Dee Regulation Scheme - Operating Rules

Operating Rules

The operating rules for the current system are agreed with all the participants and define the circumstances in which flow will be managed and precisely managed. The key measuring location is at Manley Hall, a gauging station near Chirk. This location was chosen as it was on a section of the river where flow could be readily measured and above the very flat stretch that meanders into Cheshire. The current rules state that when flow exceeds 10 m³/s no intervention is required. In practice some adjustment of Bala sluices may take place to increase storage in Llyn Celyn and conversely some releases may be made from Llyn Celyn for recreational or power generation purposes. When the flow at Manley Hall approaches 10 m³/s, additional flow is released from Llyn Tegid. If that is insufficient, flow from Llyn Celyn is used to maintain 10 m³/s at Manley Hall. In extreme situations where the flow from Llyn Celyn is insufficient to maintain the flow, releases of water from Llyn Brenig . Circumstances can arise where even this is insufficient to maintain flows and in such cases drought provisions are agreed which progressively reduce the maintained flow at Manley Hall. The overall effect of this regulation has a marked impact on the hydrograph of the River Dee. In dry years the hydrograph flat-lines at 10 m³/s whilst dry weather continues such as in 1990