In part two of a three-part series, Stuart Crisp, UK manager of Advanced Drainage Systems (ADS), examines how early design decisions, maintenance considerations and realistic assessments shape the success or failure of Sustainable Drainage Systems (SuDS).
The recently updated planning guidance on flood risk and coastal change rightly states that SuDS layout and function should be embedded at the earliest stage of design. Doing so allows projects to maximise all four SuDS pillars – water quantity, water quality, biodiversity and amenity – before subsequent ‘value engineering’ strips away critical components.
The industry has seen many schemes where treatment devices included at concept stage quietly diminish during procurement. These omissions have consequences not only for environmental performance but also for long-term maintenance requirements and cost.
Water quality, although one of the four pillars, receives only cursory attention in the guidance. Pollution loads from commercial sites, logistics hubs and roads can be significant. Without engineered pre-treatment, vegetative SuDS assets such as swales or ponds can act as unintended pollution sinks.
Over time, accumulated sediments and other contaminants can reduce hydraulic capacity, impair ecological value and complicate maintenance. Some SuDS ponds end up containing toxic sediment requiring excavation, specialist disposal and heavy plant – with the added risk of mobilising the captured pollutants – none of which may have been accounted for in the original design or cost plan.
Similarly, assumptions that natural SuDS are “easier to maintain” can be misleading. In many cases, engineered systems – when designed with accessible chambers, removable filter media or isolated sedimentation zones – can be safer, simpler, cheaper and more predictable to maintain than vegetated SuDS assets.
Technologies such as arch-shaped attenuation structures with in-built sediment capture allows routine cleaning with standard sewer-maintenance equipment, reducing disruption and lifecycle cost.
The guidance also proposes infiltration as the preferred discharge method (N.B. this is inconsistent with the recently published non-statutory National Standards for Sustainable Drainage Systems, which cites Priority 1: for rainwater to be collected for non-potable use), encouraging permeable paving, soakaways and infiltration basins wherever feasible. While logical in principle, this introduces tension with adoption requirements.
Under the Design and Construction Guidance, water companies will only adopt SuDS components that exhibit a clear inflow and outflow. Pure infiltration features do not meet this criterion, making them unattractive to adopting authorities. As a result, a scheme designed around infiltration may leave developers without a viable adopting body, complicating long-term stewardship.
Groundwater conditions add further complexity. Areas experiencing groundwater rebound, or likely to do so over a development’s lifetime, may lose infiltration capacity entirely. Designers must therefore consider future abstraction patterns, climate-driven changes and the risk of groundwater-induced surface flooding.
The guidance requires planning authorities to be satisfied that robust maintenance and adoption arrangements exist for the lifetime of the development. Given that different SuDS components vary dramatically in maintenance needs, accessibility and long-term reliability, this is a crucial but often under-scrutinised part of the planning process.
The practical issues explored here highlight the difficulty of delivering effective SuDS when key elements of policy and adoption guidance remain fragmented. Part three turns to the wider regulatory landscape – including the long-on-the-horizon Schedule 3 of the Flood and Water Management Act 2010 – and considers why statutory SuDS approval is urgently needed.
