Flood response and management in the Ballymote catchment
The Ballymote River drains a 46 km² catchment. Since 2005, 9 km² of peat bog has been planted with conifers and 3.4 km² of floodplain has become a retail park and housing. A gauging station recorded a storm of 38 mm in six hours. In 2004 a similar storm produced a peak discharge of 14.2 m³/s and a lag time of 8.5 hours. In 2024 the same rainfall total produced a peak of 23.6 m³/s and a lag time of 3.9 hours. Downstream, 46 properties flooded to 0.6 m. The council can build 2.4 km of embankments for €11.8 million or restore 12 km² of bog with 40 leaky dams for €4.1 million. Residents say embankments will move the problem downstream. A decision is needed within nine months to qualify for EU funding.
1. Using the data, explain why the lag time and peak discharge changed between 2004 and 2024. Refer to at least three processes and show the size of the changes.
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First calculate the changes. Lag time fell from 8.5 to 3.9 hours: a reduction of 4.6 hours, or 54% (4.6 ÷ 8.5 × 100). Peak discharge rose from 14.2 to 23.6 m³/s: an increase of 9.4 m³/s, or 66% (9.4 ÷ 14.2 × 100). These figures point to faster delivery of rainfall to the channel and less storage in the catchment.
- Conifer planting on 9 km² of peat bog: conifers intercept some rain, but ploughing and drainage of peat reduce infiltration and the bog's natural water-holding capacity. More water therefore moves overland to the river quickly.
- Urbanisation of 3.4 km² of floodplain: roofs, roads and car parks are impermeable. Drains and sewers convey water rapidly to the river, shortening the lag time.
- Loss of floodplain storage: building on the floodplain removes the area where floodwater would spread out and be released slowly. The river's peak is therefore higher.
The rainfall total was the same in both events, so the comparison isolates land-use change rather than storm size. The shorter lag time reduces warning time, and the higher peak puts more properties at risk.
2. Evaluate the two flood-management options for Ballymote. Weigh cost, effectiveness, timescale and the residents' concern.
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Embankments (€11.8 million, 2.4 km) could protect the 46 properties to a design standard and can be built within the nine-month EU funding window. But they are expensive, need maintenance, and fail badly if overtopped. By confining the channel they can raise water levels and speed flow downstream, which supports the residents' concern. They may also encourage further building behind the wall, increasing future risk.
Bog restoration and leaky dams (€4.1 million, 12 km²) work with natural processes. They slow runoff, increase infiltration and storage, and reduce the peak. They are cheaper and bring biodiversity and water-quality benefits. But they take time to establish, may not protect homes in a large event, and need agreements with many landowners across 12 km². Their effect on the 2024 peak would need modelling.
A judgement: embankments alone are not sustainable, but restoration alone may not protect 46 homes quickly enough. A hybrid is best - embankments where properties are most exposed, plus upstream restoration to reduce the peak. Conditions should include no new floodplain development and a monitoring plan. The €4.1 million option is better value per euro and more resilient, but the nine-month deadline means the council cannot rely on it alone.
3. Calculate the percentage increase in peak discharge and the percentage decrease in lag time. Explain one limitation of using these two events to predict future flood risk.
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Peak increase = (23.6 − 14.2) ÷ 14.2 × 100 = 9.4 ÷ 14.2 × 100 = 66.2%. Lag-time decrease = (8.5 − 3.9) ÷ 8.5 × 100 = 4.6 ÷ 8.5 × 100 = 54.1%.
Limitation: only two events are compared. The 2024 storm may differ in intensity, duration or antecedent soil moisture even though the total was 38 mm. A single pair cannot separate land-use change from rainfall variability, and the gauging station record may have changed. Predicting future risk needs a longer record and hydrological modelling, not just two events.