Satellite images show state of Napier's water post flood

HBRC satellite imagery showing the hundreds of thousands of tonnes of sediment in major rivers offshore from last week's storm. Photo / Supplied

New satellite imagery has revealed the scale of Napier's erosion problem since the recent flooding across the city.

The imagery from Hawke's Bay Regional Council's satellite shows hundreds of thousands of tonnes of sediment in some of the area's major rivers and 20 kilometres offshore following last week's storm.

HBRC group manager of integrated catchment Iain Maxwell said their science team are still analysing water samples to measure how much sediment was carried during the flood.

"These satellite images reveal a telling picture of the ripple effect set in motion when we experience a major event of this kind," he said.

The first satellite image shows the bay between Napier Port and Cape Kidnappers in October, while the following image was taken after the storm struck, on Wednesday, November 11.

It shows sediment clogging up in major rivers - Tutaekuri, Ngaruroro and Clive - and being discharged from these rivers and out to sea.

"The storm has dislodged large volumes of potentially productive soil from our vulnerable, eroding hill sides, and this has entered our rivers, and gone as far as 20 kilometres offshore," Maxwell said.

Council modelling shows 252,000 hectares of Hawke's Bay hill country is prone to erosion.

Maxwell said more has to be done to protect vulnerable hill country from continual erosion.

"As our climate changes, and storms and drought become more intense and frequent, it is important that we restore our vulnerable land, and increase the quality of our waterways," he said.

Maxwell added: "The most effective way to prevent erosion on vulnerable hill slopes is to plant trees, and that's why we are exploring how we can upscale our tree planting programme, and plant the right trees in the right places."

Sediment negatively impacts the natural ecosystems of Hawke's Bay's rivers and coastline Maxwell said.

It reduces water clarity, removes habitat for small aquatic animals and affects the ability of fish to find prey.

Maxwell said on the coast it's also problematic for algae that use sunlight for photosynthesis, and they struggle to survive, leading to a loss of habitat.

The sedimentation effects in the deeper coastal marine area are not as well-known.

A HBRC spokeswoman said they are currently supporting a University of Waikato PhD student who is creating a hydrodynamic model of Hawke's Bay.

"The model will shed light on the sources of sediment and its transport and fate in the Hawke's Bay coastal marine area," she said.