Evidence over Emotion - the value of modelling storm overflow
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| Storm overflows, or engineered overflow points, were originally designed to act as relief points on wastewater networks to prevent flooding during rainfall. They are intended to operate in line with environmental permits, which have constraints related to when and how they should spill. In the UK, the Storm Overflow Assessment Framework (SOAF) looked to improve the understanding of the impact of overflow spills on receiving waterbody water quality through undertaking investigations and modelling. This enabled the identification of overflows where a cost-beneficial investment could be developed to reduce the impact and improve water quality. In the last two years, public perception of overflows has grown significantly. Spills are regularly making the headlines and the public are reported to be outraged. The Environment Act 2021 legislates that all inland and costal storm overflows, regardless of their impact, should achieve 10 spills per year by 2050. This has generated a multi-billion-dollar investment programme, which will inevitably be funded through increased customer rates. This presentation will share some examples of where modelling has provided valuable insights into the performance and impact of storm overflows. It will then go on to explore whether public perception (the emotion) of the impact of overflows has more weight, and is therefore more valuable, than the significant modelling datasets (the evidence) that have been produced. An assessment of the performance and impact of almost 130 storm overflows in the Northumbrian Water operating area of north-east England has been completed following the SOAF methodology. This included wastewater network model upgrades and correlation with in-sewer level monitors to determine the suitability of the model for use in the assessment. Surveys were completed to evaluate spills impact in terms of aesthetic and ecological impact. The assessments identified that there are a number of factors influencing the degree of measurable impact from storm overflows, including; sewer dry weather flow, river Q95 (dry weather flow) and the presence of other overflows or continuous or other discharges. Not all storm overflows were predicted by the modelling to be causing significant ecological harm, even when spilling more than 40 times per year. For these types of overflows, providing a spill frequency reduction scheme, at significant cost, is unlikely to provide sufficient benefits for the investment to be deemed cost-beneficial. Conversely, around 20 overflows were identified where a spill reduction scheme would deliver water quality benefits. A short-list of five overflows were identified for fast-track delivery. The Environment Act amendment to reduce all overflows to 10 spills by 2050 could result in significant investment at hundreds of storm overflow locations where there will be no measurable water quality benefit. The modelling that has been completed has been largely ignored in the development of the amendment and is seen by many in the water industry as a political response to the emotive headlines around the topic. Whilst it is agreed that there is work to be done to improve the overflows situation, there is potential for the intelligence created by modelling to be overlooked. |