Morphodynamics of the Karnali Fluvial Fan

An assessment of the effects of human interventions on the morphodynamics of an alluvial river using optical remote sensing data

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Abstract

Humans have a long history of intervening in natural river systems. These human interventions affect how water and sediment interact with the bed and banks of the river. The interaction between water and sediment is called morphodynamics and is an important component of the natural ecosystem of the river and its surroundings. This research aims to investigate if human interventions have disturbed the natural morphodynamics in the Karnali fluvial fan and if this can be the cause of grassland degradation in the adjacent floodplains of the Bardiya national park. To do so, the following research question is answered: What is the relation between human interventions and the morphodynamics of the Karnali fluvial fan?

To do so, a method is developed where a time-series analysis of the morphodynamics is performed, based on discharge and optical remote sensing data. The flow regime is investigated by computing the temporal evolution of discharge, the flow duration curve, the base flow index, and the water distribution at the bifurcation in the upstream part of the fan. Morphodynamic parameters that can be described using remote sensing data concern the planform and the land cover of the river channel-belt. The planform dynamics are described by looking at the evolution of the sinuosity and braiding, and the land cover dynamics are represented by the evolution of the river's channel, sediment bar, and grassland areas. The most important human interventions are identified, after which the relation between the morphodynamics and the human interventions is assessed.

No evidence was found that human interventions have significantly influenced the evolution of the morphodynamics of the Karnali fluvial fan. Only on a small scale, changes or peculiarities in morphodynamic parameters can be attributed to human interventions. In this case study, the construction of embankments reduced lateral erosion, and the Kailashpuri dam influenced upstream sedimentation and erosion processes. A shift of the main stream from the eastern to western river branch in 2009 most likely caused a change in river-floodplain interaction in the branch closest to the Bardiya national park. The quantities of sediment and water extraction were found to be negligible in comparison to the natural water and sediment fluxes.

This research is the first step in quantifying the morphodynamic behaviour of the Karnali fluvial fan and indicates which are the most important drivers of morphodynamic changes. Besides this contribution, the study also includes the development of a methodology that could improve remote sensing-based morphodynamics studies.