Author: Thomas Cooney, SPTO Mapping and Modelling Unit, Rivers Agency During 2013 the Department of Agriculture and Rural Development (DARD) Rivers Agency constructed a £1.3 million flood alleviation scheme to protect the Co Tyrone village of Ballygawley. Rivers Agency is the statutory drainage and flood defence authority for Northern Ireland, responsible for reducing risk to life and damage to property from flooding from rivers and the sea. Ballygawley is located in south Co Tyrone at the intersection of the A5 Omagh – Monaghan Rd and the A4 Dungannon – Enniskillen Rd and has a population of approximately 600 people. The Ballygawley Water flows through Ballygawley and adjacent to St Ciaran's school, which is situated about 800m southwest of the village. The Ballygawley Water is a tributary of the Blackwater River, which flows into Lough Neagh, and drains a catchment of 55km2. Flows are recorded along the watercourse at Tullybyran Guaging Station just downstream of Ballygawley with records dating back to 1980. These were used to aid the hydrological estimation process and resulted in an estimated Q100 flow of 63m3/s. The Ballygawley Water has a very steep profile and coupled with the local geology the watercourse tends to rise very quickly. This flashy flood response has led to numerous flood events over the years with widespread damage caused during particularly severe floods in October, 1987, August, 2008, and October, 2011. In the aftermath of the October 2011 event, which resulted in properties including the local school being flooded twice in a week, a commitment was given to proceed with a new flood alleviation scheme.

Scheme design

[caption id="attachment_25995" align="alignright" width="300"]aaflood-2 Ballygawley Q100 flooded area[/caption] A HEC-RAS hydraulic model was constructed to represent the river channel, this was then used to estimate flood levels for the one in 100-year design event and calculate damages for the economic appraisal. The hydraulic model was also utilised to assess potential flood alleviation options which included upstream storage, raising embankments, construction of floodwalls and upgrading bridges and culverts. The scheme consisted of three distinct sites and following an option appraisal process construction of flood walls and embankments using various materials and methods was identified as the preferred solution. Following construction of the scheme the cost benefit ratio was recalculated based on actual out-turn figures and was found to be in excess of 10:1. [caption id="attachment_25997" align="alignright" width="300"]aaflood3 Site 1 • Remove flood embankment and palisade fence • Construct 700m of sheet piled and reinforced concrete floodwall • Construct 60m of flood embankment • Fishery enhancement measures • Install 600m of back drainage[/caption] [caption id="attachment_25998" align="alignright" width="300"]aaflood4 Site 2 • Construct 160m of reinforced concrete floodwall • 150m culvert upgrade 450mm to 600mm diameter • Installation of headwall and Tideflex valve • 115m of back drainage • Reinstatement of garden areas[/caption] [caption id="attachment_25999" align="alignright" width="300"]aaflood5 Site 3 • Construct 240m of kingpost concrete floodwall • Construct 50m of landscaped flood embankment • Install 250m of back drainage[/caption] The majority of the scheme consisted of a sheet piled and concrete floodwall with the piles which extended to 9m in places providing a cut-off to prevent seepage under the defence. Ground conditions at Site 3 necessitated an innovative design approach and a king post wall solution was selected. At all locations freeboard was incorporated into the crest level of the flood defences to allow for climate change. An overview of the works completed at each site is provided below:

Construction

The principal contractor for the scheme BSG Civil Engineering, was appointed in December, 2012, with work commencing on site in March, 2013. Prior to this, during the summer of 2012, Rivers Agency in partnership with TransportNI replaced the Tullybyran Road bridge and Richmond Lane culvert with larger more hydraulically efficient structures. Ancillary works included the rationalisation of several drainage outlets and new outfall structures. These pre-works contributed to the improved flood protection in Ballygawley and the joined-up approach to delivering this element ensured an enhanced level of protection was provided to at-risk properties prior to completion of the main works. The main contract was formed under the NEC 3 Option C Target Cost contract, which contributed greatly to the project being completed ahead of schedule and under budget. The incentive of the contractor's share ensured delays were minimised and value engineering proposals were forthcoming. The most significant of these included a reduction in sheet pile sections following trial installation drives on-site which provided a substantial cost saving. Other notable savings were delivered through the use of pre-cast wall sections, rationalisation of the reinforcement design and wall layout to minimise wastage and the utilisation of a 100 pour concrete liner to prolong the life of the formwork and reduce downtime. Time savings were achieved through a combination of good planning and at times good fortune. Early engagement with utility providers and stakeholders averted issues which could have led to potential delays if not addressed at the outset. Careful programming was also an essential element particularly working close to St Ciaran's school where the works had to be completed during the summer holiday period which left little room for error. [caption id="attachment_26001" align="alignright" width="300"]aaflood6 Completed floodwall along St Ciaran's school[/caption] Flexible subcontractors and planning to provide a fall-back option with work ongoing on three sites simultaneously also contributed to the early completion. The warm and sunny weather conditions during the summer of 2013 also had a very positive effect and allowed work to progress without disruption. It was, of course, not all plain sailing and there were numerous challenges to overcome throughout the construction stage. Chief among these was working alongside a large school and a creche where health and safety was the main priority and more than 10,000 man hours were completed with no accidents. Minimising disruption was also a primary concern, particularly during sheet-piling operations and the use of Movax vibro piling equipment achieved the desired result with no complaints from residents or structural damage to nearby buildings. [caption id="attachment_26002" align="alignright" width="300"]aaflood7 Vibro sheet piling[/caption] At all times the contractor was required to maintain the existing level of flood protection to properties adjacent to the works, which at times proved difficult. Careful planning and placing a large degree of faith in weather forecasts was required prior to the removal of the existing flood embankments. On some occasions, the sheet piles were installed at a higher level to preserve the existing flood protection prior to being installed to design level for construction of the floodwall.

