Bowerhill drainage and wastewater strategy

Bowerhill drainage and wastewater strategy

This Drainage and Wastewater Strategy covers the area served by Bowerhill Water Recycling Centre (WRC), also known as Sewage Treatment Works, including Bowerhill, Berryfield, Semington and the eastern edge of Melksham. This area is a part of the Bristol Avon Management Catchment and Wessex Water’s Drainage and Wastewater Management Plan

Catchment background

The area

Bowerhill is a small satellite town of Melksham and the catchment is formed upon mudstone sedimentary bedrock, with clay, sands, silts and gravels overlain across lower elevations. The catchment abuts Melksham town at the north and includes the eastern areas of Melksham. Much of the developed areas is used for industry. Treated effluent from Bowerhill WRC discharges to Berryfield Stream which flows westward to the Bristol Avon river.

Sewer network

Bowerhill has a predominantly separate sewer system, where wastewater, sewage from homes and businesses, is collected into the foul only sewer and is conveyed to the WRC. Storm water, rainwater collected from roofs and yards, is collected into a separate surface water sewer which conveys the rainwater to the river. However, in some situations the surface water sewer discharges to the foul sewer. In these cases, under heavy storm conditions, 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 Bowerhill is treated under normal flow condition. 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.

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 Bowerhill area has medium risk for sewer incapacity and has experienced sewer flooding due to hydraulic incapacity in the past three years. There are no frequent spilling storm overflows in the catchment.

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 Bowerhill 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 the 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 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 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 or your local 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 Bowerhill 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 adopted Wiltshire Core Strategy 2015 allocates upwards of 2240 new homes over the plan period for the Melksham area. Phased allocations will be made through the plan period and there will be significant residential growth within the catchment.

Strategic improvements to the network will need to be considered and phased to match the rate of development and growth. Bowerhill WRC is a strategic asset and the existing odour buffer zone should be retained around the works.

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

Short term

  • 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.

Medium term

  • Provide network capacity improvements to support local developments.
  • Review capacity at the WRC, considering growth in the catchment and climate change, and identify mitigation measures if required.
  • Review and update odour models to protect development and the WRC.

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.