The client’s requirements were to better understand flood risks during extreme events with up to a 1 in 1,000 year (0.1%) annual probability and an assessment of the feasibility of adopting a range of flood resilience or resistance measures. These measures needed to be practical, cost-effective, flexible and adaptive to each individual site in order to mitigate against the potential impacts of flood water ingress.
It was understood that, for some sites, an initial review by a third-party flood contractor had recommended a raft of uniform measures across the site without evidencing whether these were proportional to the flood risk by taking account of the potential route of flood water conveyance, frequency and depth of flooding.
A tailored assessment of risk
Ramboll was brought in as a flood risk expert to evaluate 65 sites and propose strategies to mitigate risk, with a further 11 locations later brought into scope. These were sites that had been flooded previously, had been identified by the client as being at risk of flooding, or presented a risk of significant potential disruption should a flood occur. The objective was for the client to be able to make reasonable effort to protect the critical assets, or the function they provide, from the growing threat of flood due to climate change.
Ramboll undertook an initial desk-based study to evaluate the baseline flood risk at each site, looking at the probability, frequency, depth and route of flooding, also taking account of the potential for increases to flood risk due to climate change.
Site inspections were then undertaken in order to assess the vulnerability of each asset to internal ingress of waters and to develop a range of mitigation strategies. The surveys focused on the site topography and external layout, including buildings structure, the presence and location of critical operational assets, existing flood mitigation infrastructure (e.g. sump pumps, and flood doors), and external features within the site boundary and surrounding land/properties of potential relevance to the flood risk appraisals (e.g. significant gradients, surface water courses, existing boundary walls or embankments serving to mitigate flood ingress).
The surveyors were able to identify potential routes for ingress of flood waters including doors, windows, air bricks, vents, emergency exit hatches and other openings to basement levels, as well as areas of buildings or features with potentially poor structural resilience to flood waters that would warrant a building/structural condition survey. Topographic surveys were also undertaken in order to determine accurate height measurements (in metres above sea level) for specific locations, including ground floor and basement levels, and possible water entry points such as air bricks and cellar hatches.
Insight-led strategy to mitigate risk
Ramboll was able to present a detailed picture to the client of where their flood vulnerabilities were across the subject sites so that funding could be sought and prioritised for certain assets. A risk ranking approach was applied to the portfolio, using a ‘traffic light’ system to rank the identified risks of internal ingress of flood waters.
We were able to indicate where and how this risk could be mitigated using measures such as watertight doors, demountable flood barriers, external walls, raising air vents or re-siting critical equipment.
Our report recommendations provided a conservative strategy to protect the assets from flood or to maintain their functionality, as well as a more ambitious approach should they seek to further reduce that risk. High level cost estimates for each strategy were put forward to help aid the client’s decision on which approach to take, with the client’s contractor responsible for providing the detailed specifications and costs during subsequent site visits.
Focusing efforts where they will make the biggest difference
Like many businesses, our client was aware that severe weather events could have a significant impact on the operational status of their assets and extreme flood events are likely to occur with more frequency due to climate change. There was an identified vulnerability across key assets within their estate, but this had not been accurately assessed and without such assessment it was not possible to make an informed decision on where and how to manage the risk.
Whilst it could have been possible for a flood defence contractor to manufacture and install measures at each asset, such an approach without initial assessment of flood risk factors could have required significant expense and resulted in the cost of flood resilience not being proportionate to the frequency and magnitude of likely flooding. Through our tailored, risk-based flood resilience/resistance strategy for each asset, the client was able to safeguard their infrastructure without disproportionate capital expenditure.