Impact Of Heavy Rainfall on Coastal Water Quality Captured in MetOcean Model
The severe weather system that hit the shores of New Zealand over the weekend of the 17th of July, 2021, caused significant flooding and damage to property and infrastructure within the Marlborough and West Coast regions. However, the impacts of heavy rainfall are not limited to what we see on land. Precipitation causes elevated river discharges and run-off from land, which can carry various contaminants into coastal waters, including the faecal indicator bacteria: E. coli and Enterococcus spp, implying elevated levels of disease-causing organisms. In fact, models openly available on MetOceanView illustrated the elevated concentrations and advection of E. coli and Enterococcus spp in the Tasman and Golden Bays in the days following the extreme weather. The models showed levels reaching concentrations that can potentially impact aquaculture and beach water quality within the region.
Elevated concentrations of E. coli bacteria forecasted for Tasman and Golden Bays, following heavy rainfall in July 2021. The forecast layer of concentrations of E. coli is freely available at Forecast2 on MetOceanView.
"What makes this model so interesting is its ability to predict the dispersal of these bacteria with higher accuracy," says oceanographer Phellipe Couto from MetOcean Solutions. Historically, measurements of water quality have been sparse; requiring site-specific physical water sampling and associated lab analysis, resulting in bacteria levels potentially not being known for days post sampling. This modelling allows robust relationships between the river discharge rates and bacteria loadings of the water to be applied in a numerical context, with information extrapolated both temporally and spatially with the area modelled.
The operational model was created in a collaborative project between MetOcean Solutions, NIWA and Cawthron Institute (who led the project), as part of the Sustainable Seas National Science Challenge in 2016. The project built an infrastructure of connected models from the land to the region's main rivers, and finally to the ‘end of the pipe’ coastal region in order to map and predict the dispersion of harmful bacteria within Tasman and Golden Bay, New Zealand.
Cawthron delivers bacteria levels based on concentrations from 11 rivers, whereas NIWA's TopNet model provides freshwater fluxes. Oceanographers from MetOcean then incorporate this information into a Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM) that spread and dilute the bacteria concentrations according to the local coastal circulation. "By considering this wide range of processes, we can provide more accurate estimates of the coastal circulation and monitor water quality around the region," says Phellipe Couto.
The water quality forecasts are highly relevant to stakeholders concerned with water quality. As such, councils can now predict which beaches are at risk of contamination and warn beachgoers. Similarly, aquaculture industries can time shellfish harvesting with optimal water qualities to ensure high-quality products.
All three organisations have recognised the value of this product and are looking at ways to keep the modelling operational well beyond the completion of the originally funded project. “We’re now entering a phase of industry engagement, led by Cawthron, to evaluate the modelling from an aquaculture management perspective. The end goal of this type of work is to apply scientific rigour to the management of offshore aquaculture, particularly with respect to forced closures, and get away from the somewhat arbitrary river flow thresholds applied by MPI currently.” - Dr. Brett Beamsley, General Manager – MetOcean Solutions.
Elevated concentrations of Enterococcus spp bacteria forecasted for Tasman and Golden Bays, following heavy rainfall in July 2021. The forecast layer of concentrations of Enterococcus spp is freely available at Forecast2 on MetOceanView.