Challenges at Chasewater28 October 2010
Addressing the twin threats of internal erosion and overtopping at Chasewater dam in the UK has presented Lichfield District Council with a multitude of challenges. The search for an improvement scheme which not only addresses safety issues but is practicable and affordable has been a big ask for a small council with limited funds and no in-house engineering expertise. Neil Turner and Lizzie Thatcher report
The primary water source for the canal network in the English West Midlands is under threat. The 200-year-old reservoir, known as Chasewater, a key piece of the region’s infrastructure, is leaking and in need of significant and extensive improvements.
The large raised, category A reservoir, which covers an area of around 1.06km2, and holds approximately 4.5Mm3 of water, is retained by two dams. The western embankments are 4m high and 1200m long, and the eastern dam rises to a height of 12m and stretches for over 560m long.
The challenge to bring it up to contemporary safety standards, for the benefit of the region, is being faced by Lichfield District Council – a small council in Staffordshire, with no in-house engineering staff or expertise, limited funds, and no ability to exploit the value of the water in the reservoir to recoup the costs of the work.
A brief history
In the late 1700s, the area of Birmingham and the Black Country was the cradle of the industrial revolution. To support the rapid expansion of industrial activity, an extensive canal network was built to provide fast and effective transport between its growing towns and cities and the markets for their products.
Initially the canal network relied on water from streams and rivers. But in 1797 work started on the construction of a reservoir in the Crane Brook valley, between Walsall and Lichfield. Although the original dam was washed away in a summer storm of 1798, the dam was immediately rebuilt and is still standing today.
In 1957, with the canal network seemingly in terminal decline, the British Transport Commission sold Chasewater and its land to the local council. Fifty years later, ownership now rests with Lichfield District Council – a small second tier local government authority.
Today the canal network carries leisure boaters and holiday makers, rather than freight, and together the canals and Chasewater support an increasingly valuable part of the region’s economic and social landscape.
But the reservoir’s main eastern dam is leaking, and it requires extensive improvements to ensure that it can withstand the next two centuries.
In his 2007 report, the council’s inspecting engineer, Rod Bridle of Dam Safety Ltd, concluded that in the ‘interests of safety’, the council needed to implement the following actions:
• Investigate the effects of water seeping through the eastern dam and quantify the dam’s resistance to internal erosion. The fear is that the velocity and volume of water flowing through the dam could lead to internal erosion, causing pipes to form which could threaten the dam’s structural integrity.
• Ensure there is sufficient overflow capacity, so that the reservoir can withstand levels of water expected during a period of very heavy rainfall.
A dam without records
The age of the reservoir, and the many changes of ownership over the years, has meant any records or drawings that were prepared in the dam’s early years have long since been lost. Not knowing its exact composition has meant that assessment of the dam’s robustness has been extremely challenging. It may have stood strong for 200 years, but there is no guarantee that it could stand for another two centuries.
Because of this lack of knowledge about the dam, a quantitative risk assessment was completed. It suggested that the probability of the dam failing in any one given year was 1 in 1700 – which is an intolerable and unacceptable level of risk. It was forecast that a sudden and catastrophic failure could release a wave of up to 5m in height, which would flow rapidly downstream towards the town of Tamworth. Without adequate notice this wave of water could lead to the loss of 80 lives, threaten the homes of around 2000 residents and cause considerable damage to major pieces of infrastructure including the M6 Toll motorway, the trunk roads of the A5 and the A38 and the Birmingham Cross City railway line.
This worrying scenario has focused the minds of the engineers and the council. Over the last three years, Chasewater has been studied from all angles, firstly to assess the problem and secondly to ensure that improvement works can be specified. Investigations have concluded the dam is best defined as an old, unzoned heterogeneous dam, with no clay core or filters. The following investigations have been carried out:
• Hydrological flows have been calculated and assessed.
• Boreholes have been drilled and trial pits dug to plot the composition of the dam and its foundations. Investigations were started with high hopes that we would encounter a clay core, or other forms of protection against erosion. No clay core, or other defence mechanism, was found.
