Heathrow Airport’s consultation on its preferred expansion masterplan – known as the Design Consent Order Project - published this week includes details of potentially significant negative impacts on the surrounding floodplain, rivers and lakes.

Commenting on the water environment, the plan says that 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. Without mitigation, expansion will result in the loss of flood plain in the catchment of the River Colne and Colne Brook.

Significant negative effects are predicted on rivers due to the length of river channels to be lost or altered, and due to changes in groundwater and on standing water (lakes) due to land take or land cover change - five lakes are due to be lost, and one is due to be altered.

However, Heathrow says that the losses would “largely be associated with the first phases of the DCO Project, as this is when the majority of land take occurs.” Following completion of the construction activities, a “likely significant positive effect is predicted for groundwater and surface waters.”

Announcing the consultation, the airport said:

“We plan to address our effects by creating opportunities for significant enhancements. We are committed to ensuring that no homes or businesses are at greater risk of flooding because of the Project, so we will replace the lost flood storage to achieve this.”

“Our proposals seek not only to protect the quality of the water environment but also ensure no increase in flood risk for local communities.”

The Masterplan preferred option sets out proposals to divert, realign or bridge several rivers and smaller streams, including:

• The Wraysbury River, River Colne and Bigley Ditch will join together north of Harmondsworth Moor and pass under the airfield in one channel through a covered river corridor to the east of the existing M25. South of Bath Road, the Wraysbury River will split from the River Colne and travel west under the existing and new M25.
• The River Colne will pass under the Southern Perimeter Road before connecting back to its existing channel north of Stanwell Moor.
• The Duke of Northumberland’s River will be combined with the Longford River and will pass under the airfield in one channel through a covered river corridor parallel to the combined Colne/Wraysbury channel.
• The Colne Brook will be diverted around the northern side of the Thames Water sewage treatment works and the northern and western side of the railhead/logistics centre, before returning to its existing channel north of Colnbrook village.

Preferred option benefits include "larger construction logistics centre and being easier to deliver"

According to the Preliminary Environmental Information Report (PEIR), compared with other options that have been considered, the proposed combination provides a good balance between:

• offering a better water environment for plants and animals;
• allowing a larger construction logistics centre that would aid delivery of the DCO Project;
• providing a better buffer zone between the Airport and local communities;
• and being easier to deliver.
• new flood storage areas within the Colne valley, to ensure no increase in flood risk from rivers
• flood defence works on the channels through Colnbrook village to manage the risk of flooding
• new and expanded drainage networks, to provide appropriate management of surface water runoff from rain water due to an increase in areas of impermeable surfaces
• subsurface features will be designed to minimise effects on groundwater levels
• development of a pollution control strategy to manage spills and other unintended releases

The expansion will also incorporate a range of flood defence measures to mitigate the heightened flood risks arising from the scheme.

The natural floodplains of the Colne Valley and Crane Valley where Heathrow is sited form corridors made up of woodland, meadows, wetlands and watercourses connected with the wider landscape linking the Chilterns with the River Thames. The area of land required to deliver the expansion covers around 2,900 hectares.

The consultation documents refer to a variety of sources of flood risk within the vicinity of the site - rivers (including the Rivers Colne and Crane and their tributaries), groundwater, surface water and artificial sources.

The masterplan includes proposals to put a range of flood storage and water quality treatment areas in place, including: 

• new flood storage areas within the Colne valley, to ensure no increase in flood risk from rivers
• flood defence works on the channels through Colnbrook village to manage the risk of flooding
• new and expanded drainage networks, to provide appropriate management of surface water runoff from rain water due to an increase in areas of impermeable surfaces
• subsurface features will be designed to minimise effects on groundwater levels
• development of a pollution control strategy to manage spills and other unintended releases                                                                                                                                   

Preferred option for wastewater treatment is existing sewer network

Currently, the option selected is to use the existing sewer network, with a pumping method for discharge. However, a number of alternative solutions are still under consideration including:

• using a new or enhanced waste water treatment works, which would make it possible to use the treated water for non-drinking water requirements in the Airport, and so reduce water consumption
• discharging into local watercourses, which would assist in flow rates; for example, the River Crane is known to have low flow issues

No additional environmental measures proposed – “relevant measures embedded into Project design”

According to the PEIR, at this stage “no additional environmental measures are proposed as the relevant and implementable measures are considered to have been embedded” into the project design and the good practice measures to be employed during construction.

The PEIR summarises the findings at this point in the Environmental Impact Assessment process for effects on the water environment - rivers, lakes and groundwater - as a result of the Heathrow expansion. This includes effects on water quality, river flows, groundwater levels, and physical changes to rivers, lakes and other water features, as well as effects on water supply or waste water networks.

The assessment incorporates five key technical areas:

1. Surface water quality assessment - potential effects on water quality in rivers as a result of discharges during construction and operation (e.g. discharging treated runoff from the airfield)
2. Groundwater modelling assessment - a numerical groundwater model to assess the effects of construction activities and land use changes on groundwater levels and flows
3. Drainage Impact Assessment - overview of the proposed approaches to managing surface water, and associated potential changes to the direction and amount of runoff
4. Preliminary Water Framework Directive (WFD) Risk Assessment - potential effects on rivers, groundwater and lakes in relation to the requirements of the WFD
5. Flood Risk Assessment - preliminary assessment of all potential sources of flood risk, including rivers, surface water, rainfall runoff, groundwater and artificial sources

The focus has been on rivers in the catchments of the River Colne and River Crane, including their tributaries and the underlying aquifers.

"At this stage of design still uncertainties that result in significant negative effect”

The PEIR says that the majority of effects are “concluded to be not significant, as a result of the measures that are embedded in the design of the permanent features and those that would be implemented during construction.” Heathrow says that monitoring will be carried out during construction and operation to identify any issues with those measures.

However, it also cautions that significant negative effects have been predicted on the Wraysbury River, River Colne, Longford River and Duke of Northumberland’s River as a result of the river diversions that would be required. This includes passing the rivers beneath the North West Runway in the Covered River Corridor.

The PEIR states:

“These changes would be implemented during Phase 1 and would affect the amount and quality of habitat available. While this will be addressed through the design of the new river channels and the Covered River Corridor, at this stage of the design there are still uncertainties that result in a significant negative effect.”

Potential for significant cumulative negative effects combined with effects of High Speed Two

The PEIR also warns that there is the potential for significant cumulative negative effects to the River Colne arising from the effects of the DCO Project with the effects of High Speed Two.

“There could potentially be an overlap in the requirements for and/or locations of additional mitigation between the two projects. The potential for a greater cumulative negative effect on the River Colne compared to the DCO Project alone, means that a significant cumulative negative effect is currently predicted”, the PEIR says.

Seven companies have been contracted by Heathrow to design the plans - the Integrated Design Team (IDT) consists of Amec Foster Wheeler, Arup, Atkins, Grimshaw, Mott MacDonald, Jacobs and Quod. The firms have each been awarded 4 year term contracts, commencing November 2016.

The consultation runs for 12 weeks - deadline to submit responses is 13th September 2019.

Heathrow will submit a final proposal to the Secretary of State for Transport in 2020, kickstarting its approvals process. The decision on whether to grant the DCO will be made by the Secretary of State following a public examination period.

Click here to access the dedicated consultation website

Click here to access the environmental documents on the consultation website

Click here to download the Preliminary Environmental Impact Report - Non Technical Summary