Hydrodynamic Flood Modelling and GIS-Based Risk Mapping for Identifying Urbanization-Safe Zones in Hanwella, Sri Lanka

Urban expansion in flood-prone regions increases exposure to hydrological hazards and makes flood-informed land-use planning increasingly important. Hanwella, located in the Kelani River basin of Sri Lanka, is particularly vulnerable to flooding due to intense rainfall, river overflow, and growing urban pressure. This study develops a two-dimensional hydrodynamic flood modelling framework based on the shallow water equations and applies a Godunov-type finite volume method with the Harten–Lax–van Leer (HLL) Riemann solver to simulate water-level dynamics in the Hanwella region. The two-dimensional shallow water equation model explicitly incorporates upstream river contributions together with overland flow contributions from the surrounding river basin to represent the major hydrological inputs. The model integrates hydrological, meteorological, and topographical data obtained from the Sri Lankan Irrigation Department and NASA datasets. Model performance was validated against observed water-level records using the Nash–Sutcliffe Efficiency coefficient, yielding an NSE value of 0.7, which indicates acceptable predictive performance. Uncertainty was examined through validation-residual statistics and a qualitative expert-based discussion of potential uncertainty sources; formal parameter-sensitivity and probabilistic uncertainty-propagation analyses were beyond the scope of this study. The simulated outputs were subsequently integrated with GIS-based spatial analysis using the Flood Inundation Calculator plugin in QGIS to generate flood-risk maps under different rainfall intensities and to identify areas with relatively lower flood susceptibility. The findings provide a practical basis for zoning, risk-sensitive land-use planning, and targeted flood mitigation in the Hanwella region.