MSc Student in GI, City University, London
Dr Kalanithy Vairavamoorthy
South Bank University,London
Conventionally the design of water distribution systems has been based on the assumption of continuous supply. In most developing countries water supply is not continuous but intermittent, and this could have been foreseen at the design stage. By trying to fit in a system that was designed to operate as a continuous one to operate intermittently results in severe supply pressure problems in the network and great inequities in the distribution of water.
The overall shortage in water availability in most developing countries necessitates intermittent supply at a low per capita supply rate. In particular they often lead to inequitable distribution of available water resources. Since these systems are water starved, consumers collect as much water as possible, quantity collected being directly related to pressure at outlets. And since pressures vary greatly in the network, quantity of water supplied is inequitable.
In addition to inequities, low pressures arise because systems are designed based on low per capita allocation and with the assumption that demand is over a 24 hours at about 2.5 times average flow rate. In reality water is drawn in a shorter duration. This implies that the system suddenly becomes undersized because flows in pipes are much greater than anticipated resulting in severe pressure losses. Hence, there is generally a low-pressure regime in the network.
In intermittent water networks the quantity of water collected by consumers will be dependent on the driving pressure heads at the outlets and hence the relationship between the pressures in the system and the demands are important, and it cannot be assumed that demand will be met under all conditions. Therefore, the application of standard methods of network analysis to intermittent flow conditions is inappropriate and hence the need for modified design for intermittent water distribution systems. The Design Guidelines formulated by Water Development Research Unit of South Bank University, London under Dr K. Vairavamoorthy is a big step in this direction and my study is based on this.
GIS in Design and Asset Management
An important component of a water supply system is the distribution network (pipes, nodes, pumps, valves and storage tanks). A water distribution network design involves optimal design of network components and all the raw data needed for analysis and design has spatial component and hence the need for GIS integration. Any water distribution system design involves the analysis of network data in the form of pipe parameters (length, diameter, roughness coefficients etc). This task is voluminous and takes up huge resources in terms of skilled man-hours in trying to build up a framework. Also for optimal network design parameters of the network components are varied and recompilation carried out. This is a mammoth task in itself and by the use of GIS helps the design engineers in not only speeding up the process but also devising better techniques for doing this. Basically there are two types of design. First case is to design for an entirely new system and second one is to augment an existing system. In both case data on network components is of great importance.
Proper Techniques for Asset Management is a huge problem faced by engineers in developing countries and GIS holds the key in helping them do this. Also by the use of web based maps of the network, the engineers in the different field offices can by cost-effective means be able to access network information. Once all the network information is stored in a GIS it makes the use of querying possible with ease and helps the planners in better understanding of the system and is able to act in a effective manner to any contingencies that may arise. Use of GIS data though Web based interfaces offers staff in a distributed environment manage the assets in a cost-effective manner.
A water distribution network design involves optimal design of network components and all the raw data needed for analysis and design has a spatial component and hence the need for GIS integration. GIS finds its role in the Analysis, Policy, Design and Implementation phases of Intermittent Water Distribution Systems and should be envisaged on this bigger picture.
GIS can be used as an integrated tool in processing of spatial data for the overall design and asset management of intermittent water distribution systems.
Figure1: Demand Area with polygon around primary nodes and distribution of the demand
The aim is of the paper is to look into ways in which GIS can be used as an effective tool to help the engineers in the design and asset management of intermittent water distribution systems. The reason I am concentrating on intermittent systems is that these systems are widespread in developing countries and as far as I am aware of the widespread use of GIS in tackling the design as well as O & M problems has not taken place. The main factors that are responsible for this situation are resource constraints and very little research concentrating on this aspect.
The objective is to highlight the key application areas and highlight how through the use of GIS will help for in both design and asset management.
The broad areas where GIS can be applied are
Data Collection: Conventionally data on Networks were collected and stored in paper format. This is not only labour intensive and error prone but less effective. With the advent of newer technologies like GPS and digital cameras data collection can be made faster and accurate and also better GIS integration of data. In areas where this digital data is not available paper plans can be scanned and used instead. For newly developed areas where no data is available aerial photographs or satellite images can be used. It is also possible to use a combination of raster and vector data.