Environmental measures

The primary aim of the project was, of course, to provide the specified standard of flood protection to properties in Ballygawley; however, another major objective was to avail of the opportunity to enhance the riverine environment and have a positive lasting effect on the Ballygawley Water. Through close co-operation with the Department of Culture Arts and Lesuire (DCAL) Inland Fisheries personnel a series of fishery enhancement measures were designed and installed along the reach of the river adjacent to St Ciaran's school. This included the creation of spawning and nursery beds along with downstream pools and the installation of D-groynes to provide fast-flowing oxygenated water within the watercourse. Prior to these works taking place electro-fishing was carried out to establish a baseline, and initial results after the first year are extremely promising with increases in both salmon and trout numbers. [caption id="attachment_26004" align="alignright" width="300"]aaflood8 Fishery enhancement measures[/caption] [caption id="attachment_26005" align="alignright" width="300"]aaflood9 Otter holt construction[/caption] During the electro-fishing process the fisheries officers identified the presence of an otter along the river bank. In consultation with Rivers Agency Environment Section an otter holt was constructed using surplus materials including reinforcement mesh and drainage pipe offcuts. Throughout the project every effort was made to protect the environment and minimise any adverse effects. All works were completed from the river bank with a 'green barrier' maintained to safeguard water quality in the river. The alignment of the floodwall was amended on-site to retain mature trees and limit damage to root systems. [caption id="attachment_26006" align="alignright" width="300"]aaflood10 Footpath along the crest of the flood embankment[/caption] Social clauses were included within the contract and the contractor was required to provide opportunities for the unemployed and following an initial 13-week placement a local engineer was retained on a permanent basis by BSG Civil Engineering. Improvements to community facilities were also incorporated where possible and a footpath was installed on the crest of the flood embankment in a local park which has become a popular local amenity. While working alongside St Ciaran's school presented many challenges but it also afforded the opportunity to engage with the pupils and staff. Working closely with careers and geography teachers, presentations were devised to both promote a career in civil engineering and complement the students’ syllabus. [caption id="attachment_26007" align="alignright" width="300"]aaflood11 Presentation to pupils in St Ciaran's school[/caption] These were very well received and proved extremely successful, with students engaging in and asking questions about the project on an ongoing basis. However, they weren't so enthusiastic when they realised there would be no more days off when the school flooded. The project was registered for the Whole Team Award under the Civil Engineering Environmental Quality (CEEQUAL) Award Scheme Version 5. CEEQUAL is a sustainability and awards scheme for civil engineering projects which celebrates the achievement of high environmental and social performance. It also aims to deliver improved project specification, design and construction of civil engineering works while encouraging performance above legal, environmental and social minimal standards. [caption id="attachment_26008" align="alignright" width="300"]aaflood12 Project team at CEEQUAL award presentation[/caption] The aforementioned environmental measures and interactions with local residents and stakeholders resulted in the achievement of a CEEQUAL Excellent Award for the Ballygawley Flood Alleviation Scheme with an overall score of 80.4 per cent. The project team identified a number of benefits from using CEEQUAL including delivery of a more sustainable project, improved public relations, enhanced team spirit and increased interest within the construction workforce about environmental issues.

Conclusion

The Ballygawley Flood Alleviation Scheme achieved its primary aim of providing one in 100-year flood protection to properties in the village ahead of schedule and under budget. It did, however, also achieve much more than this through environmental improvements, engagement with the local community and has, ultimately, had a positive lasting impact on Ballygawley. The sense of satisfaction on completion when the constant fear of flooding had been relieved for local residents made it hugely rewarding. The project was completed in October, 2013, and as of yet hasn't been fully tested but there's no doubt the day will come when it will be called into action and the residents of Ballygawley will appreciate the protection provided by their new flood defence if not the pupils of St Ciaran's.