LOCATION:
AWE Burghfield is a 225-acre site where around 4,500 staff work to assemble and maintain warheads while in service and decommission them when out of service. After a serious flooding event, AWE determined that the risks associated with the downtime and damage caused to key facilities and its ability to support the UK’s Continuous At Sea Deterrent (CASD) was unacceptable, leading to the development of this scheme.
The scheme was designed to protect the site from future flooding, upto a 1:200-year six hour critical storm event.
The nature of AWE’s work required us to strictly comply with their rigorous site regulations, safety standards and quality management.
Key features were:
- Widening of Burghfield brook along the length of the south and east boundaries through adjacent Ministry of Defence land for approximately 1.5km, from a single narrow channel to low and high flow channels, substantially increasing capacity.
- Excavation of an Upstream Storage Area (UpSA) of 5.4ha within an adjacent field to capture overland flow. Modelling of the flood event showed that the channel flow and overland flow elements of the flood combined at the AWE Burghfield site. The UpSA collects and holds back the overland element, allowing the peak flow from the upstream catchment to pass before the peak from overland flow enters the brook.
- Bunds and berms to contain and direct flood waters to the brook channel.
- Raised roadway to prevent the brook returning to its original course across the middle of the AWE Burghfield site.
- Tideflex valves installed on each outfall to prevent backflow from the brook into the surface water drainage network, thereby reducing surface water flooding and protecting the sewage treatment works.
- Pumping station to pump water from the surface water network and treated sewage into the brook at the downstream end of the brook, just before it leaves the AWE Burghfield site.
- Planting and ecological features.
- Access steps for maintenance and monitoring.
- Replacement of car parks removed during the brook widening.
- Replacement of four bridges.
- Fabrication and installation of two debris screens.
To become an approved supplier, we first had to complete a complex evaluation process. Having a high proportion of directly-employed workers helped signficiantly with the associated issues of working in this tightly secure location.
Challenges
Earthworks for the channel widening and upstream storage area generated approximately 50,000m3 of excavated material.
Other major challenges included managing high rainfall and flow events, service diversions, asbestos contaminating soils, spatial constraints, maintaining compliance with permits and licencing and ecological aspects.
In line with developments of this scale a full suite of ecological surveys were required in advance of works taking place.
Solutions
We worked closely with AWE and our consultant, Card Geotechnical Limited (a key supply chain member), to divert as much surplus excavated material from landfill as possible.
Under the CL:aire protocol and our materials management plan, all topsoil was recovered and reused within the development, with 30,000m3 of subsoil being reused on site within the scheme to construct the berms and bunds, 24,500m3 of soils used in two West Berkshire Council managed flood alleviation schemes (one in Thatcham and one in Winterbourne). A further 16,00m3 reused in a Cotswolds restoration project.
Stakeholder engagement and collaborative working were essential to the success of this multi-faceted project and included West Berkshire Council, the Environment Agency, contractors, local residents and businesses.
A year of surveying confirmed earlier estimates that there was a medium population of grass snakes Natrix helvetica present, this meant every effort would be needed to move as many snakes as possible out of harm’s way. This
involved designing a suitable translocation plan which was agreed by the Local Planning Authority.
The highly constricted nature of the narrow strip of land, squeezed between a nuclear licensed site, an explosive area and the exterior fence line, posed a challenge in terms of translocating the grass snakes.
A creative solution was required whereby two areas were carved out, one at each end of the brook, where the habit was improved allowing relocated animals to disperse into the wider environment.
Outcome
This project has successfully demonstrated how works can be undertaken with ecology, sustainability and industry best practice as priorities.
The scheme was designed to be passive and where possible maintenance free, thereby minimising resource use and manpower during operation and maintenance. Where possible, components made from recycled materials were utilised including steel reinforcing, Truckpave, dura curb and type 1 aggregate.
To further reduce site waste the project utilised modular construction methods whereby components and systems manufactured off site were brought to site ‘just in time’, including pipes, culvert and bridge sections, Redi-Rock wall units and headwalls.
Works within the brook were undertaken offline, i.e. the water was flumed or over pumped, rather than flowing in the stream bed. These methods were chosen both to minimise the health and safety risks of working alongside a water body and to minimise the mobilisation of sediment within the brook.
