Corsham drainage and wastewater strategy
This strategy covers the area served by Corsham (Thingley) 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.
Corsham is primarily in the Bydemill Brook River catchment, a headwater stream of the Bristol Avon river. The area is limestone and mudstone geology, is relative flat area with low points and within the town there is a culverted watercourse.
The Corsham 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 Corsham water recycling centre (WRC) are treated under normal flow conditions and are further treated through phosphorus removal to reduce the nutrient load discharged to the Bydemill Brook. 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 Bristol Avon 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 Corsham area has a high risk for sewer incapacity and has experienced flooding due to sewer incapacity, which has been exacerbated by highway gullies discharging surface water to the foul only sewer causing inundation, and consequently flooding, during storm events. The Corsham area also has several frequent spilling storm overflows, which we are investigating.
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 Corsham area has an above average risk for blockages because of sewer misuse and has suffered sewer flooding in the past.
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. Corsham WRC is within 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.
Corsham has met development commitments as identified in the Wiltshire core strategy (2015) up to 2026. Development has and is being built on former MOD sites; Royal Arthur Park and Copenacre and greenfield sites in the Bradford Road and Park Lane area. The emerging Corsham neighbourhood plan (2018) identifies minimal future growth. The success of existing housing schemes in Corsham could lead to further growth in the area to satisfy housing shortfalls elsewhere in the local planning area. The emerging local plan review (2018) may lead to further allocations.
To support this growth and reduce flood risk strategic improvements will be needed to ensure the network and WRC can accommodate this increase in flow and will be agreed with partners. Local improvements to the network may be required to satisfy further growth.
Climate change and urban creep
Climate change is likely to increase the intensity of rainfall leading to increased risk of flooding in the future. Frequency and severity of flooding in the future; however, the magnitude and timing of this change is highly uncertain.
An increase in severe rainfall events, like that seen in Corsham in September 2014, could lead to a rise in surface water flooding and sewer flooding especially if surface water continues to be connected to the wastewater system.
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.
We are working in partnership with Wiltshire Council, Environment Agency, Corsham Town Council and Network Rail on a flood alleviation scheme to reduce the risk of flooding in Corsham. Together we are monitoring assets, investigating the complex sources of flooding, included overland flow, surface water, highways and sewer, and identifying and implementing solutions to reduce the risk of flooding. We are also in support of the natural flood management pilot project with the Bristol Avon catchment partnership, which is engaging a wide array of organisations within this area to help 'slow the flow' as well as delivering environmental improvements.
- 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.
- Review capacity at the WRC, considering growth in the catchment and climate change, and identify mitigation measures if required.Improve capacity at the WRC by improving the screening process and treatment.
- Investigate options to reduce the impact of the WRC on the receiving watercourse, the Bydemill Brook. Carry out surveys of the sewers and modelling to assess the performance of the network and identify improvements.
- Carry out jetting of the sewers to remove any blockages or debris that could reduce capacity. Separate surface water from the foul sewer by disconnecting gullies from the foul sewer and reconnecting it to the culverted watercourses and identify lateral drains that connect the foul sewer.
- Work with customers to promote responsible and correct use of the sewer system to prevent blockages and misconnections.
- Work with partners to produce an integrated catchment model to understand how all assets can affect flood risk and potential solutions.
- Investigate localised flooding problems as they arise and are prioritised.
- Investigate and, if applicable, identify solutions for improve any frequent spilling storm overflows.
- Relocate the WRC outfall to reduce the impact on the receiving watercourse, the Bydemill Brook.
- Investigate the feasibility of a new pumping station to deliver flows from the centre of Corsham directly to Thingley WRC and improve capacity in the network.
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.