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BOC Immingham Dissolved Acetylene Project Environmental Statement: Vol. 2 Main Text
Fluvial Flood Risk
9.29 Middle Drain (see Figure 9.2) is maintained by the NELDB. Sluice gates and the pumping station
prevent the tide moving upstream towards the site, so during high tides the drain is discharged via
a pumping station, with free drainage during ebb tides. Detailed hydrological and hydraulic
modelling has not been undertaken for Middle Drain; however, NELDB has examined the
conditions in Middle Drain in the past to assess the drainage implications of large industrial
developments. As a result, Middle Drain is understood to have a capacity of 1% AEP and up to
0.1% AEP combined with flood storage and the pumping station.
Figure 9.2 Photographs of Middle Drain and the Pumping Station
9.30 Investment has been made at Middle Drain to assist in the development of the area. The pumping
station has the capacity to pump approximately 6,300 litres per second 61 .
9.31 Studies suggest that increases in rainfall are resulting from climate change and will result in an
increase in peak flow in the watercourses. Guidance within PPS 25 (based on the Defra
‘FCDPAG3’ report 62 ) suggests an increase in peak river flow of 20% should be considered and in
peak rainfall intensity of up to 30%. The proposed development site is not considered to be at risk
from fluvial flooding and there is no change to the baseline condition when climate change is
taken into account.
Groundwater Flood Risk
9.32 The development site is underlain by Estuarine Alluvium and Glacial Till which extend to between
18.2 m and 18.4 m bgl. These are underlain by the Cretaceous White Chalk Formation (Bedrock).
9.33 Groundwater in the top one metre of soils is understood to be controlled largely by land drains.
Below this, shallow groundwater in the Superficial Deposits is understood to flow towards the
Humber Estuary. Deeper groundwater is present in the Chalk Principal Aquifer; however this is
isolated from the shallow groundwater by the thick layer of clay.
9.34 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. As the
groundwater is likely to be in hydraulic continuity with the Humber Estuary the main influence on
groundwater levels within the development site will be the tidal levels.
9.35 The British Geological Survey Groundwater Flooding Susceptibility Map (see Appendix 9A)
suggests that the site is at moderate to high risk of groundwater flooding. However, there is no
record of any groundwater flooding within the vicinity of the proposed development site.
5100935.404 Environmental Statement August 2011 134