Data Storage: Data on network components can be stored in GIS for spatial querying. The main data that will be stored will be pipe and node locations and characteristics, reservoir details, valve details and depending on level of sophistication even include types and details of consumer locations. This data can be collected and inputted by field staff in spreadsheets (e.g. MS Excel) and later imported into a GIS. This helps bring down costs as technicians with minimum GIS skills can do bulk of the data handling operations.
Pipe data is held in the system in a node and line type format. This is ideally suited to the pipeline type layout with nodes and connecting pipe-work. Data can be input in several ways, either directly onto to the screen, from a digitiser, or from previously captured data that can be read into the database as long as it has a record of its grid co-ordinates and connectivity. The GIS includes an import facility for standard GIS to enable interoperability.
Data Mapping: generating the mapping of distribution system for various criteria using GIS makes everyday querying based on specific criteria quick, easy and understandable for engineers. Fundamentally the model presents information, as on a map. But it is an intelligent map in that each item that you can see is supported and described by other data that the user can access by a simple click on the mouse button. There is a great deal of flexibility in the way that data can be used. Background maps in a variety of formats can be supported.
The GIS contains a standard database structure to hold all the data that is regularly used for distribution systems, but the system is not limited to just this data. Any data set can be viewed provided that the information is given either a grid reference or a cross-reference to an item (node or line). With maps and data in the system it is easy to view and interrogate. Information can be viewed selectively, for example with different layers switched on and off as need may be. Or data can be filtered: only pipes of greater than 100mm diameter can be displayed, or even those greater than 400mm diameter viewed in blue, and those greater than 200mm diameter in red. The graphical output can be shown in GIS and other relevant features can also be shown relative to for example. Pressure variation. The model output can be overlaid on an aerial photograph of the city and also the location of consumers potentially affected shown. If the population theme is turned on, then the Select feature can select an area of interest highlighting the population at risk.
Searches can be made on various criteria. Areas for a search can be defined by boundaries such as municipality or zonal area, or an area defined on screen by drawing a line with the mouse. Reporting is also easy. Plots can be produced to replicate what is on the screen. Text reporting uses the GIS functionality to select data requested by the user, but outputs in a spreadsheet format suitable for exporting to a package such as Excel.
Analysis: Statistical analysis, data interpolation and spatial statistics to be carried out with the GIS. Due to the voluminous amount of data that has to be stored in case of any distribution network the various analysis tools in any standard GIS not only help in Spatial analysis but also in statistical analysis of data sets.
Visualisation: Multimedia data like photographs, videos and 3D panchromatic imagery to be stored for corresponding data sets to enhance real-life scenarios. This enables site condition assessments for immediate actions. Also the visualisations tools in GIS can be used to find patterns and relationships in the huge amounts of data collected like pressure and flow recordings. Also the functions in GIS to plot graphs for the data help the engineers in finding anomalies in the huge data sets that is otherwise impossible to detect. Also 'what-if ' scenarios can be generated and effects studied which helps in better and informed decision making.
GIS in Demand Modelling and Design: GIS to be used to model hydraulic conditions in spatial terms and how it can be used to help in design of systems. Performance information is so important to network design and in planning. A dynamic, hydraulic, water-distribution model helps to understand the effect of different demands on the network, and allows the right design decisions to be made to allow for future increases in demand. The performance of the network can be tested under extreme conditions. . A model helps in the assessment of which parts of the network will be prone to low pressure conditions. A model gives an estimate of pressures throughout the network, during the supply interval, while maintaining an adequate pressure
A main factor for calculating the water demand is finding the area of influence of primary nodes in water distribution system design thereby finding the demand in that polygon. Each primary node in a water distribution network can be considered as an important node in the network into which demand of many secondary nodes or individual connections are lumped. The area of influence around each primary node is that area (polygon) which can be considered to be supplied from that primary node. There are many ways to determine demand polygons, one of which being the Voronoi polygon method.
Basic Procedure for the construction of Voronoi polygons for Primary Node Demand Allocation
- Step 1: Identify primary demand nodes in the supply area
- Step 2: Draw lines to connect each of the primary nodes to their neighbouring nodes
- Step 3: Draw perpendicular bisectors to the lines connecting the primary nodes.
- Step 4: Output is the area around the primary nodes enclosed by the perpendicular bisectors.
- Step 5: Find the secondary connections (HC1, HC2, HC3, HC4 etc) falling within the primary node polygon and add the cumulative demand in each category for each primary node.
