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BOC Immingham Dissolved Acetylene Project Environmental Statement: Vol. 2 Main Text
Environment Agency Flood Map
9.22 The Environment Agency Flood Map is provided in Figure 9.1 and shows that the proposed
development site is located within Flood Zone 3, High Risk within a 0.5% annual exceedence
probability (AEP) of tidal flood risk (equivalent to a return period of 1 in 200 years) but is defended
from the Humber Estuary by embankments.
Figure 9.1 Flood Map (derived from Environment Agency Website 2010)
9.23 The Environment Agency Flood Map is based on still water tide levels and does not take into
account surge or waves (except the component of these that is inherently embedded in the tidal
gauge data around the Humber Estuary). Also the Environment Agency Flood Map does not
distinguish between the tidal and fluvial flood risk of Middle Drain; however, as the discharge from
Middle Drain is pumped into the Humber Estuary at high tides it is assumed that the flood risk
from Middle Drain is fluvial.
Tidal Flood Risk
9.24 The flow of water in the Humber Estuary is driven mainly by the impact of tides on sea water level,
which varies on an approximately semi-diurnal (twice daily) cycle. The tidal range of the Humber is
macro tidal, i.e. its tidal range is greater than 4 m (see Table 9.2 below).
9.25 The Humber Estuary is the principal source of flood risk at the site, through a combination of high
tides, storm surges and wind-generated waves. Two potential mechanisms of tidal flooding have
been considered and include overtopping during high tides in combination with storm surge and
the breach of coastal defences.
5100935.404 Environmental Statement August 2011 132