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Flooding in Hambledon - update

Southern Water is working alongside other agencies to help ease the impact of unusually high groundwater levels and torrential rain in Hambledon, Hampshire.

Our sewerage systems were working as they should and were pumping the capacity they were designed for, however, they have been overwhelmed by the sheer volume of groundwater entering the system.

The water has risen at an alarming rate – 20 metres in five days – due to the prolonged rain, which also causes surface water flooding. This groundwater inundates our sewers through cracks in joints caused by ground movements over the years, as well as through customers’ private lateral pipes. The sewers then become full of wastewater, heavily diluted by groundwater

The sewers are only designed to carry wastewater flows from people’s homes. They are not designed to cope with the enormous flows which result from surface water, groundwater and river flooding – and neither should they be as Southern Water is not responsible for such events.

Usually, the flows of wastewater in sewers are transported from properties to a treatment works by gravity or via pumping stations. These pumping stations are designed to push a specified amount of wastewater to the treatment works and the sewers are designed to carry a certain amount of waste – these amounts define the capacity of the system.

When the system becomes overwhelmed, customers may have problems using their wastewater services such as flushing toilets and using showers and sinks as the waste flows have nowhere to go.

The situation in Hambledon has been exacerbated by the rain falling on saturated ground, which is causing surface water flooding.

As the sewerage company, Southern Water is not the agency directly responsible for investigating and solving groundwater, river flooding or surface water problems. However, we must ensure our customers can continue use their wastewater services.

To ease the pressure on the sewers, enabling customers to use their waste services and to help prevent wastewater overflowing from manholes, we use tankers to suck out the excess flows which are then transported to a nearby treatment works. Overpumps are also used to take heavily diluted groundwater from the sewers, pass it through a treatment process and pump it into a local water course.
In Hambledon, we have also added extra capacity at our Bury Lodge Pumping Station by installing temporary pumps which are pumping water out of the sewers.

The pumping station, which is designed to pump wastewater flows through the sewerage network, has not failed – we have simply added new pumps so it can pump away even more water from the sewers.

However, no amount of additional capacity at our pumping station will resolve the groundwater or surface water flooding issues in the area, which is caused by rainwater flooding off the South Downs. The issue of flooding is complex with various agencies, including local authorities and the Environment Agency, responsible for different aspects of drainage.

We are committed to making improvements to the sewerage system in Hambledon and, as well as our work to update the pumping station, over the year we have invested £160,000 to survey and seal the sewers in the southern part of the village.

This resulted in repairs being carried out to 15 manholes which were found to be leaking and relining 100 metres of sewers.

Bearing in mind that the optimum time for locating leaks is when the level of groundwater is marginally above the sewer and falling, we will return to conduct further work in the northern part of the village when conditions are appropriate.

Throughout the year we have written to every customer on a regular basis to keep them up to date with the progress of our work to survey and seal the sewer network in Hambledon which forms a small but high priority part of Southern Water`s huge 40,000 kilometres (24,854 miles) network of sewers – about the same length as the circumference of the earth.

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