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BOC Immingham Dissolved Acetylene Project Environmental Statement: Vol. 2 Main Text
9.50 During construction, the operation of machinery may result in compaction of the soil which will
reduce the infiltration capacity. This could result in an increase in surface water runoff and could
lead to localised flooding and runoff into Middle Drain.
9.51 It is unlikely that any effects will be realised as a result of climate change during the 9 month
construction phase (due to the relatively short timescale). It is, therefore, considered that no
significant change in flood risk at the site will occur during the construction period.
Operation
9.52 PPS 25 sets out the recommended contingency allowances for sea level rise as a result of climate
change. The potential increase in sea level rise to 2063 xiii is 0.399 mm; this will increase the risk of
flooding to the site, especially from the overtopping of the coastal defences.
9.53 There is a potential flood risk to the staff and equipment from extreme events in Middle Drain
coinciding with tide-locking due to a failure of the pumping station.
9.54 Prolonged periods of rainfall in the surrounding catchment and high tides in the Humber Estuary
can lead to an increase in groundwater levels and contribute to groundwater flooding.
9.55 The proposed development will increase the impermeable area of the site and will result in an
increase in surface water runoff. Any surface water flooding is likely to have an adverse impact on
the operation of the site and may increase the risk of flooding at the proposed development site or
on land within the surrounding area.
9.56 Earth embankments (2 m high) will be constructed on three sides of the main process area; these
may result in displacement of flood water and changes to surface water runoff pathways
increasing flood risk to the surrounding area.
Impact Assessment
Construction
Tidal
9.57 The existing flood defences for the current situation are high enough to prevent overtopping such
that the site is defended from coastal flooding to at least a 1 in 1,000 year standard of protection.
There is, however, a risk of flooding from breach of these defences and overtopping from more
extreme scenarios.
9.58 The existing coastal defences are considered to be fair or good condition and the risk of failure of
these defences (i.e. a breach) is considered to be minimal. The EA have provided flood hazard
outputs from a two dimensional model built to simulate the coastal flooding and the overtopping of
the existing defences at the proposed development site for a 1 in 200 year and 1 in 1,000 year
event current day scenario (2006). The results of this modelling are provided in Appendix 9E.
9.59 The results of the modelling show that using the Defra report FD2320 63 the flood hazard rating is
greater than 2. The velocities and depth represent a severe flood hazard which is considered to
be dangerous for all as the breach of the defences results in deep or fast flowing water. The
impact would be considered to be of major adverse significance.
9.60 The modelling of the overtopping of the coastal defences for a 0.1% AEP present day scenario
indicates that there is no risk to the proposed development site. The impact of overtopping is
therefore considered to be of negligible significance.
5100935.404 Environmental Statement August 2011 137