Building Exposure Maps of Urban Infrastructure and Crop Fields in the Mekong River Basin
Haas, Eva Maria; Weichselbaum, J.; Militzer, J.

In the frame of the Integrated Water Resources Management (IWRM) initiative for the Mekong river basin World Bank is collaborating with the Mekong River Commission and governmental organizations in Cambodia, Lao PDR, Thailand and Vietnam to build national and regional capacities for managing the risks associated with natural disasters, such as floods, flash floods and droughts. Within 'eoworld', a joint initiative set up by ESA and World Bank to foster the use of Earth Observation (EO) for sustainable development work, a comprehensive database of elements at risk in the Lower Mekong river basin has been established by GeoVille, including urban infrastructure and crops (primarily rice paddies). In the long term, this exposure information shall be fed into an open-source multi-hazard modeling tool for risk assessment along the Mekong River, which shall be used by national stakeholders as well as insurance and financial institutions for planning, disaster preparedness and emergency management.

Earth Observation techniques can provide objective, synoptic and repetitive observations of elements at risk including buildings, infrastructure, and crops. Through the fusion of space-based and in-situ data from field surveys and local knowledge (e.g. on building materials) features at risk can be characterised and mapped with high accuracy.

Earth Observation data utilised comprise bi-weekly ENVISAT-ASAR imagery programmed for a period of 9 months in 2011 to map the development of the rice cultivation area, identify predominant cropping systems (wet-season vs. dry season cultivation), crop cycles (single /double / triple crop per year), date of emergence/harvest and the distinction between rice planted under intensive (SRI) vs. regular rice cultivation techniques.

[Caption: Mekong - Rice acreage and seasonal cultivation pattern; © GeoVille for ESA / World Bank; Image data © ESA]

Very High Resolution (VHR) optical data from SPOT, Kompsat and Quickbird were used for mapping of buildings and infrastructure, such as building footprints, residential / commercial areas, industrial buildings, main infrastructure, and other public assets. A key input to this work was data collected by the project team in the field with the purpose of scoping information about buildings including material, height (number of stories), construction technique, and floor area. A high resolution satellite based Digital Elevation Model was additionally generated to provide surface elevations of vegetation and man-made objects with a vertical accuracy of 10 m. By using this methodology thousands of buildings and infrastructure features were mapped, clearly indicating the location and characteristics of the assets.

[Caption: Mekong - Urban land use and building types; ©GeoVille for ESA / World Bank; Image data © Spotimage]

Exposure maps were complemented with the analysis of historical flood and drought events using ERS and ENVISAT-ASAR radar data for historical flood mapping alongside with vegetation index data from SPOT-VEGETATION and NOAA-AVHRR concerning drought events.

This study resulted in a comprehensive building and infrastructure inventory with over 110,000 buildings and 3,200 km of road network mapped. Only 5 % of the area used for settlementsand 1% of land for transport infrastructure. 79% of the settlement area and 88% of the road network are threatened by high-water levels in case of a severe flood similar to the 2001 event. More than 90% of the cropland would be threatened by high water levels. 99% of the cropland was exposed to the 2002 drought for a period of at least 1 month, and almost 50% of cropland was under heaviest drought conditions at the drought peak in April 2002.