Cause of the incident
The incident was caused by the extended period of hot dry weather which was then followed by significant rainfall on the 28 and 29 of July.
Prolonged dry weather in urbanised catchments leads to a build-up of contaminants from roads, in addition to organic matter such as leaves, silts, and sewage from misconnected properties.
All of these materials collect in the surface water drainage system and are released as run-off into rivers during the first few minutes of a significant rainfall event.
This ‘first flush’ caused by the rainfall is short-lived, but can have a major effect on water quality. The impact can be greater with prolonged warm temperatures and low river flows. This is because the higher temperatures increase the breakdown of organic matter and low flows provide less dilution of the run-off. The combination of these factors can result in a rapid drop of oxygen levels which, if extensive, can result in widespread fish mortality. This has been the case in the Beverley Brook catchment.
Base flows in the Hogsmill River and River Wandle were much higher before the rainfall events, therefore the impact on these two rivers was less significant.