Understanding the flood-generating mechanisms in a monsoon-dominated river basin

Abstract

This study examines changes in flood magnitude and timing across twelve stream gauging stations in the Narmada River Basin from 1972 to 2024. Flood events were identified using the Annual Maximum Series (AMS) and Peaks-Over-Threshold (POT) approaches. Flood frequency analysis employed best-fit probability distributions, with Weibull and Gamma selected for AMS based on the Akaike Information Criterion (AIC). Circular statistics were used to assess flood timing, while magnitudes were categorized into small, moderate, and large floods based on return periods. Notably, large floods were predominantly concentrated in the Sandia catchment under both AMS and POT approaches. Trend analysis using the Modified Mann-Kendall test revealed a significant decline in peak streamflow at most stations, particularly along the mainstem Narmada in the AMS dataset. The POT approach showed a shift toward delayed flood timing at Sandia, Mohgaon, and Garudeshwar. The mean flood timing across the basin typically occurred from early to mid-August. Hydro-meteorological analysis of major flood events indicated that floods were driven by extreme precipitation, catchment wetness, or a combination of both. A catchment-based assessment of sensitivity to antecedent precipitation buildup (APB) revealed that flood magnitudes in upper and upper- mid reaches of catchments were notably correlated with APB. These findings highlight evolving flood dynamics in the Narmada basin and emphasize the need for incorporating catchment-scale processes and antecedent conditions into regional flood management strategies . Keywords: Flood frequency analysis, Annual Maximum Series, Peaks Over Threshold, Circular Statistics, Trend Analysis, Flood generation mechanisms, Narmada River Basin.