Bridport Drainage and Wastewater Strategy

Bridport Drainage and Wastewater Strategy

This Drainage and Wastewater Strategy covers the area served by Bridport Water Recycling Centre (WRC), also known as Sewage Treatment Works, including Bridport, West Bay, Askerswell, Beaminster and Burton Bradstock. This area is a part of the Dorset Management Catchment and Wessex Water’s Drainage and Wastewater Management Plan.

Catchment background

The area

Bridport is a coastal town on the Jurassic Coast in West Dorset. The town is on the confluence of the rivers Brit and Asker. The area is predominantly clay geology surrounded by sandstone hills.

Sewer network

The Bridport area has both combined and separate sewer systems to convey 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

Wastewater received at Bridport WRC is treated under normal flow conditions. 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 river as a storm overflow, as permitted by the Environment Agency, having benefitted 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 or other inflows such as surface water entering the sewer system.

The Bridport area has very high risk for sewer incapacity and has experienced sewer flooding due to hydraulic incapacity in the past three years. 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 Bridport catchment has high risk for blockages and has experienced sewer flooding due to blockages in the past three years.

Asset health

Sewers are inspected to assess the 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 for 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 convey 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 or the Dorset Council’s website for more information.

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.

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

The Bridport catchment has not experienced any 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 potential sewer flooding. Best practice is to utilise Sustainable Drainage Systems (SuDS). We have a policy that surface water connections to the foul sewer system are not permitted.

The West Dorset, Weymouth & Portland Local Plan was adopted in 2015. Development at Vearse Farm, Bridport is promoted through the plan and has recently obtained planning permission for 760 new homes and approximately 4 hectares of employment land. The Joint Local Plan Review preferred options (October 2018) confirms Land at Vearse Farm “is designated to meet the long term needs of the town”. The review identifies additional sites within the original Vearse Farm allocated site which could deliver a further 170 homes.

To support growth strategic improvements to the sewer network will be agreed with partners to accommodate growth at Vearse Farm.

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 will produce models to understand how these challenges may impact the area.

Strategy

Partnership working

We have worked with the lead local flood authority, Dorset Council, to alleviate flooding in Bridport by repurposing an abandoned rising main as a highway drain to drain an area at risk of flooding. We are also working in partnership with Dorset Council, to develop surface water management plans. These plans set out how surface water will be managed in the long term to prevent flooding from sewers, drains, groundwater, land runoff and small watercourses.

Short term

  • Investigate the condition of priority sewers and reline if required to reduce risk of collapse and flooding.
  • Model and assess the impact from development on the network, considering climate change and urban creep, and identify enhancements required to accommodate development.

Medium term

  • Investigate and, if applicable, identify solutions to improve any frequent spilling overflows.
  • Investigate the condition of the Brit Valley rising main and, if required, carry out improvements.
  • Deliver enhancements required to accommodate developments.

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.