Options Report

June 2024

The Midleton Flood Relief Scheme Options Report has been made publicly available and is available to view; 

Options Report- Section 1 to 5

Options Report- Section 6 to 14

Options Report- Appendix

The contents of the Options Report is summarised below;

The first stage of the project, the Development of a number of flood defence options and the identification of a preferred Scheme (Stage I) is now complete. This stage included hydrological analysis, hydraulic modelling and analysis, and preparation of flood risk mapping. The existing flood risk and flood mechanisms are described in detail in the accompanying Hydrology Report and Hydraulics Report .

Based on the hydraulic analysis carried out as part of this study, the areas listed in the following table were identified as being the critical locations at risk.

Significant consultation with all interested parties including the public was undertaken throughout Stage I in order to give interested parties an opportunity to communicate local knowledge, how they may be impacted by existing flood risk and also to obtain feedback on the proposals from all relevant affected stakeholders and landowners who might be impacted by the Scheme. Feedback throughout the project to date has been very carefully considered by the project team and, where appropriate, has influenced decisions on the final emerging preferred scheme. This is discussed in Section 2 and Section 13 of the Options Report.

The options development commenced with an initial screening assessment of flood risk management measures. Some measures were not deemed as viable and as a result were screened out. Other measures were considered as potentially viable and these were carried forward for further assessment. This is discussed in Section 3.

Following the initial screening of the potentially viable measures, seven measures were assessed in greater depth to ensure suitability within the scheme areas. The potentially viable measures considered are discussed in Section 4.

The potentially viable measures, which were progressed through the screening stages and met the project objectives, were then combined to develop reasonable alternatives/ flood relief options. The reasonable alternatives developed for each area are described in Section 5.

An environmental assessment of the reasonable alternatives was undertaken and considered the key environmental constraints that were identified for each area and the potential environmental effects of the proposed interventions. This is described in Section 6.

An economic assessment for the reasonable alternatives was undertaken and is outlined in Section 7. This included the estimation of the project costs of the flood relief options/reasonable alternatives. An economic damages assessment was also completed to estimate the reduction in risk of flooding to property derived from the flood protection works. This risk is quantified as the expected damage to property that would occur over the lifetime of the scheme which has been assumed as 50 years. A cost benefit ratio for each flood relief option combination was then prepared.

Section 8 describes the detailed multi-criteria analysis (MCA) of the reasonable alternatives which was carried out to evaluate the performance of each option in terms of predefined technical, environmental, social and economic objectives. Multi-criteria analysis is a decision-making support tool, developed for complex multi-criteria problems that include quantitative and/or qualitative aspects of the problem in the decision-making process.

Following the multi-criteria assessment (MCA) and the cost-benefit assessment (CBA), the emerging preferred option was selected. The merits of the alternative options are summarised in Section 9 on the basis of cost, MCA score, environmental and ecological impact, climate change adaptability and consideration of the feedback which arose during the public and stakeholder consultation process.

It was concluded during the selection process for the emerging preferred option that flood relief measures upstream of the Cave system in Water Rock are not viable. This is due to the high costs/low benefit associated with the options as well the technical complexity of delivering any of these options. It is not therefore proposed to progress with the flood relief culvert/channel options to mitigate flood risk in the area upstream of the cave system. It is instead proposed to carry out further investigations into potential flood risk management measures that may provide flood mitigation, albeit not to the target standard of protection of the scheme.

The emerging preferred option in each area is as per table below.

Following feedback received at and post Public Participation Day 3 and other assessments, including tidal water level updates (Section 10) detailed freeboard analysis (Section 11) and inclusion of Climate change assumptive allowance (Section 12), Section 13 outlines the proposed refinements to the emerging preferred scheme.

On the 18^th October 2023, Midleton was severely flooded during Storm Babet. A detailed hydrology assessment was carried out after the event and this assessment concluded that the design flows for the scheme are increased by 5.2% when compared with the 2018 Hydrology report. The uplifted flows require some new defences and changes to a number of defences in the fluvially dominated area of the scheme.

While the changes required to the scheme vary between the individual defences, on average the increase in defence height is circa 100mm and the average increase in length of the defences is circa 10m. These changes to the defences are deemed to be modest and can be accommodated as part of the current scheme without any significant impact on the environmental and/or landscape heritage constraints.  

Section 14 outlines the refinements required to the proposed scheme.