Rural Support Trust – they are in regular contact with organisations that can help.
Find out more
- Pastures imperiled by seawater flooding – Radio NZ website
- Rural Support Trust
- Salt water inundation – technical advice from the Waikato Regional Council [PDF, 400 KB]
- Dealing with floods