Colin Whittingham, RSK Land and Development Engineering (RSK LDE) director of hydrology, discusses sustainable drainage systems and their role in mitigating increasing flood risk
If you’ve worked in development over the past decade, you’ll have felt the shift. Flood risk has moved from a technical appendix to a central pillar of planning. The direction of travel is clear: tighter planning scrutiny, greater emphasis on surface water and a slow but meaningful evolution of sustainable drainage systems (SuDS) from ‘nice to have’ to essential infrastructure.
What’s less clear and more problematic is how consistently that direction is being applied.
Flood risk: From rivers to rainfall
Historically, flood risk in planning was dominated by fluvial and tidal considerations, flood zones, sequential testing and defending development in areas at risk from rivers or the sea. That hasn’t gone away, but it’s no longer the whole story.
On several recent projects I’ve worked on, sites located entirely within low-risk Flood Zone 1 have still faced significant challenges due to surface water. In one case, a large residential scheme stalled at the pre-application stage because pluvial modelling revealed extensive ponding across key development parcels. The issue wasn’t proximity to a river; it was how rainfall interacted with topography and existing drainage constraints.
This reflects a broader shift in planning policy and guidance, where surface water flood risk is now being treated with comparable weight to fluvial risk.
However, the application of that policy is far from consistent. Some lead local flood authorities (LLFA) now expect detailed modelling, exceedance routing, and early integration into layout. Others remain largely focused on discharge rates and storage volumes. The result is a fragmented system in which the same site can be designed very differently depending on where it sits, and where early design decisions are often driven by perceived acceptability rather than by best practice.
The reality of ‘defended’ flood risk
Another area where practice has evolved is in the treatment of defended floodplains. Sites behind flood defences were once seen as relatively straightforward to justify. Now there is far greater scrutiny of residual risk, for example, what happens if those defences fail or are overtopped?
In a brownfield regeneration scheme within a defended floodplain, we found that relying solely on the presence of defences was insufficient. The design had to incorporate safe access and egress routes, finished-floor-level strategies, and a clear understanding of breach scenarios. The conversation with planners and stakeholders was less about “can it be built?” and more about “is it genuinely safe over its lifetime?”
Even here, though, there is inconsistency. Some authorities push hard on residual risk and emergency planning, while others are more comfortable relying on the existence of defences. That variability creates uncertainty and, in some cases, encourages a more optimistic interpretation of risk at early stages than is later borne out through detailed assessment.
Brownfield complexity: Groundwater and contamination
Flood risk is rarely a single-issue constraint, particularly on brownfield land. On a former industrial site, groundwater emerged as a critical factor, not only in terms of flood risk but also in terms of contamination.
The challenge was balancing infiltration-led SuDS (strongly encouraged by policy) with the risk of mobilising contaminants. Ground investigations showed variable conditions, with some areas suitable for infiltration and others requiring lined systems.
The eventual drainage strategy was a hybrid: targeted infiltration where safe, combined with attenuation and controlled discharge elsewhere. It’s a good example of how SuDS design has become more nuanced, less about applying a standard solution and more about responding to site-specific constraints.
It also highlights a wider tension in policy. National guidance promotes infiltration as a priority but often fails to fully reflect the realities of brownfield delivery. In practice, demonstrating why infiltration isn’t feasible can be as significant a task as designing the drainage system itself.
The evolution of SuDS standards
The National Standards for SuDS have been around for some time, but their application has matured. Early on, compliance could be fairly basic: demonstrate greenfield run-off rates, provide some attenuation and include a token swale or basin.
That approach is no longer sufficient.
Authorities now expect SuDS to deliver multiple benefits: water quantity, water quality, biodiversity and amenity. On a recent scheme, what began as a purely functional attenuation basin evolved into a multifunctional landscape feature, incorporating wetland planting, informal play space and accessible green infrastructure.
But again, delivery is uneven. Some authorities actively push for multifunctional, above-ground systems integrated into the layout and public realm. Others remain content with underground storage solutions that meet runoff criteria but offer little else.
The standards themselves are not the issue – they are broadly sound. The challenge is that they are interpreted through a planning system that still leaves significant room for negotiation, particularly regarding land take and viability.
Planning applications: Raising the bar
All of this is feeding into a more demanding planning process. Flood risk assessments and drainage strategies are being interrogated in greater detail, often earlier in the process.
Pre-application engagement has become essential. On multiple schemes, early dialogue with LLFAs has helped identify issues, whether that’s surface water constraints, groundwater behaviour or adoption challenges, before they become blockers at the application stage.
However, even this stage is not immune to inconsistency. Pre-application advice can vary significantly in scope and certainty, and positions taken early on are not always maintained through determination. That lack of continuity can leave applicants navigating shifting expectations rather than working to a clear, stable set of requirements.
There’s also a growing expectation for clarity and transparency. It’s not enough to present a compliant design; applicants need to clearly explain how risks have been identified, assessed and mitigated.
Where next?
Looking ahead, the trajectory is unlikely to reverse. Climate change allowances are increasing, rainfall patterns are becoming more intense, and planning policy continues to emphasise resilience.
But if the system is to genuinely improve outcomes, consistency needs to catch up with ambition.
For developers, that means recognising that flood risk and drainage cannot be left until late in the design process. For authorities, it means applying policy more consistently and earlier, reducing the scope for negotiation on issues that are, ultimately, fundamental to site safety and performance.
For practitioners, it’s an opportunity. The projects that work best are those where flood risk and SuDS are not treated as constraints to overcome but as frameworks to design within.
Done well, they don’t just manage water, but they create better places. But when done inconsistently, they create uncertainty and, too often, compromise.
Colin Whittingham is a director at RSK Land and Development Engineering (RSK LDE). He has 25 years’ experience in leading hydrology, flood risk and drainage projects from major Nationally Significant Infrastructure Projects to small-scale developments. He is a Chartered Water and Environmental Manager through the Chartered Institution of Water and Environmental Management (CIWEM) and a Practitioner member of the Institute of Sustainability and Environmental Professionals (ISEP), demonstrating commitment to professional standards and best practice in water and environmental management.
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