Managing the Environmental Risk from Reservoir Draw Down

Pollution incidents caused by uncontrolled discharges of sediment rich water from reservoirs can impact on an owners' reputation and performance measures. Such incidents can also affect programme and have significant cost implications.Scottish Water is developing a Reservoir Operation and Maintenance Strategy (ROMS) comprising work procedures and instructions, similar to those that it has implemented to control activities on its water distribution network. This paper describes the development of processes to identify the risks and manage the controlled drawdown of reservoirs for the purposes of maintenance and capital works, ranging from nominal water level lowering over a period of a few weeks for minor repairs to complete reservoir emptying for the purposes of dam breaching. It includes lessons learned from previous incidents, an outline of the approach to identifying the threats within the reservoir and the receptors in the downstream environment and hence the level of risk that the drawdown may pose. It describes the suggested minimum mitigation and monitoring measures to be incorporated in the drawdown plan dependant on that level of risk.