Chapter 9: Road drainage and the water environment

5.1.2 Mitigation for Increased Runoff Volumes and Rates
In order to deal with increase runoff from the highways without increasing the
discharge to the neighbouring watercourse, namely the River Avon, five
attenuation ponds have been proposed.
The Environment Agency was consulted in the development of the Contractors
drainage proposal. The EA design parameters adopted at this stage of design are
as follows:
• Where attenuation is to be provided the proposed discharge rate is to be
limited to 60% of existing (existing discharge rates based on a 2 year
design storm 30 minute rainfall event). This parameter has been adopted
to achieve the EA objective to reduce the total existing discharge flow
within the limits of the scheme by a minimum of 20%. It also provides a
conservative basis from which to determine land take and allows for
further design development during PCF Stage 5 (Construction
Preparation).
• Existing greenfield run-off rates are to be restricted to 5l/s/ha
• Attenuation ponds are to be designed to accommodate a 100 year design
storm plus an additional 20% for possible climate change
Furthermore, the highways drainage system has been designed in accordance
with the Design Manual for Roads and Bridges (DMRB) to the following criteria:
• Carrier drains have been designed to accommodate a 1 year design
storm in bore and checked to ensure no flooding in a 5 year storm
• Filter drains have been designed to accommodate a 1 year design storm
in-bore and checked to ensure no surcharging above formation level or
sub-formation level in a 5 year storm
• Networks have been designed to accommodate an additional 20%
increase in rainfall intensity for possible climate change without flooding in
a 5 year design storm.
Additionally, the attenuation ponds will require a regular maintenance regime to
maintain the effectiveness of the attenuation ponds. This is described in detail in
the Contractor’s Drainage Proposal (see Appendix B).
5.1.3 Further Mitigation Requirements
The above describes the measures required to mitigate against increasing flood
risk from the development. It should also be noted that the embankments,
bridleways and approach ramps are at fluvial flood risk and as such should be
designed with the relevant design scenarios in mind (e.g. rapid drawdown, scour,
hydrostatic loading etc.). Furthermore, the bridleway crossings and approach
ramps will be submerged during high flows. The bridge and approaches should
be designed to minimise the obstruction to the flow. Additionally, signage should
be considered to inform users that the crossing is impassable during high flows.
5.2 Mitigation of Groundwater Flood Risks
Certain parts of the proposed road infrastructure are below existing ground levels
and have been identified as being at risk of groundwater flooding.
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