Abstract:
The analysis of extreme hydrological events in river basins and rainfall trends is crucial for reducing vulnerability to flooding and developing flood mitigation strategies. Understanding the correlation between flooding and rainfall patterns helps identify flood-prone areas within the region, plan infrastructure, and improve early warning systems, ultimately mitigating the negative impacts of future flood events and protecting communities and resources. The objective of this study was to determine the relationship between rainfall and anomalous flood occurrence in the Nilwala River Basin (NRB) using statistical analyses. The study was conducted in the southern province of Sri Lanka.While NRB is characterised by monsoon-induced flooding, an unusual flood event was reported recently. Thalgahagoda, Panadugama, Pitabeddara and Urawa represent the four main gaging stations (GS) of NRB. Water level (m) and rainfall (mm) data were collected from each GS for September, October, November, and December over four consecutive years (2021, 2022, 2023, and 2024). The study investigated differences in water levels and rainfall among stations, and the correlation between rainfall and flooding patterns, by analysing the collected data using One-way analysis of variance (ANOVA) and Duncan's multiple range test (DMRT). All stations recorded a significant increment of their highest peak water levels in 2023, compared to previous years: Thalgahagoda (1.69 ± 0.47), Panadugama (5.35 ± 0.99), Pitabeddara (1.84 ± 0.68), and Urawa (1.17 ± 0.51) indicating a potential rise in flood intensity while rapid recovery of water levels was reported in every GSs (1.15 ± 0.39), (4.40 ± 1.07), (1.22 ± 0.55) and (0.63 ± 0.34) respectively in 2024. Results revealed no statistically significant differences in rainfall amounts across the years considered, with only slight variation in mean rainfall values. However, Pitabeddara consistently recorded the highest rainfall in all three years, peaking at 26.46 ± 33.15 mm in 2023. The findings indicate that although interannual rainfall was not significant over the period considered, the downstream section of the basin has been affected by the observed anomalous flood levels. However, the results show a weaker correlation between rising water levels during flood events and consistent rainfall patterns, indicating the potential influence of anthropogenic activities. Overall, the study emphasizes the urgency of enhancing flood management strategies and investigating potential anthropogenic factors, particularly in flood-prone areas such as Thalgahagoda and Panadugama.