• Groundwater surveys have been completed.
• The mining history of the area has been collated and plotted against the dam’s foundations.
• The chemistry and temperature of the water in the lake, the dam and downstream has been sampled and compared.
• Silt samples and water quality tests have been undertaken.
• Core samples of the dam have been tested as to their resistance to internal erosion.
But it’s not that straightforward. If the engineering problems were not challenging enough, project managers and engineers are further hampered by a series of other complications and factors.
Complex land ownership arrangements, the presence of European protected species, high public interest, and the availability of funding to a small district council, have all bought additional complexity to the project.
Parts of the eastern dam are in the control of two other public bodies – British Waterways and Staffordshire County Council. This in itself ought not to be too troublesome because all parties are part of the public sector and are committed to ensuring public safety, but what is difficult is that a large sector of the downstream face is occupied by three privately-owned domestic properties. These homes are built into the dam, which increases the difficulties in establishing a means to protect against internal erosion along the entire length of the dam. The team has also had to work closely with the residents, to ensure that major civil engineering operations can be completed on the cherished gardens.
Chasewater Country Park is a high profile venue, popular with residents and visitors. Because of this, the works will be completed under the closest public scrutiny. And, with a circumference of approximately 6.4km and numerous entry points, it is impossible to prevent public access to the park – and neither would we want to, because people come to the park to relax in the natural peace and quiet, to participate in water sports and to enjoy a ride on Chasewater Railway, a growing heritage steam railway.
Since the ending of mining and heavy industrial activity in the area, the park’s ecological and bio-diversity value has rejuvenated spectacularly. Now large parts of the reservoir are designated as sites of special scientific interest (SSSI). Of particular interest to ecologists is the presence of the European protected species of great crested newts, little ringed plovers and floating water plantain. Because of this, all engineering plans have had to be designed and implemented in a way which meets with the approval of Natural England, and which minimises the impact on the area’s ecological value.
We also knew that to complete any works to the toe of the dam, the water level of the reservoir would need to be lowered significantly, if not completely emptied. But the area’s industrial and coal mining heritage has left a legacy of pollution, which could be mobilised as the level drops, and the reservoir bed becomes increasingly exposed. Initial tests suggested that the risk of discharging heavily polluted water was minimal, but more confidence was needed to confirm that water of poor quality would not be pumped into the canal and hence into a Special Area of Conservation further downstream.
Overshadowing all of these issues is that fact the Lichfield District Council is a small organisation with limited finance to meet the costs of bringing the reservoir to an acceptable standard. Moreover, it inherited the reservoir as a result of a local government boundary change, and had no real desire to own it. The council also has limited opportunity to exploit the water to their benefit to cover the cost of any works, as the rights to the water in the reservoir remain with British Waterways.
Lichfield District is home to just 40,000 council taxpayers. The possible repair bill of £5M (US$7.9M) would cost each household £125 (US$199) – which would represent a council tax rise of approximately 80%. With the UK government forcibly preventing rises greater than 5%, there is little opportunity for the council to meet the costs of the improvement works without it significantly impacting upon the provision of core services offered by the council. In consequence, any expenditure incurred and planned comes under the closest political scrutiny.
Lichfield District Council takes its responsibility for maintaining Chasewater extremely seriously, but as a small local authority, it understandably has no in-house engineers and other specialists. Given the complexity of the project, the council has appointed a highly skilled and experienced team to ensure that action taken to reduce the risk of failure are proportionate and practicable.
The team is led by Ron Bridle of Dam Safety, who has appointed Ian Carter of MWH, Alan Brown of Jacobs, and Dr Jean-Jacques Fry, Head of Reservoir Safety at edf, France to a specially formed Chasewater review panel.
The panel provides technical insight and academic challenge, and is supported by geo-technical, geological and water chemistry expertise from Adrian Jones of MWH, plus Simon Hake and Rob Reuter of Wardell Armstrong.
Bringing all this together and bridging the academic and technical gap between engineers and the council has been Rob Travis of Travis Baker Associates, the council’s consulting engineer. David Brown of British Waterways acts as the reservoir’s supervising engineer.
