Rainfall-Runoff Simulation and Modelling Using HEC-HMS and HEC-RAS Models: Case Studies from Nepal and Sweden

Floods are probably the most frequent, widespread and disastrous hazards of the world. Almost every year, Sweden and Nepal are affected by floods though their nature and impact differs due to complete different geographical and hydrological setting. This study aims to develop Hydrologic Engineering Centre’s Hydrologic Modelling System (HEC-HMS) and Hydrologic Engineering Centre’s River Analysis System (HEC-RAS) models for Kankai River basin of Nepal and Kävlinge river basin of Sweden to analyse the effects of rainfall on surface runoff and peak discharges of these rivers and ultimately produce flood inundation levels to assess the flood risks in both areas. Being a rainfed catchment, the flood discharges of Kankai River are highly affected by the extreme rainfall events whereas Kävlinge River has a significant influence of ground water. Apart from some extreme events, HEC-HMS modelling using Soil Moisture Accounting (SMA) loss method, manage to simulate flood discharges of Kankai River more accurately than Kävlinge River. The flooding impact of Kävlinge River is significantly less as the catchment is small with defined flow route while the Kankai River being a large catchment with braided river form, inundates vast downstream flood plain region during high flood level. With numerous people living along the river and downstream plains, flood risk is predominately high in the Kankai River catchment.