Highways Agency

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 provides 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 16 and 23.

The Alder Bourne

The Alder Bourne passes through Junction 16. It is a meandering river at this point, culverted under the slip roads of the Junction. The bed is predominantly gravel and sand with silt and pebbles. Both banks are earth and dominated by a mixture of rank grassland, scrub and trees. The channel vegetation is dense and diverse and includes branched bur-reed (Sparganium erectum), water mint (Mentha aquatica) and yellow iris (Iris pseudacorus).

The River Misbourne

The River Misbourne is a gently meandering river with a slow to moderately fast flow and good water quality. It is culverted under the carriageway (Chainage 4,250). Generally the banks are wooded with pedunculate oak (Quercus robur), ash (Fraxinus excelsior) and hazel (Corylus avellana).

The River Chess

The River Chess is crossed by the carriageway three kilometres upstream of its confluence with the River Colne. This reach has been modified to a straight cut-through channel. Downstream this channel merges with the Solesbridge Lane Loop and finally ends at a weir.

The River Colne

The carriageway crosses the River Colne at the upper end of its catchment, where the catchment area is quite small. Because of the low topography, this is a slow flowing and meandering section of river. The water is generally turbid, with a substrate consisting of silt and gravels.

The River Gade

The section of the River Gade at the Scheme crossing has been canalised to form part of the Grand Union Canal. It is a straight concrete sided channel containing a slow flowing river 4-5 metres wide and approximately 1.5 metres deep. The water is generally turbid and the substrate not usually visible.

The River Ver

The River Ver is a chalk stream with clear water and a substrate of silt and pebbles. The channel at the motorway bridge has previously been over-widened and considerable deposition of sediment has occurred in this section as a result.

The Catherine Bourne

The Catherine Bourne is a tributary of the Mimmshall Brook, which it joins between the motorway crossing and its disappearance underground. Main watercourses like this are considered highly sensitive to change.

Mimmshall Brook

At the M25 the meandering Mimmshall Brook is culverted under the motorway in a concrete lined channel approx 4-5 metres wide. Downstream of the M25 the moderate flowing meander continues with a width of approx 1.5 metres and varying depths. The water is slightly turbid (cloudy).