Chapter 9: Road drainage and the water environment

Mitigation measures to reduce the impact of groundwater flooding locally (on the
scheme) are given in Section 0: Proposed Mitigation Measures.
4.3 Impacts on Flood Risk from Surface Water Runoff
The scheme involves some major earthworks particularly in the construction of
the M6-A14 link and the M1–A14 link, this is shown in Figure 3. The impact of the
proposal will be to reduce the total area contributing to surface water runoff which
would ordinarily have found its way into land drains and subsequently into the
River Avon. The existing proposal of cuttings, embankments and highways will
divert this surface water runoff into the proposed drainage system. Therefore the
scheme will have an effect of reducing the surface water runoff and the
associated risk. However, it is noted that this surface water will be managed
differently in the current proposal i.e. transferred to the artificial drainage system,
this is considered in the following section.
4.4 Impacts on Flooding from Artificial Drainage Systems
The volume of runoff picked up by the artificial drainage system is set to increase
following the proposed scheme because it will:
• collect groundwater for the substantial cuttings
• collect surface water which would have ordinarily infiltrated or have been
intercepted by land drains
• collect from a larger impermeable area
The runoff from the existing drainage system discharges into nearby water
courses which subsequently flow into the River Avon. The impact of the
increased volume of runoff from various sources conveyed by the artificial
drainage system may increase peak flow and thus flood risk in the River Avon.
Arguably the time of concentrations from a catchment wide event would be very
different between the natural catchment and the impermeable catchment. Despite
this, mitigation measures should be considered (see Section 5).
4.5 Impacts on Flooding from Infrastructure Failure
Construction of the scheme (particularly around the Swinford Road cutting and
A14-M1 link) may damage the water supply pipelines through the site. Damage to
these pipes could cause overland flows towards the site. Whilst, this flooding is
not likely to cause a risk to highway users or the scheme in general, it could
cause short term nuisance. Mitigation will be required on site to avoid any
damage to these pipes.
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