The World Commission on Dams’ thirteenth guideline refers to updating contingency plans for the operation of reservoirs in extreme flood events. This is an area which requires attention as there are some widespread misconceptions concerning the operation of spillway gates.
I have before me a press article on the Indian floods of autumn 2000 which refers to flooding being ‘made worse by engineers releasing water to prevent dams bursting’. There have also been similar articles in relation to the repeated terrible floods in Mozambique.
Spillway gates have to be opened in a big flood despite the flooding that will result although, if the gates are correctly operated, the flooding should always be less than it would have been if the dam had not been there. This may not, however, be understood by people downstream who may have got used to the flooding benefits which the dam has provided over a number of years when there have been only small floods.
When partly empty reservoirs absorb large floods it is appreciated by few people. A non-event is unlikely to seize the headlines and more often than not goes noticed. The absorption of 1.5km3 of water by the El Cajon reservoir in Honduras during Hurricane Mitch seems to have attracted little attention, and there are other examples.
Unfortunately, there is a real potential for the mis-operation of spillway gates both, because of inadequate gate operation procedures, and because there may be political pressure from downstream on operators to keep gates closed when dam safety requires that they be opened. This sometimes leads to gates being kept closed until opening becomes imperative. The gates may then have to be fully opened, releasing a larger flood than would have been the case if the gates had been partially opened at an earlier stage. The best guard against this is to follow J J Cassidy’s advice that ‘detailed rules for the operation of spillway gates should always be displayed in the operating area at the spillway’.
The WCD report refers to using flow forecasting to optimise reservoir operation. This has a place, particularly in large and well funded schemes, but caution is needed because such systems can be complicated and need to be fully tested and validated if they are to be a reliable working tool. We should beware of thinking that a system must be good just because it is complex. Indeed the very complexity may represent a hidden danger.
The position of gate operators is not an enviable one as people, with the benefit of hindsight, will always be able to argue that the gates could have been better operated for a particular flood. A priority for engineers involved with large gated spillways must be to ensure that the operators have absolutely clear and written instructions as to what they should do in every eventuality. Nothing less will do.