Performance of Models for Flash Flood Warning and Hazard Assessment

Performance of Models for Flash Flood Warning and Hazard Assessment: The 2015 Kali Gandaki Landslide Dam Breach in Nepal

Bricker, J.D. (TU Delft Hydraulic Structures and Flood Risk; Tohoku University)
Schwanghart, W. (University of Potsdam)
Raj Adhikari, B. (Tribhuvan University)
Moriguchi, S. (Tohoku University)
Roeber, Volker (Tohoku University)
Giri, S. (Deltares)

2017

The 2015 magnitude 7.8 Gorkha earthquake and its aftershocks weakened mountain slopes in Nepal. Co- and postseismic landsliding and the formation of landslide-dammed lakes along steeply dissected valleys were widespread, among them a landslide that dammed the Kali Gandaki River. Overtopping of the landslide dam resulted in a flash flood downstream, though casualties were prevented because of timely evacuation of low-lying areas. We hindcast the flood using the BREACH physically based dam-break model for upstream hydrograph generation, and compared the resulting maximum flow rate with those resulting from various empirical formulas and a simplified hydrograph based on published observations. Subsequent modeling of downstream flood propagation was compromised by a coarse-resolution digital elevation model with several artifacts. Thus, we used a digital-elevation-model preprocessing technique that combined carving and smoothing to derive topographic data. We then applied the 1-dimensional HEC-RAS model for downstream flood routing, and compared it to the 2-dimensional Delft-FLOW model. Simulations were validated using rectified frames of a video recorded by a resident during the flood in the village of Beni, allowing estimation of maximum flow depth and speed. Results show that hydrological smoothing is necessary when using coarse topographic data (such as SRTM or ASTER), as using raw topography underestimates flow depth and speed and overestimates flood wave arrival lag time. Results also show that the 2-dimensional model produces more accurate results than the 1-dimensional model but the 1-dimensional model generates a more conservative result and can be run in a much shorter time. Therefore, a 2-dimensional model is recommended for hazard assessment and planning, whereas a 1-dimensional model would facilitate real-time warning declaration.

Nepal
earthquake
landslide dam breach
flood
HEC-RAS
Delft-Flow
steep mountain stream
OA-Fund TU Delft

http://resolver.tudelft.nl/uuid:6dc5d3ce-2359-4b32-a2c4-51bf7c479010

https://doi.org/10.1659/MRD-JOURNAL-D-16-00043.1

1994-7151

Mountain Research and Development, 37 (1), 5-15

Institutional Repository

journal article

© 2017 J.D. Bricker, W. Schwanghart, B. Raj Adhikari, S. Moriguchi, Volker Roeber, S. Giri