Divergent options to cope with vulnerability in subsiding deltas

Net subsidence of most major deltas in the world and related vulnerability are thought to be increasing, and this is often linked causally to human activities. This paper examines this causality against a range of co-varying factors. We do so with a principal component analysis of co-variability of a range of geophysical and socio-economical indicators of 33 deltas mainly derived from the DIVA tool. Land potentially lost and people at risk of flooding are our indicators of vulnerability. The former correlated positively with maximum surge height and negatively with net sea level rise. The latter correlated positively with delta area, average river discharge, and maximum surge and negatively with net uplift (or subsidence). Thus, variation in societal vulnerability across deltas depends on short-term, instantaneous risks linked to lowland area, river discharge and storm surges rather than on longer-term, slow, net sea level rise. Delta management should focus on precautionary spatial planning, and on maintenance or restoration of historical sediment delivery and accretion rates. Especially larger deltas with high population densities combine a high risk with the potential to accommodate flood water and mitigate flooding risks. The deltas of the Yangtze-Kiang and Ganges-Brahmaputra share these characteristics. Here space should allow engineering of flood retention, sedimentation and diversion channels as well as refuges and safe economic hotspots. At the other end, in deltas with a high population density and limited space, like the Chao Praya, means for adaptation must be sought outside the delta proper. In deltas with low population densities, such as the Lena, Yukon or Fly, natural delta dynamics can prevail.