Management of Water Levels on the River Shannon

Water levels are managed on the River Shannon through the operation of gates and sluices at the outlets of the three major lakes and the operation of sluices and weirs at Jamestown, Roosky, Tarmonbarry, Meelick and the New Cut. These operations influence the levels on the Shannon but due to the shallow gradient of the river and the many natural restrictions along its course, the levels cannot be fully controlled especially when the Shannon is in flood. The river level is primarily the result of the amount of rain that falls on the river catchment area.

The water level on Lough Allen is managed by the ESB through the operation of the sluices at the outlet of the lake at Bellantra.

The water level on Lough Ree is managed by the ESB through the operation of sluices at Athlone, although in times of flood, most water flows uncontrolled over the 170m weir alongside the Athlone sluices. Waterways Ireland provides manual assistance at Athlone, working to instruction from the ESB.

The level of Lough Derg is managed by the ESB for the purpose of electricity generation, through operation at Ardnacrusha and gates at Parteen Weir. The operation of Ardnacrusha and the gates at Parteen Weir does not affect the levels on the River Shannon above Meelick Weir.

Levels in Lough Allen and Lough Ree are managed to assist with navigation, to ensure minimum navigation levels in the river during dry periods and to ensure that floods are passed safely. ESB always operates to its Regulations and Guidelines for the control of the River Shannon. The levels in between the lakes are managed for navigation purposes.

Waterways Ireland manages the sluices and weirs at Jamestown, Roosky, Tarmonbarry and Meelick and the New Cut. The function of these weirs is to maintain water levels in the summer periods to allow boats to use the waterways. When all gates at the weirs are open, there is no further action to prevent further rising water level. At this stage the water is running through all open gates and over adjacent weir.

The ESB has a statutory role to generate electricity at Ardnacrusha. The relevant legislation is the Shannon Electricity Act 1925, Electricity Supply Act 1927 and Electricity Supply (Amendment) Act 1934. In broad terms, this legislation permits the ESB to generate electricity at Ardnacrusha and to manage the weirs, sluices and other works that are part of the Shannon Scheme.

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Waterways Ireland is a navigation authority established under statute. It is responsible for the management, maintenance and development of those waterways principally for recreation purposes.

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There is daily communication and a co-ordinated approach to management of the water levels between Waterways Ireland and the ESB. ESB manages the levels for electricity production, ensuring the safety of the embankments and dams. Waterways Ireland manage the levels for navigation.

ESB generates electricity at Ardnacrusha by diverting water from the River Shannon downstream of Lough Derg. As such, the operation of Ardnacrusha generating station alleviates flooding in the lower Shannon by diverting flood waters away for the “old” river. If there was no generating station at Ardnacrusha the full flood would be flowing down the “old” Shannon river and downstream flooding in these areas would be more severe.

Ardnacrusha generating station uses up to a maximum of 400 cubic metres per second (tonnes per second) of water, and so up to this amount of water is diverted away from the “old” river. This water therefore bypasses areas that historically have been at greater risk of flooding. In a flood with 800 cubic metres per second (such as happened in December 2015) ESB can divert nearly half of the water through Ardnacrusha and away from the “old” river.

ESB does not restrict the flow of water though the river, but rather once it flows through Lough Derg, ESB diverts the water to Ardnacrusha to generate electricity. During large floods, excess water that exceeds the capacity of Ardnacrusha will flow down the “old” course of the Shannon, controlled by the operation of the gates at Parteen Weir. These gates have a very large capacity. As such, water does not “back up” the river as a result of the operation of Ardnacrusha.

While ESB has an influence on water levels on some parts of the river, the ESB does not control the water level overall on the River Shannon, nor was the Shannon Scheme designed for that purpose. The river level is primarily the result of the amount of rain that falls on the river catchment area. While the ESB uses the water flowing in the river to generate electricity when it reaches Ardnacrusha, the rate at which the water travels to Ardnacrusha is primarily a result of the natural topography of the river and the natural restrictions in the river. It takes many days for water to travel down the Shannon. There have been times when there was flooding in the Callows area south of Athlone yet only enough water in Ardnacrusha to operate the generating station at less than half maximum output.

ESB does not store water on Lough Allen or Lough Ree for the purpose of electricity generation. Following the construction of the Shannon Scheme ESB was interested in lake levels for electricity generation because Ardnacrusha was the main source of electricity in Ireland in its early days. However, as newer generating stations were built the significance of the lake levels lessened, so that for the last number of decades ESB has not stored water on Lough Allen or Lough Ree for the purpose of electricity generation.

In the case of Lough Derg, ESB diverts most of the water flowing out of Lough Derg down the manmade headrace canal to Ardnacrusha generating station, so this water flow determines that amount of electricity generated. The amount of water that flows is determined by the water level in the lake.

While ESB uses the water flowing in the river to generate electricity when it reaches Ardnacrusha, the rate at which the water travels to Ardnacrusha is primarily a result of the natural topography of the river and the natural restrictions in the river. It takes many days for water to travel down the Shannon.

The primary influence on lake levels is the rainfall on the river catchment. The operations of the gates at Lough Allen and Lough Ree have little or no effect when the Shannon is in heavy flood.

The primary influence on the water level in Lough Ree is rainfall on the catchment upstream of Athlone. ESB exercises some influence over the water level in Lough Ree when the river is not in flood with the operation of the sluice gates at Athlone weir, just south of the town. These sluice gates are operated to attempt to reduce the level in Lough Ree towards minimum safe navigation level in the lake in summer; however, this operation must also take account of the water levels in the Callows area downstream of Athlone weir, which can be subject to waterlogging in wet weather.

