Abstract
The Nilwala river basin is prone to frequent flooding during the southwest monsoon and second intermonsoon periods. Several studies have recommended coupling 1D and 2D models for flood modelling as they provide sufficient descriptive information of floodplains with greater computational efficiency. This study aims to couple a 1D hydrological model (HEC-HMS) with a 2D hydraulic model (iRIC) to simulate flooding in the Nilwala river basin. Hourly rainfall and streamflow data of three flood events were used for calibration and validation of HEC-HMS. The model performed exceptionally well considering the Nash–Sutcliffe coefficient, percent bias, and root mean square error. The flood event of May 2017 was simulated on iRIC using the streamflow hydrographs modelled by HEC-HMS. An overall accuracy of 81.5% was attained when the simulated extent was compared with the surveyed flood extent. The accuracy of the simulated flood depth was assessed using the observed water level at Tudawa gauging station, which yielded an NSE of 0.94, PBIAS of −4.28, RMSE of 0.18 and R2 of 0.95. Thus, the coupled model provided an accurate estimate of the flood extent and depth and can be further developed for hydrological flood forecasting on a regional scale.
Highlights
A significant consequence of climate change is the increased frequency and intensity of extreme weather events [1]
A past study conducted within the Nilwala river basin confirmed that the Snyder transformation method provided the most accurate results [46]
Another study conducted in the Attanagalu Oya basin using Hydrologic Engineering Center—Hydrologic Modelling System (HEC-HMS) concluded that the Snyder unit hydrograph simulated flow with greater reliability than the Clark unit hydrograph and SCS Curve Number methods [23]
Summary
A significant consequence of climate change is the increased frequency and intensity of extreme weather events [1]. Germanwatch described Sri Lanka as being among the top ten most affected countries due to climate change in 2018, with a global climate risk index of 19.0 [2]. Regional studies conducted on rainfall trends in Sri Lanka have indicated both increasing and decreasing patterns [3,4,5]. It was found that there was a higher tendency for short-duration high-intensity rainfall events to occur within the island [4], which often resulted in water-related natural hazards. Floods are destructive and occur more frequently than other natural disasters [6], with Asia being one of the most affected regions [7]. The Fifth Assessment Report of the IPCC states with medium confidence that the magnitude and frequency of recent floods are comparable to or surpass historical floods [1]
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