Chapter 9: Road drainage and the water environment

3 Flood Risk to the Proposed Development
3.1 Fluvial Flooding – Main River
Fluvial flooding refers to flooding from rivers, streams and other inland natural
watercourses. It is usually caused by prolonged or intense rainfall, generating
high rates of runoff which overwhelm the capacity of the river or channel. When
this occurs, excess water will spill onto low-lying areas of land adjacent to the
channel. This area is known as the floodplain.
In England, the term ‘Main River’ is given to strategically important waterways
that drain a catchment. They are defined by lines drawn on a statutory map held
by the government (Defra).
Main Rivers are not always the biggest rivers in a catchment but they are
hydraulically important to how water drains from a catchment. Consequently, they
also tend to be associated with the greatest flood risks.
The Environment Agency has permissive powers, but not a duty, to carry out
flood defence works on Main Rivers.
3.1.1 Initial Investigation
The nearest Main River is the River Avon, which flows from east to west. It
passes under the A14 via a formal bridge crossing.
The widening of the A14 slip roads will require embankments which encroach on
flood zone 3a (see Figure 2 and Figure 8 in Appendix B). The embankments will
therefore experience flooding in the 1 in 100 (1%) annual probability event,
however since the A14 road level is significantly above the 1 in 100 (1%) annual
probability level the consequence of flooding is thought to be minimal i.e. does
not represent a tangible risk to road users. Flood risk to the A14 embankments
(shoulders) from the Main River is considered low.
The river continues to the west between the A14 and the M1 where it passes
under the M1 under a formal bridge crossing. There is no proposal to change the
existing highway bridge crossings therefore the water levels will remain the same.
The M1 and A14 are both significantly elevated (5.2m and 1.4m respectively)
above the River Avon 1 in 100 (1%) annual probability levels therefore the flood
risk to the M1 and A14 highways from the Main River is considered very
low.
Upstream of the A14 and between the A14 and the M1 there are proposed
bridleway crossings shown in Figure 2. The proposed bridleway bridges
upstream of the A14 will be replacing an existing bridge of simple timber
construction. Whilst the bridleway bridge between the A14 and M1 will be a new
bridge. The deck level and approach ramps of the proposed bridges will be below
the 1 in 100 (1%) annual probability flood level. This design has been agreed with
the Environment Agency. The structure will be at risk from flooding, furthermore
users of the bridge will be at risk from flooding. Flood risk to this bridleway
bridge is therefore considered to be high and some degree of mitigation
will be required (see Section 5).
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