Chapter 9: Road drainage and the water environment

M1 JUNCTION 19 IMPROVEMENT
ENVIRONMENTAL STATEMENT VOLUME 2
CHAPTER 9 – ROAD DRAINAGE AND THE WATER ENVIRONMENT
9.9 SUMMARY
9.9.1 An assessment of the effects of the proposed junction improvement, in relation to surface
water, groundwater and flood risk, has been undertaken following DMRB methodology,
specifically developed for the assessment of the impacts of highway schemes, and based
on advice from the EA. Impacts for the Catthorpe Viaduct have been indicated separately
and included in combination with an overall assessment of the junction improvement.
9.9.2 Baseline conditions have been reviewed. The site is underlain by a mixture of non-aquifer
and minor aquifers. The distribution of the latter are associated with the location of the
various surface watercourses. The site lies within the catchment of the River Avon, a main
river, and the highway crosses the River Avon at a number of locations. There is one other
main river, the Clay Coton Yelvertoft Brook. There are numerous other ordinary
watercourses / drainage ditches and ponds.
9.9.3 There are some groundwater abstractions, including private potable supply, located within
1km of the highway, presumed to be sourced from the minor aquifer. There are no surface
water abstractions, although there is an important public water supply abstraction some
seven kilometres downstream on the River Avon.
9.9.4 The main rivers are of good water quality. However, the existing highway drainage
undergoes little treatment prior to discharge and, in addition, these receiving watercourses
are very vulnerable to pollution incidents.
9.9.5 Without mitigation, construction work has the potential to adversely impact the water
environment. The main potential impact is pollution of surface waters. It is assumed that
good site practice as incorporated within and fundamental to the CEMP would be followed
during the construction phase. The overall assessment score for the construction phase is
of a Moderate Adverse effect – however, this is considered to be conservative as it reflects
the inclusion of potential pollution of the River Avon drinking water supply. This would be a
temporary risk, and of very short potential duration as the treatment ponds would be
constructed early, providing mitigation for accidental spillage.
9.9.6 For the proposed junction improvement, the drainage scheme for the highway operation
would upgrade the existing arrangements with spillage containment, water treatment and
attenuation where required. It is assumed that maintenance and management of the
operational scheme would be undertaken as necessary. The overall assessment score for
the operational phase is of a Slight Beneficial effect.
9.9.7 With respect to groundwater, there is a potential for construction of cuttings to lead to a
lowering of the water table within the minor aquifer. However, this effect would be localised
and of Neutral significance. Other impacts on groundwater are relatively insignificant, and
few mitigation measures are proposed or required. This results in the residual impact being
little changed from the current situation.
9.9.8 In terms of flood risk, the widening of the A14 embankments and the creation of two
bridleway bridges would reduce the storage capacity in the River Avon floodplain.
However, flood compensation would be provided and so there would be no net loss of flood
storage. Attenuation ponds would reduce the discharge rate for highway runoff across the
scheme by at least 20%, and would therefore have a Slight Beneficial effect on flood risk.
9.9.9 The junction improvement would satisfy the requirements of DMRB for routine runoff and
pollution incidents from accidental spillages. Details of the DMRB calculations are included
in Appendix A.
B0531000_ID_70-EIA Vol 2 Chap 9 Road Drainage and the Water Environment Final Rev 4.doc Page 47