Locating Sediment Evacuation Zones – A Prefatory Action for Early Warning System Development in Mountainous landscapes

The past decade has seen an alarming rise in the number of extreme events, most of which are high magnitude hydrological events triggered by focused precipitation, glacial lake outburst or both. During such event large amount of debris is mobilized and get deposited in downstream reaches. Studies have quantified the volumes of debris exported by the events and have shown them to possess potential for future hazard (e.g., Hooke, 2019; Sarkar and Singh, 2022; Westoby et al., 2023). However, a pressing question that remains unaddressed is regarding the identification of storage sites of these sediments prior to the event.

We have employed the concept of index of connectivity (IC) to locate sediment stored in the landscape. We have altered the relationships of the upslope and downslope components of the basic framework of index of connectivity (Borselli et al., 2008), and normalized the values to obtain a dimensionless storage potential index (SPI) that indicates the proneness of a point to arrest sediment flux and disrupt the routing process. Using the SPI and normalized IC, we have formulated a Sediment Evacuation Susceptibility Index (ESI_S), the values of which ranges between -1 to 1; lower ESI_S values indicate stable zones with higher thresholds of evacuation, and vice versa.

The model has been tested in a small catchment (~93 km²) known as Pranmati catchment in NW Himalayas, India. Our results show that significant volume of sediment gets arrested along the margins of land cover units that have contrasting impedance to sediment transportation. Sediment flux also gets arrested in isolated pockets (e.g., grassland patches) within forested land. Croplands tend arrest and store sediment due to intense anthropogenic modification of hillslopes. Landslide talus deposits are a potential sediment storage unit. Mid-slope regions of hillslope transects tend to have high storage potential. These sites get connected during extreme hydrological conditions and release the stored sediments. Landslides debris deposits are found to be highly stable. However, parts of the hillslope in the vicinity of the stream network have a very high susceptibility to evacuation. The results have been validated in field with reference to two major local high magnitude flash flood events. The evacuation susceptibility assessment can be the first step for risk identification, development of an early warning system for flood hazards and disaster mitigation.

References

Borselli, L., Cassi, P., & Torri, D. (2008). Prolegomena to sediment and flow connectivity in the landscape: A GIS and field numerical assessment. Catena, 75(3), 268-277.

Hooke, J. M. (2019). Extreme sediment fluxes in a dryland flash flood. Scientific Reports, 9(1), 1686.

Sarkar, A., & Singh, V. (2022). Characterisation and Assessment of a Flash Flood in the Himalaya: Understanding the Significance of High Magnitude Events in Sediment Mobilisation. Journal of the Geological Society of India, 98(5), 678-686.

Westoby, M. J., Dunning, S. A., Carrivick, J. L., Coulthard, T. J., Sain, K., Kumar, A., ... & Shugar, D. H. (2023). Rapid fluvial remobilization of sediments deposited by the 2021 Chamoli disaster, Indian Himalaya. Geology, 51(10), 924-928.ter, Indian Himalaya. Geology, 51(10), 924-928.