SuDS - Holding Back the Floodwaters

The National Infrastructure Commission (NIC) has published a report into mitigating surface water flood risk in England, which offers a sobering assessment of the preparedness of large parts of the country to flooding, and the levels of protection currently offered to vulnerable communities.

The report states that a combination of climate change and increased urbanisation means that, if no action is taken, 600,000 homes and businesses in England are likely to experience surface water flooding in the next 30 years.  It calls for urgent action, including £12bn investment in above- and below-ground drainage infrastructure as well as stricter controls over new property developments and better, more joined-up, governance and management strategies.

It suggests that the primary reasons for a rise in flood risk are a combination of: extreme weather caused by climate change; rising pressure on existing drainage systems due to creeping urbanisation; and the spread of impermeable surfaces including paved garden areas. All these factors, the report warns, will pushing thousands more homes into the high-risk category.

The delay to the implementation of Schedule 3 to the Flood and Water Management Act 2010, enacted by the Government in response to the Pitt Report, is also raised within the report as a cause for concern.  Schedule 3 made sustainable drainage systems a legal requirement for most new developments.  It also amended the right to connect to public sewers.  The report’s authors recommend the government implements Schedule 3 by the end of 2023 and updates its technical standards for sustainable drainage systems.

Surface water flooding (also known as pluvial flooding, or more commonly ‘flash’ flooding) is typically caused by heavy rainfall or storm water that overwhelms the existing drainage networks. 

The impact of flash floods can be both extremely costly and very destructive, causing lasting damage to local communities.  And, because surface water flooding routinely causes sewers to overflow, the water is likely to be contaminated with pollutants which then pass into rivers. 

Floodwater management policies aim to protect at-risk areas from storm water through containment, safe removal, and by creating suitable spaces for the water - recognising that rainwater is also a valuable source of freshwater. 

The NIC’s report includes recommendations around mitigating the impact of urban spread by upgrading the existing drainage systems, both above-ground (such as green gullies, rills and ponds) and below-ground (typically pipes and sewers).

The acknowledgement of the value of both above- and below-ground drainage solutions and their roles in supporting the sewer system is welcome.  Both solutions have a key role to play in flood management schemes, and while natural solutions have become increasingly popular in recent years we believe that decisions as to what techniques should be employed should be directed (first and foremost) by the requirements of each site - including the ease of service and maintenance - rather than ideology.

That is why we recommend that any new drainage system is designed by qualified drainage professionals.  Similarly, we suggest that the design of a system should start at an early stage in a build project (rather than at the end), to allow drainage engineers to make reasonable adjustments as required to accommodate expected loads and available space.

Sustainable drainage (SuDS) schemes are designed to manage peack storm water flow rates by containing the excess flow at source before releasing the water at a specified reduced rate, during and after the period when the rain eases.

A sustainable management train (aka SuDS train) is a system of linked drainage products or elements (above- and/or below-ground) where each element performs a slightly different function of slowing or containing the water temporarily until it is discharged into the ground. 

A SuDS train is also designed to clean the run-off water, removing any pollutants and hence improving its quality.

The report also highlights the importance of ongoing servicing and management to maintain drainage systems.

All drainage techniques, whether natural or proprietary, should have a management plan in place to ensure each individual technique remains effective over the lifetime of a development (>100 years). 

This isn’t a step that can be passed over.  It might be tempting, for example, to think of natural solutions as self-managing, but that’s not the case. Swales, rills, ponds, gullies etc. all require regular clearing, desilting, mowing and cutting-back of vegetation if they are to be effective in managing large quantities of water.  Meanwhile, proprietary products such as attenuation tanks or oil separators, need routine emptying and de-silting.

Primary responsibility for servicing and maintaining drainage systems is usually the remit of the individual or organisation that has ‘adopted’ them.  Unfortunately, the report’s authors advise that “planning applications often lack clear maintenance agreements for sustainable drainage systems, and local authorities can lack the resources to monitor and enforce compliance”.  In other words, systems that should support at-risk communities are failing because responsibility for managing these assets is either never properly established, or those who have responsibility are perhaps unwilling or unable to fulfil their obligations.

In our view, ease of management and maintenance are two of the most crucial steps in designing and developing a long-term flood management strategy for any community, and no technique should ever be regarded as fit and forget….