Blackheath Drainage and Wastewater Strategy

Blackheath Drainage and Wastewater Strategy

This Drainage and Wastewater Strategy covers the area served by Blackheath Water Recycling Centre (WRC), also known as Sewage Treatment Works. This catchment serves rural settlements located along the A35 highway, north of Wareham town including Tolpuddle, Affpuddle, Briantspuddle, Bere Regis, Morden and Lytchett Matravers. This area is a part of the Dorset Management Catchment and Wessex Water’s Drainage and Wastewater Management Plan.

Catchment background

The area

The west of the catchment is formed on chalk bedrock with sand and gravel deposits found in river settings. The east of the catchment is more varied with sedimentary sand, clays and silts in coarse to fine grained bedrock deposits. The River Piddle flows through the western areas to Poole Harbour, whilst Blackheath WRC discharges to the Sherford River north of Wareham Town. Settlements form smaller sub-catchments pumping onwards to Blackheath WRC located in a central position. Dorset Council replaces the former local councils of Purbeck & West Dorset.

Sewer network

The Blackheath WRC has both foul only and separate sewer systems, 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 can discharge 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. Storm overflows, permitted by the Environment Agency, can operate to prevent sewer flooding.

Water recycling centre

Wastewater received at Blackheath WRC is treated under normal flow condition. 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 Blackheath area has high risk for sewer incapacity and high groundwater levels from prolonged rainfall periods affect areas of this catchment. The catchment has not experienced sewer flooding due to hydraulic incapacity in the past three years but there is a frequent spilling storm overflow in the catchment for which we are following the Storm Overflow Assessment Framework methodology to investigate and make improvements.

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 Blackheath catchment has 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 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 unauthorised wastewater spills or leaks, misconnections (when wastewater from household is incorrectly connected to the surface water sewer), or storm overflows.

The Blackheath 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 Purbeck Local Plan submission will guide development until the new Dorset Local Plan is adopted from 2023 onwards. There are land allocations at Lytchett Matravers (150 new homes) which are scheduled for early completion by 2025, and at Bere Regis (105 new homes), with nominal growth predicted for the remaining settlements.

To support this growth improvements will be needed to ensure the network 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 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

  • 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.
  • Prioritise investigations in sub-catchments affected by infiltration.
  • Investigate and, if applicable, identify solutions to improve the frequent spilling overflow.

Medium term

  • Reline sewers to reduce the risk of groundwater infiltration and sewer flooding.
  • Deliver enhancements required to accommodate local development.
  • Identify and prioritise strategic network improvements if required to maintain compliance with permit conditions.

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.