Following attendance at the susdrain hosted visit to the UK’s largest retrofit SuDS scheme in Mansfield, Jacqueline Diaz-Nieto, Associate SuDS Engineer, shares her thoughts on the successes and challenges of this vast, multi-supplier project.
Severn Trent don’t do things by halves, and this was evidenced at the open event this week, where SuDS enthusiasts and professionals were invited to learn about the successes and challenges of the UK’s biggest SuDS retrofit scheme. In just three and half years, Seven Trent achieved delivery of getting 353 small and distributed SuDS interventions, through a supply chain that involved engaging numerous consultants, contractors and universities: no small feat herding that many big cats along with the smaller ones!
The day started with a visit to see a humble 3% of the SuDS interventions delivered throughout the town. Our group, led by SuDS champion nominee, Angus Smith, started with the scheme in the town centre. With nature coping well with the dry weather, and witnessing local workers enjoying the public space, our group were enlightened by Zac Tudor, the landscape architect behind Sheffield’s Grey to Green, about the secrets to the success of the vibrant and thriving planting scheme. It was clear that these city centre SuDS had turned a redundant car park, whilst intercepting drainage from the bus station, into a functional and well-loved public space.
From there we went to a raingarden build-out in a spot that was previously notorious for water to pool and cause a nuisance to highway users, as well as flooding the nearby low-lying garages. Our guide for this raingarden, the contractor from Kier, showed us photos of the historic flooding precisely at the location of the raingarden, and no one can negate that this opportunity to change a space where water pools and is a nuisance, into an attractive build-out by the highway, that can capture the nuisance water as part of its function, is a no-brainer.
The morning took us to see a number of larger interventions ranging from detentions basins that took advantage of housing estates with separate systems going into combined systems, that were plumbed into a retrofit basin, through to those collecting primarily highway drainage alongside green spaces converted into functional SuDS off the highway, to solutions which won the buy-in of allotment tenants by providing enhancements to their experience.
Heading back to the council chambers for an afternoon of talks, the air was charged with excitement. Witnessing firsthand what had been accomplished in Mansfield, and with several flood risk management authorities in attendance, there was a palpable sense of inspiration. Everyone seemed eager to return to their own regions and apply the same drive to make a difference.
With notebooks open and pens poised to take down the gold nuggets of success, we learnt more about the commercial design and delivery processes, key metrics, and the innovation and research resulting from this project. Reflecting on what I heard, my personal view is that Mansfield, as the demonstrator project that it was, has proven the possibilities and clear benefits of catchment wide retrofitting of SuDS.
These small interventions, dispersed throughout a huge watershed, all add up to provide a noticeable cumulative effect. To quote former SuDS champion, and in my eyes one of the founders of the SuDS movement in England, Sue Illman, Mansfield “got nibbling”, and by goodness did it nibble away at the flood risk by delivering a whopping 31 million litres of storage, by finding the undeniable opportunities to intercept overland flow and use up that storage.
The only saddening aspect about it, is the cost and funding stream. Mansfield was funded by a one-off funding stream created by Ofwat following the pandemic, to build-back better and
greener, known as Green Recovery. This funding stream allowed a bespoke cost benefit assessment to be developed which enabled the delivery at pace by speeding up the initial project stages of evidencing a business case before construction can commence.
Financial success and governance were provided by a cost-benefit ratio which was seen to be quick and dirty, and called the “rule of thumb”, which negated the need for a long, drawn-out process developing reliable hydraulic models to demonstrate upfront that the interventions will provide flood risk alleviations. In Mansfield, a pragmatic approach was adopted using the “rule of thumb” which in essence was an excel based SuDS volume calculator. If it could be demonstrated, using knowledge of the topography and how water will flow, that an intervention can intercept a minimum amount defined by a cost beneficial threshold of overland flow, then the SuDS intervention could proceed.
I found myself asking, and posed the question to the panel, whether this pragmatic approach should be advocated and allowed within the flood risk funding decision makers such as the FCERM (Flood and Coastal Erosion Risk management) funding streams provided by Treasury. It could fast track projects into delivery by avoiding excessive model-building by the rightly so, perfectionist hydraulic modelling community, who often take great pride, at the expense of time and cost, in developing the best and closest model they can to represent reality, before they put their name to it and allow decision makers to use it as a basis to unlock precious construction funding.
Finally, the only downside to the Mansfield project I heard discussed, amongst the excitement of these beautiful green and functional spaces, was the cost. You could the sense the cash-strapped local authorities’ hearts sink as they learnt about how inflation in recent times had seen the total cost of these scheme soar.
Is there anything in the commercial strategy adopted by a multi-national listed company, that maybe the smaller authorities don’t necessarily have to follow, to deliver more cost-effective solutions. As mentioned in my introduction, Severn Trent engaged a large number of supply chain partners, with multi-tier contracting and complex project management and governance arrangements. I am hopeful that this aspect of Mansfield doesn’t necessarily have to be emulated by smaller authorities wanting to copy Mansfield’s approach.
Could an alternative approach be to work through a single source supplier, potentially with a local connection, appointed to a dedicated single intervention or group of interventions connected by the community (e.g. a street). Could this offer a cost-effective solution that would also go some way to addressing one the points raised by Mansfield about the need for successful community engagement and buy-in?
I look to the future with great anticipation on how the learnings from Mansfield will influence the future of retrofit SuDS. I’m committed to continuing to pursue the effective implementation of SuDS alongside like-minded colleagues and feel that there is no better time than now to really push the boundaries.
