Compound flood hazard assessment and analysis due to tropical cyclone-induced storm surges, waves and precipitation: a case study for coastal lowlands of Kelani river basin in Sri Lanka

The co-occurrence of multiple flood drivers resulting in compound flooding has received increased attention during the past decade subsequent to several catastrophic flood events in coastal areas around the world. However, most coastal flood risk assessments generally account for one of either oceanic or inland flood sources even in situations where the joint probability of occurrence of more than one flood driver is high. Accordingly, this paper examines the compound flooding potential due to tropical cyclone-induced storm surges, waves and precipitation for a major river basin in a low-lying coastal city of Sri Lanka. The physical processes associated with compound flooding in the coastal lowlands of the river basin, i.e. hydrodynamics of the storm surge including wave effects, river hydraulics as well as precipitation-induced direct surface run-off, have been numerically simulated by employing an array of loosely integrated models to compute space- and time-varying flood distributions for 14 scenarios of cyclonic storms and riverine floods covering a range of return periods. The selected scenarios cover a range of combinations of relative strengths of oceanic and inland flood drivers. The model simulations have been performed for three cases: storm surge (S) only, river flow (R) only and compound flooding (C) with S and R combined. The precipitation has been added in the cases of river flow and compound flooding events. The analysis of model simulations provides further insight into the temporal and spatial variation of the nonlinear interaction of storm surge and riverine flow that occurs in a transition zone between the respective fully dominant regions. The analysis of time-varying flood levels indicates a lesser interaction between storm surge-induced and riverine flood waves when one component is more dominant than the other whilst nearly equal magnitudes resulting in a greater interaction and rise in instantaneous compound flood level compared to flood levels if either component were to act separately. The effect of the phase shift of storm surge and riverine flood waves on the peak flood levels and extents as well as on the degree of nonlinear interaction of the component flows in the compound region are also examined.