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Integrated InfoWorks CS modeling unravels the causes of complex Birmingham flooding

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Courtesy of Innovyze

Atkins Group has effectively modeled two trial catchments that highlight the potential of MWH Soft’s InfoWorks CS to improve understanding and communication of the issues and potential solutions for urban drainage.

The consultancy is undertaking a landmark research project for UK Water Industry Research (UKWIR) on the effects of climate change on sewerage networks (see box below) and has developed two fully-integrated urban drainage (IUD) models to unpick the complex causes of serious flooding in Birmingham.

InfoWorks CS and InfoWorks 2D were used to create the models for the Birmingham Water Group (BWG), which was established during the Defra Upper Rea IUD pilot. The BWG, which consists of Birmingham City Council, the Environment Agency and Severn Trent Water, is building on the partnerships created in the pilot project to provide a holistic approach to flood management across Birmingham.

The Wood Brook model

The first model represented the Wood Brook, a small heavily urbanised catchment in the Selly Oak district of Birmingham.  While the name suggests an idyllic setting, it is locally known as the “sewer-brook”, which highlights that the issues faced are not simply flood-related.  The tool therefore had to be able to represent the foul sewer system as well as the surface waters of the brook in one integrated hydraulic model.

The area has a history of flooding problems, and the current study was driven by an event at the end of the summer of 2008 when a deep area of low pressure slowly moved north-eastwards across the Midlands. In Birmingham, the resulting exceptional summer storm caused severe flooding in the Wood Brook area. The causes were complex and attributed to a mix of overtopping from the catchment’s surface waters, surface waters flowing down roads, and surcharging from the wastewater system.

Atkins mapped the extent of the flooding based on reports from residents and historic flooding locations, which were obtained from a database that Atkins had helped Birmingham City Council to compile. The consultancy undertook more detailed analysis on the larger of the two catchments, the River Cole.

The River Cole catchment

The River Cole is Birmingham’s second largest river and a tributary of the River Tame.  While the catchment is not totally urbanised, as is Wood Brook, previous studies have shown that the effects of urbanisation and the associated drainage infrastructure dominate Birmingham’s flow regime. 

This study originated from a serious flood event in mid-2007, when much of the UK experienced exceptional wet weather. A band of rain that extended across the Midlands to the north east of the UK left the River Cole catchment very vulnerable to further rains that fell on 20 July. This combination of wet weather events caused the River Cole to swell and burst its banks; because the drainage system was unable to discharge into the river, surface water flooding also occurred. In all, 60 people had to be evacuated from the area.

Fully IUD modeling approach

As InfoWorks CS had been effectively used to develop a fully integrated model for Defra’s North Brent IUD pilot project, it was decided to incorporate the recommendations from the integrated modeling benchmarking study and the lessons from North Brent into the Birmingham project using MWH Soft’s InfoWorks CS and InfoWorks 2D.

Atkins chose the integrated InfoWorks CS/2D solution because InfoWorks CS is able to represent open watercourses and flows in pipes, modeling the open channel flows and below-ground assets using a fast and stable 1D mathematical solver, while InfoWorks 2D models the above ground flows using a 2D unstructured mesh.  As both the 1D and 2D components of the software are fully integrated within a single user environment, the seamless linkages allow modelers to accurately represent the interactions of all of these elements.

A fully integrated urban drainage model of the Wood Brook had been developed (for more details see “Managing surface water - demonstrating an integrated solution” ) and the same methodology was also used to develop the IUD model of the River Cole - the only major difference was the need to represent the Cole’s more rural upper watershed. Because the runoff models and methods were similar to those traditionally used in river models, the existing one-dimensional ISIS model’s ReFH inflow units were input into the IUD model as inflow hydrographs feeding into the upper catchment.

Topographic survey data was used to represent the river channel, and the overland surface was characterized using a DTM based on 1m LIDAR data from the Environment Agency.

This approach is in keeping with the recently published IUD modeling guide, and proves that holistic approaches to urban flooding can succeed both in highly-urban catchments and those with more rural elements.

Simulating the 2007 event

Given that InfoWorks CS is not a traditional modeling platform for investigating fluvial flooding, confidence in how it represented this had to be explored as part of the project. This was achieved by simulating the rainfall data recorded for the 2007 event. 

To give complete confidence in the in-bank hydraulics, the flood height at one location was compared with the wrack mark level calculated from a photograph taken during the 2007 event. The company also mapped the flooding reported in the flood surveys - again the results tied in well with photographs taken at the time, which confirmed the accuracy of the out-of-bank hydraulics of Infoworks CS.

The InfoWorks CS River Cole model also accurately identified the locations of residents who reported flooding during the event. Looked at in time slices, the model showed that the drainage flooding occurred well before the arrival of the fluvial peak – as the event progressed, the river became fuller and the drainage network backed up, causing flooding to the properties. Combining the 1D modeling of the drainage network and 2D modeling of overland flows enabled surface water flooding from the sewer network to be identified and traced through the urban setting, as well as the extent and direction of the fluvial flooding – something a traditional fluvial model could not have achieved.

The model simulation was found to closely represent actual events reported by the Environment Agency – a level of accuracy that makes it an extremely powerful and realistic tool.

To provide further confidence in the River Cole model, Atkins ran a 100-year design event through the model and was able to replicate the Environment Agency’s record of flooding in a particular zone. The modeling identified additional and quite extensive surface water flooded areas, although the roads and buildings had not been added to the model at that point, which will undoubtedly alter the flow routings and outcomes.

Next steps

Since reporting the Wood Brook study, the IUD model has been calibrated and a Surface Water Management Plan (SWMP) is now being developed for the catchment. Likewise, for the River Cole, a SWMP is being developed through the inclusion of additional partners (Solihull Metropolitan Borough Council and Worcestershire County Council) and will be used to assist BCC with reservoir assessments that are needed within the catchment. When this is complete, the outputs of the UKWIR research project will be used to assess the future risk of climate change in these catchments. If this work takes place, it will unravel the conflicts between urban and rural hydrology reported in the Defra IUD pilots, as well as the suitability of the climate change allowances currently used.

Conclusion

MWH Soft’s InfoWorks CS and its 2D module have been shown to be an effective platform for fully-integrated modeling. The solution provides an ideal way to test alleviation measures and scenarios that can then be demonstrated in 3D to the client, giving everyone involved an improved understanding of the options.

Atkins recommends that hydrologists and engineers should carefully consider using InfoWorks CS for fully integrated studies - such studies are data intensive, but provide a much-needed way to holistically investigate complex flooding in urban areas. The River Cole project demonstrates that with the help of appropriate software, flood risk management strategies - and in particular SWMPs - can achieve many of the Pitt Review’s recommendations.

This article is based on an original presentation by Kristian Ravnkilde and Adam Cambridge of Atkins Group to the MWH Soft User Conference in September 2009.

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