Chapter 9: Road drainage and the water environment

4 Impacts of the Proposed Development on Flood Risk
Elsewhere
4.1 Impacts on Fluvial Flood Risk
The effect of the M1 Junction 19 improvements proposal on fluvial flood risk can
be summarised as follows:
• The expansion in impermeable area will result in a net increase in the
volume of highway runoff and also in the peak runoff flow rate. If this
runoff is allowed to discharge freely into the adjacent land drains and
ditches (and subsequently into the River Avon) it would have the effect of
increasing fluvial flood risk.
• The widening of the embankments along the A14 will have the effect of
displacing the floodplain and reducing the total volume of storage
available to the river in times of spate. This would have the effect of
increasing water levels and therefore increasing fluvial flood risk.
Therefore, it is important that any increases in flow volumes and displacement of
the floodplain are carefully managed in line with Environment Agency
requirements (discussed in Section 0, Proposed Mitigation Measures).
Limited information regarding the bridleway bridge crossings is available at this
time. A simplified Manning’s assessment of the bridges based on an assumed
rectangular cross section measuring 5m wide and 1.5m deep indicates that the
bridge will be surcharged at low return period flows i.e. approximately 1 in 2 yr or
less. The low deck height of the bridleway bridge is required due to the proposed
use of the bridge (a low gradient is required to improve accessibility to all users).If
the bridge was to be raised out above the 1 in 100 (1%) annual probability flood
levels, the height required would be significant. This would also require large
lengths of the bridleway to also be raised which would have a greater impact on
flood risk than the bridge alone.
The impact of bridges will be to reduce the conveyance of the river channel
sections at these two locations. This may have an impact of backing up flows and
raising water levels upstream of the bridges. However, as the bridges will remain
low this impact will be minimised. The bridges may have the effect of flooding the
adjacent fields more frequently, and therefore increasing the risk to farmland
marginally. At higher return period events the bridge and approach ramps will be
overtopped and bypassed.
4.2 Impacts on Groundwater Flooding
The scheme proposals include provision for deep excavation below the adjacent
ground water level for the Swinford Road underpass and the A14-M6 link. Slope
face drainage has been proposed in the Contractor’s Drainage Report to control
groundwater seepage. Such a system of drains will have the effect of lowering
groundwater levels locally, with a decreasing effect further away from the
scheme. At this broad level of assessment it is not thought that the scheme will
increase the groundwater levels or the groundwater flood risk elsewhere.
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