Weymouth drainage and wastewater strategy

Weymouth drainage and wastewater strategy

This strategy covers the Weymouth and Portland area, including Upwey, Preston and Osmington, which is served by Weymouth water recycling centre (sewage treatment works). This area is a part of the Dorset management catchment and Wessex Water's drainage and wastewater management plan.

Catchment background

The area

Weymouth is a seaside town in Dorset in the River Wey catchment which discharges to Weymouth Harbour. To the north of the catchment is the chalk ridge of the South Dorset Downs and to the south, the tied limestone island of Portland. The rest of the catchment is sand and clay geology with areas of Weymouth being very low lying and some below sea level.

Sewer network

The Weymouth and Portland area has a predominantly combined sewer system but in places there are separate sewer systems for wastewater, sewage from homes and businesses, and storm water, rainwater collected from roofs and yards. Under heavy storm conditions, where the sewer conveys both wastewater and storm water, sewer capacity can be exceeded and built in safety valves called storm overflows, permitted by the Environment Agency, can operate to prevent sewer flooding.

Water recycling centre

Flows to Weymouth WRC are treated under normal flow conditions and discharged to the sea via a long sea outfall. Under heavy storm conditions, flows into the WRC can exceed its capacity. These excess flows will first overflow to storm storage tanks. If these tanks become full, they in turn spill to the sea as a storm overflow, as permitted by the Environment Agency, having benefited from screening and a degree of settlement within the storm tank.

Current performance

Sewer capacity

Hydraulic incapacity is when the drainage network cannot convey the runoff from heavy rainfall and can lead to sewer flooding. It can be exacerbated by groundwater entering the sewer systems. The Weymouth and Portland area has a high risk for sewer incapacity and under high groundwater conditions, groundwater enters the sewer and capacity is reduced further. The area also has several frequent spilling storm overflows which we are investigating.

Sewer misuse

Sewer misuse includes flushing anything other than the three Ps (pee, poo and toilet paper) down toilets. Wet wipes, nappies and sanitary products should not be flushed regardless of their labelling. Fats, oils and grease should not be poured down sinks in the kitchen, as this creates 'fatbergs'. Sewer misuse can lead to blockages which can cause sewer flooding. The Weymouth and Portland area have a high risk for blockages because of sewer misuse.

Asset health

Sewers are inspected to assess their condition using a risk-based approach, using the likelihood of it failing and consequence of failure. The sewers in the worse conditions are prioritised for more frequent inspection or rehabilitation. The risk profile for the condition of the sewers in this area is at high risk compared to the rest of the Wessex Water region.

Surface water flooding

Surface water flooding occurs when very heavy rainfall overwhelms drainage systems. Responsibility for surface water flooding is complex, but in summary Wessex Water is only responsible for sewer flooding. Where heavy rainfall occurs, overland flow collects runoff into rivers or low points on the ground. See the Environment Agency flood maps for more details.

Water recycling centre

Each WRC has a permit, as agreed with the Environment Agency, for how much water is treated under different weather conditions and the quality of the water that is discharged to the environment. Weymouth WRC is within capacity to meet its permit.

Water quality

When untreated wastewater is discharged to a watercourse it can affect the downstream environment including the river and coastal areas. This could be from:

  • pollution
  • unauthorised wastewater spills or leaks
  • misconnections (when wastewater from household is incorrectly connected to the surface water sewer)
  • storm overflows.

Future challenges in the catchment

Growth

New developments can cause an increase in wastewater requiring conveyance and treatment. Improvements to the foul sewer system to support new development will be assessed by Wessex Water developers' group and infrastructure charges paid by new developments will fund required upgrades to ensure sewer flooding risk is not increased. 

Developments can also increase the area contributing to rainwater runoff to the urban drainage networks, whether it is a surface water or combined sewer, causing an increased risk in surface water or combined sewer flooding. Best practice is to utilise sustainable drainage systems (SuDS). We have a policy that surface water connections to the foul sewer system is not permitted. 

The local planning authority of Weymouth and West Dorset are undertaking a review of their adopted local plan (2015). 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 paper (2017), forming part of the review, 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 in Portland. Options are explored to maintain or vary the recent growth in Weymouth and Portland of 150 and 45 dwellings per annum respectively. 

To support this growth and reduce flood risk, local improvements will be needed to ensure the network can accommodate this increase in flow. Strategic improvements to the STW may be required to accommodate this growth as the developments become more defined.

Climate change and urban creep

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. 

Urban creep can also pose a challenge for managing our drainage and wastewater networks. This is when existing households extend or build over gardens for 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.

Strategy

Partnership working

We are working in partnership with Dorset county council, West Dorset district council and Weymouth and Portland borough council to resolving flooding issues in the catchment and ensure solutions are holistic.

Short term

  • Relocate Radipole outfall to reduce flooding risk of sensitive areas and increase capacity for development.
  • Implement local improvements to the network to accommodate development in the area.
  • Monitor Portland pipebridge
  • Reline priority sewers to reduce risk of collapse. Model and assess the impact from future development on the network, considering climate change and urban creep, and identify enhancements required to reduce the risk of flooding.
  • Assess flooding incidences and potential solutions to reduce flood risk.

Medium term

  • Investigate the condition of Radipole and Wyke Regis Hillcrest rising mains and, if required, carry out improvements. 
  • Implement solutions for Watery Lane and St. Andrew Avenue to reduce flood risk.
  • Investigate and, if applicable, identify solutions for improving frequent spilling storm overflows.
  • Investigate and identify options for improving the resilience of the Portland pipebridge.
  • Review capacity at the WRC, considering growth in the catchment and climate change, and identify mitigation measures if required.

Long term

We are developing long term options that address and mitigate for climate change, development, urban creep and other future challenges as a part of the drainage and wastewater management plan process.