Melksham drainage and wastewater strategy

Melksham drainage and wastewater strategy

This Drainage and Wastewater Strategy covers the area served by Melksham Water Recycling Centre (WRC), also known as Sewage Treatment Works, including majority of Melksham, Whitley and Broughton Gifford. Berryfield, Bowerhill and Semington are served by Bowerhill WRC and are not included in this strategy. This area is a part of the Bristol Avon Management Catchment and Wessex Water’s Drainage and Wastewater Management Plan

Catchment background

The area

Melksham is a town in West Wiltshire on the Bristol Avon river. One of the larger market towns within the county, the A350 running north-south provides regional transport links to Trowbridge and Chippenham. The local geology is mudstone with a shallow gradient with hills to the north at Whitley. 

Sewer network

The Melksham area 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 Melksham are treated under normal flow condition and flows 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 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 Melksham area has a high risk for sewer incapacity. The catchment has experienced sewer flooding due to hydraulic incapacity in the past three years but 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 Melksham catchment has a 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 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 the Wiltshire Council 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 and was expanded in 2010 – 2015 to accommodate and treat increase in flows from developments.

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 Melksham catchment has experienced a significant pollution event 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 is not permitted.

The Wiltshire Core Strategy (2015) up to 2026 allocates an indicative requirement of 2,240 homes within the Melksham town area. Development has and is being built out to the east of the town and on the site of the former George Ward School. Future proposed development at Bowerhill and the Spa will generate local sewer improvements.

The emerging Local Plan Review (2018) may lead to further allocations and the need for future improvements.

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

  • Investigate and identify solutions for alleviating flood risk at priority locations.
  • Trial a change in permitting approach in the Bristol Avon, known as Catchment Permitting, to reduce the nutrient loading to the Bristol Avon river in a more efficient and effective way.
  • Investigate and if viable, implement first time sewerage for Beanacre.
  • 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

  • Continue to monitor storm overflows in the catchment and if required, investigate and, where possible, implement solutions to improve storm overflow operations.

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.