Water Resources Research Report

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Summary

The main objective of the research project currently under way is to provide an engineering assessment of the vulnerability of London’s public infrastructure under projected rates of climate change with special emphasis on flooding. An original systematic procedure is used to gather and examine available data in order to develop an understanding of the relevant climatic effects and their interaction with municipal infrastructure. Assessment of climate change impacts on municipal infrastructure requires floodplain maps and inundation that will correspond to examined climate change scenarios. This report presents the results of hydraulic analyses used in floodplain mapping under changing climate.

Combined, climate and hydrologic modeling, were used to generate input flow data for hydraulic modelling. Standard computer software HEC-RAS is used for hydraulic computation of water elevation. The existing HEC-RAS models of the Upper Thames River basin are not georeferenced and therefore they cannot be used for hydraulic modeling under climate change. Consequently, it was necessary to develop new HEC-RAS models for the rivers and creeks of London that were considered in this project.

Geometric input data for new HEC-RAS models were created using HEC-GeoRAS software, which is an extension of ArcGIS computer package for spatial analysis. In the preprocessing phase the HEC-GeoRAS is used to create a digital terrain model from the contour lines shape file provided by the city of London. In the next step the following geometric data layers were generated: river center line, bank lines, flowpaths, cross sections, and bridges. Required attributes were assigned to each of the layers. In the last step of the pre-processing stage the input file for the HEC-RAS hydraulic analysis was prepared. The hydraulic analysis starts with the geometric data import, followed with the preparation of the hydraulic structures data and flow data. A very detailed quality control was performed on the cross sections data generated during the pre-processing phase. The roughness coefficient values were determined using the existing HEC-RAS models and aerial photography of the basin. Data on bridges, taken from the existing models and drawings were integrated with the rest of the data.

Two climate scenarios (historic and wet) developed by climate and hydrologic modeling (Eum and Simonovic, 2009) were used and water surface elevation profiles were calculated for 100- and 250- year return periods. The computation results were used to assemble the HEC-RAS GIS export file for floodplain mapping. The Arc Map software package was used to create water surface GIS layer. Overlaying this layer with the terrain provided for calculation of floodplain boundaries and inundation depths. The floodplain maps generated using this process are used in vulnerability assessments of London’s public infrastructure to climate change currently in progress.

The results of water surface profile computations are presented in tabular form for the 250- year flood under historic and wet climate scenarios. The final floodplain maps along Main Thames for both scenarios show minor deviation of the floodplain boundaries when compared with the existing floodplain lines. However, the water depth difference is up to 50 cm. The area upstream from the culvert on Pottersburg Creek (close to the intersection of Trafalgar St. and Clarke St.) is identified as critical due to the high extent of flooding. The flooding at this location is caused by insufficient culvert opening that creates a backwater effect. Areas of special concern are identified where the floodplain mapping results are not sufficiently accurate due to inaccuracies in the contour lines. The main recommendation based on the work presented in this report is that new georeferenced cross sections should be surveyed in order to increase the accuracy of the floodplain mapping process. The hydraulic analyses should be repeated with more accurate input data and the resulting floodplain maps should be revised accordingly.

ISBN (Online)

978-0-7714-2847-0

ISBN (Print)

978-0-7714-2846-3

Publication Date

9-2009

Publisher

Department of Civil and Environmental Engineering, The University of Western Ontario

City

London, Ontario, Canada

Disciplines

Civil and Environmental Engineering

Notes

Report no.: 069

City of London: Vulnerability of Infrastructure to Climate Change. Background Report #2: Hydraulic Modeling and Floodplain Mapping

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