View All Articles 207 - How does the Flood Defence Barrier work? Read the FACTS here The Penstocks at the Yarrow Flood Defence Barrier , Eccleston basin

How does the Flood Defence Barrier work? Read the FACTS here

4th of January, 2025

Many thanks to members of Lower Yarrow Flood Action Group for the attached detail. 

Preamble

  • The purpose of the dam is to keep the river level as it flows through the village,  low enough to allow surface water landing on or arriving in the village,  to continue to drain into the Yarrow. If the river level gets too high , the water will level off with it and flooding of roads and properties may occur. 
  • The dam achieves this by 'impounding' water in the valley ( known as the basin) upstream of it ( this is in Eccleston) . The dam has two gates ( known as Penstocks) which are lowered in stages when flooding is threatened. 
  • These Penstocks work automatically from readings taken on the river level at Castle Bridge and the way to find out what that river  level is , and the level at the 3 other measuring stations on the Yarrow, is to go onto the Environment Agency ( EA) website - River and Sea Levels in England - Location Croston - River Yarrow - River Yarrow at Castle Bridge , Croston. 

Dam Closing Operation 

  • The Dam will start to operate when the River Yarrow level rises to 3.36 meters. The first of the 2 Penstocks will move to fully closed. Nothing more will happen for 90 minutes but if the Yarrow level then is 3.42m or higher, the second Penstock will close by the first of 3 steps to 33% closed. 
  • Again , nothing more happens for 90 minutes,  but if then the river level is still 3.42m or higher , the second Penstock will close by another step to 66% closed. If after another 90 minutes , the Yarrow level is still 3.42m or higher, the second Penstock will move to fully closed. 
  • In all, it takes a minimum of 4.5 hours for the Dam to go from fully open to fully closed. Fully closed , water is impounded in  the basin in Eccleston. 

Raising Operation 

  • The Penstocks start to be raised when the Yarrow level at Castle Bridge drops to 3.31m. The first Penstock will then be raised by 1 stage . Nothing more will happen for 90 minutes but if the level is still 3.31m or lower, the first Penstock will be raised by another stage. 
  • Providing the Yarrow level remains 3.31m or lower, this process will continue every 90 minutes until both Penstocks are raised to fully open. In all it takes 7.5 hours for the Dam to go from fully closed to fully open. 

Capacity of the Basin in Eccleston 

  • The basin will hold 1,300,000 cubic meters of impounded water , up to the point when water would then start to run over the  "spillway" on top of the dam. At that point , the river level at the Dam would be 8.1m . 
  • The highest level so far recorded at the Dam since the Dam became operational in late 2016,   is 7m on 1/1/25. The previous highest level was during the Flood Event of  18-21 /1/2021 at 6.7m when it was using slightly less than 50% of the basin's capacity. 

Sources of Details Quoted 

  • Penstock Closing / Raising Procedure - email from James Chapman ( EA) dated 5 June 2023 
  • Basin Capacity - Graph received in email from Chris Tomlin ( EA) dated 14 December 2022 

Acknowledgement : RIG  25 February 2024