Weymouth and Portland sewers unable to cope with demand from new housing developments

That's according to Wessex Water, which manages Dorset's sewer systems. The issue of sewer incapacity has been brought to light after recent flooding and heavy rain saw raw sewage discharged at swimming spots and rivers along Dorset's coastline, including at Weymouth Beach.

Under heavy storm conditions sewer capacity can be exceeded and built in safety valves, called storm overflows, permitted by the Environment Agency, can operate to prevent sewer flooding.

According to the water utilities company the Weymouth area has medium risk for sewer incapacity, and there are frequent spilling storm overflows through screens into Weymouth Bay.

Wessex Water map showing the number of spills from sewage overflows last year

Wessex Water is following a methodology to investigate and make improvements - however the company has warned that new developments proposed in Dorset Council's Local Plan are likely to put additional pressure on the system - and that upgrades will be needed to the sewer infrastructure, paid for by developers.

• Click to view interactive map showing where sewage has been pumped into local rivers

A Wessex Water spokesman said: "The Local Plan allocates large site developments at Chickerell and Upwey and further development in the town centre and sites at Redland and Broadwey.

"Redevelopment at the former Osprey Quay and Hardy complex have contributed to the housing needs for Portland. The Issues and Options consultation (2017), explores further development in Chickerell, Wyke Oliver Farm, Upwey and South of Wey Valley and the eastern end of Western Street and south of Southwell on Portland. Options are explored to maintain or vary the recent growth in Weymouth and Portland of 150 and 45 dwellings per annum respectively.

• Read more: Do not swim! Sewage emptied into water at 12 Dorset beaches

"To support this growth and reduce flood risk, local improvements will be needed to ensure the network can accommodate this increase in flow."

Developments can also increase rainwater runoff into drainage networks, causing an increase risk in surface water and potentially sewer flooding. "We have a policy that surface water connections to the foul sewer system is not permitted," the spokesman added.

Sewage was emptied into bathing spots across Dorset's coastline last week (Surfers Against Sewage)

Wessex Water also said that "strategic improvements" may be needed to Weymouth's waste treatment centre to accommodate this growth.

The spokesman added: "Climate change is likely to increase the intensity of rainfall leading to higher risk of flooding in the future; however, the magnitude and timing of this change is highly uncertain.

• Read more: Sewage emptied into Weymouth Bay

"Urban creep can also pose a challenge for managing our drainage and wastewater networks. This is when existing households extend or build over gardens and create impermeable area for use such as car parking. This type of growth increases the area contributing to fast runoff to the urban drainage system and can increase the risk of flooding.

"As a part of the Drainage and Wastewater Management Plan process, we are producing models to understand how these challenges may impact the area."