The spill thresholds currently applied are shown in the table below and depend on the number of years of Event Duration Monitoring (EDM) data available. As more data becomes available a tighter threshold of spills per year is adopted.
In the period 2020-2025, 727 storm overflows are currently being investigated. As the provision of storm overflow monitoring data increases more overflows are being identified as crossing the spill thresholds and will be put forward for the WINEP. If the investigation shows that it is cost-beneficial to reduce spills, a scheme would be secured through the WINEP.
The Environment Agency also investigates storm overflows based on evidence of environmental impact and/or non-compliance with permit conditions.