In June 2022, the Department for Infrastructure (DfI) in Northern Ireland commissioned Amey to review feasibility options for a flood alleviation scheme in Drumahoe and take this through to design and construction.
This County Londonderry village has been subject to repeated flooding from the Burnagibbagh River. For this reason, the river was identified as a potential flood risk in in the DfI’s North Western River Basin Flood Risk Management Plan 2015 – 2021.
Flood events particularly impact the village’s Ivy Mead development. Flood waters threatened properties there four times in a five-year period between 2008 and 2012 - with flood calls being made in 2008, 2009, 2011 and 2012 – and a significant flood event occurred in 2017.
This 2017 event was caused by prolonged rainfall and associated high flows in water courses around the Burnagibbagh leading to fluvial (river) flooding. These flows – estimated at Q330, or a once in 330 years event – were above the current standard level of protection. This is set at Q100 + 20% Climate Change, a Q100+CC is a 1% AEP (Annual Exceedance Probability) event which means there is a 1% chance of the flood occurring every year for 100 years. An additional 20% is then added to the flows to account for Climate Change. Significant Pluvial flooding (surface water) contributed to the duration of the flooding which lasted approximately 12-15 hours. As the Fluvial flooding had already caused high river levels/out of bank flooding in the area, the surface water drains and sewers were unable to outfall and instead flowed over the land causing significant damage to local infrastructure.
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Amey’s proposals included constructing eight new defences along the Burnagibbagh River and replacing the Church Brae culvert, as outlined in Figure 1 below.
A subsequent flood event in in 2022 precipitated further investigation and new proposals to protect people and property in Drumahoe.
The challenge
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Working with specialist engineers JBA consulting, Amey reviewed existing modelling and carried out an economic appraisal of the costs and benefits associated with the proposed defences. A ground investigation (GI) showed that the proposed Type 2 and Type 4 flood defence walls were not the best solution because they would necessitate significant costly seepage control. An example of Type 2 and 4 flood defence walls are shown in Figure 2 below.
The Project Engineer and Geotechnical team modelled types of defences that would be most effective and identified the depth of seepage control required. As a result, four proposed defences were updated to resilient and cost-effective kingpost designs and one to a sheet pile design. This has a low footprint but the capacity to hold back large volumes of water. Three proposed defences remained as Type 2 or 4 floodwalls.
July 2022 Flooding
A further flood event in Drumahoe on 23 – 24 July 2022 caused flooding in seven separate locations. Immediately after this – on 25 and 29 July – Amey visited the area to fully understand the nature and impact of the flooding. This involved observing the aftermath and talking to residents and businesses to understand their experiences and hear first-hand accounts.
In August 2022, Amey Consulting were commissioned by the Department for Infrastructure (DfI) Rivers to investigate the source of this flood event. We established that the flooding was predominately caused by blocked grilles upstream of the Burnagibbagh River, also a key contributing factor in the 2017 flood event. This had not been picked up by earlier feasibility studies and had not been included in the review of the modelling undertaken in June 2021.
Responding to these findings, DfI tasked Amey with revising the modelling to include these blockage scenarios and to find a solution that would include upstream tributaries. Flooding occurred in the below locations, shown in Figure 3.
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Our approach
An optioneering report for the Glendermott Stream upstream tributary identified five possible options.
- Option 1 – Slope Stabilisation and Grille Replacements (036-018 & 036-015)
- Option 2 – Culverting the Glendermott Stream
- Option 3 – Culverting from Belt Road through Glenmore Park
- Option 4 – Culverting down Belt Road discharging to Trench Drain
- Option 5 – Culverting to Church Brae via Belt Road
Options 3-5 were not presented to the client as they all were significantly more expensive culvert runs, they were located on main roads so would have high impact on local services, they would not have picked up other services entering the Glendermott Stream so the risk of blockage would not be fully mitigated against, and finally in some cases the fall would be a challenge leading to deep excavations during construction and the potential use of pumps post construction, adding maintenance and operational costs.
Amey put forward Option1 and 2 as preferred solutions to DfI as they were the most economically viable and focused in on the cause of the flooding.
Option 1 - Slope Stabilisation and Grille Replacements (036-018 & 036-015)
This involves stabilising the banks along the Glendermott Stream and upgrading the two inlet grilles (036-018 and 036-015) to CIRIA C786 guidance, the current industry standard. The new grilles will be designed to allow for 67% blockage, in combination with the bank stabilisation this will reduce the risk of full blockage due to the land slippage that occurred in 2017 and 2022 (resulting in near 100% blockage at the grilles). The location of Grille 036-015 is outlined in Figure 5 below.
Option 2 – Culverting the Glendermott Stream
This proposes removing open watercourse by culverting approximately 300m of the stream. This runs from the upstream outlet (036-014) to the inlet (036-018). Culverting and removing these grilles reduces the risk of blockage, allowing the river to flow freely without obstruction. In addition, this solution involves the upgrading of inlet grille 036-015, outlined in Figure 5.
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Benefits
Amey’s upstream options 1 and 2 will give greater protection to homes, businesses, and infrastructure in Drumahoe and are economically robust, they both deliver a viable BCR. However, Amey have chosen Option 2 as their preferred option.
This scheme is currently awaiting instruction to progress to Detailed Design.