Weathering the storm13 May 2020
Extreme weather events are challenging water managers across New South Wales in Australia. After prolonged drought and catastrophic bush fires, long awaited rainfall has led to an increased focus on water quality in dam storages. Suzanne Pritchard reports.
Australia has experienced its fair share of extreme weather events in recent months. Regions of the country have suffered from one of the worst prolonged droughts in history, followed by unprecedented bush fires which continued to burn throughout the beginning of 2020.
By February 2020 the Bureau of Metrology (BOM) had announced the likelihood of significant rainfall events, and although this was welcome news, it also led to even greater challenges for water companies such as WaterNSW.
Following the BOM announcement, WaterNSW said it was anticipating inflows into all metropolitan dam storages, including the Warragamba Dam which, on 7 February 2020, was at 42.8% capacity. It was expected that some debris and ash was likely to be transported into the water storage following the bush fires.
On 14 February WaterNSW confirmed that its team was working hard to maintain water quality into Sydney’s largest catchment, after the welcome rain had washed “significant amounts” of ash and debris into Warragamba Dam.
During December 2019 and January 2020, more than 320,000 hectares of the Warragamba Catchment was ravaged by fire. Since then, WaterNSW has sought to understand the potential consequences of such significant fire damage on water quality.
Heavy rainfall commonly pushes sediment and debris into Lake Burragorang which is impounded by Warragamba Dam. However, the addition of unprecedented amounts of bushfire ash and lack of ground cover to prevent soil erosion, resulted in increased levels of debris making its way into the lake. By mid-February sediment, ash and debris were clearly visible on the surface of Lake Burragorang.
Warragamba Dam is part of the water supply scheme for Greater Sydney in Australia
WaterNSW CEO, David Harris, said on 14 February: “Our people are the best experts in the area of water quality. We are taking a range of precautionary measures on site, including the deployment of a third floating boom in the Warragamba Gorge. Raw water quality at Warragamba is improving, however more inflows may cause further deterioration in water quality at the dam wall. The intrusion is behaving exactly as our staff predicted – it is staying in the upper section of the water column. This means that we can safely draw good quality water from much further down in the water column if we need to supply from Warragamba Dam - which we are not doing at the moment.”
Supply to Sydney Water’s Prospect Water Filtration Plant continued to run steadily without any issues but, instead of drawing water from Warragamba Dam, it was drawn from Prospect Reservoir and the Upper Canal. WaterNSW had taken the “considered decision” to draw upon Prospect Reservoir as a precaution in case of water quality issues.
Harris said that the monitoring was being carried out by highly experienced water quality scientists who are using sophisticated, real-time technology pioneered by WaterNSW, and which can predict any change in the dam storage’s water quality.
Actions taken by WaterNSW to mitigate the potential risk to dam water quality included:
- The establishment of an expert incident response team who constantly monitored water quality.
- Installation of containment booms/silt curtains at three locations on Lake Burragorang to limit the amount of ash and debris near the dam’s supply off-take point.
- Development of joint event response monitoring plans in consultation with Sydney Water and NSW Health to characterise changes in water quality.
- Erosion modelling to determine areas of highest risk and inform further mitigation strategies.
- Online monitoring and hydrodynamic modelling to pre-emptively assess ash movement and provide information on the duration, magnitude and location(s) of such an event.
- Joint operational contingency plans to maintain supply in the event of poor water quality, e.g. offtake reconfiguration/supply from Prospect Reservoir and/or Metropolitan storages.
- Assessing weather predictions daily and identifying rainfall triggers that will activate additional management actions.
- Maintaining contact with other water authorities who have had recent experience managing post-bushfire water quality risks, such as Melbourne Water.
The drought gripping the state is evident in the low water level at Copeton Dam in New South wales which supplies local irrigation, stock and household needs. July 2018
On 18 February 2020 WaterNSW’s CEO confirmed that the precautionary measures “enacted in the wake of unprecedented bushfire damage and the largest rainfall event since 1990” had been successful in protecting Sydney’s dam water quality. Warragamba Dam also resumed supply of raw water to the Prospect Water Filtration Plant.
“Since December last year WaterNSW has been quite clear in saying that we wholly expected to encounter water quality challenges if large rainfall fell on bushfire-damaged parts of the Warragamba catchment, and that we were well-prepared with scientific expertise and operational contingency plans if that’s what eventuated,” Harris said.
