Wastewater networks: Risk and service
Adam Dean of Ewan Group explores the issues and solutions in wastewater network modelling
As performance standards for our wastewater networks are progressively increased and serviceability becomes the driving factor, the ability to confidently predict what will happen in terms of risk and service is more critical than ever.
Good models, properly used, have much to offer in understanding serviceability for the asset owner, the regulator and other stakeholders who are affected by the performance of the asset.
From a wastewater network perspective, the key performance indicators are flooding, environmental impact and above- or below-ground structural failure of the network, though in the case of a structural collapse, the service failure would probably be flooding.
The majority of such indicators are based on historic performance and normally depend on the system having failed, and the failure being detected or reported accurately. In situations such as this, where field data reports may be scarce or unreliable, modelling provides an ideal means of assessing qualitative aspects
of risk and service.
Therefore, models have a crucial role to play in assessing the impact of altering levels of service and understanding the trade-off between risk and serviceability to the customer. Modelling is therefore likely to play an increasing role in regulation. There are a number of other solid reasons for this increasing likelihood:
n Models can be used to look into the future to predict levels of service way beyond those for which good historic data is available. The model can be used to assess the risk of failure in high return period events or look at the consequence of pipe failure for use in capital maintenance assessments
- Models can take account of changes in the catchment, such as development or changes to the system and predict problems before they occur. This is instead of only responding to problems that have already occurred several times
- Models can be used to predict the effect of changing climate, before the changes happen
- Models can be used to identify interactions between systems under the control of different undertakers (for example sewer and highway drainage systems, culverted watercourses), so that responsibility is clearly identified and costs can be shared
- Models can be used to achieve an optimum balance between capital and operational expenditure and to reduce flooding from other causes
- With new environmental drivers such as the Water Framework Directive, and Defra’s Making Space for Water consultation document, models will increasingly be used to support a more complete understanding of the water cycle. The interaction between catchment, collection systems, treatment plants and receiving waters will become clearer to understand and to benchmark performance.
Application of models in this way would form a key part of the management system. Models would have to be continually updated, through model maintenance programmes, to reflect changes to asset data delivered through the periodic review process.
It would, however, be wrong to give the impression that the current model platforms and the current model inventory are entirely suitable to meet all these future challenges. Significant improvements to both software, increased investment to model building and quality of model building standards would have to continue to improve if models were to become the hub of the asset owners’ management system for understanding risk and service.
Ewan Group has made significant investment in developing processes and tools to bring added value in addressing the issues identified. Some of these key tools are FastNett V1; FastNett V2; Flood Risk Mapper; Pipe Risk Assessment; and Short-term Urban Flood Forecasting (STUFF).
Since the development in 2003 of FastNett V1, a rapid sewer simulator and optimiser, which was the result of collaboration between Ewan and five of the UK’s major water companies, significant strides have been made. FastNett marked a significant advance in wastewater planning and management. It has the ability, using optimisation technology, to analyse thousands of options, to arrive at the most cost-effective flooding solution for a catchment.
In the 2004 APM4 final determination, Ofwat allocated £1.08B for water companies to spend on reducing the risk of flooding from sewers, but imposed an efficiency target of reducing cost by 10%. FastNett was developed to help water companies achieve efficiency targets such as this and on some studies achieved savings in the order of 26%. In some of these cases, the saving is set against the drainage area plan scheme costs, where costs are sometimes conservatively assessed due to lack of insufficient cost data (utilities etc). In this case, a saving is easily identifiable. However, in other cases, FastNett has demonstrated a significant saving against detailed designed schemes.
One such catchment is located in the north of England. The catchment contains a mix of residential, commercial and heavy industrial development. The population served by the sewer system is 15,000. The catchment contains a number of DG5 and Area flooding. In addition, a CSO adjacent to the flooding locations was to be provided with 6mm screens. Supplied with a verified InfoWorks model of the existing foul and SW sewer networks containing about 700 nodes, the areas within which flooding was to be eliminated were identified. The client also specified the return period of protection required for different parts of the catchment.
FastNett successfully optimised the solution for the combined sewer system. Checks carried out on the optimised solution in InfoWorks showed that the scheme objectives had been met, while CSO spill had not increased, and there was no increase in flooding elsewhere. The cost of the FastNett solution was £880,000.
Following submission of the study, as-constructed drawings for the scheme that had actually been built were made available. Costing this design with the cost model used for optimisation demonstrated that FastNett’s solution introduced savings of £420,000.
