Piddle Valley drainage and wastewater strategy

Piddle Valley drainage and wastewater strategy

This strategy covers the area served by Piddle Valley (Piddlehinton) water recycling centre (sewage treatment works) including the villages of Alton Pancras, Piddletrenthide and White Lackington. This area is a part of the Dorset management catchment and the overall Wessex Water's drainage and wastewater management plan.

Catchment background

The area

The Piddle Valley is a chalk catchment on the headwaters of the River Piddle which flows to Poole Harbour. The valley is steep sided and during winter conditions or during prolong rainfall, can experience high groundwater levels.

Sewer network

The Piddle Valley has a foul only sewer system, where all sewage from homes and businesses is collected into the foul only sewer and is conveyed to the water recycling centre (WRC). However, in some situations the surface water sewer, which collects rainfall, 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

Piddle Valley WRC is a small water recycling centre and all flows received are treated.

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 Piddle Valley under normal conditions does not suffer from sewer incapacity, however under high groundwater conditions, groundwater enters the sewer and the capacity is reduced.

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 Piddle Valley has a below average risk for blockages but has suffered occasionally from blockages, but this has not caused flooding.

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 below 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. Piddle Valley 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:

  • 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 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. 

The West Dorset, Weymouth and Dorset local plan (2015) is the development plan for the Piddle Valley area. No significant growth is proposed within the catchment, therefore no improvements to the network or WRC to accommodate growth are expected.

Climate change and urban creep

Climate change is likely to increase the frequency and severity 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 through our catchment management team with Dorset County Council and Poole Harbour catchment initiative to reduce surface water flooding through natural flood management through changes in land management practices.

Short term

  • Continue the inflow management plan programme to reduce groundwater infiltration including surveying of the sewer network and where required, relining and renovation of the sewers.

Medium term

  • Install phosphorus removal at the WRC to reduce nutrient loading to the River Piddle.

Long term

  • If flooding issues cannot be managed through the inflow management plan, investigate alternative solutions such as a construction of an interceptor sewer or additional storm overflows.

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.