Predicting the impact of climatic and geomorphic changes on flood hazards in Central Himalayan Rivers. An environmental modelling framework to predict potential flood hazards in the Karnali River in Nepal.

Abstract

Flood hazards that originate in the Himalayas threaten the lives and livelihoods of millions of people in the downstream Ganga plain. The Himalayas are a region with high geomorphic activity that is sensitive to anthropogenic climate change. The development of effective flood management strategies, therefore, needs to predict the impact of geomorphic and climatic changes on the flood hazards which is not possible with currently existing hazard modelling frameworks.

This thesis introduces an environmental modelling framework to predict potential flood hazards for the projected climatic and related topographic conditions that can be used to adapt to future flood hazards. This framework links hydrological, statistical, morpho- and hydrodynamic models to predict: i) the climate change impact on the flood discharge into the floodplain; ii) the topographic evolution of the floodplain for the projected flood flows, and; iii) the potential spatial flood hazard characteristics in the floodplain for the projected flood discharges and new topographies. This modelling framework is developed and implemented for the Central Himalayan Karnali River in Nepal.

The predictions of the potential hazards provide evidence that flood hazards in the Karnali River will intensify with increased greenhouse gas emissions. The current 1-in-100-year flood discharge into the plain (1975 – 2014) is projected to increase by 40% (medium emissions) and by 79% (high emissions) at the end of the century (2060 – 2099) (median predictions). The return period of the current 1-in-100-year event is predicted to decrease to 11 – 16 years (medium emissions) and 2 – 7 years (high emissions). The projected increase in the flood discharge intensifies the magnitude of topographic change in the downstream floodplain.

The projected increases in the flood discharge cause deeper inundations that extend further. The current 1-in-100-years event (1975 – 2014) inundates 39% of the simulated area of the floodplain. This extent is projected to increase to 51% (medium emissions) and 60% (high emissions) at the end of the century (2060 – 2099) (median predictions). The predictions indicate that topographic change alters flood hazards on the local scale and that river and flood engineering potentially intensifies the downstream hazard.

The proposed modelling framework allows continued assessment of flood risk enabling the flood management strategies to be developed for future conditions. Recommendations are provided to improve the predictions by adjusting the modelling framework and targeted investments in surveys.