Wellington Road is a residential street in the town of Greenfield, within the borough of Oldham. This area of Oldham has a history of surface water and sewer flooding, with flood events in 2016 and 2018, owing to water discharge from the Golburn Clough ordinary watercourse.
Our approach to developing the Outline Business Case started with a site visit and gap analysis of the existing data held by the Council. This exercise was used to develop the scope of a CCTV survey of the culverted sections of the Golburn Clough ordinary watercourse that contributed to the historic flooding.
We used the site investigation and data supplied by Oldham Council to develop a hydraulic model that was used to quantify the probability and consequence of flooding within the study area. An economic assessment was subsequently undertaken with the outputs from the hydraulic modelling that was used to quantify the monetary damages that could be expected to result from flooding over the next 50 years. The assessment accounted for both present day conditions and the anticipated effects of climate change.
Community and stakeholder engagement
As part of our work, alongside Oldham Metropolitan Borough Council we developed working relationships with key stakeholders, including local landowners, affected residents, United Utilities, and the Environment Agency.
Our options appraisal took a strategic review of all potential hard and soft engineering solutions and integrated these with stakeholder suggestions in a visual format for public engagement.
The options included Natural Flood Management and Sustainable Drainage Systems designed to attenuate flood flows and were evaluated using a Multi-Criteria Analysis (MCA). Options that scored highly in the MCA and were considered to meet the wider project objectives were taken forward on the shortlist for further hydraulic modelling, economic, social and environmental appraisal.
“AECOM has worked on a number of business cases for Oldham Council over the past few years and I could not recommend them enough. The level of service is second-to-none.”
– Jason Denniston, Project Manager, Unity Partnership on behalf of Oldham Council, April 2022
A lifecycle-based approach
The appraisal of the potential flood risk management measures developed a leading ‘Do Something’ option that would deliver £4,623k in benefits with a 1:40 year Standard of Protection. The scheme was determined to have a whole life cost of £570k, with a benefit-cost ratio of 8.1. Further stakeholder support was obtained to secure support for the leading option. Outline design was undertaken and whole life cost estimates were prepared for the short-list options using information from SPONS.
Risk provisions and optimism bias were factored into the whole-life cost estimating to confirm the selection of the Preferred Option. On completion of the technical, economic and environmental options appraisal, we prepared the Outline Business Case for the Preferred Option in line with the five-case business model for public spending proposals.
Securing funding to safeguard a community
We supported the Council with a successful bid for local levy funding from the North-West Regional Flood and Coastal Committee (RFCC). The Outline Business Case was subsequently independently assured by the Environment Agency and granted funding approval for detailed design and construction.