Safety modifications at Lower Laithe reservoir

Lower Laithe reservoir is located near Stanbury in West Yorkshire. The reservoir retains a volume of 1 275 000 m³ of water when full to its top water level, and as such falls under the legal obligations of the Reservoirs Act 1975. A statutory inspection of Lower Laithe reservoir was carried out in 2008 in accordance with the Act. This inspection revealed potential inadequacies with the ability of the existing overflow arrangement safely to convey the design flood. Mott MacDonald Bentley has subsequently been employed by the reservoir owner to investigate these inadequacies. A flood study and physical model have shown that during the design flood, spill flows in excess of 72 m³/s could be expected, with velocities exceeding 19 m/s. Flows of this magnitude would overtop the walls of the spillway and stilling basin and would be likely to cause extensive damage to the existing masonry lined channel. Remedial works have subsequently taken place at Lower Laithe to protect the overflow arrangement from the design flood. Improvement works to the existing spillway have included lining the structure with concrete and raising the masonry walls. Innovative modifications have taken place in the stilling basin to direct flows to the river channel, thereby protecting the toe of the embankment. In addition it has been necessary to raise the level of the clay core above the design flood. This paper describes these safety improvement works, as well as the stringent flood contingency measures which were required to maintain reservoir safety while completing works on an existing spillway structure.