Sewer Flooding: The Next Step

By Phil Wildbore, WRc Utilities

Since privatisation, water and sewerage utilities in the UK have been able to deliver almost year-on-year reductions. This applies to both the risk of properties flooding, and also to the actual incidents.

Prior to the summary in table 1 taken from the detailed Ofwat Levels of Service report for 2002/3 actual sewer flooding has reduced by a half in a decade. (Comparative numbers for 1992/3 report 4,811 sewer flooding incidents due to hydraulic overload and 6,000 due to other causes). So the industry has done well but the customer still expects more. The reports for 2003/4, issued by Ofwat on 28 October, indicate startling progress, with a further 37% reduction in sewer flooding on the 2002/3 figures.

Clearly even the 3,350 properties flooded by sewage in 2003/4 are not the best indicators of a developed nation. Soon the Ofwat Level of Service performance figures for 2003/4 will be released. Feedback from some utilities indicates that these improvements on 2002/3: mainly benefit from the weather conditions from 1 April 2002 to 31 March 2003, or has real progress been made?

Some utilities are stating that reductions in actual flooding since 2002/3 has been delivered, but at the cost of pre-emptive cleaning. (This process of pro-active maintenance was practiced extensively in the past, but became an easy target for cost-cutting. Unless adequate records are kept, together with holistic ‘what-if’ operational knowledge, then being pro-active is difficult to justify on a system that works largely by gravity, as well as being out-of-sight-out-of-mind).

So to maintain even reduce sewer flooding incidents, the industry is facing three main challenges. First, expenditure must be focused into key problem areas. Not only those of today, but also those which will become problems in the future. Sewers will deteriorate over time. Products used by customers will change, and it is unfortunate that large sectors of the population regard toilets as a waterborne waste disposal system for all manner of household items.

Second, gathering correct and accurate data which facilitates processing into information. Information to reduce sewer flooding at both an operational level and at the strategic asset management level. Also, information to better educate customers and product suppliers of disposable products.

Finally, how low can you go? What is an acceptable level of sewer flooding? We are rapidly approaching diminishing returns with current find-and-fix approaches. Ofwat are also citing ‘external flooding’ as well as the internal flooding figures reported above. This is undoubtably going to be a tough future need for utilities to manage. Ofwat currently estimate a seven-fold multiplier of external flooding over internal: that is, around 23,500 incidents per year. Is there a predict-and-prevent alternative to manage and mitigate this sewer flooding?

Most sewer flooding occurs when it is not raining

An unscientific review of both current and historic performance figures asks an interesting question. Actual sewer flooding incidents due to ‘Other Causes’ always seem to be higher than those for ‘Hydraulic Overload’. This means that most of sewer flooding in the UK occurs when it is not raining. (However, linking other causes to the word flooding implies a rainfall response not necessarily improved sewer network management).

The latest 2003/4 Levels of Service report from Ofwat identifies that 73% of all sewer flooding is attributed to these ‘other causes’. Unfortunately, this pattern is true for nearly all the 10 private water and sewerage utilities, with two of them report that 90% is attributable to other causes. (Other Causes are described by Ofwat as ‘temporary problems’ and include items like blockages, collapses, equipment failure, sedimentation). Also the two Ofwat sewer flooding risk assessments only look at hydraulic overload; so anyone’s true risk of sewer flooding is being vastly understated given that most of the actual sewer flooding stems from other causes.

It would appear that there might be some knowledge gaps:

  • What are the effect of sewer gradient on blockages;
  • What are the effects of pipeline defects on blockages;
  • What role do fats, oils, greases and tree roots play;
  • How are such factors weighted to give a risk of blockage;
  • When does the risk of blockage become a risk of flooding, and how quickly.
  • How is the operational data for ‘other causes’ collected and managed;
  • In taking over private drains and sewers in England and Wales are all these interactions confidently understood?

    WRc has recently offered to the industry through its Portfolio research program a two-year project which looks to develop in-system sewer flow monitoring to the next level. (As an analogy, think of this as a pipeline application of the pole-mounted roadside movement sensors which provide information on traffic hold-ups). WRc has already completed successful monitoring at unmanned treatment works, and applied this to the monitoring of combined sewer overflows. It is now believed that this technology can be taken to the city, residential and industrial streets to provide an early warning system of changes to the flows within sewers. It can:

  • Map historic flow patterns;
  • Collate flow changes due to many reasons other than blockages: seasonal inflow/infiltration; sedimentation; partial collapses/obstructions anything which causes a change to normality;
  • Respond to blockages before they become sewer flooding incidents;
  • Target pro-active maintenance in a timely and effective way;
  • Become the artificial intelligence gathering tool for holistic sewerage management.

    By combining this approach with the latest information on Sewer Jetting Best Practice, the modern-day sewer system manager will have access to a world class management and application process.

    For further technical details, please contact:

    Phil Wildbore, Head of Sewer Flooding on 01793 865161 or [email protected]
    For further information on the WRc Group visit

    More about Flow/Blockage Monitoring in Sewers

    Low cost monitoring technology, simple low power sensors combined with economical wireless communications, is now being widely used by Utilities to improve operational effectiveness. Recent WRc and UKWIR projects have theoretically explored the merits of approaches to blockage monitoring. WRc has demonstrated low cost approaches to level and imaging, and the use of combinations of sensors such as level and rain sensing to generate information. Using these data to detect incipient blockages, and other possible sewer problems will break new ground. For example, changes due to sedimentation and seasonal flow variations due to infiltration are also able to be monitored at low cost. This practical trial has five main objectives:

  • To provide an overall methodology for applying sewer blockage monitoring and implementing good practice;
  • To provide information on other extraneous factors which impact sewer flow capacity; e.g. sedimentation; infiltration; partial collapses, which enable the optimisation of sewer maintenance OPEX;
  • To find reliable methods of data processing for incipient blockage detection, and the other possible factors – such as inflow and infiltration, sedimentation – which may be extracted from the data;
  • To trial and assess how the monitoring data can be integrated effectively with sewerage operations;
  • To provide real cost data on the different methods of sewer monitoring so that utilities can maximise the benefits of the technology.
    Benefits to Participants

    The overall benefit is primarily to enable clients to prevent blockages and consequential sewer flooding incidents at minimum cost. This will be achieved and maximised through several subsidiary benefits:

  • Reduced cost of preventative maintenance programmes (e.g. sewer jetting by optimising visit cycles);
  • A practical methodology for using low cost blockage monitoring of sewer networks to prevent sewer flooding applicable to ‘other causes’ and ‘hydraulic overload’;
  • Experience from the trial on good blockage monitoring practice / possible pitfalls and how sewer monitoring data can be integrated with operational practice, e.g. alarm generation and alarm levels, information access, response times, natural cleaning cycles, seasonal changes due to infiltration, etc;
  • Understanding the merits of a range of technologies to enable effective purchasing for different duties and whole-life cost data to enable cost effective implementation;
  • Reduced risk in implementing a comparatively new technology and improved monitoring products for the industry by using the combined influence of the client group with suppliers.

    It may be possible to use the data as clear evidence of the need for capital investment in a sewer network with the regulator. This has long been a difficulty due to the ‘out of sight’ nature of the sewer network. It is intended that systems will be readily portable to facilitate a survey pattern of use.

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