Changes in the magnitude and frequency of late Holocene monsoon floods on the Narmada River, central India

摘要

During the past three decades, the Narmada River of monsoon-dominated central India has undergone extraordinarily large floods that rank among the highest recorded rainfall-runoff discharges per drainage area in the world. The floods on this river are a direct result of intense tropical cyclones embedded within the summer monsoon circulation. The cluster of extreme floods in the past few decades represents an anomalous increase in both the magnitude and frequency of large floods when compared with the >1700 yr record of paleoflood deposits on this river. Sand deposits from recent floods consistently blanket older flood deposits at numerous slack-water paleoflood sites along a 15 km reach of the Narmada River. At the site of the highest flood deposits, 4-5 sandy flood deposits yielding post-a.d. 1950 14C dates cap an underlying sequence of 7–10 silty flood deposits with a minimum 14C age of 650 ± 70 B.P. and a maximum age older than 1720 ± 185 B.P. The post-a.d. 1950 floods are thus the largest in at least the past several hundred years. An undisturbed surface archaeological site of microlithic artifacts <0.5 m above the post-1950 flood sands provides further evidence that no significantly larger floods have occurred in at least the past 3000 yr. Incipient soil development on the buried surfaces of some of the higher paleoflood deposits indicates long intervals in the past when no floods reached the highest slack-water site. The largest flood in the 1951-1991 gaged record of the Narmada River at Mortakka (55 323 m3s–1 in 1961) is equivalent to a 1000 yr flood in a probability distribution based on the 1700 yr paleoflood record, but is less than a 50 yr flood based on the 1951–1991 gaged record alone, demonstrating the enormous recent increase in the frequency of severe floods. This cluster of severe floods could reflect changes in either climate or land use. A number of paleoflood studies in tropical-storm regions show a similar increase in high-magnitude floods within the past four decades, suggesting a widespread climatic cause for this pattern.