The Flood Operations Manual contains procedures to be used for releasing water from the gated dams during flood events, or to achieve a temporary or reduced full supply level of water. Any releases made outside of these reasons are considered operational (for example, releases through a cone valve to assist with downstream flows for the environment).
The objectives of the Flood Operations Manual are to protect the structural integrity of the dam during flood events, mitigate downstream flooding, reduce environmental impacts, minimising disruption to transport and maintain a full drinking water supply at the end of the event.
The procedures are designed to minimise the risk to human life and safety by prioritising the structural safety of the dam. An appropriate balance is needed to manage the different objectives, and care must be taken not to hold back too much water and risk the structural integrity of the dam.
Every flood event is different, and the variation in initial dam level, catchment conditions (wet or dry), rainfall (depth, intensity and location) and inflows from upstream tributaries will change for each wet weather event. The gated dams can mitigate or reduce flood flows passing through the dam by temporarily storing a portion of the flood inflows and by restricting outflow. This can reduce and delay the contribution of floodwaters generated upstream of the dam to flooding along the river downstream of the dam.
The Flood Operations Manual is based on current (or observed) rainfall (on ground) and guides the Duty Senior Flood Operations Engineer in making decisions on when to, and how large, flood releases should be made from the dams. A flood release plan is formed to guide future operations based on the varying factors observed during a flood and constantly reviewed throughout a flood event as further rainfall occurs and conditions change.
Seqwater empty the dam of floodwater after the flood has peaked downstream. In emptying the flood storage compartment the flood manual requires Seqwater undertakes these releases at a rate that does not increase the downstream flows and produce greater flooding. The flood storage compartment needs to be emptied as soon as possible after a flood event has peaked, so the floodwater compartment remains available to manage inflows if another flood occurs shortly after the first event.
The Flood Operations Manual is a legislated requirement and approved by the relevant Minister in accordance with the provisions of the Water Supply (Safety and Reliability) Act 2008 (Qld). It has been produced from a range of supporting studies and incorporates the lessons learned from previous flood events.
