Articles & Case Studies

InfoWorks WS modeling at Bristol Water

Posted: Wednesday 27th May 2009

Bristol Water has been using Wallingford Software’s water network modeling package, InfoWorks WS, to model its network for a number of years, producing a comprehensive suite of calibrated models. Currently the models are being used extensively as part of the Periodic Review process (PR09) and as an important tool in the ongoing mains renovation and replacement regime.

The utility is a large water-only company, located in the southwest of England. Bristol Water supplies just over a million customers, its network comprises 480,000 connections, 6,100km of distribution mains and 520km of trunk mains over an area of some 2,400 km2. The area supplied is a mix of rural and urban zones with a varied topography.

There are multiple drivers for modeling within the company, in addition to basic renovation and replacement planning. These include identifying bottlenecks or lack of capacity in the system and ensuring sufficient capacity for future developments. The company also uses its GIS system to focus its mains rehabilitation work based on various inputs.

Modeling drivers
The company has included work on improving security of supply in preparing for the next asset management plan period, AMP 5, and has also used its InfoWorks WS models to help operational staff to implement the remaining engineering schemes from AMP 4. Examples include the creation of district-metered areas (DMAs), installation of pressure-reducing valves (PRVs) and design work.

The company’s leakage department has recently extended the use of PRV zones within the area of supply. Network models have been used to verify that the proposed schemes will maintain levels of service during peak periods.

Operationally, the team utilises its modeling expertise and system knowledge to assist with the day to day smooth running of the network, and is always on hand to help out during a major incident to help maintain supplies.

Bristol Water will increasingly use its models for power optimization and future system management. InfoWorks Live, in which the modeling will be linked to telemetry, is also being developed with Bristol Water as a key player.

History of modeling
Up to 1995 there were two sections in Bristol Water undertaking modeling work. These teams were combined in 1995 and WESNET was chosen as the modeling package. In 2000 the company migrated to InfoWorks WS from WESNET using tools it had developed to extract models from its SmallWorld GIS system. As of 2007 the company had eight InfoWorks WS v9.0 licences based on a Citrix ‘thin client’ server. It is currently migrating its JET databases into its Oracle database to centralize all of its models.

Key factors in successful operational use of models
Bristol Water has always undertaken modeling for operational purposes. Key factors in its success include involving operational staff in all stages of model development and encouraging staff to participate in all aspects of the Section’s modeling work. For example, the model building team also runs mains flushing and mains rehabilitation applications, and when technicians join the section they are given training in network modeling and work on models to increase their skill levels. Each of the modelers has a specific geographic area of the company that they are particularly familiar with, but are able to rotate and use each other’s models as needed.

The models are detailed ‘all mains’ models, apart from the supply pipes and cover 100% of the supply area. The models are now all constructed at treatment works zone level, so that all of the zones supplied by the individual treatment works are covered. Some of these are joined so that it is possible to model the hydraulics across a number of systems. When models are being built the controls associated with the pumps and valves are reproduced as accurately as possible. This ensures that when an incident occurs or operational planning is needed, the model will run as the system would in reality.

The operational staff and engineering personnel are involved at an early stage in the model build, so they can contribute a great deal of valuable network knowledge. Having a good rapport with operational staff is essential, as is their constant inclusion in the studies.

Case studies:

The annual Oldford derogation test
The Oldford water treatment works is supplied from a borehole in the Mendip hills from which groundwater is pumped, treated and put into supply. The nearby Whatley Quarry could potentially affect the amount of water that can be obtained from the borehole because of the geology of the strata. This means the company must test the borehole every year to monitor the impact of the quarry.

The test involves undertaking initial clearance pumping, running the borehole pumps for 12 hours at a constant rate. The aquifer is then rested by turning the pumps off for a further 12 hours, which means no water can be treated or put into supply during that period. After this shutdown the company goes through six stages of increasing abstraction from the borehole to the full licensed abstraction.

The supply system feeds water from the borehole via the water treatment works and on into distribution and several service reservoirs. Water can be pumped into the system from a boundary with another treatment works area providing partial support. When the operational staff assessed the test programme, they were concerned that one of the reservoirs would empty during the rest or low pumping periods.