Ryszard Kawak of Townsend and Renaudon has accepted the most difficult task of estimating and controlling the construction costs of the emerging designs.
To ensure that the council operates within conservation legislation and to minimise the negative impact on the area, Penny Anderson Associates has been appointed to advise and implement appropriate mitigating actions.
The engineering proposals
To meet contemporary standards, Chasewater needs to be able to withstand the effects of a 1 in 10,000 storm. A report by Jacobs suggested that in the event of a probable maximum flood, flows into the reservoir would peak at around 77m3/sec, compared with an existing discharge capacity of just 10m3/sec, causing the water to rise uncontrollably, and overtop the reservoir’s western embankments and eastern dam.
To improve the overflow capacity, an extra culvert is to be installed in the railway causeway that divides the two parts of the reservoir. This should remove the threat to the western embankments.
Further downstream, the main overflow structure is to be extensively remodelled and extended into the SSSI to ensure that it will be able to discharge the volume of flow expected during a period of heavy rain.
These works ought to ensure that the reservoir will be able to withstand the worst weather imaginable in this part of the UK.
But the solution for internal erosion has been more difficult to determine, so a series of detailed tests were conducted. These have included two erosion measurement tests on core samples taken from the dam. The hole erosion test (HET) and the jet erosion test (JET) were carried out by Geo-Consult in France, under the supervision of Dr Jean-Jacques Fry. The results showed the dam is an unzoned heterogeneous earthern embankment comprising mixed alluvial and glacial materials (including sands, gravels clays and some peat) and colliery spoil. This knowledge has helped the team to predict the behaviour of the dam, and importantly provide an insight into how it might erode and fail.
The results also showed that pore pressures in the dam, and hydrological gradients were not too high, which has given the team confidence that the dam is stable enough to have the improvements work carried out.
The proposed solution is to install sand filters that will trap the smallest particles expected to be present, supplemented by relief wells, along the entire length and height of the eastern embankment. This will involve scraping the top soil and vegetation from the surface, and installing a 200mm layer of graded fine filter material, covered with a second layer of coarser graded filter material to a depth of no less than 200mm, and then surfaced with the replaced topsoil. This will also see the gradient of the downstream face reduce from a typical 1 in 3 slope to 1 in 2.5.
Where filters cannot be installed, for instance around the properties, sheet piled walls are to be driven into the dam to provide an impermeable barrier and to direct the water towards high volume filters.
All this work will potentially leave the pipe between the reservoir and the canal as the remaining weak link. As part of the works, the pipe will be relined, so it can operate satisfactorily.
The council has undertaken a tendering exercise to appoint a suitably skilled and experienced contractor to carry out these works. Appointment was expected during July 2010, and the works are expected to take around six months to complete.
In preparation for the works, the valves to the reservoir were opened in February 2010, and the water has now reached canal level at 144m AOD. Before the works to the downstream face can begin, the remaining water will be pumped out.
Facing up to responsibilities
Ensuring the safety of Chasewater remains a most difficult and burdensome obligation for an organisation like Lichfield District Council. But despite the multitude of challenges and complexities, Lichfield District Council has faced up to its responsibilities and is nearing a position to carry out the required works that will safeguard the structure of the dam, the reservoir, the local wildlife, and nearby communities in years to come.
The lack of historical records has presented our engineers with a unique challenge both to appreciate the condition of the dams and to design an improvement scheme which not only addresses the twin threats of internal erosion and overtopping, but which is proportionate, practical and affordable.
Through this work, Lichfield District Council will also ensure that Chasewater continues to supply the canal network that runs through the West Midlands with water thereby supporting thousands of businesses, jobs and communities for many decades to come.
Neil Turner and Lizzie Thatcher, Lichfield District Council, Frog Lane, Staffordshire, WS13 6ZD England
Readers can keep up-to-date with progress at Chasewater dam via the project blog –www.lichfielddc.gov.uk/chasewaterdamblog