Open MI: linking models

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Courtesy of Courtesy of Innovyze

Traditionally sewers and rivers have been modelled separately using different software applications, with outputs from one model being fed into the other. However this does not fully represent the continuous interaction between the two modeled elements. Assessing the impact of sewerage discharges and overland flows from urban areas to watercourses and the subsequent effects of watercourse levels on the sewerage system requires an holistic approach to modeling.

Combining all of the required elements of both urban and river catchments in one software application may not be practical. However being able utilize and link existing models, taking the strongest and most relevant aspects of each one, enabling them to exchange data as they run and allowing interactions to be represented within the simulations being undertaken is currently an achievable goal.

In response to the need created by the Water Framework Directive from its introduction of integrated water management and to take a large step towards the goal of total catchment modelling, the HarmonIT project was set up.  HarmonIT developed the Open Modelling Interface (OpenMI) to allow models to exchange data, thus enabling linking of models and data interaction. OpenMI is not a product – it is a standard for data exchange. Although this began and was funded as a European initiative, it is being adopted world-wide.

Below a case study shows how the results of InfoWorks CS and RS models are linked by OpenMI to reproduce the interaction of rivers and urban areas.  The rusts show that the river ingress and outfalls blocked by high river levels, lead to a more accurate representation of urban flooding, CSO spills and river characteristics.

Case study

A project for the mainly-residential market town of Kenilworth in the affluent west Midlands, which had a history of flooding issues from various sources, utilized both InfoWorks CS and InfoWorks RS, linked via Open MI. There are three main open channels in the town – Finham Brook; Fishponds Brook and Tanyard Stream, as well as the urban drainage system, which interacts with the rivers, mainly discharging to either the Tanyard Stream or Finham Brook.

An InfoWorks CS model of the system was created, with points indicating the outflows to the rivers, a wastewater treatment plant (WWTP) and its outfall downstream. The Tanyard Stream is represented within the model. This accepts large urban stormwater discharges and transfers them to its confluence with another watercourse. The only CSO prior to the area’s sewerage strategy being undertaken was an overflow from the storm tanks at the terminal pumping station. As part of this strategy, a further overflow was added, which had to meet strict Environment Agency criteria.

The main feature of the foul/combined sewerage in Kenilworth is a triple sewer system, which roughly follows the line of the Finham Brook. This comprises the original town and Abbey system, a later, duplicate system, and relief sewers. A terminal pumping station transfers flows to the Finham wastewater treatment works on the outskirts of Coventry. The three sewers are interconnected along their lengths, with many branches. As a result the Kenilworth sewers form a hydraulically-inadequate looped network, in which the flows to be modeled are complex.

Originally there were three separate models representing the town’s water courses and sewer system – an InfoWorks CS model of the sewers, and hydraulic models of the Fishponds Brook and Finham Brook. Before OpenMI became available, the river model would be run and the resultant levels copied into the InfoWorks CS model to obtain an estimate of their impact. In order to utilize OpenMI and obtain a more accurate estimate of the interactions between the watercourses and sewers, the hydraulic models had to be converted into a compatible solution, for which InfoWorks RS v7.0 was chosen.


The InfoWorks RS and Infoworks CS models were linked using OpenMI, with the links established between data outputs and inputs.  Data outputs in CS and RS are the simulation results, with the flow at a link being either the output from the CS model or the stage at a river section in RS.  Data inputs are defined when the model is set up.  In CS, the input used was the level at an outfall.  In RS, the input was flow at a flow/time boundary.

The interactions were established as bi-directional links, in which data was exchanged to and from both the CS and RS models. Within the Open MI environment it is also possible to add a model to monitor data, which usefully enables modellers to examine the values passing between the models.


In the Kenilworth model, the results of the linked models showed that the downstream flows remained backed up for longer than had been predicted when the models were run separately. The downstream river levels in the RS model were found to stay higher for longer than expected, under the influence of the overflow discharges, rather than as represented in the separate models where the sewer overflows began to spill into the river only when its level fell, and no feedback was represented in the model.


The OpenMI is a useful tool in linking different hydraulic models.  The Kenilworth study showed that OpenMI could combine InfoWorks CS and RS in an urban environment to represent flooding interactions within one simulation run.  The linking of models increased the ability to understand the complex hydraulics.  It is possible to model this interaction through a manual transfer processes, however unless a laborious iterative approach to this manual transfer is adopted not all feedback interactions are accounted for. 

The technology’s potential reaches far beyond modelling the interactions that cause flooding; it can be used to represent and integrate other types of models such as treatment works, estuarial and coastal flows. New sources of data such as ground contour models can also be integrated, allowing overland flood routes to be accurately shown.

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