Bristol drainage and wastewater strategy

Bristol drainage and wastewater strategy

This strategy covers the Bristol area including Yate and Avonmouth, served by Bristol (Avonmouth) water recycling centre (sewage treatment works). This area is a part of the Bristol Avon management catchment and Wessex Water's drainage and wastewater management plan.

Catchment background

The area

The Bristol area is part of a limestone landscape spanning from the Mendip Hills to the Cotswolds, with the rivers cutting through the limestone to the underlying clay. It is located on the Bristol Frome and Bristol Avon rivers which discharges to the Severn Estuary at Avonmouth.

Sewer network

The Bristol 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

Flows to Bristol water recycling centre (WRC) are 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 Severn Estuary as storm overflows, as permitted by the Environment Agency, having benefited from screening and a degree of settlement within the storm tank.

The WRC receives high trade loads, including discharges from the onsite organic waste and food waste facilities. In addition, Bristol WRC receives sludge imports from other WRCs for treatment, which produces additional liquor loads for treatment through the WRC process.

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 Bristol area has above average risk for sewer incapacity. The Bristol 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 Bristol area has above average 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 above average 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. Bristol WRC is approaching 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 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 is not permitted. 

The Bristol area is in the local planning authority areas of Bristol City, South Gloucestershire, North Somerset and Bath and North East Somerset councils. Significant development is proposed in these areas including large sites emerging through the West of England joint spatial plan (2017), non-strategic rural growth promoted through the proposed new South Gloucestershire local plan (consultation document February 2018), and the advancement of the Avonmouth Severnside Enterprise Area by the West of England Local Enterprise Partnership. 

To support this growth and reduce flood risk, strategic improvements will be needed to ensure the network and the WRC can accommodate this increase in flow. Local improvement will also be required to the network to reduce flood risk.

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 Bristol City 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 and runoff from land and small watercourses. 

We are also working in consortium with Bristol City Council and other European organisations on project RESCCUE, an initiative to evaluate climate change implementations and intervention strategies.

Short term

  • Complete the North Bristol Strategic sewer project (£65m) to support future development and reduce flood and pollution risk.
  • Review capacity at the WRC, considering growth in the catchment and climate change, and identify mitigation measures if required.
  • Work with the West of England Combined Authority to twin the sewer network as a part of the Metrobus scheme, to improve performance and resilience of the network. 
  • Assess flooding incidences and potential solutions to reduce flood risk.

Medium term

  • Increase capacity at the WRC to accommodate development in the catchment.
  • Investigate the condition of Shirehampton Lamplighters rising main and, if required, carry out improvements.
  • Reline priority sewers to reduce risk of collapse.
  • Investigate and, if applicable, identify solutions for improve any frequent spilling overflows.
  • Separate surface water from the foul sewer at Ashton Avenue and reconnect to the watercourse to reduce the risk of flooding.
  • Work with Bristol City Council to modify surface water sewer network and outfalls where required to protect against tidal flooding.
  • Assess the impact of the Metro West transport scheme on the sewer network and identify solutions.

Long term

  • Produce and implement a South Bristol strategy to support future development and reduce flood and pollution risk.

We are developing further 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.