PRIORITISING FLOODING INVESTMENT IN THE NORTHUMBRIAN WATER REGION
Richard Woodhouse 1 and Nicholas Anderson 2
1 Northumbrian Water 2 MWH, UK
(free)
During the period April to October 2005, 6 major rainfall events resulted in over 600 properties
in the Northumbrian Water region suffered sewer flooding for the first time. This is 14 times
the average for the previous 7 years. Northumbrian Water has a legal obligation to report
both the flooding mechanism and frequency to OFWAT.
As part of the annual reporting process to OFWAT, NWL is required to report both existing
and first time flooded properties against DG5 indicators. Where hydraulic overload is the
confirmed flooding mechanism, the properties are to be added to the DG5 ‘At Risk’ Register
under an appropriate category, 2 in 10, 1 in 10 or 1 in 20 years, indicating the potential
frequency of risk.
MWH was engaged to support NWL in the delivery the flooding investigation programme
through the delivery of a number of Cause Report Investigations. These investigations are a
rapid assessment method focusing on the risk of service level failure.
The extents of the Cause Reports included collating all existing information, gathering
additional data to undertake an abridged hydraulic analysis with a view to producing an output
with the necessary level of understanding to allow for inclusion in the annual report submitted
to OFWAT and to allow NWL to focus investment to provide best value to customers. The extent of each Cause Report comprised the following:
• Collation and review existing asset data
• Collection of addition survey and customer experience data
• Hydraulic modelling including calibration
• Simulation and hydraulic analysis
• Flood frequency assessment
• Identification of possible mitigation measures
• Identification possible site specific construction constraints
• Identification of other potential risk areas
• Indicative cost estimate
The paper will describe the Cause Report process and how the development of a standard
approach was vital to ensuring a ‘level playing field’ was applied to all outputs and the benefit
to NWL. It will describe the process of collating data, how flooding mechanisms were
investigated, where possible mitigation measures were considered and how a costing
methodology was developed.
KEY WORDS
Flooding prioritisation, sewer flooding, flood risk, cause reports
