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Ensuring safe discharges into recreational water
In addition to the local population, flocks of tourists enjoy sea-sides, rivers and lakes across the world each year. Tourism is the world’s third largest industry and the prime economic sector in some regions. It cannot be compromised by health incidents or concerns. Meeting water quality standards and providing easily accessible and timely information to the public is a very real and current need.
Contaminant Plume Tracker. © DHI
Sewage outfalls, storm water overflows, plant releases, agricultural production effluents—water bodies in urban areas are subjected to many polluted discharges. When these water bodies are also used for recreational purposes, the safety of the population is at risk. Ensuring safe discharges into recreational water is therefore crucial for human health.
A central Waitematā Harbour water quality forecast model, underpinned by a 3D hydrodynamic model was developed to provide water quality forecasts to inform risk for recreational activity from overflows at central Auckland beaches from Point Chevalier to St Heliers Bay. Predictions from the model are presented as part of Safeswim. Safeswim is a collaboration between Council Group, Surf Lifesaving Northern Region and the Auckland Regional Public Health Service.
Calculating the concentration of Enterococci bacteria
The system constantly monitors the harbour’s water and predicts the concentration of the indicator bacteria Enterococci at specified locations along the water course in the harbour. To help forecast frequent pollution threats, DHI also collects meteorological data from forecast suppliers and runs hydrodynamic models to retrieve data. All of this information is then used to create models to demonstrate predictive forecasts using MIKE Powered by DHI software, by utilising a high-resolution 3D hydrodynamic model, MIKE 3 FM. The model simulates tidal and wind driven currents within the harbour.
During a rainfall event, calculated loads of Enterococci are discharged within the hydrodynamic model and the resulting transport, dilution and inactivation of the Enterococci plume are simulated as a tracer. Within the Waitematā Harbour, the main sources of pollutants to the harbour are combined wastewater and stormwater outfalls as well as freshwater inflows.
An animation used by the Auckland Council to predict water safety and quality for its SafeSwim website, showing raw sewage sloshing along Auckland's beaches for days after rain.
Providing forecasts ahead of time
The model is currently configured to provide forecasts three days into the future and was developed to address some of the short comings of traditional recreational water quality monitoring. MIKE OPERATIONS is the platform which operates the forecast system. It is a software product designed for model-based forecast services and for online operational control of river systems, water collection systems and water distribution systems.
Every six hours, MIKE OPERATIONS downloads data from required sources. It then converts the data to a MIKE format, generating the flow and bacteria concentration time series based on the observed and predicted rainfalls time series.
Forecast wind and solar radiation data are provided and a three day precipitation forecasts with an hourly interval are provided by Met Service for selected locations within the Auckland region. Observed rainfall for appropriate rain gauges is provided by the local council authority.
Water quality information displayed at Mission Bay Beach, Auckland. © DHI
Model simulations are carried out every six hours using the latest three-day forecasts of precipitation, solar radiation and wind. To ensure that the model captures significant rain events, the model carries out a simulation using measured rainfall data (a ‘nowcast’) before proceeding into the forecast for the next three days. This method ensures where a rainfall event was forecast but did not occur, the predicted bacteria load to the harbour is not included in the next forecast.
Greater information about water quality is now available, which in turn may help to reduce the impact of pollutant discharges, meet regulatory standards, monitor water quality and provide users with timely information.
Water quality predictions are also accessible for beach safety throughout 84 beaches within the Auckland Region over the summer bathing season. The majority of these are through a regression type model forecast, with nine central Waitematā Harbour locations from Point Chevalier to Saint Helier’s targeted through DHI’s modelling engines.
The software utilises actual information on meteorological forcings and simulates the fate of the indicator bacteria with precise hydrodynamic models. Hence, the model system can also be used as a highly effective tool to assess and identify the best method to address solutions to maintain water quality and reduce risk of pollution.