Located on the lower Williams River, Seaham Weir is a key component of the Grahamstown Dam scheme which provides 40% of the drinking water used by Hunter Water customers.
Seaham Weir provides a barrier between fresh water flowing down the Williams River and the salt water estuary, thereby enabling Hunter Water to harvest fresh water and pump it into Grahamstown Dam.
What does this project involve?
Seaham Weir consists of the rock weir, a concrete spillway, flow control gates and a fishway (fish ladder).
Built in 1967, Seaham Weir is now in need of redesign and modification to meet environmental outcomes which were not considered when the original weir was designed. The design for the new structure is currently being developed and the project is scheduled for completion in 2020.
The project will involve construction of four three-metre overshot gates on the south-east bank of the river, adjacent to a new ‘vertical slot’ fishway.
How does the weir currently function?
Seaham Weir is managed in accordance with strict operating rules that specify target upstream water levels depending upon river flow rates.
Water flows past the weir in three ways:
- Water flows over the weir during flooding and high river flows (uncontrolled flow).
- Water is released under two large flow control gates when they are opened.
- Water continually moves through the fishway.
The flow control gates are operated to control the weir pool level and prevent salt water intrusion. The gates are opened during flood events, to allow flushing and to release environmental flows. The gates are closed irrespective of flows when the tide level exceeds the upstream water levels.
Existing flow control ability
The existing gates are an older-style ‘undershot’ design. They are only able to provide coarse flow control because they must be either fully opened or fully closed, and are not suitable for partial opening to provide a lower flow rate. When the gates are open water is released at a rate of approximately 5,000 ML/d.
When there is a requirement for a low flow release, the gates need to be fully opened for a short period of time, the result being the release of a high-flow ‘pulse’. For example, a requirement to release 20ML/d is achieved by opening the gates for one hour every 10th day.
Existing fish passage
The existing fishway is an older-style ‘orifice’ design. Compared to newer designs, the existing fishway is not effective. Fishway designs have improved greatly since 1967 when the existing fishway was installed.
To enable fish to travel upstream it is necessary to have a well-designed fishway, and also to provide freshwater releases, known as ‘attraction flows’, through flow control gates located next to the fishway. The existing gates at Seaham Weir, because they are only able to provide large volume releases and ‘pulses’, are not able to provide continual low-volume ‘attraction flows’.
Fish travelling downstream need to cross the flow control gates, however the existing gates do not facilitate downstream fish travel as they are closed most of the time.
Why is this project needed?
The Lower Hunter Water Plan (LHWP) was developed in 2014 with consultation between Hunter Water, community members and other stakeholders. The LHWP sets out measures to ensure there is enough water to supply the needs of the Hunter Region while also recognising the needs of the environment.
To meet the requirements of the LHWP, modifications to Seaham Weir need to be made to:
- Improve flow control; especially to improve releases of water in low flow conditions, and to provide flows more similar to a river’s natural flow.
- Improve fish passage both upstream via the fishway and provide attraction flows, and downstream using the flow control gates.
Weir pool depth
An objective of the project is to maintain the weir pool level within the current range. This will minimise changes to the river’s banks and ecosystems and impacts to properties upstream of the weir. Improved fish passage A modern style vertical slot fishway, together with ‘attraction flows’, will greatly improve fish passage upstream across the weir over a much wider range of river flows. The new flow control gates, as well as providing improved flow control, will also enable fish to travel downstream through the gates.
Improved flow control
Four new three-metre wide overshot gates will be constructed. One or more of these gates will be lowered to release the required amount of water, greatly improving flow control.
These gates will also manage the weir pool level and assist fish passage downstream. The gates will be closed, irrespective of river flows, when the tide level exceeds the upstream water level to prevent salt water flowing upstream into the weir pool.
Construction of the new flow control gates will enable Seaham Weir to meet its new environmental requirements by achieving the following:
- Release a 500ML ‘fresh’ environmental flow approximately once per year
- Provide ‘translucent releases’* (based on river flow measured from Glen Martin) according to the following rules:
- Release 30% when no water restrictions apply.
- Release 20% when moderate restrictions (Level 1 and 2 as defined in the LHWP 2014) apply.
- Release 10% when severe restrictions (Level 3 and 4 as defined in the LHWP 2014) apply.
- Provide a 20ML/day transparent release** (relative to river flow at Glen Martin), while maintaining weir pool levels within current range.
* A translucent release is a continuous release that is a specified proportion of inflow.
** A transparent release is a continuous release equal to inflow up to a specified maximum.