Bath drainage and wastewater strategy
This strategy covers the Bath area, including Bathampton, Batheaston and Saltford, served by Bath (Saltford) 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.
Bath is within the steep sided River Avon Valley, south of the limestone hills of the Cotswolds Area of Outstanding Natural Beauty and is the largest city in Somerset. Bath is in the West of England Combined Authority (made up of three local authorities: Bath & North East Somerset Council, South Gloucestershire Council and Bristol City Council) which is responsible for regional planning, infrastructure and investment.
The Bath area has a combined sewer system, collecting all wastewater, sewage from homes and businesses, and storm water, rainwater collected from roofs and yards, in one sewer in the town centre and a separate sewer system serving newer developments on the city outskirts. 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 Bath water recycling centre (WRC) is treated under normal flow conditions and are further treated through phosphorus removal to reduce the nutrient load discharged to the Bristol Avon River. 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 benefited from screening and a degree of settlement within the storm tank.
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 Bath area has an average risk for sewer incapacity, however, at certain locations and under high groundwater conditions, groundwater enters the sewer and the capacity is reduced. The Bath area also has several frequent spilling storm overflows which we are investigating.
Sewer misuse includes flushing of 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 Bath area has an above average risk for blockages because of sewer misuse.
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 water recycling centre (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. Bath WRC is approaching its capacity to meet its permit.
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)
- storm overflows.
Future challenges in the catchment
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.
Housing development outlined in the Bath and North East Somerset core strategy and placemaking plan is being built with ongoing significant development within the central area (Western Riverside) and the redevelopment of former Ministry of Defence land at Foxhill, Warminster Road and Ensleigh.
The local plan review is running alongside the production of the West of England joint spatial plan (2017). The review is considering how additional small-scale development may be accommodated, with greenbelt release identified as a possible option for further evaluation. Some of the locations to be evaluated (Bath, Bathampton, Batheaston and Saltford) fall within the Bath WRC catchment.
To support this growth and reduce flood risk, improvements will be needed to ensure 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 urbanisation 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.
We are working in partnership with Bath & North East Somerset 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.
- Model and assess the impact from future development on the network, considering climate change and urban creep, and identify enhancements required to ensure the risk of flooding does not increase.
- Review capacity at the WRC, considering growth in the catchment and climate change, and identify mitigation measures if required.
- Protect surface water sewer network at Bristol Road from river flooding.
- Install a new screen at Kensington Meadows storm overflow.
- Install flap valves on outfalls in Bath as a part of the Bath Quays flood improvements, working in partnership with Bath & North East Somerset Council.
- Increase capacity at the WRC to accommodate development in the catchment.
- Investigate and identify solutions for improve any frequent spilling storm overflows to reduce the impact of the drainage network on the River Avon, during wet weather conditions.
- Twin the Bathampton/Batheaston sewer to improve resilience to the network.
- Reline priority sewers to reduce risk of collapse.
- Replace Saltford rising main from Midland Road.
- Produce and implement a strategy for frequent spilling storm overflows in Bath to reduce the impact the network could have on the River Avon.
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.