Tauranga Harbour sediments in 2051

What will future land use and climate change do to sediments entering southern Tauranga Harbour? To find out, Environment Bay of Plenty commissioned NIWA to model the sources and fates of sediments under various scenarios over a 50-year period.

The results will aid decisions on land use and development in the surrounding catchment, and target mitigation measures where they’ll be most effective.

Sediment runoff from the surrounding catchment is having adverse effects in the harbour, including loss of seagrass, infilling of embayments with mud, and spread of mangroves. A team led by NIWA coastal scientist Dr Terry Hume applied our Urban Stormwater Contaminants Model to identify the sources and fates of sediments in the southern harbour over a 50–year period (2001–2051). The team looked at three scenarios:

  • baseline - current-day land use and weather
  • land use change - urban growth in the catchment and current-day weather
  • climate change - urban growth in the catchment and weather under climate change.

Climate change dominates

Climate change is predicted to become a more dominant driver of harbour sedimentation than urban growth in the catchment. Under climate change, there will be longer and hotter spells without rain, but during storms, the rainfall intensity will increase, which will increase sediment runoff in response. Averaged over the whole catchment, the model predicts that catchment sediment runoff to the harbour will increase by 40% by 2051. Under urban growth alone, the model predicts catchment sediment runoff will reduce by 5%, as high-yielding pasture is replaced by lower-yielding urban land use.

Non-linear response to sediment increases

With the slight reductions in sediment runoff under urban growth, the sedimentation rate in the harbour reduced slightly, but the increases in sediment runoff under climate change caused a disproportionate – or “nonlinear” – change in the sedimentation rate. This means that even slight increases in sediment runoff can result in much greater increases in sedimentation. “This was an unexpected result,” says NIWA coastal scientist Dr Malcolm Green, “and is most likely due to the harbour’s ‘self-cleansing’ processes being overwhelmed by the increased sediment inputs.”

Identifying mitigation targets

NIWA scientists have used the results of the modelling to identify parts of the harbour most at risk of adverse ecological effects and where opportunities for mitigating sediment runoff from the catchment are greatest. “Where the two intersect are the best opportunities for management to focus,” says Dr Green. The Waimapu Stream subcatchment, for example, contributes 99% of the sedimentation in Waimapu Bay and 26% of sedimentation in Welcome Bay. “Since both of these bays are at high risk of adverse ecological effects, this makes Waimapu subcatchment a priority candidate for intervention.”

Dr Green will present the results of this study to the New Zealand Coastal Society Conference in Whitianga in November on behalf of the study team.

Contact: Dr Malcolm Green