Chapter 9: Road drainage and the water environment

M1 JUNCTION 19 IMPROVEMENT
ENVIRONMENTAL STATEMENT VOLUME 2
CHAPTER 9 – ROAD DRAINAGE AND THE WATER ENVIRONMENT
9.5.14 Where the local roads are in cutting, kerb and gully drainage is proposed on the low side of
the carriageway to drain the carriageway run-off, and filter drainage is proposed on the high
side of the carriageway to collect the run-off from the earthworks slopes. The water from
these areas would drain into the treatment ponds. Swinford Road would be in a particularly
deep cutting beneath the M6. This water would therefore be incorporated into the mainline
drainage for the M6-A14 via deep pipes, and would outfall via drainage pond 7.
9.5.15 As set out in the introduction the proposals for Catthorpe Viaduct Replacement will be to
match the existing drainage arrangements as far as possible, with only negligible
differences to the locations of pipes and gulleys. There will also be some reduction in the
highway area to be drained as the replacement viaduct will be narrower than the existing.
9.5.16 No provision of drainage ponds or petrol interceptors as described for the junction
improvement above is included for the viaduct replacement works.
9.5.17 There will be some widening as a precautionary measure of approximately 400 metres of
an existing drainage ditch along the eastern side of M1 to the north of the River Avon.
Mitigation measures will be used to minimise the risk of any suspended solids reaching the
River Avon, including phasing the regrading to reduce the duration of disturbance.
Flood Compensation
9.5.18 Proposed works to widen the A14 embankments would displace part of the floodplain of the
River Avon, and thereby reduce flood storage capacity. In addition, two bridleway bridges
would be built in the floodplain, and would also reduce the capacity. To mitigate these
impacts, several flood compensation areas would be excavated to provide 587m 3 of
additional flood storage capacity. The locations are illustrated on Figure 3.13.
9.5.19 As set out in Development and Flood Risk – Guidance for the Construction Industry Report
C624 by CIRIA 36 , compensatory flood storage must become effective at the same point in a
flood event as the lost storage would have done. It therefore has to provide the same
volume and be at the same level relative to flood level as the lost storage. This requirement
is often referred to as “level for level” or “direct” compensation.
9.5.20 The various areas shown on Figure 3.13 have been selected to be at the appropriate level.
Some of the proposed compensation areas would be returned to agricultural use. Others
have been sited to coincide with proposals for habitat creation and these are described in
detail in Chapter 3 Ecology and Nature Conservation.
9.5.21 Unlike the drainage ponds described above, the Highways Agency does not intend to take
permanent title to the areas required for flood compensation. The intention is to hand them
back to the original landowners on completion of the works. However, as a precaution the
areas will be included in the CPO to ensure certainty that they can be provided. Upon
completion the areas would become part of the flood plain regulated by the EA.
River Channel Regrading
9.5.22 Figure 3.13 also shows sections of the river bank between the A14 and M1 which would be
graded to a shallower profile. The objective of these works is not hydrological but
ecological, and the measure is described in more detail in Chapter 3 Ecology and Nature
Conservation. Enlarging the channel would not provide any direct flood compensation. The
locations have been agreed with the EA and the intention of the regrading is to encourage
the development of marginal vegetation to provide greater cover for otters and to improve
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