Toddbrook Reservoir

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Toddbrook Reservoir
Toddbrook Reservoir (northern end) - geograph.org.uk - 77856.jpg
Looking southeast towards the dam, from the northern corner
LocationHigh Peak, Derbyshire
Coordinates53°19′30″N 1°59′37″W / 53.32500°N 1.99361°W / 53.32500; -1.99361Coordinates: 53°19′30″N 1°59′37″W / 53.32500°N 1.99361°W / 53.32500; -1.99361
Typereservoir
Catchment area1,700 ha (4,200 acres)
Basin countriesUnited Kingdom
Surface area14.6 ha (36 acres)
Max. depth24 m (79 ft)
Water volume1,288,000 cubic metres (283,000,000 imp gal)[1]
Surface elevation185.69 m (609.2 ft) [2]
IslandsNone

Toddbrook Reservoir, a feeder for the Peak Forest Canal, opened in 1838.[3] It is sited above the town of Whaley Bridge in the Derbyshire High Peak area of England. The reservoir is a Site of Special Scientific Interest (SSSI) providing habitats for herons, ducks and other animals and fish, while rare mosses and liverworts grow on its shores, particularly short-lived species that grow on seasonally exposed mud.[4] The reservoir is used for sailing[5] and angling. The reservoir is owned by the Canal & River Trust and, like the nearby Combs reservoir, is a feeder reservoir for the Peak Forest Canal. The feeder runs through Whaley Bridge, and with the Combs feed enters the canal system in a pool close to the transhipment shed at the Whaley Bridge canal basin.

The reservoir is fed from the Todd Brook, a stream which has a catchment area of around 1,700 hectares (4,200 acres) including the moorland on Shining Tor and farmland around Kettleshulme village. Water enters the reservoir on its north bank via a small waterfall. The first several inches of water do not flow into the reservoir but flow down the reservoir's run-off into the River Goyt. The reservoir often receives little or no inflow during periods when rainfall does not allow the level to exceed the barrier, and this often affects the reservoir's water level, particularly in the summer months.

The reservoir's dam is built from earth with a puddle clay core. In August 2019, concrete panels on its spillway were dislodged after heavy rain, putting the dam at risk of collapse and prompting the evacuation of parts of Whaley Bridge and the surrounding area.

Dam works[edit]

The 24 metres (79 ft) high dam was constructed from clay with sands and gravel in 1840-41. Investigations in the late-20th century led to doubt as to whether it was constructed as laid out on original drawings – with a puddle clay core and embankments of more porous material on either side – or with more porous core material. The peak of the dam wall is supported either side by embankments with a slope of 1:2.[1] Underneath the dam, the geology includes glacial sands and gravel, glacial till overlying mudstones, sandstones, shales and coal measures.[6]

It was known that local coal mining had been a threat to the dam's integrity for many years, with complaints dating from 1880 that water was leaking into coal mines.[3] The builders were forced to purchase the block of coal below the dam to ensure mining did not cause structural issues.[citation needed]

Leakage was observed from the foot of the dam and investigated in 1931. The leak coincided with a hollow in the upstream slope of the dam which was filled in. In November 1975, low water revealed that another hollow had formed in the same area which led to regular monitoring of the dam. In 1977 it was noted that there had been 12 centimetres (4.7 in) of subsidence since 1975. The reservoir was drained and a four metre wide hole was discovered, partly filled by silt and a tree which appeared to have been drawn in by the flow of water into the hole. Between 1978 and 1980, further investigations included drilling bore holes into the affected areas. In 1981 a 1.2-metre (3 ft 11 in) diameter masonry culvert was discovered which had probably been built into the dam wall at the time of its construction to divert ground water, which can undermine a dam from below. Leaks had developed around it, as had happened to other dams where pipework or drainage had been built through a clay dam wall creating a weak spot.[1] Repairs were made and a layer of waterproof clay was laid over the weak spot on the upstream face of the dam. The clay core was reinforced by injecting grout along a 60 metre length and the reservoir was refilled in December 1983.[1]

The 1969 dam wall spillway in 2005. Note channel for original spillway in foreground.

In December 1964, high rainfall resulted in damage to the dam’s spillway. The water level rose to one metre above the overflow spillway at the side of the reservoir and remained high for 24 hours but did not rise above the main dam wall and the spillway prevented the water flowing over it and washing the earth embankment away. However, the high volume of water damaged the lower part of the spillway channel and escaping water started to erode the right bank at the base of the dam. The damage was repaired in 1965 but flood studies judged the spillway to be inadequate. It was noted by the Environment Agency that damage had occurred to nine other dams of similar design where the spillway delivered water to the foot of the dam.[1]

