Bournemouth drainage and wastewater strategy
This strategy covers the Bournemouth area, including Boscombe, served by Bournemouth (Holdenhurst) water recycling centre (sewage treatment works). This area is a part of the Dorset management catchment and Wessex Water's drainage and wastewater management plan.
Bournemouth is a large coastal town in Dorset, and with nearby Poole and Christchurch, forms the south-east Dorset conurbation. The area is characterised by dry, sandy soils, and naturally supports a heathland environment, although relatively little of this survives to this day. The River Bourne flows through the town centre to the beach, whilst the River Stour to the north east of Bournemouth flows to Christchurch Harbour.
The Bournemouth area has predominantly a combined sewer system but in places there are separate sewer systems for 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 received at Bournemouth water recycling centre (WRC) are treated under normal flow conditions and are disinfected by UV treatment to reduce the bacteria load to the downstream bathing water. 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 Stour 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 Bournemouth area has a high risk for sewer incapacity although there are no frequent spilling storm overflows in the catchment.
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 Bournemouth 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 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. Bournemouth 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.
Bournemouth core strategy (2012) nominates Bournemouth town centre as the location for supporting the main residential growth in the local planning area. The town centre area action plan (2013) allocates a number of brownfield sites for redevelopment. Bournemouth local planning authority has recently commenced a local plan review (2018) which seeks the views from residents for possible additional locations to meet revised housing targets.
To support this growth and reduce flood risk, local improvements will be needed to ensure the network and the WRC can accommodate this increase in flow.
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.
Wessex Water works closely with the lead local flood authority, Bournemouth Borough Council, on the local flood risk management strategy to understand the causes of local flooding, identify solutions, reduce flood risk through sustainable development and assist local communities to become flood resilient.
- Construct a new inverted siphon to the WRC to improve resilience of the network.
- Install an actuated penstock to control flows between the sea outfalls and increase capacity during rainfall events.
- Carry out sewer surveys to assess the condition and performance of the network.
- Reline priority sewers to protect against corrosion and reduce risk of collapse.
- Object to new developments where the build would be near our priority sewers and would increases the risk of collapse.
- Review capacity at the WRC, considering growth in the catchment and climate change, and identify mitigation measures if required.
- Implement solutions to reduce flood risk in areas at risk.
- Reduce spill frequency from storm overflows to reduce impact on bathing waters.
- Install phosphorus removal at the WRC to reduce nutrient loading to the River Stour.
- Investigate and assess the impact the sewerage system and WRC have on the downstream bathing waters at Bournemouth and Christchurch.
- Work in partnership with Bournemouth Borough Council to investigate the possibility of using surface water ponds to reduce the risk of flooding.
- Work in partnership to investigate options to reduce the impact of the sewerage network on the bathing waters such as the possibility of a new long sea outfall.
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.