The role of the reservoirs
At the upper end of the Water of Leith catchment, Harperrig, Harlaw and Threipmuir Reservoirs play an important part in the Water of Leith Flood Protection scheme. Approximately 50% of the Water of Leith flows can be from the Pentland Hills and the other 50% can be surface run-off from the area within the city bypass. Telemetry systems check the river flows and when required, close reservoir valves to reduce reservoir discharge flows at the start of heavy rainfall. There is a fine balance between flood storage, amenity and avoiding drought and so we pay careful attention to the operation of this system.
The reservoirs have been included in the design of the flood protection schemes, based on them being full at the start of a flood event. Normal operation will keep levels lower through the winter months, which provides additional storage.
After a flood event, when the river flows drop below certain levels, the reservoir outlet valves will reopen automatically to allow reservoir levels to drop. This can take several days and can result in higher river flows when this happens. We can do this manually to start the process earlier, depending on weather forecasts and current river flows.