FastNett V1 technology is currently being used in the US, where Ewan’s engineers are using the tool to set target costs for new extensive first-time sewerage schemes (40,000-plus population), and to look at the trade-off in both capital and operational costs in either laying gravity or pumped systems. FastNett V2 is the next development bringing together the hydraulic simulator capabilities of version one with other tools which have been developed by Ewan.
Flood Risk Mapper
Defra’s consultation document, Making Space for Water, looks at the management of flood risk by implementing an integrated approach to catchment floodwater management in order to reduce the flooding threat to people, their property and the environment. To assist this vision, there is a need for tools to support the integrated catchment management approach. As a direct consequence, Ewan is currently developing Flood Risk Mapper.
Flood Risk Mapper is based on the principles of flood-path mapping. It can be used catchment-wide to identify areas which may be at risk from flooding and to help understand the potential implications of extreme rainfall and overland flow for a specified catchment or property within a catchment.
The technology has already been used by one client to identify who is responsible for the flood water – for example highway drainage, critical open water course and so on – and how the water company can mitigate its risks. Flood Risk Mapper, in combination with a hydraulic sewer model, can be used in determining the exceedance of sewerage systems and the affect this has on customers. Later versions of the technology will also be able to calculate flood water depth which will be useful for planning operational flood mitigation measures when sewerage systems fail.
There are also obligations on developers and councils in determining the flood routing associated with new developments.
Pipe level risk assessment
The deterioration of sewer systems is a complex process influenced by many factors. There is not generally a simple relationship between use and condition. In other words, sewers do not simply wear out with use in the way that, for example, a pump or a vehicle does.
Two further considerations are important. Firstly, sewer deterioration can often be triggered by unpredictable external events and, secondly, in the majority of cases there is a dearth of knowledge about the circumstances that tend to lead to faster or slower deterioration.
The large number of potentially significant factors affecting deterioration and the paucity of repeat CCTV over a sufficiently long period currently available, make methods that rely on observations of changes in specific pipes over time, uncertain. This is especially true when the method uses changes in condition grade which are themselves a blunt instrument for describing condition.
What is possible however, is to predict the likelihood of collapse of pipes within a particular population. This has been demonstrated by the work that Ewan has been carrying out in the 2005-6 UKWIR Deterioration Rate of Sewers project. Here, the population was defined as the sewers in a particular geographic area, defined by having a similar age of development. The method was able to successfully develop equations that predicted the likelihood of blockage and collapses in two pilot catchments.
This method is now being extended to operate at the level of individual pipes. Risk is the product of likelihood and consequence. The likelihood of individual pipes collapsing or blocking is derived using data-driven modelling techniques on large amounts of historic performance data, correlated with asset attribute data (age, size, depth, gradient, etc) and environmental data (soil conditions, traffic loading etc). Consequence is derived by fast simulation techniques coupled with flood routing and GIS, which allow the effect of any pipe failing to be predicted in terms of flooding and pollution impact. The risk models produced predict current performance but, by changing the age term, the change in performance as the population ages can also be predicted.
Furthermore, the impact of making interventions can be predicted. Crucially, therefore, the equations are not merely a means of predicting deteriorating performance but can be used as part of the asset management planning toolkit for determining future intervention.
Short-term Urban Flood Forecasting (STUFF)
STUFF, which is currently under development at Ewan, is a combination of all the above tools, and will be used in combination with live rainfall data to advance predict the hydraulic performance of the sewerage system. STUFF is made possible because of FastNett’s rapid hydraulic simulator, which runs on average 50 times faster than an equivalent-sized conventional hydraulic model. Such technology will allow clients to manage storm risk more efficiently and deploy operational strategies more quickly in order to protect customers from flooding. Effective right down to street-level forecasting, STUFF should be able to predict which part of the sewerage network within an operational region will experience the worst failures.
With such advances being made, Ewan believes that the modelling of wastewater systems and the broader environment promises much both for the regulator and the regulated. To provide tools suitable for delivering maintenance and quality investment in line with the common framework, will require imagination and investment into more advanced modelling tools and processes. Modelling platforms must be developed so that asset owners can analyse risk in greater detail and continue to deliver serviceability at an affordable cost to customers. The goal must be to develop modelling technology that will enable intelligent and creative management of wastewater systems to be planned with confidence.
The company’s Strategic Asset Planning team is currently developing criticality models for PR09 and has recently been awarded UKWIR’s 21st Century Sewerage Project, which will look at issues affecting water companies over the next 80 years. The goal must be to develop modelling technology that will enable intelligent and creative management of the UK’s wastewater systems.