- Step 6: Add up the total demand for a whole demand area by summing up the demand of individual primary nodes falling within the demand area.
For each primary node create a polygon i.e. area of influence for each primary node and then generate a demand polygon overlay map (Figure 1)
Figure 1 shows a Demand Area ' A ' with 5 primary nodes. Each of these 5 primary nodes has a number of secondary nodes connecting to it. The total demand in each Primary node can be calculated and all this summed up to give the total demand in the given demand area (greyed area)
In the conventional methods of water distribution design, manual methods based on the given steps were employed which is tedious, time consuming and very difficult for adapting changes. The use of GIS coupled with Voronoi techniques is a boon for water supply design engineers as it helps in building a design model in a much quicker and in a rational way. This whole process of generating voronoi polygons and lumping demand can be done using a GIS very effectively. This is just one of the many applications in Network Design that can be automated by using GIS
Asset Management: Asset Management is the systematic process of maintaining, upgrading and operating physical assets. In the case of a water distribution systems physical assets includes the network components such as pipelines, storage reservoirs, pumps, valves etc. The key advantages of Asset management is that it enhances the knowledge of the engineers of the various assets and their details, helps in logical decision making process, it acts as a framework that provides a measure of the network performance and links it to both short term and long term planning thereby helping in optimal improvement of assets at least cost.
Proper record keeping of the network components is a big task and GIS offers the tools to successfully implement that. Engineers require information on various aspects of the network for administrational as well as maintenance needs. Also updating of information is a integral part in the successful implementation of O&M tasks. This part is focussed on creating User Friendly Interfaces including option of Web Based Facility to enable the engineers at different offices to have access to basic network information using familiar internet tools. Web based map services enable accessible better service of data more efficiently. The users can select the data they want by criteria when they want it in a cost-effective manner.
GML: The Geography Mark-up Language (GML) is an XML encoding for the transport and storage of geographic information, including both the spatial and non-spatial properties of geographic features. This specification defines the XML Schema syntax, mechanisms, and conventions that provide an open, vendor-neutral framework for the definition of geospatial application schemas and objects, enable the creation and maintenance of linked geographic application schemas and datasets, support the storage and transport of application schemas and data sets, increase the ability of organisations to share geographic application schemas and the information they describe.
Implementers may decide to store geographic application schemas and information in GML, or they may decide to convert from some other storage format on demand and use GML only for schema and data transport.
Geography Mark-up Language (GML) can be used to enable the effective data sharing and interoperability of data collected and stored of the network components. A huge benefit that the advance of information technology has brought to operations and planning is through the widespread use of networked office systems on the desktop. These have let people throughout the organisation readily share and understand information. But to do this they must make it easy to exchange data with corporate databases, Geographic Information Systems, desktop mapping, word-processors and spreadsheets. Integrated use of GIS enables the complete appreciation of the network characteristics and provides the most progressive way towards enhanced management of intermittent water distribution systems.
The application of GIS technologies for the effective design and management of Intermittent Water Distribution Systems is the need of the hour and utilities and governments in developing countries should begin to tap-in the benefits offered by the use of GIS in this regard. This is particularly true for big developing countries like India and China where the huge rate of population increase coupled with severe water shortage has forced engineers to rethink effective ways for managing water utilities and GIS application holds the key to an integrated approach to help solve this problem.
- Chen, C., 1999,A raster based method for computing Voronoi diagrams of spatial objects using dynamic distance transformation, International Journal of Geographic Information Systems, Vol 13, 209-227
- Li C, J.Chen, Li Z, Gold C, 2001, A Voronoi-based 9-intersection model for spatial relations, International Journal of Geographic Information Systems, Vol 15, 201-220
- Vairavamoorthy, K., 2001,Guidelines for design of Intermittent Water Distribution Systems, DFID, UK
- The Voronoi Web Site (2001) Available https://www.voronoi.com [30 April 2002]
- The GML WebSite (2002) Available [2 May 2002]
- Terry A.Slocum, 1999, Thematic Cartography and Visualisation, New Jersey, Prentice Hall
- Kraak, M. and Omerling, F., 1996, Cartography: Visualisation of spatial data, Harlow, Longman
- Longley P, Goodchild M, Maguire R, Rhind D (2001) Geographic Information Systems and Science, London, Wiley.