Weymouth drainage and wastewater strategy

Weymouth drainage and wastewater strategy

This Drainage and Wastewater Strategy covers the Weymouth and Portland area, including Upwey, Preston and Osmington, which is served by Weymouth Water Recycling Centre (WRC), also known as 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 overflow through screens for discharge to the sea via a long sea outfall as permitted by the Environment Agency

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 or other inflows such as surface water entering the sewer system

The Weymouth area has medium risk for sewer incapacity. The catchment has experienced sewer flooding due to hydraulic incapacity in the past three years and there are frequent spilling storm overflows in the catchment, and we are following the Storm Overflow Assessment Framework methodology to investigate and make improvements.

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 catchment has medium risk for blockages and has experienced sewer flooding due to blockages in the past three years

Asset health

Sewers are inspected to assess their condition using a risk-based approach, using the likelihood of it failing and the 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 medium 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 surface water sewers, which conveys rainwater from roofs and yards to the river. 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, from misconnections (when wastewater from household is incorrectly connected to the surface water sewer) or from storm overflows.

The Weymouth and Portland catchment has not experienced significant pollution events in the past three years.

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 increase risk in surface water and potentially 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 new unitary authority of Dorset Council is preparing a Local Plan, which will replace the 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 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 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 WRC 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 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.

Strategy

Partnership working

We are working in partnership with Dorset Council to resolve 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. This could include the use of sustainable drainage systems, new technologies and working with partners more to align long term plans and schemes.