Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Age-old problem

Sewer collapses - Sophisticated computer modelling has been harnessed to improve South West Water's sewer maintenance record. Mark Hansford reports.

Water regulator Ofwat has taken a no-nonsense approach with South West Water (SWW), giving it until 2008 to reverse a steady increase in sewer collapses, floods, smells and related pollution incidents.

In its June 2008 return, SWW will have to demonstrate how it is going to tackle the problem and justify how it will spend any extra cash. It will be hoping to provide a more detailed approach than it did at its last periodic review.

'We couldn't persuade Ofwat that we knew where to invest before collapses occurred, ' confesses South West Water wastewater planner Steve Prosser. 'It weakened our argument. We were asking for a substantial uplift in spending and Ofwat wanted to know we were going to target that investment.' But SWW faces a challenge before its next meeting with Ofwat: its maintenance record has to be improved with almost no reliable data on critical factors such as sewer age, material, condition and life expectancy.

The company has a total of 9,000km of sewerage, and deciding where to focus its inspection and rehabilitation efforts to date have been ad hoc.

'It has been purely based on the most critical sewers, such as those running under motorways, and always decided on the cost to the water company, not the consequence to the customer, ' explains James Jorgasson, project manager at FaberMaunsell, the consultant brought in by SWW to help it sort out its problems.

FaberMaunsell commended sewerage location modelling, a practical methodology for planning capital maintenance. 'It is a risk-based approach, built up by assessing the likelihood and consequence of a collapse, ' says Jorgasson. 'On top of that we have a prioritisation model and that leads to a plan of investigation and rehabilitation.' In assessing the likelihood of collapse age is the key factor, although some work was done with national soil agency Silso to correlate between ground water levels and collapse rates. Size and material are also important.

But finding out the age of the sewers was the most vexing problem. 'Knowing the age of a sewer isn't as easy as looking at a catalogue, ' says Jorgasson.

'As a surrogate for age we purchased historic mapping from the 1800s to the present day. This was used to identify development.' The assumption is that where there is development, there are sewers.

'We came up with seven eras, and you can see a clear ring effect, with the city centres the oldest. There are other methods out there - WRc has done a postcode-based age proxy, but that turned out to be very poor in comparison.' Consequence of collapse was assessed by identifying what lay above the sewers - highly trafficked roads, hospitals and schools are most critical.

Also considered are buildings generally, environmentally sensitive areas such as SSSIs, and rivers.

The outcome is a holistic view of risk. 'You can apply the methodology to each individual sewer, or a square kilometre, or a catchment, ' explains Jorgasson. 'Or you can pull out the highest risk sewers, so can package work holistically. So if the priority was to do a quick fix, you would do work ineffi ciently by doing all the worst sewers fi rst.' Prosser adds: 'The attraction for SWW is we get an instant overview without widespread CCTV surveys. It gives a view of risk we have never had before.

With CCTV you need to be fairly sure that you are targeting areas where it is going to be worthwhile.' Jorgasson adds: 'During an initial pilot study we tested the methodology by doing some CCTV. Historically, SWW gets up to a 20% hit rate, where for every 100m identied, 20m would need rehabilitating. Using location modelling doubled that.' With an accurate model of the state of the network today, further work was done to allow SWW to predict into the future.

The base data here was historic CCTV survey data, although nding surveys carried out preprivatisation was a problem.

Data from 25,000 surveys was found and inputted, and due to SWW's previous penchant for surveying only critical sewers, there was a lot of repeat data which meant deterioration over time could be mapped.

This was used to create ve categories of condition, with category ve representing immediate state of collapse and one being good as new. From this SWW can now produce a graphic, validated, demonstration of the effect of no interventions.

'That's what ticks the box for the regulator, ' says Jorgasson.

'We see the outputs from this modelling as driving our active sewer rehabilitation programme in the next ve years and that is something we have not been able to do in the past, ' says Prosser.

'We have been focused on existing problems.' To illustrate the importance of this point Jorgasson shows a graph of capital expenditure divided into proactive and reactive maintenance. At present almost all spending is reactive; the plan is for that to rapidly change providing much more certainty of investment.

So where will SWW start?

'There are an awful lot of red (high risk) sewers, ' says Jorgasson. 'We are weighting the probability and consequence with reported collapses, ooding events and odours - making the best use of existing information.' Work can be prioritised on an asset or catchment level, but it is more likely - as it is more efcient - to do it on a catchment level. 'So we have ranked all 630 catchments and have a top 10 to start on, ' says Jorgasson.

In total, 134km of sewers have been identied in the 10 catchments. A sample of these is to be CCTV surveyed in 2007/08 to determine the effectiveness of the overall location modelling approach.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.