Anthony McCloy recently presented at the CIWEM (The Chartered Institution of Water and Environmental Management) Northern Ireland conference on the practicalities of using nature based SuDS features such as Rain gardens to reduce Combined Sewer Overflow (CSO) overflow frequency.
For those who have not been following the recent press coverage; or are not familiar on how our sewerage pipe systems were installed up to the mid 1900s’; CSOs occur in areas where stormwater and sewage are combined in a single pipe system. During heavy (and in many places during small to moderate) rain events, the system becomes overwhelmed, leading to untreated wastewater discharges into natural water bodies.
The situation has gotten so serious that Water companies plan “a £11bn to reduce overflow spills, and remove more than a million tonnes of phosphorus from rivers…which would help reduce spills by more than 140,000 annually by the end of the decade”. https://tinyurl.com/58ec83xa
As many CSO overflows occur during small rainfall events, i.e. rainfall which occurs more frequently, it would logically follow that targeting day to day rainfall has the potential to greatly reduce spill frequency.
Rain gardens, which are designed to manage stormwater runoff, can help alleviate CSO issues. Rain gardens are typically designed with permeable soil and vegetation that promote water infiltration, but they can also be installed on clay soils with inclusion of an slow release outlet to the sewer. This helps to retain and slow down stormwater, allowing it to recharge the groundwater instead of overwhelming the sewer system. This reduction in stormwater can help prevent the sewer system from reaching capacity and overflowing.
The effectiveness of nature based solutions such as rain gardens to reduce CSO spill will depend on various factors. By strategically placing rain gardens in key locations, within enough coverage in catchments of influence, cities can reduce the frequency and resulting impact of CSO events.
As with any piece of infrastructure we build, whether it be grey or green; benefits are not a given and the effectiveness of rain gardens in reducing CSO spill frequency will be influenced by design. If they are poorly designed or maintained (or gardened!), water may not be slowed down and you will not realise the full benefit for CSO spill reduction. As importantly, if not maintained, raingardens will not be accepted by the general public, particularly if they degrade the visual quality of the area – how it looks can be as important as how it operates!
The key advantage that nature based SuDS has over the conventional grey infrastructure methods is their innate ability to deal with the ‘day to day’ rainfall that targets CSOs that are frequently overflow due to small rainfall events. These rainfall events are potentially more impactful to the environment when compared with larger volume spills due to the lack of dilution in the sewer and the receiving watercourse.
The benefit of nature based SuDS is their ability to deliver interception losses. This is where all of a ‘small rainfall event’ or, the ‘first part of a larger rainfall event’, naturally soaks into the raingarden, ensuring water from this rainfall event does not reach the sewer.
In the engineering sector, we tend to be relatively conservative and where a benefit or parameter cannot be guaranteed every time it rains, worst case scenario is generally assumed in design calculations and this is seen as an additional factor of safety in the hydraulic design calculations. In Anthony’s view, this misses the point in promoting and celebrating the hydraulic contributions of green blue infrastructure in dealing with day to day rainfall along with recognising the benefits to the combined sewer. In reality, the benefits will be there for the vast majority of rainfall events and this should be a greater driver for installation, rather than ignoring the benefit. From review of monitoring and research the hydraulic benefits can be much greater than the 5mm of rainfall which is normally considered for interception.
Complementary Measures: Rain gardens can be complementary to a wider CSO spill reduction strategy, which may include; infrastructure upgrades, real time control within the sewer system such as CENTAUR® (utilising sewer capacity already inherent within the sewer system) and the implementation of other sustainable stormwater management practices such as rain harvesting.
In summary, rain gardens along with the wider suite of nature based SuDS can be an instrumental tool in reducing CSO spill frequency by targeting smaller rainfall events. Their effectiveness depends on proper design, strategic placement and rollout over the catchment of influence on the CSO. They can also form an effective as part of a broader strategy to address CSO issues, and can work particularly well in dealing with day to day rainfall!