Gillingham Drainage and Wastewater Strategy
This Drainage and Wastewater Strategy covers the area served by Gillingham 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.
The drainage catchment serves the market town of Gillingham and the smaller settlement of Motcombe located 3km at the east. The area sits upon Kimmeridge Clay mudstone overlain with coarse to fine grained clays, silts, sand and gravel sedimentary deposits. The town has formed around river valleys of the Stour and Lodden with tributaries Shreen Water and Fern Brook. The River Stour forms a confluence with the Lodden at the south of the town before flowing southwards to Blandford and the coast. The town of Gillingham operates as a major service centre for North Dorset.
Gillingham 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 Gillingham WRC is treated under normal flow conditions and are further treated through phosphorus removal to reduce the nutrient load discharged to the River Stour. 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.
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 Gillingham area has a high risk for sewer incapacity and has experienced sewer flooding due to hydraulic incapacity in the past three years. There is also a frequent spilling storm overflow in the catchment and degree of infiltration occurs in the Motcombe 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 Gillingham catchment has a medium risk for blockages and has experienced sewer flooding due to blockages in the past three years.
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.
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 Gillingham catchment has not experienced significant pollution events in the past three years.
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 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 North Dorset Local Plan adopted in January 2016 makes provision for 2200 new dwellings over the plan period to 2031. Allocated sites at the south of the town will require a co-ordinated drainage strategy with partnership schemes to enhance network capacity.
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.
- Co-ordinate with the local planning authority, developers and landowners to deliver phased strategic sewer schemes to enhanced sewer capacity and accommodate development.
- Investigate and, if applicable, identify solutions to improve any frequent spilling overflows including infiltration reduction measures.
- Provide additional phosphorus removal and enhancements at the WRC to improve river water quality.
- Monitor infiltration in vulnerable sub-catchments at Motcombe and if necessary investigate and identify solutions.
- Review capacity at the WRC and in the network, considering growth in the catchment and climate change, and identify mitigation measures if required.
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.