As the water level in the Callows downstream of Athlone rises, the sluice gates are closed and no further control is exercised on the water levels in Lough Ree. The vast majority of the water flowing from Lough Ree flows uncontrolled over the 170m weir alongside the sluice gates.

In drought periods when there is very low flow in the river, the gates are operated to augment river flow downstream of the Athlone weir.

In summary, ESB can influence the level of Lough Ree during non-flood periods but ESB’s operations have no impact on the Athlone area during times of flooding. ESB co-operates closely with Waterways Ireland in the operation of these sluice gates.

At the onset of a flood, water flows into the lakes at an increasing rate, but with narrow river channels particularly at the outlet of Lough Ree and Lough Derg the flow of water out of these lakes only increases as the lake levels rise. As more water flows into the lakes, the levels continue to rise until a “balance” between inflow and outflow is achieved. For a large flood, the lake levels will be quite high by the time this balance is achieved, regardless of the lake level at the start of the flood.

Studies have shown that lake levels in Lough Derg and Lough Ree at the start of a flood have negligible effect on subsequent peak flows and peak water levels in the lakes for large flood events.

In summer, the Athlone sluice gates are operated to attempt to reduce the level in Lough Ree towards minimum safe navigation level in the lake in summer. This operation must also take account of the water levels in the Callows area downstream of Athlone weir, which can be subject to waterlogging in wet weather and if the flow from Lough Ree is too high. The water level in the Callows is very dependent on rainfall. Also, the Rivers Suck and Brosna flow into the Shannon in the Callows and add significantly to the Shannon flow at these points. These tributaries have a significant impact on water levels in the Callows.

As the water level in the Callows rises, the sluice gates at Athlone are closed. In flood conditions, the sluice gates will always be in the closed position and the vast majority of the water flowing from Lough Ree flows uncontrolled over the 170m weir alongside the sluice gates.

The water level in Lough Derg does not influence water levels in the Callows above Meelick Weir which is near Banagher in Co. Offaly. ESB Ardnacrusha and Gate Movements at Parteen Weir have no effect above Meelick Weir. Increasing the discharge at Parteen Weir will have no effect on water levels in Athlone.

Parteen Weir, which is downstream of Lough Derg, allows the ESB to divert most of the water normally flowing in the Shannon via a manmade canal to Ardnacrusha for the production of electricity.

ESB diverts as much water as possible through Ardnacrusha generating station for the production of electricity. During large floods, however, the amount of water flowing in the river is greater than the capacity of Ardnacrusha, and in these circumstances the excess water will continue to flow down the “old” course of the Shannon. This is achieved by operating the gates at Parteen Weir. ESB Ardnacrusha and Gate Movements at Parteen Weir have no effect above Meelick Weir.

Ardnacrusha generating station was designed for a water flow of up to 400 cubic metres per second (or tonnes per second). This maximum flow of water that can be put through Ardnacrusha is mainly related to the difference between the water level at the upstream end of the manmade canal and the water level at the generating station itself. To allow more water flow through Ardnacrusha would require a larger water level difference between these points and therefore a higher water level in Lough Derg. Such a higher level in Lough Derg would be unsafe because of the risk of flooding at the lake and also the risk that the water level at various embankments along the river near Parteen Weir would be too high.

Parteen Weir is a weir with a number of gates built across the River Shannon downstream of Lough Derg. The purpose of Parteen Weir is to enable ESB to divert most of the water flowing in the Shannon via a manmade canal to Ardnacrusha for the production of electricity.

Under normal river flow conditions, most of the water flowing out of Lough Derg is diverted through the manmade headrace canal to the generating station at Ardnacrusha.

When the River Shannon is in flood, Ardnacrusha station is put on maximum output to take as much diverted water as possible. However, when the flow of water exceeds the maximum that can be diverted to Ardnacrusha the excess water will flow down the “old” course of lower Shannon, controlled by the gates at Parteen Weir.

The operation of Ardnacrusha provides a flood alleviation benefit to the lower Shannon by allowing a large percentage of the water to be diverted away from vulnerable areas downstream of Parteen Weir.

ESB Ardnacrusha and Gate Movements at Parteen Weir have no effect above Meelick Weir.

It is not possible to store water at Parteen Weir in a flood situation as there is no capacity there to hold any significant amount of water. If water is not allowed to pass through the weir, then water levels will rise quickly upstream and there is a risk to dam and embankment safety and an even greater risk of flooding.

At the onset of a flood, water flows into Lough Derg at an increasing rate, but with a narrow river channel at the outlet of Lough Derg the flow of water out of the lake only increases as the lake level rises. As more water flows into the lake, the level continues to rise until a “balance” between inflow and outflow is achieved. For a large flood, the lake level will be quite high by the time this balance is achieved, regardless of the lake level at the start of the flood.

If the ESB tries to extract more water at Parteen Weir or Ardnacrusha than the channel allows, the level at Parteen Basin will fall quickly below the minimum stability level of the earthen embankments and this creates a risk of dam failure.

As the water comes from Lough Derg into Parteen Basin, ESB diverts as much water as possible through Ardnacrusha generating station for the production of electricity. During large floods, however, the amount of water flowing into Parteen Basin is greater than the capacity of Ardnacrusha, and in these circumstances the excess water will be diverted down the “old” course of the Shannon.

Studies have shown that lake levels in Lough Derg at the start of a flood have negligible effect on subsequent peak flows and peak water levels in the lake for large flood events.