Southern Water has told customers that the £30 million improvements undertaken over the last few years on its five stormwater outfalls into Langstone Harbour has resulted in a 50 per cent reduction in the number of days when releases have occurred between 2014 and 2015.

The water company was explaining the work it is doing to protect the environment around the Harbour - which it described as “a complex and emotive issue” - to assure people it has gone to great lengths to minimise the releases.

The overflows are required to protect homes and businesses from flooding. Combined wastewater treatment networks, which carry both wastewater and rainwater such as in Portsmouth, cannot operate as closed systems. Without this release valve, flooding of homes, businesses and highways would be inevitable during heavy rainfall.

Of the 49 outfalls that release stormwater – wastewater heavily diluted by rainwater, into the harbour, five are owned by Southerh Water and the releases from them are regulated and consented by the Environment Agency.

The utility has the capacity to store 47 million litres of stormwater at its existing storage tanks Fort Cumberland and Budds Farm. The stormwater is only released into the harbour, via the outfalls, in order to prevent flooding if the rain persists once the tanks are full.

Southern Water said the releases are always a last resort in order to prevent flooding and it is continuing to make improvements to minimise them and improve the way it reports them when they are required.

The £30 million improvements undertaken over the last few years has resulted in a 50 per cent reduction in the number of days when releases have occurred between 2014 and 2015. This includes a significant 85 per cent reduction in releases from Fort Cumberland.

In particular, Southern Water’s recently completed £20 million surface water separation scheme is capable of diverting up to 6,400 litres a second of rainwater to sea – reducing the burden on the network by a third and minimising the need for releases into the harbour.