Staffordshire County Council took over responsibility for carrying out a major programme of restoration works to Chasewater’s reservoir from Lichfield District Council earlier this year (2011).
The works are helping to make sure the reservoir remains safe in years to come, by limiting the effects of heavy rainfall and water erosion.
The Eastern dam is an earth embankment, and it is common place to carry out ongoing improvement works to a dam of its size, age and construction.
Throughout the project, LDC and now SCC have been working closely with ecologists and local wildlife groups to help plan and carry out the works with the minimum impact on local wildlife, flora and fauna, including newts and rare plants. Find out more on our wildlife page.
Throughout the works Chasewater Country Park is still open, and visitors can enjoy the mini-golf and boating lake on sunny days, and rides on Chasewater Heritage Railway, walks and refreshments throughout the season. We are very grateful to the clubs who use Chasewater Reservoir for their understanding and patience during the works.
So what is SCC doing to make Chasewater Dam safer?
Chasewater’s main (eastern) dam is over 210 years old, and was built when understanding of how dams work was not as advanced as it is now. If we were building Chasewater dam today, we would use different materials and designs that incorporate the latest safety features.
Our engineers regularly inspect the condition of the eastern dam and reservoir, and have identified where and how improvements need to be made.
There is a very low risk of the dam failing, and by carrying out the works, and maintaining a programme of proper maintenance, we can help to drastically reduce the chances of the dam failing.
That said, it’s important to remember that, like any structure, no reservoir is ever completely safe. This is because events like major storms, mining subsidence or earthquakes can always affect the strength of a dam.
Our engineers have told us we need to complete two pieces of work, which are detailed below.
Protect against a massive storm
The reservoir’s overflow system was tested by the floods of July 2007, and thankfully it passed with flying colours.
That said, unlike modern reservoirs, our engineers have told us that Chasewater’s overflow system was not built to withstand a massive storm, the type of which is rarely seen, and that we might experience once in every 10,000 years!
We need to make sure that if such a major storm took place over Chasewater, the dam’s overflow system would be able to cope. That said, storms of this magnitude are thankfully incredibly rare in the UK, but they have happened unexpectedly in other parts of the world.
To make sure the overflow system could cope, we are strengthening and widening the spillway, weirs and culverts. This will mean that if a big storm takes place, excess rain water will be able to flow down the spillway and safely away from the reservoir, rather than over the top of the dam.
We need to prevent water flowing over the top at all costs, as if this were to happen, it could wash away part of the dam, and cause a massive failure.
Protecting against erosion
We suspect that when the dam was built in 1799, the builders used whatever material they could lay their hands on. Unfortunately there are no records that describe what materials were used and how the dam was built, so we’ve had to carry out tests.
Whilst we know the dam contains large rocks and stones, these do not actually give it its strength. Instead, fine particles in the dam (of less than 0.2 microns in size – a micron is 1000th of a millimetre!) give it its strength. As you can imagine however, these tiny particles can be easily washed away by water passing through the dam.
Many modern dams have impervious barriers to prevent water flowing through them. Often earthen dams are built with a clay core, which slows down the flow of water through the dam, and stops fine particles from being washed away. We have drilled into Chasewater’s dam to see if it has a clay core, but it does not.
This means there is little in the dam to prevent the fine particles from being washed away. As such, if the speed and volume of the water that passes through Chasewater’s dam increases, more and more of the fine particles will be washed away, and the dam will become weaker.
Our engineers have told us that internal erosion could lead to a natural pipe being formed through the dam, through which water would flow, causing further and possibly irreparable damage to the dam. Over time (which could be very quick) more and more the dam erode away, until the dam failed, or the pipe collapsed onto itself.
Unfortunately we cannot simply put in an impervious barrier into the eastern dam, because if we get it slightly wrong, it could concentrate a leak into a specific weak point. Instead we need to install sand filters along the entire length of the dam.
These filters are layers of very fine sand will be laid onto the downstream face. The sand will allow water to pass through, but will trap the fine particles, and prevent erosion from starting.
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