Repairs at Rowallan Dam8 March 2016
Unprecedented repairs have taken place at Rowallan dam in Australia. What made this project so special is that deep excavations exposed the heart of the live dam while it was still impounding the 130,000 megalitres of Lake Rowallan. Hydro Tasmania and Entura explain how they used flood forecasting techniques to help protect the integrity of the structure and maintain the safety of downstream communities.
For hydropower businesses, being able to confidently predict the likelihood, timing and level of flooding is vital for managing day-to-day operations and dam risks. But, when a dam undergoes major refurbishment, reliable and timely flood warnings are particularly important, buying time to implement emergency safety plans to avoid damaging and costly inundation of the construction works or even breaching of the dam.
Over the summers of 2012-13 and 2014-15, Hydro Tasmania successfully achieved a technically complex and challenging feat: excavating Rowallan Dam from crest to foundation on both sides of the central spillway, exposing the heart of the dam.
"What made this project unprecedented in Australia was that it was conducted on a live dam, holding back Lake Rowallan, with its capacity of 130 000 megalitres, and this meant high exposure to flood risks for the duration of the work," said Chris Topham, Hydro Tasmania's Dam Safety Manager.
Rowallan Dam is a 43m high central-core earth and rockfill dam in the Mersey-Forth Power Scheme in northern Tasmania, which contributes up to 14% of Tasmania's total energy production per year. Constructed in 1967, it comprises two embankments either side of a central concrete-lined spillway. The spillway walls retain up to 15m of earth and rockfill.
As Rowallan Dam is a headwater storage of the Mersey-Forth Power Scheme, the consequences of dam failure would be significant, and include failure of a downstream dam, lengthy interruption of power production, and major flooding of downstream communities.
Piping incident and repair
With its central-core earthfill construction, Rowallan Dam was susceptible to internal erosion of the core, a failure mode commonly referred to as 'piping'.
Following the first filling of the storage, a small subsidence (approximately 300mm) was observed in July 1968 adjacent to the right-hand spillway wall. In November 1968, a 1.5m diameter, 1.3m deep sinkhole was observed on the downstream edge of the crest adjacent to the right-hand spillway wall. The sinkhole was the only outward sign of piping.
Lowering of the reservoir via the penstock bypass valve and through the power station began immediately, along with investigations and repair works (in December/January).
The 1968 investigations, via excavation of a deep shaft in the affected location, revealed that the contact earthfill (highly plastic clay placed against the spillway wall) had eroded into the downstream filter and a piping pathway had formed at several elevations. Had this process continued, the eroded pathways had the potential to enlarge sufficiently to breach the embankment adjacent to the spillway, which could have caused an uncontrolled release of the storage.
The 1968-9 repair backfilled the investigation shaft, reconstructed the upper embankment, and duplicated this repair on the left spillway wall interface. The dam has since performed without incident and with no adverse surveillance observations.
"Despite the 1968 piping incident, Rowallan Dam has provided nearly fifty years of good service, and a review in 2011 of the dam's condition and performance indicated that it was meeting expectations for a dam of its design, construction and age,"Topham said. "However, since it was built, engineering standards have changed, and we needed a major refurbishment to bring the dam into the 21st century, install permanent repairs for the original piping incident, and reduce risks that were unforeseen in 1967."
21st century refurbishment
The first stage of the 2013-15 Rowallan Dam refurbishment upgraded the existing spillway training walls. These were overly slender and had deformed during construction, contributing to the piping incident. The upgrade works used the existing training walls as formwork for new reinforced concrete walls, built to modern standards, on the inside of the existing spillway.
In the second stage of the upgrade works, all earth and rockfill material next to the spillway walls which had been impacted by the piping incident was removed to foundation level (a depth of 15m) and replaced with new and re-used materials to modern design standards.
The second-stage works also removed and replaced the top 7m of embankment materials across the entire length of the embankments to address piping risks in the upper portion of the dam. As part of this element of the work, the top of the core (which previously stopped 1.5m below the crest) was raised to the top of the dam to better protect against overtopping of the dam in extreme floods.
"Rowallan Dam will now withstand extreme floods, of the severity only estimated to occur once in around 20,000 years," Topham added.
Managing risk during construction
Such extensive excavation work called for a rigorous programme to mitigate construction risks.
"It was an optimisation problem," Project Manager Brian Daws says. "We needed to make the worksite accessible to trucks and large equipment, but we had to have a plan in place to protect the work against inundation from a flood resulting from high inflows during construction.
"How much material can you remove from the wall of a live dam to give yourself enough room to do what you need to, but not so much that you can't put it all back in a hurry? And how do you predict with confidence whether and when a flood will hit, and how high the water will be? he asked."
To reduce the amount of time that the dam was exposed to greater flood risk, the deep excavation work was undertaken using 24-hour, continuous shifts.
An emergency plan to backfill the worksite was developed that would be robust, able to be undertaken quickly in heavy rain, and that considered and accommodated all the stages of the proposed excavation and reconstruction works.
"The sensitivity of when to trigger emergency action was a particular challenge," Daws said. "If backfilling was not carried out in time and a flood breached the construction works, the dam would be at risk of failure, with serious consequences for the storage downstream and potential impacts on communities."
On the other hand, triggering an emergency backfill unnecessarily would be costly.
"Backfilling is expensive and would risk extending the construction period into the wet season and the following summer," he commented, "which would mean extra construction costs and losing water that could otherwise have been used to generate power."
