Wimborne

Wimborne Drainage and Wastewater Strategy 

This Drainage and Wastewater Strategy covers the area served by Wimborne Water Recycling Centre (WRC), also known as Sewage Treatment Works. This area is a part of the Dorset Management Catchment and Wessex Water’s Drainage and Wastewater Management Plan.

Catchment background 

The area 

The Wimborne catchment is formed from sand, silt and clay sedimentary bedrock with overlain river terrace sands and gravel deposits. The River Stour flows from a westerly direction with the River Allen tributary joining at the south of the town, continues north of Bournemouth and onto Christchurch Harbour. The Wimborne catchment occupies areas within the former East Dorset District Council and Poole Borough Council. Unitary Authorities of Dorset Council and Bournemouth, Poole and Christchurch Council were formed in 2019. The catchment includes the market town of Wimborne and settlements of Colehill at the north, Oakley, Merley and Canford Magna south of the River Stour.

Sewer network 

Wimborne 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 Wimborne WRC is treated under normal flow conditions and is disinfected by UV treatment to reduce the bacteria load to the downstream bathing water at Christchurch Harbour. 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 Wimborne area has high risk for sewer incapacity and high groundwater levels from prolonged rainfall periods affect areas of this catchment. The catchment has experienced sewer flooding due to hydraulic incapacity in the past three years but currently no frequent spilling storm overflows have been identified 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 Wimborne catchment has a 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 convey 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 growth or permit conditions. 

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 Wimborne 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 Local Plans of East Dorset and Poole provide for allocated sites and upwards of 1760 new homes over the plan periods, within the catchment. Site allocations extend the urban boundaries at the north, south, east and west.

Local improvements to increased capacity in the network will be required to serve these new neighbourhoods as well as improvements to the WRC. A phased programme of works to match the rate of development can be considered over the short and medium term.

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  

  • Install phosphorus removal at the WRC to improve river water quality in the River Stour.
  • 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.
  • Review mitigation measures to reduce infiltration in the Clapgate sub-catchment.

Medium term 

  • Review and, if necessary, deliver phased strategic network improvements to accommodate development and reduce sewer flood risk at Poole Road pumping station and Canford Magna pumping station.

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.