Chapter 9: Road drainage and the water environment

3.7 Flooding from Infrastructure Failure
This type of flooding occurs as result of the failure of infrastructure which exists to
retain, transmit or control the flow of water. The failure can be caused by
structural, hydraulic, mechanical, geotechnical or operation problems.
The risk of flooding comes from infrastructure such as canals, dams, water
mains, flood defences, pumping station and blockages of pipes.
Investigations of the Severn Trent infrastructure suggest that there are water
supply pipes that pass directly through the site. Inspection of the OS background
indicates that there is a reservoir upstream of the site.
(a) Reservoir Failure
From Ordnance Survey data it is observed that Stanford reservoir lies
approximately 4km upstream of the site and feeds the River Avon at the
upstream limit.
Severn Trent has a comprehensive and ongoing mandatory inspection and
maintenance regime of their reservoirs under the Reservoirs Act. Any structural
problems with this reservoir would be identified by this regime and rectified. A
sudden failure of the reservoir is considered to be a very low probability event.
A dam break analysis has been performed for Stanford Reservoir, and an
inundation map was produced as part of this. Severn Trent’s reservoir
supervising engineer 3 has stated that the M1/M6 junction was part of the detailed
analysis and the report concludes that "The M1/M6 motorway junction and wide
bridge opening across the valley between the dam and the disused railway
embankment, at 4.8 km downstream, would have a negligible effect on the flood
wave". The remainder of the dam break information is considered a confidential
nature at present.
A request for the inundation map has been made but has not at the time of
writing been made available.
(b) Water Supply and Sewage Infrastructure Failure
The existing data obtained from Severn Trent does not indicate the size of the
pipes, depth of the pipes or what they convey. The designers should ensure that
the water supplier (Severn Trent) is contacted and details of the pipes obtained.
Where necessary an application for diversions will have to be made.
The failure of any of these pipes during construction may lead to overland flows
towards the low spots such as Swinford Road crossing and the A14-M1 link. The
water supply pipes are unlikely to represent a serious risk to the scheme or
highway users. The flow rates are likely to be low and the water would be
intercepted by the drainage network.
The risk of flooding from identified infrastructure failure is considered to be
low.
3 Email (13/07/09) from Mr. Francis Simba Chigara, Reservoir Supervising Engineer, Severn
Trent. email: francis.chigara@severntrent.co.uk
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