In 1969 an additional concrete spillway was built on the downstream face of the dam along a length of 75 metres (246 ft). This new weir was formed by adding concrete capping to the earth embankment to protect it from flowing water and was set above the existing spillway so that excess water would initially overflow along the original path.[1][3] In the 1980s, British Waterways carried out significant repair works to deal with leaks involving the mine shafts around the dam and a culvert was constructed under the current beach/launching area of the reservoir. A stone marker on the main beach showed its location for many years, but was relocated to the footpath opposite Toddbrook Lodge during access work for the 2009 draining.[citation needed]

The reservoir was partially drained in 2009 for inspection[7] before being fully drained for repairs in 2010. The £500,000 project entailed fish stocks being moved to other locations before repairing pitching to the wall of the dam. Other work involved repairs to the 100-year-old pipework used to feed water stored in the reservoir at a controlled rate into the Peak Forest Canal and included; clearing silt and debris from pipework inlets, removing old valves and connections to pipework, cleaning, inspecting and repairing old pipework, re-lining old pipework with a resin “sock” liner cured under pressure using ultraviolet lights, replacement of valves, testing of the new system and replacement of screens to intakes.[8]

2019 incident[edit]

A view looking down the spillway in 2014 before the concrete slabs to the left of the picture were damaged in 2019

On 1 August 2019, a major incident was declared and 1,500 residents were evacuated from parts of Whaley Bridge, Furness Vale and New Mills after concrete slabs on the 1969 overflow spillway were partially dislodged by high volumes of water following several days of heavy rain.[9][10][11] The Environment Agency issued a 'danger to life' warning due to the possibility of the dam collapsing. High-volume pumps were deployed to take water from the reservoir to prevent it from overflowing and reduce pressure on the dam. An RAF Chinook helicopter dropped 400 tonnes of aggregate into the damaged area[11] and specialist contractors added concrete grouting between the bags of ballast to bind them together to support the spillway.[12]

On 6 August the Canal & River Trust confirmed that the target reduction in water level of 8 metres (26 ft) had been achieved and that the reservoir was down to 25% of its capacity, which would allow a full inspection of the damage to the structure to be made.[13] Fifty residents were allowed to return to their homes on the evening of 6 August and, following a final safety inspection on 7 August, Derbyshire Constabulary declared that it was safe for the rest of the residents to return also.[14]

It was reported on 7 August that a representative of the Canal & River Trust had told residents, at a meeting on 6 August, that the whole of the reservoir, including the dam, would be completely reconstructed, in a project that would span a number of years.[15]

References[edit]

  1. ^ a b c d e f Charles, J Andrew; Tedd, Paul; Warren, Alan. "Delivering Benefits Through Evidence Lessons from Historical Dam Incidents". Environment Agency. p. 140. Retrieved 1 August 2019.
  2. ^ Toddbrook Reservoir update, Canal & River Trust, accessed 2. August 2019
  3. ^ a b c Labrum, E.A, ed. (1994). Civil Engineering Heritage: Eastern and central England. Institution of Civil Engineers. pp. 9–10. ISBN 072771970X.
  4. ^ "Toddbrook Reservoir [SSSI citation]" (PDF). Natural England. 1981. Retrieved 12 July 2012.
  5. ^ Toddbrook Sailing Club accessed 16 May 2008
  6. ^ Horgan, Rob. "Engineers battle through night to prevent catastrophic dam failure". New Civil Engineer. Retrieved 4 August 2019.
  7. ^ Toddbrook Reservoir forum posts at whalyebridge.net, 14 September 2009
  8. ^ Toddbrook Reservoir refilling to start in new year, 5 January 2011
  9. ^ "Whaley Bridge dam collapse: Town evacuated over Toddbrook Reservoir fears". BBC News. 1 August 2019.
  10. ^ Slater, Chris; Wilkinson, Damon; Britton, Paul; Fitzgerald, Todd; Yarwood, Sam (1 August 2019). "Thousands of Whaley Bridge residents evacuated as 'danger to life' warning issued over fears dam could burst". Manchester Evening News. Retrieved 3 August 2019.
  11. ^ a b "Whaley Bridge dam collapse: RAF Chinook brought in". BBC News. 2 August 2019. Retrieved 2 August 2019.
  12. ^ "Whaley Bridge dam: Threat of storms as repairs continue". BBC News. 3 August 2019. Retrieved 4 August 2019.
  13. ^ "Water level target reached at Whaley Bridge dam". 6 August 2019 – via www.bbc.co.uk.
  14. ^ Rucki, Alexandra (7 August 2019). "Thousands of people evacuated in Whaley Bridge can FINALLY return to their homes after Toddbrook Reservoir deemed safe". Manchester Evening News. M.E.N Media. Retrieved 7 August 2019.
  15. ^ Scheerhout, Britton, John; Britton, Paul (7 August 2019). "Toddbrook Reservoir in Whaley Bridge to be completely rebuilt after dam emergency, evacuated residents told at public meeting". Manchester Evening News. M.E.N. Media. Retrieved 7 August 2019.