Hydrodynamic Modelling of Chorabari Glacial Lake Outburst Flood - 2013

Abstract

Extreme climate events have risen significantly across India since the turn of the millennium. Floods, droughts, and cyclones have become increasingly common in many regions across India's diverse climatic zones. Kedarnath flash floods were one of the worst disasters which took thousands of lives and livestock. A deadly combination of an extreme storm event coupled with the failure of the Chorabari lake had decimated vast areas of Uttarakhand. Hydrological modelling is a cumbersome process owing to the involvement of divergent watershed parameters. But at the same time, it is imperative to understand the watershed hydrological response due to rainfall. In the present study, we used the HEC-HMS model to simulate the runoff response due to extreme storm event in the Mandakini catchment. The Mandakini watershed was discretized into 19 sub-catchments, considering the drainage pattern of the watershed to represent drainage area contribution on generated runoff realistically. We used TRMM daily Rainfall data garnered from the EarthData website as the onsite telemetry stations washed off during the torrential downpour. Initial and constant loss rate, SCS-UH and Muskingum Cunge methods were chosen for accounting losses, the transformation of excess rainfall hyetograph into runoff, and routing the runoff through the channel network, respectively. Flood hydrograph was simulated for each sub-basin for the period of 10-20th June using the developed HEC-HMS model. Calibration and validation of the hydrologic model could not be performed because of the non availability of observed discharge values for the extreme flood event. Therefore, the hydrological simulation of the extreme event was performed by assigning model parameters based on values reported in literature and knowledge of catchment characteristics. The peak discharge simulated by the model was 2299.3 m3/s at Rudraprayag (before the confluence with Alaknanda River). The flood hydrograph for all sub-basins obtained from the model. Further, along with Geometric data, these discharge data were used to study flood's dynamics in the Mandakini River using the HEC-RAS 2-D model. The inundation depth of the flow varied from a couple of meters to 15 m. Flow velocity ranged from 3 m/s to 25 m/s. When cloudburst's effect alone took into account, there was a substantial decrease in inundation depth throughout the 27.7 km chainage up to Rudraprayag. From this study, we can corroborate that although the causes for these floods were cloudburst and GLOF, the latter paid more destruction to Kedarnath and its surrounding areas.