Project Red Dragon Overview

Project Red Dragon is an £80 million Integrated Fast Jet Maintenance Facility, located at RAF St Athan, South Wales. The site is approximately 5 miles west of Cardiff International Airport. Laing O’Rourke were the main contractor tasked with constructing the project for the client, the Defence Aviation Repair Agency (DARA). Arup were lead consultant for the project, providing a multi-disciplinary design service from inception to completion.

Completed in December 2004, the site comprises a 48-bay superhangar, support building, paint spray and strip facilities and ancillary support buildings. The development of the site meant that approximately 30 hectares was covered in hard surfacing, covering what was effectively a greenfield site. The drainage of this facility required careful consideration and also had to facilitate potential future expansion of the site. It was recognised at an early stage that on-site flow attenuation would be required to limit flows conveyed to the receiving watercourse (the Nant y Stepsau) to the Environment Agency’s (EA) permitted discharge rate. This rate was assessed as being comparable to the Greenfield run-off rate (GFR) of the catchment prior to development. A detention basin was identified as being the most appropriate solution, given the size of the site and unsuitable ground conditions making an infiltration solution impossible.

Detention Basin

The detention basin had to be located downstream of the main facility to allow flows to be collected from the site and passed forward towards the outfall in a single 1350mm diameter pipe. Due to the size of basin required, suitable locations were very limited. The chosen location influenced the shape of the basin as it had to be designed around an exclusion zone associated with an existing ammunition store on the site. This shape, and the designed grading of the basin, is shown on the attached drawing.

The detention basin is provided off-line, allowing flows below the discharge limit to bypass the basin and discharge directly to the Nant y Stepsau. The need to provide it off-line was driven by the client’s brief to discourage birds from the area. It would, therefore, have been difficult to include a permanently wet feature in this location, given the sensitivity of operations within the new facility. The basin has been designed to store the 1:30 return period rainfall event without flooding. It has a volume of approximately 10,000m³ and has been sized to accommodate flows from future expansion of the site.

Flow Control

A flow control penstock located upstream of the basin throttles flows, allowing flows in excess of the GFR to spill to the basin. The penstock position has been set to allow a maximum flow rate of 190 l/s through under a head of 3.86m, slightly lower than the allowable discharge associated with the EA’s specified GFR. The basin has been designed with a sloping invert to allow attenuated water to flow back into the discharge system once flows have subsided to an acceptable level.

Pollution Protection

The basin also provides pollution protection to the site and receiving watercourse in the event of an emergency, such as a fire suppression sprinkler activation in one of the hangars. An emergency actuated valve, located in a chamber downstream of the basin, can be closed remotely allowing flows to be impounded in the basin for inspection.

The actuator is housed in a GRP kiosk above the chamber and has the facility to be manually operated if necessary from above ground. The actuated valve is linked to the site-wide BMS and can be closed remotely should this ever be required. When closed, flows will be impounded in the attenuation basin. This open water body can then be tested for pollutants before a decision is made to pass to the outfall or pump off-site. The 10,000m³ capacity of the basin has been assessed as sufficient to contain a large spillage or prolonged sprinkler activation within the hangars.