Grahamstown Dam

Overview

Grahamstown Dam

Catchment area

115km2

Dam volume

182,305ML (million litres)

Average depth

9 metres

Surface area

2800 hectares

Annual rainfall

1125mm (Williamtown)

Areas supplied

Lower Hunter, approximately 40% of volume

Land use breakdown of catchment

39% Hunter Water freehold, 39% rural, rural residential or urban, 22% State Forest.

Grahamstown Dam was constructed between 1955 and 1965 as the Hunter’s third water supply source and is the Hunter’s largest drinking water supply dam.

The primary function of the dam is to provide a sufficient volume buffer to maintain water supply in a severe drought. The dam provides about 40 per cent of the drinking water requirements on an ongoing basis, but would supply a much higher percentage in a drought. The dam and its delivery and treatment systems are also critical components for meeting peak day demands, and can meet up to 75 per cent of supply requirements on a peak day.

Grahamstown Dam is an off-river storage that is primarily used to store water extracted from the Williams River. As well as rain, the dam also receives inflows from its own catchment. The other key components of the Grahamstown Dam supply scheme are:

  • Seaham Weir, which limits the upstream movement of tidal saltwater
  • Balickera Canal and pumping station, which are used to transfer water from the Williams River to Grahamstown Dam.
  • Campvale Pumping Station
  • George Schroder Pumping Station and delivery mains, which are used to deliver water from the dam to the water treatment plant.
  • Grahamstown Water Treatment Plant.

How It Works

Seaham Weir, which is located on the lower Williams River near Seaham, is used to separate the downstream tidal estuarine salt water from the upstream fresh water. The weir structure includes a fishway for fish migration, and gates that are used to control the upstream water level. The canal intake is located immediately upstream of Seaham Weir at Boag’s Hill and includes a radial gate, trash rack and a water quality monitoring station.

Balickera pump station and canal are used to transfer water from the Williams River to Grahamstown Dam. This conveyance system includes around 5 kilometres of canal and a pump station part way along that lifts the water around 15 metres. The decision of when to pump water is based on availability of water in the river, space in the dam, and the quality of the water in the river. Pumping is suspended if nutrient or algae cell levels at Boag’s Hill intake exceed pre-determined limits. While the average flow in the Williams River is around 1,000 megalitres per day, the distribution of this flow is highly variable, with flows being below 100 megalitres per day around 50 per cent of the time but also having capacity to be more than 100,000 megalitres per day. The operation of the pump station is therefore opportunistic by necessity, with pumping only possible following rainfall in the catchment when there is sufficient flow and appropriate quality in the Williams River.

Balickera pump station has six large pumps with a combined capacity of 1,350 megalitres per day - a small proportion of possible flows in the river, which can exceed 100,000 megalitres per day in a major flood. The Balickera pumping concept is therefore designed to transfer flows from the very high flow periods that would otherwise just make their way out to sea.

Hunter Water is currently investigating the opportunity to increase the capacity of Balickera pump station to make greater gains from these very high flows. Water stored in Grahamstown Dam is accessed for supply to customers at George Schroder pump station. This pump station delivers water through twin parallel mains to the Grahamstown water treatment plant at Tomago. All water from Grahamstown Dam is fully treated before distribution to customers.

History

The storage was formed by building an embankment across the outlet of a natural depression known as the Grahamstown Moors. Construction began in 1955, and although all the elements of the entire scheme were not completed until 1965. Water was first supplied in 1960 during the severe drought from 1960 to 1963.

Upgrades

Two major modifications have been made since the dam was completed:

  • 1973: a bentonite clay core 2,020 metres in length was installed in the central section of the Grahamstown Embankment to provide a watertight seal, the 1.5 metres thick core has an average depth of 21.3 metres
  • 1985: the FSL (Full Supply Level) of Grahamstown was reduced from 11.1 metres to 10.6 metres as a temporary measure to reduce the risk of damage to the main embankment from major flooding
  • 1994: further work was undertaken to raise the level of the clay core to road level over the full length of the embankment and rock armouring of the main embankment was also undertaken
  • 2005: Stage 2 Augmentation works. These works increased the Full Supply Level to 12.8 metres and provide 50 per cent more storage capacity. These works involve construction of a larger spillway at Irrawang and discharge channel under the Pacific Highway. Work was completed in December 2005.

The Catchment

Grahamstown Dam is the Lower Hunter's major water supply, typically providing 40% of our water. The proportion of water supplied is much higher in times of drought.

Transfers from the Williams River

Grahamstown Dam is classified as an off-river storage facility, storing raw water pumped from the Williams River at Seaham Weir through the Balickera Canal, raised approximately 15m at the Balickera Pumping Station and then flows through the Balickera Canal and Tunnel into the northern end of Grahamstown Dam. On average, 50% of inflow to Grahamstown Dam is pumped from the Williams.

Hunter Water monitors water quality in the Williams River for nutrients before transferring water to Grahamstown Dam. Like most Australian rivers, the Williams is highly influenced by climatic conditions and is consequently highly variable in flow and water quality. Flow and water quality are assessed against pumping rules to minimise the nutrient load transferred to the dam.

The Grahamstown catchment

Grahamstown Dam's small catchment contributes the remaining inflow. The catchment is primarily located on the northern and eastern shores of the dam.

To the north of the catchment Seven Mile Creek fills the dam with runoff from small farms and other minor developments. Approximately 75% of total catchment runoff comes from the northern part of the catchment.

Runoff from the east comes directly from the urban settlement of Medowie through the Campvale Swamps. Water is pumped into the dam via the Campvale Pump Station and finally spills at the Irrawang Spillway.

Hunter Water works closely with landowners and residents in the Grahamstown catchment area to improve the quality of water draining into Grahamstown Dam.