The world's population living on low-lying deltas is increasingly vulnerable to flooding, whether from intense rainfall, rivers or from hurricane-induced storm surges. High-resolution SRTM and MODIS satellite data along with geo-referenced historical map analysis allows quantification of the extent of low-lying delta areas and the role of humans in contributing to their vulnerability. Thirty-three major deltas collectively include ~26,000 km 2 of area below local mean sea level and ~96,000 km 2 of vulnerable area below 2 m a.s.l. The vulnerable areas may increase by 50% under projected 21st Century eustatic sea level rise, a conservative estimate given the current trends in the reduction in sedimentary deposits forming on the surface of these deltas. Analysis of river sediment load and delta topographical data show that these densely populated, intensively farmed landforms, that often host key economic structures, have been destabilized by human-induced accelerated sediment compaction from water, oil and gas mining, by reduction of incoming sediment from upstream dams and reservoirs, and from floodplain engineering.
IntroductionClose to 0.5 billion people live on, or near, world deltas, inclusively in many mega-cities (1, 2). Ten countries (China, India, Bangladesh, Vietnam, Indonesia, Japan, Egypt, USA, Thailand, and the Philippines) account for 73% of the people that live in the world's coastal zone, defined as within 10 m a.s.l. (3). 20 th -century catchment developments and population and economic growth within subsiding deltas have placed these environments and their populations under a growing risk of coastal flooding, wetland loss, shoreline retreat, and loss of infrastructure (4, 5). It is estimated that more than 10 million people per year experience flooding due to storm surges, and most of these people are living on Asian deltas (6). Using new, globally-consistent and highresolution topographic data, three hypotheses are tested: 1) deltas are rapidly sinking, often to below local sea level, 2) the lack of sediment getting to delta floodplains is the main reason so many deltas are sinking, and 3) human activities are largely responsible for the present vulnerability of deltas. For a representative suite of deltas, Shuttle Radar Topography Mission (SRTM) data are applied to evaluate delta topography, in relation to mean sea level. Historical maps are geo-referenced against detailed topographic data to map morphodynamic patterns and quantify how rivers once flowed through deltas. Visible and near-infrared Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images are used to assess flooding in modern deltas and investigate whether such flooding is mainly from river runoff or instead from coastal storm surges, and whether present river suspended load is sufficient to maintain delta plain aggradation and stability.
