Salisbury drainage and wastewater strategy

Salisbury drainage and wastewater strategy

This strategy covers the Salisbury area including Wilton, Harnham, Alderbury, Laverstock and Old Sarum, served by Salisbury water recycling centre (sewage treatment works). This area is a part of the Hampshire Avon management catchment and Wessex Water's drainage and wastewater management plan.

Catchment background

The area

Salisbury is a cathedral city in south Wiltshire. The area is in a valley of chalk geology at the confluence of the Rivers Nadder, Ebble, Wylye and Bourne, to the Hampshire Avon. The Hampshire Avon is designated for national and international importance for its habitat and wildlife.

Sewer network

The Salisbury 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 water recycling centre (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

Flows to Salisbury WRC are treated under normal flow conditions and are further treated through phosphorus removal to reduce the nutrient load discharged to the Hampshire 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 entering the sewer systems. The Salisbury area has an average risk for sewer incapacity, however, under high groundwater conditions, groundwater enters the sewer and the capacity is reduced further. The Salisbury area also has a frequent spilling storm overflow 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 Salisbury area has a high risk for blockages from 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 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 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. Salisbury 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, infrastructure charges paid by new developments will fund required upgrades to ensure sewer flooding risk is not affected. 

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) identifies significant allocations for growth in the Salisbury area. These include strategic sites to the north west of Salisbury at the UKLF site and Fuggleston Road and to the north east at Longhedge/Old Sarum. Within the city centre growth will be delivered through brownfield redevelopment. The Wiltshire local plan review including Swindon & Wiltshire joint spatial framework is currently in preparation with the initial consultation looking ahead to plan for the period to 2036. 

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 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 Wiltshire 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.

Short term

  • Improve the inlet to the WRC to accommodate growth and improve resilience.
  • Carry out sewer relining in Wilton to reduce the risk of infiltration from groundwater. 
  • Monitor and investigate sewers at risk from blockages. 
  • 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. 

Medium term 

  • Increase capacity at the WRC to accommodate development in the catchment and monitor compliance. 
  • Investigate and, if applicable, identify solutions for improve any frequent spilling storm overflows.

Long term 

  • Review capacity at the WRC, considering growth in the catchment and climate change, and identify mitigation measures if required.

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.