The expansion of Heathrow will extend the airport into the Colne Valley, affecting the routes of a number of watercourses and areas of existing floodplain.
We need to protect the quality of the water environment and work with the Environment Agency and other stakeholders to deliver appropriate solutions. Our plans for green infrastructure are set out here. We will ensure that expansion does not put any homes or businesses at greater risk of flooding.
The Colne Valley has three principal rivers namely the River Colne, the Colne Brook and the Wraysbury River. The Duke of Northumberland’s River and the Longford River are also important watercourses that flow around the existing western airport boundary.
The area is very flat and one of the challenges when diverting a river is to ensure its flow is maintained. By raising the proposed new runway and its taxiways by 3 to 5 metres above current levels we create the opportunity for some of the rivers to flow underneath it before reverting to their existing channels south of the runway.
We have considered a range of options (see below) and recognise that it may be possible to divert some rivers under the runway and some around the runway in new channels. The exact alignments require further development. Our current thinking for these watercourses is as follows:
- Longford River and Duke of Northumberland’s River – with their easterly alignment it is not feasible to divert them around the runway and maintain their flows. The only viable option is to divert both rivers under the runway;
- Colne River and Wraysbury River – diverting around the runway in an open channel would require more land, involve complex construction sequencing as well as the need to cross the M25 and M4 motorways. Diverting these rivers under the runway is less complex to deliver, requires less land and keeps the rivers as close as possible to their existing alignment. Further work is required on the potential effects to habitat connectivity;
- Colne Brook – with its current western alignment, the option to divert around the west end of the runway would be less costly and help provide habitat connectivity and landscape value.
Options to create a new channel west of the new runway are also being investigated to understand the potential to provide habitat connectivity, landscaping and amenity value.
The new runway will be built partly in the flood plain of the Colne Valley and as a result replacement flood storage will be needed. Any changes proposed to the flood plain will not increase the existing risk of flooding.
We have short-listed two approaches for the re-provision of flood storage:
Approach 1 – On-airport storage: This would require building flood reduction measures under the new airfield which could reduce the amount of land needed, but these structures are costly to build, require complex phasing and may compete with other operational land uses such as taxiways and aircraft parking.
Approach 2 – New upstream storage: Flood storage options outside the airfield would need land to be lowered or existing watercourses widened to increase flood storage capacity. These options (shown below) are located closest to the areas lost from the existing floodplain, provide opportunities to improve the landscape, avoid effects on the airport operation but would require more land outside the airport.
Please note that not all of these sites will be required for flood storage and we are likely to seek an appropriate balance between using on airport and new upstream storage capacity for water storage if a flood event occurs.