Moneymore Flood Defence Scheme
Moneymore is a small village located on the outskirts of Magherafelt in County Londonderry, Northern Ireland. Within the village there are two housing developments which are bordered by the Coltrim and Ballymully Rivers. Through recent years the Ballymully River has over-topped its banks on several occasions during periods of prolonged and heavy rainfall. This combined with upstream run-off from Slieve Gallion, poses the potential risk of serious flooding to some 100 dwellings.
In February of 2008, DAWSON WAM Ltd were appointed as the main contractor by the Department of Agriculture and Rural Development Rivers Agency (DARDNI) to construct new flood defences and reduce the risk of flooding to a 1 in 100 year event. The works were to include:
- The supply and installation of 510 lin/m of steel-sheet piled floodwall
- 230 lin/m of contractor-designed flood barrier
- Installation of back drainage
- Supply and placement of 1500 tonnes of rock armour for toe protection
- Installation of various fishery measures, including areas of spawning gravel, 'D'-shaped groynes, plunge pools, etc
- Reinstatement of 40 riparian gardens
There were several environmental and construction-related challenges to overcome that required strategic design, programming and implementation:
- All tree felling and riverbank clearance had to be carried out between the 1st September and the 29th February to avoid the bird nesting season
- No work was permitted within the river corridor between 1st November and 14th April to avoid the spawning season
- Routine water course sampling had to be conducted throughout the course of construction to monitor silt/sediment
- Piled flood defences had to be installed as close as 300mm to existing dwellings
- Piles had to be toed into sandstone to depths of up to 5m
- Vibration & noise monitoring had to be carried out during all piling operations
- All work had to be carried out within a 3-5m wide wayleave along the top of each riverbank
Innovative alternative design proposal
Due to the construction restrictions on this project, DAWSON-WAM Ltd submitted an alternative design proposal to construct a secant piled floodwall with a cast-in-situ concrete top wall in lieu of steel sheet piles. Through an integrated team approach and early contractor involvement the alternative design proposal was accepted by the client as it was deemed to have the following advantages:
- Using a pile press/crush system it was doubtful that the mandatory pile depths into the underlying sandstone specified by the client could be achieved. Utilising DAWSON-WAM's secant piling system guaranteed that the piles could be cored to the required depth.
- The secant piling operation could be carried out using a silent & vibrationless rig which could install piles as close as 50mm from dwellings without causing any damage.
- There was no procurement time for materials allowing the contract to be implemented within the restricted working times during the same calendar year.
- DAWSON-WAM proposed the use of a 'stone look' formliner to construct the cast-in-situ top wall. All riparian's were approached on an individual basis to canvas opinion, and it was widely preferred for its aesthetics as opposed to the painted sheet-piled wall.
- There were substantial energy savings through both material choice and construction methods adopted.
Raising the bar
Prior to commencement, the construction team produced a range of detailed and extensive plans in order to establish a high benchmark from the outset. These plans included, but were not limited, to:
- Green Travel
- Energy Management
- Air Quality
- Sediment & Erosion Control
- Pollution Contingency
- Site Waste Management
- Customer Care
Through working collaboratively with CEEQUAL and the client, the bar was further raised throughout the project with all team members committed to going the extra mile. Examples of such measures were:
- Electrofishing of the watercourse to remove small fry and restock in the local hatchery further downstream prior to commencement of the works
- Recycling of spawning gravel from upstream gravel traps for the construction of the new fishery measures
- All 1500 tonnes of rock armour used for toe protection was recycled material
- All felled trees were logged and stacked along the riverbanks on completion to create habitats for invertebrates
- All plant & equipment used bio-degradable oils in case of accidental spillages
- 98% of inert and non-hazardous waste material was sent for recovery in a waste to energy plant
- All labour, sub-contractors and material suppliers were sourced within an 18 mile radius to minimise the carbon footprint of the site
- Extensive silt control mechanisms were installed within the river to prevent migration of silt downstream while installing the toe protection and fishery measures
Customer care & community liaison
DAWSON WAM's contracts manager was appointed as the contractor's public liaison officer. Prior to commencement each riparian was individually visited outside working hours to discuss the scheme and ease any concerns. Through dealing promptly and effectively with the smaller concerns, residents developed the confidence and trust that bigger problems would be addressed in a similar manner. Regular visits, letter drops, public meetings and a 24hr emergency contact number all aided in making this a 'pain-free' project. During unprecedented flooding one Saturday evening, contracting staff were onsite during the event. Staff remained late into the evening until the peak of the flood had passed, providing assurance to residents by monitoring the works and aiding with sandbagging.
On completion of the project no complaints had been received in what was deemed to be an extremely high-public-profile scheme. Instead, several letters of commendation were received from residents, the Police Service of Northern Ireland (PSNI), Road Service and a number of charitable and community organisations, praising the way the scheme was implemented. This was reinforced further through the presentation of a 'Silver' Considerate Constructor's Award.