Using hydraulic modelling to mitigate impacts on rivers and coastal waters

Millshaw Combined Sewer Overflow - Discharge Reduction

Working as part of the Yorkshire Storm Alliance—a strategic partnership with Yorkshire Water and Ward and Burke—we helped deliver £1 billion of improvements across the county to reduce storm overflows and improve river water quality.

Storm overflows act as a relief point for the sewer network during periods of heavy rainfall. They discharge when the system is at capacity to prevent water backing up and flooding homes and gardens, but this has environmental impacts on river and coastal waters. Looking to reduce the frequency of overflow discharges, Yorkshire Water asked us to create outline designs for an upgrade of the Millshaw combined sewer overflow (CSO) in Beeston.

Yorkshire Water proposed building a new water storage tank and asked us to assess how the CSO would perform once it was in place. The main challenge was making sure that when the new tank was at full capacity, it would not cause detriment to the operation of the CSO nor lead to premature discharges. This required a clear understanding of how flows and water levels behaved across the network, confirming if the design would work as intended or if it needed changes.

We used our in-house steady state Hydraulic Analysis and Design System (Hades) to assess the Millshaw CSO. While CSOs operate dynamically, using steady-state analysis software allowed us to model key parts of the CSO and proposed storage tank (and relevant connections) and conduct a detailed and conservative analysis of the system, allowing the risk level to be accurately captured. The modelling helped us identify hydraulic constraints, check performance limits, and confirm that the proposed tank would not have a detrimental effect on the existing CSO.

We completed the hydraulic assessment in March 2025 and confirmed that the proposed design was effective. The scheme is now under construction, with the modelling giving Yorkshire Water confidence to move forward with the design, meet regulatory requirements, and deliver environmental benefits through reduced overflow discharges.