Modeling revealed that the level in one reservoir would indeed drop quite dramatically and then pick up again at around 11am on the day of the test. This information was relayed to the operations room, which monitored the telemetry closely and found that the model predicted very accurately the time at which the levels in the reservoir would begin to increase.

For the other reservoir the modeling showed that under average demand conditions the water levels would drop slowly, but generally it would be able to operate well on the day of the test. However, if demand rose by a further 10%, after about 36 hours the reservoir would empty.

The model also showed that if testing were undertaken during a peak summer demand period or if demand suddenly rose dramatically the reservoir would empty around 6pm on the second day. The modeling underlined the relative sensitivity of that particular system to demand, and showed that any increase in temperature or demand would mean the test would have to be cancelled immediately.

Durdham Down reservoir
Durdham Down is a small, hilltop, single-compartment service reservoir feeding part of Bristol and is supplied by fixed-speed pumps. As a result of a flow balance exercise, the leakage department were able to pin down a significant leak to the reservoir. The challenge was to take the reservoir out of service to effect repairs without damaging the mains in the area during the night, or losing supplies completely during peak demand periods.

The team used the model to look at the differences between day and night pressures. It showed that the pressure could increase by around 6m, but there was concern that this could be sufficient to cause a great many bursts. Conversely, during peak demand the model showed that if the zone was fed directly from the fixed-speed pump there would be insufficient pressure to pump the water to the reservoir site.

To resolve this, a pressure-reducing valve was inserted downstream of one of the pumps, which maintained the pressure during off-peak periods. The valve was set to the same pressure gradient as the reservoir at the top of the hill, but the model showed that during peak demand this would still mean pressure would drop below an acceptable level. To avoid this, staff monitored pressures at the top of the hill, switching on a second fixed-speed pump gradually to meet peak demand as needed.

Old Market trunk main analysis
This study took part in an extremely old area of Bristol, where the mains generally date from the 1840s to early 1900s. In the west, Victoria reservoir acts as the controlling head for the system. Towards the north east is an elevated, mainly residential area.

Modeling the additional demand for a new development highlighted a low pressure area when run under average day, peak week demands. The model identified high velocities, and headlosses, in the 10” cast iron trunk main feeding the area.

The operational staff were not convinced of a potential problem until they were shown a longitudinal profile of the pressure gradient resulting from the new development. One section in particular had a headloss of around 40m per kilometre, causing low pressures downstream. The team proposed replacing the main with a new 355mm PE pipe, which modeling had shown would improve pressures to the level of service expected in the problem area. This proposed main has now been successfully laid and commissioned.

Kingswoodcap zone
This is a mainly residential suburb located on a ridge of high ground to the east of Bristol city centre. The 12,200 connections in the zone are supplied via variable-speed pumps that pump water directly into the zone from a reservoir, creating a ‘cap zone’. The zone was being operated as one open zone, with the reservoir and pumps in the north.

There were many waste district isolation valves within the zone, but most of them were kept open to maintain adequate pressures. The Leakage Department approached the modelers because they were experiencing difficulties locating leaks and bursts in the area. Night flows had been increasing steadily over a number of days and the department was aware there must be a burst, but could not identify its location. The department wanted the modelers to split the zone model in two, which would halve their work in locating the leak as they would be able to focus on the half of the zone where night flows increased significantly.

Running normally as an open zone at peak demand, the model showed that pressures within the network were adequate. An existing low-pressure area in the network is permanently monitored to ensure it does not drop below the company’s minimum standard. New developments identified as due for construction by 2025 were also added, and a new pumping station created to cope with the additional demand. Given all this information, the modellers found the network operated well as an open zone.

The modellers divided the zone into two using the proposed east west split, closing valves to create two DMAs. When the model was re-run it could be seen that the western side would experience significant problems - there were a number of areas where pressures dropped below 10m. This meant that the zone could not be split simply by shutting valves - extra works to ensure the supply to the west side would be required.

Again, the company used a head loss theme to highlight the bottlenecks. Focusing more closely, the main supplying the west zone was analyzed. The modellers proposed a 250mm diameter replacement, which modeling confirmed would achieve the desired result. While this work was taking place, the company’s Development Services Department had been liaising with a local developer. The result was an agreement to lay a 180mm diameter main, which meant the modellers had to re-evaluate their plans. There was not sufficient room in the street to lay another parallel main.

