Building better integrated network data
Drawing on experience from the UK, Tony Andrews of Wallingford Software believes that the integration of GIS technologies with hydraulic modelling software is a powerful solution to help water authorities meet their responsibilities relating to river catchments, urban drainage, and water distribution.
However, new data extraction and cleansing techniques together with better linkages between GIS and modelling software, have produced good quality geographic data enabling modellers to build bigger networks faster and more reliably
There is growing evidence that the closer integration of GIS with hydraulic modelling software can assist water authorities to meet regulatory requirements, achieve financial targets, carry out design work, and improve the operational and environmental management of rivers, water distribution networks and collection systems.
The basic system architecture of an Integrated Network Model links data storage using a GIS to an hydraulic modelling software suite such as InfoWorks1.
Specific data requirements are different between the three modelling areas of drainage, water supply and rivers, but the maintenance, versioning and auditing of static asset data are fundamental requirements of an Integrated Network System. GIS vendors and the many specialist asset information database suppliers now provide data models that can be adopted in the drainage, supply and river sectors
Typical GIS data requirements for a wastewater hydraulic modelling study comprise: Network asset data (i.e. con duits, manholes and ancillaries).
- Sub-catchments (contributing areas)
- Surface area breakdown (road/roof polygon areas) from impermeable area study for area take off calculation.
- Population data (address point).
- Rainfall profiles from Thiessen polygon analysis.
- Viewing geographic information data types and image formats as background mapping layers.
An excellent example of the integration of GIS and wastewater modelling can be seen in the surface area breakdown from an impermeable area study for area takeoff calculation. Impermeable area surveys are conducted to establish an understanding of the distribution of impermeable and permeable areas in catchments in order that the correct surface type can be assigned to features in the urban environment. This is typically carried out through a survey of the catchment, and represented digitally in a GIS using a combination of data acquired from the UK Ordnance Survey and aerial photography. The analysis of the different areas is conducted using GIS, with the hydraulic modelling software providing area take-off tools to calculate the runoff surface areas and the contributing area for a subcatchment using the data imported from the GIS (see Fig.1).
Water supply systems
In the water supply environment, GIS and GI data assist modellers through the incorporation of sup porting asset information (pipe condition, class, material, age etc.). It provides the functionality to assign elevation to nodes and customer points using digital elevation data, and to associate spatial information such as bursts and customer complaints with hydraulic data.
Hydraulic modelling software is designed to streamline the modelling process by automating the most repetitive tasks and providing flexible links to all the source data. The functionality of modelling software has extended well beyond just simulation; examples include:
- GIS data cleanup and connectivity checking.
- Links to logger and telemetry in their own formats.
- Automatic demand allocation.
- Automatic setting of elevations.
- Look-up tables to set asset attributes (e.g. pipe diameter and roughness).
The main purpose for providing the direct link between GIS and modelling software is to facilitate model building and automatically allocate demand using a combination of GI data, Microsoft Office data files such as Access or Excel, and text files (e.g. commaseparated variable). The most fundamental requirement of any hydraulic modelling package is the provision of tools to enable the:
- Automatic derivation of elevations at all nodes, spatial data (e.g. bursts, complaints) and customer points.
- Automatic allocation of demand at any node and/or pipe using georeferenced seed point information such as address point (see Figure 2)
- Incorporation of geo-referenced information to support the model ling process such as customer com plaints and pipe bursts. These can then be allocated to the nearest main and pipes graded by structure as well as hydraulic condition.
However, it is the process of allocating demand on the water supply network that has benefited the most from the incorporation of GIS technology and GI data in the network modelling products. This is where the water supply modelling community have been able to automate tasks and save precious resources in building demand into the model.
Figure 2 illustrates the incorporation of address point data to allocate demand at nodes. The address point data was imported into InfoWorks WS from GI data, having been prepared using a GIS. A base demand is applied for unmetered customers in appropriate units (e.g. litres per property per day) or for metered consumption demand can be extracted from the billing data.
The key data requirements for river models are the cross sectional profiles and elevation data relating to the river flood plain. Profile data represented by a series of x, y, z-values (z representing elevation) does not have to be managed and served to the modelling system using a GIS, but the preparation of a digital elevation ground model of the flood plain is perhaps the clearest example of the necessity for integrating GIS technology.
Digital elevation data will be familiar to most hydraulic modellers and should be familiar to all GIS specialists. Elevation is represented as a matrix of points or more commonly in a regular grid raster pattern. In order to analyse, display terrain features and fit surfaces to the elevation data, the grid data is converted using GIS technology to a triangulated irregular network (TIN) dataset. GIS specialists will be familiar with this data, but to river modellers TIN data will be even less familiar than the grid dataset.
A TIN dataset represents a surface derived from irregularly spaced sample points and breakline features, with the points comprising x, y, and surface or z-values, and a series of edges joining these points to form nonoverlapping triangles. The triangular mosaic forms a continuous faceted surface. TINs offer an alternative to the raster data model for representing surfaces.
Using TIN/GRIDDED elevation data in modelling software such as InfoWorks RS enables the direct takeoff of elevation data to facilitate the extraction of model sections and floodplain storage properties based on overlaid section locations and boundaries.
The TIN/GRID is also used to generate and display ground level contours, and forms the basis for dynamic flood mapping. River modelling products now have full flood-mapping capability based on sophisticated floodinterpolation models overlaid onto a TIN/GRID based ground model.
The floodinterpolation model enables:
- Instantaneous flood mapping of any simulated event, typically including the additional ability to replay dynamic results in animation, or display flood maximum extents.
- Contouring of flood depths.
- Flood graph of water level and depth at any point within the flooded envelope.
- Interaction with imported georeferenced seed point data (addresspoint) to produce reports of flooded-depth and duration at specified locations.
Hydraulic Network Modelling software in the UK has been essential in meeting expected levels of service and in reducing costs. Clearly, closer integration of GIS and modelling software will advance the capability of water authorities to achieve regulatory requirements, meet financial targets, carry out design work, and improve the operational and environmental management of rivers, water distribution networks and collection systems.
Recognising this, GIS service implementers and middleware providers have formed
partnerships with businesses in the water industry, whilst GIS vendors dedicate
teams of specialists to focus on the water business. Likewise, hydraulic modelling
vendors continue to add new features and tools to automate much of the previous
manual work of building models, and to ensure that their product is a component
of the integrated network modelling strategy.