Changes in the magnitude and frequency of Holocene monsoon floods on the Mahi River, western India with special reference to Anthropocene

Abstract

Over the past seven decades, the monsoon-dominated Mahi River in western India has experienced extraordinarily large floods, ranking among the highest recorded rainfall-runoff discharges per drainage area in the world. The large magnitude floods on this river are direct outcomes of severe tropical cyclones/storms implanted within the Indian summer monsoon. The cluster of extreme floods in the past few decades represents an inconsistent strengthening in both the magnitude and frequency of large floods when compared with the 17 ka record of palaeoflood deposits in the basin. Palaeoflood records have been reconstructed from high-water marks (scour line, shrub line, trimline) and palaeostage indicators such as slackwater deposit (flood deposit benches, eddy bars, and deposits at tributary mouths). An investigation of relative magnitude for modern, historical and palaeofloods shows that the present and second half of the last century is characterized by very large magnitude floods. The study demonstrates that most historical floods and palaeofloods, for which there is evidence, were smaller in magnitude compared to modern floods. The examination further reveals that the floods are clustered in distinct time intervals. There is evidence of clustering in five different periods - (a) 9000 - 7000 BCE, (b) 3000 - 0 BCE, (c) between 1520s and 1720s, (d) Aaround 1920s, and (e) between 1960s and 1990s. The largest floods in the modern record of the Mahi River at the Wanakbori site between the 1960s and 2020s is 120-yr flood, while its tributary, the Som River at the Rangeli site, is 720-yr flood according to the Log-Pearson III probability distribution. A comparison between palaeofloods and gauged record demonstrating the enormous recent increase in the magnitude and frequency of severe floods. This cluster of severe floods from the current and last century could be due to changes in climate, land use, or the construction of dams, thereby indicating the influence of human activity. These post-1950 CE floods are thus the largest at least in the Holocene. Numerous palaeoflood studies in tropical storm regions reveal a similar increase in high-magnitude floods within the past seven decades. This pattern suggests that widespread climatic changes, land use modifications, and the construction of large dams are contributing factors, indicating an increase in the magnitude and frequency of floods during the Anthropocene.