Environmental Impact Assessments
The assessment of environmental impact of our storm overflow discharges is a principal element of investigating and classifying our storm overflow performance.
Where overflows have an adverse impact on the receiving environment, we will prioritise delivery of improvement schemes to address this as part of our AMP8 and future investment programmes. More detail on Storm Overflows is available here.
Our environmental regulators specify the criteria and methodology for investigating the environmental impact of our storm overflows. For AMP7, requirements were specified within the Storm Overflow Assessment Framework (SOAF) document. As a result, environmental impact assessments were completed for a significant proportion of our high frequency spilling storm overflows. For AMP8, SOAF will remain the agreed approach for investigating high frequency spilling sites within England. In Wales, the investigation and classification of storm overflow performance will be completed considering the requirements outlined within Natural Resources Wales (NRW) "How to Classify Storm Overflow Performance" guidance note, referred to as GN066.
Both SOAF and GN066 incorporate an environmental impact assessment stage that focuses on evaluating the effects of our storm overflows on the receiving environment. They require a comprehensive evaluation of aesthetic, biological, and water quality factors, including in situ surveys, sampling, and desktop modelling assessments. An overview of these key components is provided below:
1. Aesthetic impact:
Conduct site surveys to assess the presence of sewage-derived litter items, sewage fungus and review incident and complaint records related to the storm overflow. Alongside an amenity value and score is applied and aesthetic impact is categorised.
2. Biological impact:
Assess small organisms living on river and stream beds as ecological indicators. These organisms provide detailed insights into ecosystem health and samples are collected upstream and downstream of overflows. Seasonal samples offer a comprehensive assessment of river health over time. The Whalley Hawkes Paisley Trigg (WHPT) method evaluates the condition of benthic invertebrates, which are sensitive to changes in their environment. It uses parameters like the Number of Taxa (WHPT-NTAXA) and Average Score Per Taxon (WHPT-ASPT) to assess ecological health. Scoring is based on these parameters.
3. Modelled Water Quality Impact:
Conduct initial screening based on dilution to identify overflows unlikely to cause water quality issues. If dilution criteria are not met, perform water quality modelling to quantify the impact on parameters like total ammonia, BOD, and dissolved oxygen. Use varying complexity of modelling approaches based on the problem's complexity and potential solution costs, ranging from simple statistical mixing models to detailed hydrodynamic simulations. Impact scoring considers 99 percentile quality and Fundamental Intermittent Standards (FIS) – see Urban Pollution Management (UPM). The worst score from these assessments is used for classification.
The worst score from these assessments determines the water quality impact classification, ranging from "No impact" to "Severe."
To access existing available environmental impact assessment information please download the available spreadsheet and locate the relevant asset of interest. A link to a summary document, where available, is provided within the sheet. We are continuing to deliver further and updated ecological harm assessments and new information will be made accessible intermittently where available.