Running the model with the 180mm main alone resulted in poor pressures as expected. So the company modelled the installation of a further section of 180mm pipe, connecting into the proposed 180mm section. Re-running the model showed that this would bring pressures back up to acceptable levels. Detailed modeling also meant the company was able to phase its investment: an initial length of180mm main has been installed and further lengths will be added as development takes place.

Banwell
Bristol Water identified a potential water quality problem in its Banwell zone. Banwell treatment works supplies the coastal holiday town of Weston-super-Mare, and there are a number of water storage reservoirs within the system.

There was concern that the treatment works may have contributed to this.

Using the InfoWorks WS water quality module, the modellers introduced a conservative pollutant and observed its passage through the system. Because of the time taken for the pollutant to reach the reservoir, it was possible to eliminate it as the cause of the problem.

Security of supply scheme
The Northern Strategic Support Scheme project began in 2003 and enables the company to provide backup for a large zone supplied by a treatment works to the north of its area. At that time, had the treatment works failed for any reason, it would have been difficult for the company to find an alternative supply for some 200,000 customers.

This meant it was vital to devise a scheme that could provide security of supply for this eventuality. The scheme being considered was the provision of a pumping station and storage reservoir at a point adjacent to another treatment works, allowing water to be pumped back into an existing trunk main to provide a supply north to the vulnerable area.

The team first used a trunk main model, starting with an old model of the entire system that was updated and used for the initial runs to gain an idea of the size of mains and pumping stations that were required. Then the work moved to an all-mains model that shows the three main treatment works supplying the zone - probably the company’s largest model with 106,000 nodes and 111,000 links.

As this is such a large model it takes a relatively long time to run - 20 minutes for a single, 24 hour, run compared with seconds for most. A recent InfoWorks WS feature that allows a subset of a model to be run proved extremely useful in reducing the run time. The model is used almost daily to assess strategic options and for various operational situations - the areas are divided frequently with regular rezoning for water quality or power optimization purposes.

The model is used not only to design mains diameters and pump duties, but also to work out control parameters in close liaison with the company’s control engineer. The model enabled a control mechanism to be designed which allows the network to be rearranged so that particular pumps can be activated to provide flows where and when they are needed. The charting options in InfoWorks illustrate pump performance, rise and fall in demand, power usage characteristics and pressures, which help to analyse the results of proposed controls.

The model has also been used during staff awareness sessions so that departments such as customer services can understand the work of the modeling team. The ability to export models to Google Earth is seen as an excellent way to make the modeling more easily understandable for non-technical staff.

Sizing of new reservoir
Modeling was also used to calculate the size of a reservoir to ensure it could act as a buffer for initial high demand, allowing a treatment works to slowly ramp up to increase its output. Various sizes of reservoir were considered, from 15Ml to 50Ml, and the results overlaid on a chart. This was later used in discussions with senior management as it provided an excellent graphical representation of the various options. The models give confidence in aspects of design when the company is sizing very large construction projects.

Mostar
Some years ago the company undertook some rapid water network modeling for Mostar, a town in Bosnia-Herzegovina sited on extremely hilly terrain. The Bristol team measured flows and pressures, particularly on the outlets of the reservoirs, using ultrasonic flowmeters and pressure loggers to obtain good data to calibrate the model.

Because of the recent war the town had few records of its water network, apart from a few paper plans. It was therefore necessary to produce records of the system on AutoCAD in a short space of time, recording the various sizes and materials on different layers within AutoCAD.

The company was able to produce a base Geoplan of the model that included elevations, pipe diameters and pumping stations within a few weeks. The calibration proved good despite the lack of data and the team were able to recommend zoning and mains reinforcements for the town.

InfoWorks WS attributes
This particular exercise underlined the fact that InfoWorks, with its data import and cleansing features, can be used to build a model very quickly even when there is a shortage of available data. Bristol Water has found in general over the years of using InfoWorks WS that it has good functionality and is capable of modeling most of the challenges encountered within the company.

This article is based on a paper presented at the Wallingford Software International User Conference in September 2008 by Kevin Henderson, Network Planning Manager and Richard Foster, Network Modeling Engineer at Bristol Water.




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