Open data flood mapping of Chao Phraya River basin and Bangkok Metropolitan Region.

Aims: To examine the utility of open data for flood mapping of the Bangkok Metropolitan Region and Chao Phraya River basin. The region is particularly vulnerable to flooding, having experienced recurrent major flooding events, including the some of the most extensive and prolonged in 2011. Study Design: Novel methodologies were innovated utilising open spatial data and open source geographical software to generate flood extent/hazard maps of the Bangkok Metropolitan Region and Chao Phraya River basin. Key geospatial data were sourced from the Thai Geo-Informatics and Space Technology Development Agency and NASA’s Shuttle Radar Topography Mission. Methodology: Given limited resources for conducting detailed hydrological-hydraulic analyses, two alternative approaches were examined for flood extent/hazard mapping of the basin and city. The first method made use of publicly available historical flood data to produce an up-to-date composite flood extent/hazard map. The second approach, using the latter output as a reference source, examined the utility of a modified topographic index for delineating flood-prone areas, as integrated into the r.hazard.flood module of the open source GRASS GIS application. Results: Compilation of multi-year historical data enabled generation of a relatively finescale (~100m spatial resolution) flood extent/hazard map for the basin and city. The optimal tau threshold for delineating flood exposed cells from the modified topographic index was linearly related to the sub-basin mean slope. The four most northerly subbasins of the Chao Phraya basin, those with higher mean slopes, gave lowest total errors, ranging from 17.5 to 35.9 percent. Conclusions: Open data in the form of multi-year spatial flood layers were effectively Original Research Article British Journal of Environment & Climate Change, 4(2): 186-216, 2014 187 combined to generate a relatively fine-scale flood extent/hazard map for the Chao Phraya River basin and Bangkok Metropolitan Region, and the modified topographic index showed promise as an alternative means for identifying flood exposed areas.