Highways Agency

Water Quality

Overview of Effects

Because the Scheme increases the paved area to be drained, the volume of motorway runoff and amount of contamination generated are expected to increase proportionately.

The new highway drainage collects runoff from the whole motorway and incorporates treatment, containment and attenuation through the use of a number of sustainable drainage features.  These features counteract the adverse impacts from the increase in highway area and where possible provide an improvement. 

Consequently, the Scheme is expected to have no additional impact on the quality of road runoff discharging to watercourses and to ground, compared to current conditions.  

The inclusion of accidental spillage containment throughout the Scheme provides an improvement compared with the existing drainage system. 

The Scheme has no impact on flood risk in the receiving watercourses flood plain storage or flood channels. 
Overall, based on the surface and groundwater design, the Scheme has a slightly beneficial effect on the water environment.

The following describes the principal watercourses crossed by the M25 between Junctions 27 and 30.

Brookhouse Brook

The Brookhouse Brook flows beneath the carriageway from north to south towards the east of Junction 27.  The brook flows directly into the River Roding after passing beneath the carriageway.

There are seven outlets associated with the Brookhouse Brook and high levels of nitrates within in the water system, associated with the sewage works upstream.

At present the brook contains no designated fish species such as carps and minnows.

River Roding

The river is a slow flowing, lowland river with a bottom of silt over clay. It flows to the southwest under the M25.

On the clockwise side of the M25 carriageway the river flows between fields managed as arable but a strip of grass sown along the bank prevents soil being washed into the water and decreases the flood capacity of the channel. 

On the anticlockwise side of the M25 carriageway there are pastures on either bank and the right bank is poached where cattle come down to drink.

Despite some enrichment the river is of a high quality with a good diversity of plants and animals. There is an excellent range of wetland plants growing by and in the river.

Bourne Brook

The Bourne Brook is the name given to the Headwaters of the River Rom, which then becomes the Beam River prior to its discharge into the River Thames.  It flows northeast to southwest, west of Junction 27.  It is crossed by the M25 in the vicinity of Middle Brook Farm.

Chemical water quality is currently of a ‘fair’ standard, and biological quality is ‘fairly good’.  This is not a designated fishery and not prone to flooding.

Weald Brook

Weald Brook flows almost parallel to the M25 and crosses under the motorway at four locations.  It is the main source branch of the River Ingrebourne.

The high, steep banks are dominated by shrubs and trees and there is little channel vegetation. No management was evident and deer using the channel were reported as the only fauna of interest. Water voles were recorded on the banks, which are lined with bramble, hawthorn, young sycamore and alder.

Ingrebourne River

The Ingrebourne River flows in a south easterly direction towards the A12 after Junction 28.
At the lower end is a nationally important site of naturally-formed freshwater habitats, known as Ingrebourne Marshes.

The main source branch of the Ingrebourne River is known as the Weald Brook, which runs parallel to the M25 and crosses the motorway on four separate occasions.
There is a Wastewater Treatment Works (WTW) discharging to the Ingrebourne River and evidence supports a clear impact on water quality.

The river corridor supports woodlands, grasslands and areas of fen and evidence to support Kingfishers nesting within its steep banks has also been identified.

River Mardyke

The River Mardyke is a meandering tributary of the River Thames flowing through mainly agricultural land in the north and more urban/industrial land to the south.

The chemical water quality is assessed as fair but there is considerable impact on the river from a variety of sources including agricultural and industrial runoff. 

The Mardyke area is expected to be prone to flooding.

Running Water Brook and Long Pond

Long Pond is located on either side of the carriageway. On the east side it runs through Oak Wood and Belhus Park East before it is culverted under the carriageway. Upstream is a residential area and downstream the river feeds into what is known as Running Water Brook.

The east side is home to a plantation of Oak wood and Ash.  Most land in the area is designated as agricultural land, with occasional tracts of woodland.

The Brook drains off into Aveley Marshes adjacent to the River Thames and part of the Inner Thames Marshes Sites of Specific Scientific Interest (SSSI). 

Some land downstream of the M25 is prone to flooding.