“We had a month’s supply in Prospect Reservoir available so we could draw on it until our expert monitoring team could fully understand the effects of the prolonged drought and extensive fire followed by heavy rainfall, and confirm their confidence in the water quality in Warragamba. We spent the last week monitoring the intrusion into the Warragamba system and are now confident in resuming supplying raw water to Prospect Water Filtration Plant.”
Harris went on to explain that Prospect Reservoir is regularly used to supply raw water to Prospect Water Filtration Plant. It is described as being a critical part of the system and is often used when other parts of the supply system are offline for planned maintenance.
“This is a standard part of operating Greater Sydney’s water supply network. Our experts regularly re-configure our network to deal with these issues and do so with minimal interruption to Sydney’s drinking water supply or quality,” Harris added. “With safe raw water supply available from Prospect Reservoir, the people of Sydney would rightly have considered it irresponsible if we had not taken every precaution with our decisions around raw water from Warragamba.”
Wyangala Lake on the Lachlan River in New South wales. More than 60,000ML of inflows from 10-19 February 2020 increased storage from 8.4% to 13%. WaterNSW plans to raise the dam leading to a 53% increase in storage capacity
Professor Stuart Khan, from the School of Civil & Environmental Engineering at the University of New South Wales in Sydney, says that maintaining drinking water quality is a major challenge for water managers during and after bushfires.
He explained that the fires had severely and extensively burnt major drinking water catchments for Sydney and the Shoalhaven region in NSW. The loss of trees and groundcover make the soil more vulnerable to erosion so when the next big rainfall event comes along (a downpour of around 100mm in a short period of time would qualify as such an event), runoff will wash a lot of that ash into waterways and dams. Rain can also wash in other contaminants, such as building debris, dead animals and pollutants from fire retardant.
In bushfire-affected areas, Professor Khan said the destruction of water infrastructure was a more immediate concern. Drinking water treatment plants in fire zones had lost power and were damaged, while some had even lost the ability to pump or properly treat water. An urgent boil water alert was sent to residents in Eden and Boydtown in early January 2020 after a loss of power to chlorination facilities at Ben Boyd Dam.
Professor Khan added that the immediate threat of water contamination was a cause for greater concern and although the long-term problems are going to be a real challenge for water utilities, he doesn’t think that such challenges are completely insurmountable.
In an effort to support its members and the industry through their bushfire recovery, the Water Services Association of Australia has been working with water utilities to share bushfire and water quality knowledge and experience.
SA Water has provided information about its experience and lessons learned when managing sediments in water catchments following bushfires in 2015. The company provides water services to around 1.7 million South Australians and maintains and operates ten major metropolitan reservoirs.
SA Water says that it learned that larger more intense fires create more ash which can be up to 100mm deep, and is loose and highly mobile. Their advice was to also retain bushland sediment as it contains seeds that will start to hold the catchment together once it germinates, and will contribute to water quality protection in the long term.
TasWater recounted its experience of numerous bushfires which started across Tasmania in January 2019 following extensive dry lightning strikes; some of which continued to burn until April 2019. Approximately 25% of the Huon River catchment area was burned. TasWater learned that:
Bushfire preparedness planning needs to include catchment risks in addition to asset risks and needs to begin earlier.
Access to impacted catchments needs to be negotiated earlier to assess the potential impact on water quality.
Brief drought relief
On 2 March 2020 the New South Wales Rural Fire Service announced that, for the first time since early July 2019, there was currently no active bush or grass fires across the country's most populous state.
On the same day, WaterNSW announced that several dams hardest hit by the ongoing drought had experienced a welcome reprieve with recent rainfall resulting in the first inflows in years, and a minor increase to dam levels.
Keepit Dam - which services the township of Walgett as well as other needs on the Namoi River – saw the best of the increases, rising to 11.2% from 0.6% of capacity just weeks before.
Welcome flows also hit Copeton Dam, bringing levels up to 10.8% capacity, an increase from 9.3% a week earlier. Burrendong Dam rose from 1.6% to 4.2%.
While these storage increases provide brief respite, WaterNSW cautions that much of the state is still in drought. As of 25 February, 98.9% of the state remained in drought with 23.6% experiencing “intense drought”.
“Some parts of the state hardest ravaged by ongoing drought have seen very welcome water flow into dwindling storages,” WaterNSW Executive Manager of System Operations, Adrian Langdon, said. “While these inflows are not drought-breaking, they will provide much needed relief for several communities. WaterNSW will continue to forge ahead with projects that will improve the state’s drought resilience,” he added.