Australians are coastal people with more than 85% living within 50 kms of the coastline (and within an estuarine catchment).  As such, tidal rivers (or estuaries) are an integral way of our life.  However, these systems are facing a rapidly changing climate, matched with major population increases and an uncertain future.

Our multi-disciplinary research team is addressing this issue by understanding the critical concerns and developing novel solutions. Our aim is to work with others to solve the most pressing questions and management issues related to climate change in our estuaries.

There are 3 major reasons why estuaries and the ecosystems they support are particularly vulnerable to climate change:

  1. Double whammy impacts
    Estuaries are at the interface between coastal rivers and the ocean; this means they are impacted by climate change from upland catchments (rainfall, heat) and oceanic (sea level rise, temperature, pH, etc) shifts.
  2. Vulnerable eco-hydrology
    Due to their unique hydrologic setting, which often includes shallow, protected, dynamic and nutrient rich brackish waters, estuaries are one of the most productive ecosystems on earth.
  3. Increasing pressures
    Many estuaries are already facing increasing development and population stresses as coastal populations grow, resources are depleted, nutrients increase and flow regimes change. When combined with climate change impacts, these pressures can result in ecological tipping points, drainage declines and forced changes to adjoining land-sue.

Assessing the impacts and effect of climate change in estuaries is complex. To help estuarine managers and coastal communities navigate this complexity we have developed a multi-report guideline that details climate change, it’s role in estuaries, existing trends/pressures and the potential impacts. Titled “Climate change in estuaries: State of the science & guidelines for assessment” this report and database is freely available online. The guide provides a summary of the relevant climate, ocean and ecosystem science along with best-practice frameworks for prioritising risks.

This research was undertaken in collaboration with scientists at the NSW Office of Environment and Heritage (OEH) and Macquarie University. The research was funded via the NSW Government’s Adaptation Research Hub’s Coastal Processes and Response Node led by the Sydney Institute of Marine Science.

Risk assessment guide

Assessing the impacts of climate change in estuaries is a complex task. To help navigate this complexity, a multi-report guide has been designed for estuarine managers, scientists, practitioners and coastal communities. The guide provides a summary of the relevant climate, ocean and ecosystem science along with best-practice frameworks for prioritising climate risks in estuaries. The guide consists of 8 module reports that cover different aspects of the risk assessment process and are designed to be read together or independently.

  • This module provides an introduction to various aspects of climate change in estuaries. The module addresses the following questions:

    • Why assess climate change risk in estuaries?
    • How do estuaries function?
    • What are the main types of estuaries in NSW?
    • How does climate change affect estuaries?

    The module contains an overview of the remaining reports and introduces the framework for assessment, a conceptual road map for assessing climate change risk in estuaries, as shown below.

    Download Module 1: "Climate change in estuaries - State of the science & guideline for assessment"

  • Cold water pollution is a serious environmental issue observed downstream of large storage dams. Dams are commonly fitted with deep offtakes, which withdraw water from below the thermocline. As a result, water discharged into the downstream river environment is often much colder than the natural river temperature. In Australian reservoirs, stratification can commonly result in up to 8 to 12 °C difference between the surface and bed temperatures in a reservoir. In deeper reservoirs, these variations can exceed 12 °C.

    This can have significant negative impacts for hundreds of kilometres downstream of the reservoir. Most notable are the ecological impacts on native fish, where unnatural temperature variations can affect their physiology, behaviour, reproduction, recruitment and even lead to mortality. 

    Cold water pollution is particularly impactful downstream of large irrigation storage reservoirs, where significant volumes of water are consistently released during irrigation (summer) seasons, when stratification is most prevalent.

     

  • As well as cold water pollution, stratification in reservoirs can lead to a variety of other water quality issues.

    Dissolved oxygen (DO) is an essential part of the reservoir environment, as it contributes to the metabolism of all aerobically respirating organisms. In a stratified environment, where little to no mixing between the hypolimnion and epilimnion occurs, hypolimnion waters (below the thermocline) can become anoxic due to the biological oxygen demand (BOD) at the sediment-water interface at the reservoir bed. Anoxic waters can lead to a number of ecological impacts, including decreased heart, ventilation and metabolic rate in fish, and in some cases mass fish kills.

    Low DO levels can facilitate the release of undesirable soluble iron and manganese. These metals can impact drinking water quality, increasing turbidity, darkening the water colour and causing an undesirable taste. As well as this, high concentrations of both have been linked to ecological impacts on fish populations, including oxidative stress, gill damage and haematological impacts.

     

  • Reservoirs are inherently conducive to algae growth, due to the high residence time of water and periods of reduced mixing due to stratification. Cyanobacteria (more commonly known as “blue-green algae”) blooms are of particular concern to water quality. This algae type produces cyanotoxins which can cause illness in humans and can be fatal to fish and other organisms.

