Flooding induced by development (VIDEO)

The Cahercalla stream originates near the Cahercalla hospital and creates a small lake within the grounds. Historically, the stream flowed through the wooded area and reached the entrance of the Ennis Rugby Club before disappearing underground. In the past, the Willsgrove green area served as the stream’s floodplain, but when the Willsgrove estate was built in the late nineties, the floodplain was relocated due to concerns about exacerbating flooding problems in the nearby town. To manage the stream’s flow and reduce the risk of flooding, a 1200mm diameter pipe was installed along the boundary between the Willsgrove and Cahercalla drive estates. This has been effective so far, but if the pipe were to become blocked during heavy rainfall, the water would have nowhere to go.

The Cahercalla stream can carry large volumes of water; in 2009, it recorded a flow rate of 1 cubic meter per second after 40mm of rainfall in a single day. To anticipate a blocked pipe scenario, a predictive flood model was developed, showing estimates of flood water coverage after 1, 2, and 3 hours. The situation has become more complex with the proposed construction of garden walls to contain flood water in the Willsgrove and Cahercalla drive estates.

An existing cross-section of the pipe and structure at Willsgrove, in the wooded area, and at Pairc na Coille is as follows: water enters the pipe at the boundary of Willsgrove and travels 100 meters before discharging into a tank percolation structure in the wooded area. For years, excess water has percolated into the wooded area through cracks and holes in the tank structure during times of flood. The new development proposal would cover the tank structure and fill the surrounding floodplain area, effectively extending the length of the pipe through the tank structure and through a further pipe in Pairc na Coille, to a total distance of 430 meters. This would significantly slow down the flow rate of the stream, potentially reducing it by as much as 50% in times of flood.

With the proposed development, the tank structure will no longer distribute flood water into the wooded area. This change will have an impact on flood modeling. If a flood occurs with a flow rate of 1 cubic meter per second and no blockage, the tank structure will fill quickly, and the pipe in Pairc na Coille will overflow into the showgrounds. The speed of the water in the pipe will be slowed down substantially, potentially taking less than 50% of the water volume it previously did. Modeling shows the effects of this scenario after 2 hours, 4 hours, and 48 hours.

This situation is similar to several incidents in other locations, where undersized pipes have restricted flood water from reaching town centers, causing damage and disruption. These incidents often go unreported, but they occur frequently.



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Mary Coleman

Very worrying situation in Ennis. Flooding a real problem and the developers couldn’t care less. Action needed!!

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