What will be the impact of the Flood Relief Scheme on properties outside the scheme area?
Protecting properties in Castlefinn will displace some flood water i.e. that which currently floods properties in Castlefinn. The Project Team have assessed the impact of this on properties across the Finn Valley and found the impact to be small. Flood extents across the Finn Valley will not change significantly although there may be an increase in flood depth in the design 1% Annual Exceedance Probability (or 1 in 100 year) event of 1mm to 10mm. For more frequent events, this impact will be lower, reducing to 0mm in the 50% Annual Exceedance Probability (or 1 in 2 year) event.
Why is the Scheme taking so long?
Even though the outline of a potential flood relief scheme for Castlefinn was identified in OPW’s Flood Risk Management Plan in 2018, the analysis underpinning the Plan was not of sufficient detail to allow the development and design of a Flood Relief Scheme (FRS) through the statutory planning process and to construction.
A project brief was developed in 2019/20 to meet the detailed needs of a FRS for Castlefinn. Specialist engineering and environmental consultants were appointed in July 2020. Since then, a range of engineering and environmental surveys have been completed to ensure the Scheme design has sufficient detail and is up to date in terms of conditions on the ground. Complex hydrological and hydraulic analysis reflecting best practice across Ireland and the UK was required to determine to a sufficient degree of accuracy the design parameters for the scheme. Detailed multi-year environmental studies are required to ensure a robust assessment of this scheme’s impact on the environment. A range of potential flood relief options were assessed and the preferred option for the Scheme has now been identified. The preferred scheme is a comprehensive solution to the relatively complex scenario whereby flood risk presents not only from the River Finn, but also from the streams that flow through the town towards it. The feasibility design in the Flood Risk Management Plan of 2018 did not consider the flood risk from these streams in detail. We have made sure that the preferred scheme design addresses this issue. It is also important that the scheme is designed to be adaptable to climate change. We have analysed the impact of climate change on flood risk in the area and included for this in scheme development. Analysing and developing this more robust scheme has taken longer than originally programmed.
Why is the scheme not proposing to increase the capacity of the River Finn?
A detailed assessment has determined that increasing the capacity of the River Finn cannot provide the full Standard of Protection to properties at risk in Castlefinn. That is to say, dredging and/or cleaning out the River Finn will not protect Castlefinn from the 100 year return period flood. Flood levels in the area are partially driven by flood waters from the Foyle coming back up along the River Finn. Increasing channel conveyance measures such as dredging have low effectiveness when this is the case.
Dredging the channel would have a very significant environmental impact on aquatic life and water quality on the River Finn.tt would have to be repeated periodically to maintain any benefit. Such a proposal is highly unlikely to receive consent and pursuing such a measure would be a lengthy process with no sustainable flood relief benefit at the end of it.
When can I expect to be protected?
The current project programme shows that Stage IV, construction will be completed in 2026. Elements of the scheme which could provide partial flood protection may be in place earlier in the construction period.
The project programme is subject to change outside the Project Team’s control and will be updated periodically. The project programme is available on the Project Website at:
https://countydonegalfrs.ie/castlefinnfrs/index.php/timeframe-2/
Will the Scheme alleviate all flooding?
The Scheme is being developed to alleviate all flooding from rivers and watercourses up to the 1% AEP (1 in 100 year) event for properties within the Scheme Area. This means that Castlefinn properties currently at risk will be protected from a flood that has a 1% chance of occurring in any given year.
The scheme is not being developed to alleviate flooding from other sources such as pluvial (directly from rainfall) or out of sewer flooding. However, the upgrade of key watercourse culverts within the scheme area will have the effect of improving some of the drainage systems and may reduce flooding caused by surface water.
Why am I not being protected by the scheme?
The scheme is being developed to protect properties at risk of river and watercourse flooding from the 1% Annual Exceedance Probability (AEP) event, or the equivalent of a 1 in the 100-year return period. There are a number of reasons why a property may not be included to be protected by the scheme including:
- the project has not determined the property to be at risk from flood events up to the 1% AEP event even though it is at risk in more extreme events;
- the property is farmland or has another use which is compatible with being located in the floodplain;
- the property is at risk from a different source of flooding, e.g. pluvial or direct rainfall runoff or flooding from the drainage system or;
- the property is outside the Scheme Area as defined in the Flood Risk Management Plan.
If you think your property should be protected based on the criteria above please contact the Project Team to discuss.
Will the scheme protect against climate change?
The analysis includes a full assessment of the level of present-day flood risk and flood risk under two future climate change scenarios (mid-range and high-end projections). There is significant uncertainty in all of the scenarios. It is proposed the FRS is designed for the present-day scenario with an extra allowance for uncertainty, referred to as ‘freeboard,’ which is typically provided as additional height on a wall or embankment. A climate change adaptation plan has been prepared. The plan will ensure that the performance of the FRS is monitored into the future and adapted if certain trigger points are reached. The trigger points represent the point in the future where there is no longer sufficient freeboard on the scheme. If this happens, the actions included in the plan will be taken to ensure the original Standard of Protection of the scheme is maintained as and when the effects of climate change start to occur . The FRS is not being designed for climate change levels from the outset as there is significant uncertainty in what these are and the impact of the scheme could be unnecessarily large e.g. overly high walls.
Where it does make sense to design the scheme for climate change now, e.g. installing a larger pipe from the outset to allow for potentially larger flows, the climate change adaptation plan will recommend this.
Will interim measures be delivered in the meantime?
No interim measures have been identified for Castlefinn which can be delivered ahead of the main construction contract.
Will the Scheme alleviate the problem of the foul sewer system overflowing, particularly at the pumping station at Grahamsland?
The scheme is not being developed to alleviate foul sewer issues in Castlefinn. Uisce Éireann is the agency responsible for waste water services including the foul sewers. The Project Team is liaising with Uisce Éireann as project stakeholders and have made them aware of this issue.