Maintaining optimum lake levels
To help minimise the risk of flooding, the works were conducted over summer, when the region expects low rainfall. As well, Lake Rowallan was lowered to its normal minimum operating level to provide a buffer against unexpectedly high levels of rainfall or inflows.
Like the complexity of determining the appropriate sensitivity of the backfilling trigger, establishing the optimum water levels for Lake Rowallan throughout the construction period also required a fine balance to enable safe work without losing too much valuable water and revenue by releasing more water than necessary through bypass valves rather than through the power station.
"Releasing water via the bypass valve is physically easy," said Chris Topham, "but all the calculations that go into determining what the lake level might do - evaporation, losses, what rainfall is on the way - are highly sophisticated."
Effective flood forecasting
A key factor in the suite of risk mitigations was an innovative flood detection and warning system that could reliably inform both the flood preparations to protect the construction works and the optimum water levels for Lake Rowallan. To develop such a system, Hydro Tasmania drew on the expertise of its specialist power and water consulting business, Entura.
"The Rowallan flood forecasting system needed to forecast whether or not imminent weather systems posed a threat to the construction site, as well as continually forecast whether lake levels were likely to reach the emergency trigger level, and if so, how long they would take to reach that level," Kim Robinson, Entura's Senior Hydrologist and project manager for the Rowallan flood forecasting system, explained.
The trigger level varied depending on the stage of construction works but maintained the same level of flood risk at all times. Entura's flood forecasting system combined measured rainfall data from a network of seven telemetered rainfall gauges surrounding the catchment, with rainfall forecasts from weather forecasting agencies into a hydrologic model of the catchment.
The Rowallan catchment covers 344 km2 with mean annual rainfall of 1900 mm. Once the catchment is saturated, high rainfall can bring on steep increases in inflows within a couple of hours. To manage the construction flood risk for this project, Hydro Tasmania's existing inflow and lake level forecasting system for Rowallan needed considerable upgrades.
"The project required a model that adopted best practice, and offered optimised predictive performance that was well understood," Robinson commented.
Rainfall inputs were reviewed and updated, the inflow model was rebuilt and calibrated, and a lake level operation model was developed based on the results of the inflow model and predetermined lake operating rules.
The entire modelling process ran automatically and provided an updated forecast every two hours during the construction period, with forecasts extending seven days ahead. Plots were developed showing the best-estimate forecast of rainfall over the catchment, inflows and the resulting lake levels. The operators and construction managers used these plots to guide the operation of the valves to keep the lake at appropriate levels and to ensure appropriate preparation for emergencies.
The model status was routinely communicated to the site team via twice-daily emails of data currency reports and forecast plots. These were also available on a website. Flood alerts were set to trigger if the forecast exceeded predetermined critical levels. Flood alerts were disseminated via the SCADA system to Hydro Tasmania's general control desk (operated 24 hours, 7 days per week), as well as via SMS to the dam safety team and site team.
"The system was made more robust by building in redundancies in the modelling and developing a stand-alone system at the dam site that did not require continuous connection to the database. This could be used if communications with the dam site were lost," Kim Robinson explained.
Confidence was critical
To adopt a safe yet 'just-in-time' approach to triggering emergency backfilling required high levels of confidence in the accuracy and reliability of the flood forecasting system, and clarity of the level of certainty of the forecasts.
A plan was put in place to ensure that critical rainfall and flow gauges for the modelling were operational and that any maintenance required was undertaken within strict timeframes. Routines were also developed to automatically check all input data for errors or missing data, and send alerts if inconsistencies were detected.
When the telemetry actually did fail on a critical rain gauge, the failure was quickly detected and a helicopter was dispatched to fix the gauge. This had previously been identified as a risk, and the remedial measures were well understood and readily implemented
The flood forecasting outputs included indications for operators of the level of certainty of both the rainfall forecasts and the modelling.
"We developed the rainfall forecast uncertainty measures by comparing forecast rainfall to rain-gauge data over a historical period. The model uncertainty was estimated by running the model over a historical period with historical rainfall as input, and comparing outputs with measured lake levels", said Robinson.
As forecast rainfall was the greatest source of uncertainty in modelling, a process was established for obtaining further detailed information from the Bureau of Meteorology on the forecast rainfall or complex weather systems if a flood alert were issued, including a briefing from a meteorologist.
"This extra advice and information was important for increasing the level of confidence in the forecasting," she added.
Before the extensive construction works began, the model and the flood management response were exhaustively tested in an exercise involving the State Emergency Service and police, the dam safety regulator, and other stakeholders. A specialist external reviewer also provided assurance of the validity and defensibility of the flood forecasting system and emergency management plans.
The flood forecasting system developed for the Rowallan Dam upgrade project performed well through the varying flow experienced through the actual construction period. Inflows were close to average over the period, but the last two months included a very dry April and an unusually wet May.
Learnings from this project are being rolled out to progressively update the flood models for all of Hydro Tasmania's extensive water storages.
"The forecasting system has permanent ongoing value to Hydro Tasmania," says Chris Topham, "and is being transitioned into the ongoing inflow and flood forecasting system used by the business to safely and sustainably manage power generation and dam safety risk."
In August 2015, Hydro Tasmania's Rowallan Dam upgrade project was awarded the Tasmanian Project of the Year in the Construction/Engineering category by the Australian Institute of Project Management.