Chapter 9: Road drainage and the water environment

5 Proposed Mitigation Measures
5.1 Mitigation of Impacts on Fluvial Flooding
Sections 3.1 and 4.1 have identified that the site is at direct and indirect risk from
fluvial flooding. The direct risk is that some of the proposed embankments are
within Flood Zone 3. The indirect risk is that the scheme would result in
increasing the rate and volume of surface water runoff into the receiving
watercourses.
5.1.1 Mitigation for Loss of Floodplain
The EA have stipulated in their communication 7 regarding the scheme that:
1. Details on any loss of flood plain (Flood Zone 3), and resulting flood plain
compensation proposals. The proposed fill and compensation areas assessment
should be undertaken on a like for like level basis in 200mm slices, from (below)
the modelled 100 year flood event level, and presented in plan and tabular form.
The floodplain of the River Avon is defined by 1 in 100 (1%) annual probability
event design water levels from the Environment Agency’s hydraulic model of the
River Avon. The floodplain level at the sites of interest vary and ar given in the
table below.
Calculations have been performed in AutoCAD to identify the volume loss per
200mm level interval for all affected sections within the flood zone. There are
three distinct areas where flood plain loss occurs shown in the table below.
Area of Flood Plain
Loss
River Avon Crossing 1
(North of A14)
A14 embankment
widening
River Avon Crossing 2
(Southeast quadrant)
100 yr flood level
(mAOD)
Volume loss of flood
plain (m 3 )
- floodplain
compensation
required
98.2 83
97.6 405
96.6 99
Table 8: Summary of sites flood plain loss
The distribution of volumes and locations for floodplain compensation areas for
construction purposes are given in Appendix D.
7 EA letter ref: LT/2009/107833/01-L01 (26 th May 2009)
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