A method for evaluating flood hazard and flood risk of east Bangkok plain, Thailand

Abstract

A new method for estimating flood hazards was developed in this study for flat and low-lying areas wherein floods have low flow velocity, large depth and long duration. The hazard was computed as a function of flood depth and duration, not only depth as in the past. The study area of 624 km2 in suburban Bangkok, Thailand, was divided into small grids, each 200 m by 200 m. Because flood depth and duration have different dimensions, this new method categorised the depth and duration into four groups of small, medium, large and largest magnitudes represented by the integers 1, 2, 3 and 4, respectively. The depth integer and duration integer of each grid were weighted by the factors μ and (1 − μ), respectively, and summed up to obtain the flood hazard indicator. The factor μ was assumed to be between 0 and 1 and adjusted by trial and error until the computed hazard satisfactorily satisfied the hazard estimated from field and questionnaire surveys. For flood damage vulnerability, an established methodology was applied considering the vulnerability as a function of population density and land use assets at risk. By multiplying the computed hazard and vulnerability, the flood risk maps of three return periods were constructed.