Assessing the recent subsidence of the central coastal Delta of Bangladesh by dating submerged kiln
Final Report Abstract
Assessing the recent subsidence of the central coastal Delta of Bangladesh by dating submerged kilns The densely populated low lying Ganges‐Brahmaputra Delta is highly vulnerable to the global sea‐ level rise. In order to estimate the subsidence of the delta, we examined submerged salt‐producing kiln sites in the coastal Sundarbans. These kilns were built just above the previous winterly spring high‐tide level, but are currently located ~155 cm below the corresponding modern level. According to optically stimulated luminescence (OSL) dating, the kilns were ultimately fired ~300 years ago and salt production was terminated by a catastrophic event, which affected the kiln sites at different levels and locations. AMS‐14C ages of charcoal at the kiln’s base and associated mangrove stump ho‐ rizons support the OSL dates. Based on the elevations and the ages, the 300‐year‐average rate of sinking of the outer delta is 5.2 ± 1.2 mm/a, which includes 0.8 mm/a of eustatic sea‐level rise. Reasonably postulating that the sub‐ sidence rate will not change during the next few decades and accepting the estimates of current sea‐ level rise of 1.8–3.0 mm/a or 2.7–7.1 mm/a, a RSL rise of 6.4 ± 1.7 mm/a or 8.9 ± 3.3 mm/a, respectively, must be assumed along the Sundarbans coasts. Modern 137Cs‐based sediment accumulation rates of ~3.7 mm/a in average (locally ranging at 0.0– 11.3 mm/a) are comparable to Holocene accumulation rates of 2–5 mm/a (1.3–7.0 mm/a). This long‐term aggradation process was obviously in overall balance with land subsidence. Our new land lowering rates for the central coasts of Bangladesh in addition to the present and future ESL rise of similar range as well as with additional adverse anthropogenic damming and dike effects suggest that the future defense of the lower delta plain will be a great challenge. The southern Ganges–Brahmaputra Delta is densely populated (143 million inhabitants in the entire delta, 2.5 million in the Sundarbans surroundings, and 2.2 million in the two coastal districts Barguna and Patuakhali located east of the Sundarbans). Subsidence of this specific coastal area by 5–6 mm per year in combination with the future sea‐level rise will lead to enhanced saltwater intrusion, worsen the effects of storm surges, and reduce the arable areas as well as the drainage during mon‐ soon flooding. This study has documented the rapid burial and abrupt abandonment of a former large, industrial coastal complex. The present pro‐ tection of the coastal delta zones (except the Sundarbans) by dikes has almost completely stopped new land aggradation inside the polders, whereas the land continues to aggrade in the undiked Sundarbans. In areas of high subsidence rates, such as the study area, and on time scales of 100 years, it will be increasingly difficult to drain the polders as well as protect them against storm surges. Thus, a controlled sedimentation system is needed to keep the southern part of Bangladesh above the sea level and in a habitable form.
