The race to create sewer space

Optimising hybrid solutions to utilise Sustainable Drainage Systems (SuDS) more effectively.


Hybrid solutions are being increasingly considered by Water and Sewer Companies (WaSCs) in their approach to reducing combined sewer overflow (CSO) spills.

These solutions are a combination of traditional grey infrastructure (storage tanks, sewer diversions etc) and green solutions such as Sustainable Drainage Systems (SuDS).


Understanding SuDS

SuDS aim to utilise natural processes to attenuate and store surface water runoff, reducing drainage to the sewer system. This at-source surface water management can fully or partially disconnect impermeable area from the drainage system. SuDS are therefore now another arrow in WaSCs’ arsenal to achieve the targets set in the Storm Overflows Discharge Reduction Plan (SODRP), to reduce the 300,000+ CSO spills per year and continuously reduce the amount of surface water connected to their combined sewer network. 

SuDS solutions can satisfy other commitments too, increasing environmental pledges and commitments by WaSCs can be mutually achieved through their use e.g., moving towards net carbon zero. They are also scalable above-ground solutions which can increase the resilience drainage systems, providing extra storage for uncertain climate change scenarios. Additionally, they will soon be a mandatory solution for new developments, with the Water and Environment Act’s Schedule 3 preventing these flows from being connected to the sewer system. 

The challenges

WaSCs must meet these CSO spill targets quickly to avoid significant fines, for example the SODRP dictates that “Storm overflows will not be permitted to discharge above an average of 10 rainfall events per year by 2050”. The capacity must be increased quickly, creating a race to create sewer space. Building large scale hybrid schemes with SuDS systems is relatively in its infancy. However, given the time pressure to meet CSO spills targets, there is limited time to learn about how to optimise the combination of solutions and monitor their performance. Therefore, there is a risk in the level of investment required.

In this race, CSO assessments and their given solutions are being produced on mass, and designs are sometimes rushed and disconnected from the outcomes they are trying to achieve. WaSC-built SuDS, in principle, are aiming to prevent surface water runoff from impermeable areas (IA) draining to sewers. Therefore, in SuDS design there should be an understanding of what IA can drain to each intervention. 

To assist in large-scale SuDS planning, some opportunities mapping tools are available. However, these have poor hydraulic properties to understand how they may reduce runoff from catchment-specific IA and therefore reduce demand on the urban drainage system. A method to understand, plan and model the optimal cost-effective hybrid solution for a catchment is therefore not currently available. 

Introducing Bloom

RPS are rising to this challenge, we have developed a new tool, Bloom, which can not only map SuDS opportunities but also relate specific local impermeable area and system types to each intervention. Cost effectiveness, determined by an intervention predicted hydraulic impact, can be predicted with greater accuracy. In combination with RPS’ automatic storage sizing tool, PLATO, a cost-optimised hybrid solution can be determined with confidence for any CSO. 

This tool will also be able to find the optimal solution based on a range of factors, such as land ownership and return period design standard, and how they are screened or prioritised based on user preference. Furthermore, to ensure green solutions are accurately optimised to reflect their overall benefit, natural capital values are also attributed to each intervention. Natural capital can also be assessed based on user preference to ensure only the environmental targets the user wishes to be accounted for in the scheme are optimised, whilst still determining the lowest cost hybrid solution.

For example, a WaSC may have a commitment to create 0.5ha of green SuDS, in which they want to prioritise net carbon zero. The automated optimised solution will therefore include a storage tank and a minimum of 0.5 ha of specific SuDS types which create green space and maximise carbon sequestration to reduce a CSO’s spill frequency to a specified annual target e.g., 10. 

Bloom will provide a specialised hybrid optioneering approach to find viable solutions which satisfy hydraulic needs, budgets, feasibility, and wider environmental goals before refined model testing and design.  


Water is vital to human life, health, and prosperity. Managing it is one of the most complex issues that faces our world today. We work alongside world-renowned water custodians across the globe, providing technical and commercial expertise to sustainably utilise, manage, and conserve water resources.

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