    Cyanobacteria blooms are most common in warmer months when a reservoir stratifies. They are capable of regulating their buoyancy through gas vesicles, which allows them to dominate the warmer surface layers in reservoirs during periods of low wind and high solar irradiance. This provides a competitive advantage over less toxic phytoplankton such as diatoms and green-algae, by increasing daily light dose and reducing sedimentation losses.

    Nutrients released in anoxic waters during periods of stratification contribute to the growth of algae blooms.

     

  • Stratification and cold water pollution remains, to this day, a significant issue for larger storage reservoirs. A number of solutions have historically been employed to minimise the impacts of stratification in reservoirs, with varying degrees of success.

    WRL’s Brett Miller and Farid Chaaya conducted an exhaustive literature review of the previous work that had been completed to attempt to mitigate cold water pollution and reservoir stratification in general. From this, two main strategies were recognised as the most feasible: selective withdrawal and artificial destratification.

     

  • Selective withdrawal involves withdrawing water from different depths of the reservoir using multi-level offtake (MLO) infrastructure. An MLO could be used to withdraw water through one (or more) of a number of offtakes at varying depths in the reservoir for the purposes of cold water pollution mitigation (by withdrawing from above the thermocline) or mitigation the release of cyanobacteria downstream (by withdrawing from below the water surface). 

    Compared to artificial destratification, one of the distinct advantages of an MLO are the seemingly minimal operational costs. Conversely, one of the disadvantages are the large capital costs, which increase significantly with an increasing reservoir size. 

    A distinct disadvantage of selective withdrawal are the conflicting cold water pollution and cyanobacteria protocols. Where cold water pollution required withdrawal from above the thermocline, mitigating downstream algae release requires water to be withdrawn from below the surface and often below the thermocline. Currently, algae release mitigation protocols trump cold water pollution mitigation operations, which can have significant impacts downstream due to the sudden introduction of cold water to the previously warmed downstream environment (referred to as “cold shock”).

     

  • Artificial destratification involves artificial mixing a stratified reservoir to break natural stratification and maintain a destratified reservoir. This would theoretically mitigate the previously outlined issues associated with stratified reservoirs, including cold water pollution and anoxic waters.

    Artificial destratification is commonly achieved through one of two methods: bubble plumes or mechanical mixers. Bubble plume destratification is achieved by pumping compressed air to a diffuser located at a deep point in the reservoir, and released as bubbles through small nozzles. These bubbles rise, entraining cold hypolimnion waters to the surface. These waters detrain at the surface, sink to a level of neutral buoyancy (based on density differences) and propagate away from the plume. This sets up large circulation cells, which eventually break down the thermocline and mix the reservoir.

    Mechanical mixers utilise propellors to jet water either from the surface-down or bed-up. At the end of the jet, water sinks or rises to a level of neutral buoyancy (based on density differences), much like the bubble plumes. Mechanical mixers at the surface are often accompanied by a draft tube, which directs the jet down to a particular depth.

    As part of this review, a library of 125 international examples of destratification was collated. Extensive review of these resources led to the conclusions that in most cases, bubble plume destratification was the more viable option. Mechanical mixers can result in more localised mixing effects, and applications are often limited due to the necessity of a draft tube and the difficulty associated with long draft tubes in very deep reservoirs. 

    Compared to selective withdrawal, artificial destratification presents significantly lower capital costs. Operational costs, however, are significantly higher, increasing with the size of the reservoir. One of the main advantages of artificial destratification is the potential benefits to the in-reservoir water quality while concurrently mitigating cold water pollution. By nature of destratifying, DO levels are increased throughout the water column, increasing the health of the reservoir and decreasing the release of undesirable soluble metals and nutrients. Artificial destratification also has the potential to reduce algae growth, by artificially removing the buoyancy advantage it has in a stratified environment. In a deeper reservoir, cyanobacteria should theoretically be mixed to a depth at which it is light deprived, further limiting growth.

  • With the insights gained out of the literature review, we are looking to the future of mitigating cold water pollution. A number of potential projects will hopefully take us one step further to solving this significant environmental issue, including investigations into:

    • Potential feasible sites for in-situ trials of an operational bubble plume destratification systems
    • Physical and numerical modelling work to optimise destratification operations
    • Renewable energy sources for large scale destratification operations 
       
Please contact:

Brett Miller | Director, Industry Research | b.miller@wrl.unsw.edu.au

Farid Chaaya | Project Engineer | f.chaaya@wrl.unsw.edu.au

F C Chaaya and B M Miller (2022) “A review of artificial destratification techniques for cold water pollution mitigation”, WRL TR 2021/17, February 2022, UNSW Water Research Laboratory.