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BOC Immingham Dissolved Acetylene Project Environmental Statement: Vol. 2 Main Text
Pluvial Surface Water
9.85 Land raising is proposed as part of the development, especially in relation to the ramping up of the
bridge crossing over Middle Drain. This may result in displacement of flood water and changes to
surface water runoff pathways increasing flood risk to the land adjacent to the facilities and
increasing flood risk in the development site. Flooding is not considered to affect neighbouring
areas of land and the impact would be considered to be of minor adverse significance.
9.86 The land raising and contouring of the site has been engineered to ensure runoff is directed
towards existing drainage channels and outfalls to Middle Drain to ensure there is no flood risk
from surface water runoff and overland flow. Consideration has also been given to reducing the
residual risk to buildings by providing flow paths which divert flows away from the buildings.
9.87 The proposed development will increase the impermeable area of the site by approximately 1.5 ha
and will result in an increase in surface water runoff. In the absence of control measures surface
water flooding would be considered to have a moderate 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. Appendix 9A provides an assessment of the likely increases in surface water
runoff resulting from the increase in impermeable area.
9.88 The rainwater from the Cylinder and Bundle Filling and Generator Building roofs will be collected
and then stored within an above ground Rainwater Harvesting Tank for use in the manufacturing
process. The remaining rainwater will be collected by the Site’s Surface Water Drainage System.
Paved areas (i.e. roads and hardstanding) have been designed so that rainwater will fall away
from buildings and be collected by gullies and channels prior to entering the underground network
of surface water drains.
9.89 The Drainage System will discharge to Middle Drain via a pumping station. The pumping station
will contain three pumps with variable speed drives capable of discharging to Middle Drain at flow
rates between 25 l/s and 300 l/s. The peak flow rate from the site during a 100 year storm, with
20% allowance for climate change, is approximately 409 l/s. Flows in excess of 300 l/s will
overflow into the Deluge Lagoon and ultimately be returned to the pumping station via a low level
return.
9.90 The standards of service relating to flooding used in the design of the surface water drainage
network are based on a storms of a 1 in 30 year (3.33 AEP) return period and a 1 in 100 year (1%
AEP) return period (the latter with 30% allowance for climate change). There will be no flooding
from the drainage system during a 1 in 30 year (3.33% AEP) storm.
9.91 Any on site flooding caused by storms with return periods longer than 30 years (3.33% AEP) will
be contained on site. The water will be allowed to flood the paved surfaces in a controlled manner
such that water is shed away from buildings. Any floodwater will either be stored locally to return
to the drainage system when the storm abates, or will be allowed to flow through the site towards
the outfall and will be routed to avoid buildings. The system will be designed in such a way as to
prevent flooding of adjacent agricultural land up to the 100 year return period (1% AEP) storm
(with 30% allowance for climate change) and for storms of longer return period if it is practicable to
do so.
9.92 Outputs of the models developed to design the Drainage System are provided in Appendix 9G.
9.93 With the application of the control measures described above, the operation impact for pluvial
surface water is considered to be negligible.
5100935.404 Environmental Statement August 2011 141