Reconstructing the magnitude and characteristics of the 2013 Kedarnath disaster using its geomorphic signature

In 2013, a cloudburst event devastated the town of Kedarnath in the Indian Himalaya. This widespread, extremely intense burst of rainfall triggered thousands of landslides and debris flows in several hours. Simultaneous rapid snow melt, an abundance of landslide debris and heavy rainfall led to the catastrophic breach of the Chorabari Tal lake, sending a sediment-laden flood wave through the town and propagating down valley, killing over 5000 people and causing around US$1 billion in damage. The catastrophic damage at Kedarnath means documentation and reconstructions of the event have focused largely along the Mandakini River. Furthermore, the spatial extent and intensity of cloudburst events is often difficult to ascertain due to events being highly localised and difficult to capture using satellite datasets or rainfall gauges. However, the effect of the cloudburst extended much further across the entire Alaknanda catchment, with several sediment-rich flow events also triggered in the neighbouring valley of Badrinath. Since sediment-rich flows typically occur individually, this event presents a unique opportunity to consider controls on the magnitude and characteristics of sediment-rich flows triggered under similar tectonic and climatic conditions.

Here, we present a manually mapped inventory of debris flows and sediment-rich floods for the high elevation regions of the Alaknanda catchment. By manually mapping debris flows and sediment-rich flood deposits using high-resolution imagery, we can document the geomorphic signature of the 2013 Kedarnath disaster in both Kedarnath and Badrinath. We use this inventory to determine controls on the magnitude and occurrence of sediment-rich flows within the Indian Himalaya, exploring the importance of topography, channel characteristics and sediment supply. We will simulate mapped flows using the model LaharFlow to evaluate controls on the size and triggering conditions of the flows. We will supplement our modelling analysis with metrics such as debris flow densities to better constrain the intensity of the cloudburst event across the full Alaknanda basin. This research will identify first-order controls on the magnitude and frequency of sediment-rich hazards triggered during the same cloudburst event. As cloudbursts are likely to increase in frequency and/or intensity with climate change, this research is time-critical.