We operate and manage 43,000km of sewer networks in north west England, many of which were constructed by the Victorians. These sewers vary in diameter depending on the volumes of sewage and surface water run-off the sewer is expected to carry.
Foul Flooding
Sewer or foul flooding as it’s sometimes referred to describe cases where sewer contents have backed up the waste pipes of homes or properties and spilled out onto the property’s floor or garden. There are several causes of sewer flooding:
- Overloaded sewers, e.g. insufficient hydraulic capacity during times of high flow
- Blocked sewers, e.g. tree roots
- Collapsed sewers
- Equipment failure, e.g. pumping station failure
We are monitored on the performance of our pipes and sewer networks by Ofwat the economic regulator of the water industry in England and Wales. A number of foul flooding measures are reported to Ofwat:
- The number of properties at risk of internal flooding from sewers due to hydraulic overloading more than twice in ten years, more than once in ten years, and more than once in twenty years.
- Properties that are internally flooded from other causes- The Ofwat category of ‘other causes’ includes flooding due to blockages, collapsed sewers and equipment failure. Of these the largest single cause is blocked sewers which may be caused by anything from tree roots to accumulations of fat from cooking.
- Any properties considered to be at risk due to hydraulic overloading are placed on an ‘At Risk Register’. The number of properties on this register and the number of properties removed, due to capital investment, are both reported.
During 2008 469 2 in 10 year properties and 521 1 in 10 year properties were on the ‘At Risk Register’ with 132 removed due to our capital investment programme, as described in the foul flooding case studies. 415 properties were at risk from sewer flooding caused by hydraulic overload and 1204 properties from flooding other causes.
In its annual Service and Delivery – performance of the water companies in England and Wales 2007-08 Report, (published in October 2008), Ofwat highlighted concern about our processes for dealing with sewer flooding information. During the past year we have reviewed the information we hold and the way we investigate incidents to improve our understanding of the risk of sewer flooding. This has led to an increase in the number of events reported in our flooding register in the categories ‘two in 10’ and ‘one in 10’ (flooding which occurs on average twice and once in any 10 year period), which is reflected in the performance reported to Ofwat. The impact on the regulated businesses 1 in 20 register is small and changes are within confidence grades already reported in prior years’ returns to the regulator.
This review has also lead to enhanced processes for the review and auditing of flooding other causes events and these have led to an increase in the reported numbers captured through our business systems. Accordingly, this is reflected in our data returned to Ofwat.
During 2008/09 the regulated business has entered into an undertaking to solve sewer flooding problems in the Penketh Catchment of Warrington. There are four key outputs associated with the undertaking involving capital schemes at Pyecroft Road, in Penketh North and South and for mitigation in Penketh East. The outputs are on track for delivery with the exception of Pyecroft Road, where legal delays have impacted on the milestone delivery date. The regulated business is working with its customers to resolve the issues as soon as is practicable, and as a short term measure has offered to mitigate those customers affected.
Unsatisfactory Intermittent Discharges
In times of heavy rainfall sewers sometimes reach their full capacity and overflow into designated watercourses. These overflows are termed intermittent discharges. Some have been classified as unsatisfactory by the Environment Agency, the water industry wastewater compliance regulator, for either visual / aesthetic reasons or water quality reasons.
