High discharges at Inniscarra as a result of successive rain events
Met Eireann records that Cork Airport measured 220% of the long-term average for February rainfall, the seventh wettest February on record. There were four rain events in succession from the 17th February up to the wettest day of the month on the 23rd, as shown in the figure below. The final two rain events resulted in the second and third largest inflows to Carrigadrohid reservoir since February 2021, each greater than 300 tonnes of water per second for a period of many hours.
While none of the rain events were named storms by Met Eireann, the cumulative effect of having one rain event after another resulted in high discharges at Inniscarra reservoir and significant flows in the River Lee downstream. The week demonstrates that while the ESB reservoirs provide some flood water storage, the reservoirs are small relative to the size of the Lee Catchment, and they cannot prevent flooding from occurring downstream. If there is enough rain for long enough, the reservoir levels rise, the storage is used up and downstream flooding will occur.
Figure 1: Rainfall across the period from 17th to 25th February
ESB Lee Stations
ESB Lee Stations watches the Met Eireann weather forecast and lowers the levels in both Carrigadrohid and Inniscarra reservoirs when significant rainfall is forecast. The green line in the chart below shows the level in Carrigadrohid reservoir at its minimum level at midday on 17th February in advance of the first band of rain.
Figure 2: Carrigadrohid reservoir level across the period from 17th to 25th February
After the rain, as the flow in the river decreased downstream, and in the knowledge that another band of rain was forecast, Lee Stations was able to increase the discharge out of Inniscarra reservoir to bring the level down in advance of the rain. However, the starting level was now higher than for the first band of rain. Similar steps were taken following the second, third and fourth bands of rain. On each occasion there was less storage capacity available in the reservoir for additional rain and larger discharges were needed to reduce the reservoir level. The red line shows the increasing discharges out of Inniscarra necessary to bring the reservoir levels down to create storage for the next rain event. Personnel from the Lee stations communicate with the relevant local authorities and people living downstream of the dam during these rain events.
Figure 3: Inniscarra reservoir discharge across the period from 17th to 25th February
Discharges from Inniscarra
The discharge from Inniscarra forms only part of the total flow in the River Lee as it enters Cork City. Thirty percent of the Lee Catchment is located downstream of Inniscarra dam. The uncontrolled flows from downstream tributaries, such as the Rivers Shournagh and Bride, make a very significant contribution to the risk of flooding in the city. Lee Stations considers the downstream tributaries when discharging from Inniscarra. As successive rain events fill the reservoirs, the storage capacity of the reservoirs reduces and with it the ability to wait for the downstream tributaries to abate before discharging from Inniscarra.
For this sequence of rain events, only on 25th February did the rainfall forecast show consistent settled conditions. If there had been another rain event forecast, discharges would have increased again and there could have been flooding downstream.
Lee reservoirs are small relative to the size of the Lee Catchment and whilst they do have the capacity to provide some flood alleviation, they cannot prevent flooding from occurring downstream if too much rain falls or if the rain falls for too long.
For more details on our hydrometric information, please go to this link on our website.