The content for this article was contributed by Jean-François Mercier, Tecsult Inc. and Energy Infratech ltd.
The Teesta III Hydropower Project located in Sikkim, India includes a 60 m Concrete Face Rockfill Dam (CFRD), located in a steep narrow Himalayan valley. This valley experiences high seismic activity and its steep slopes are subject to landslide. It was feared that a landslide into the reservoir upstream of the future dam could induce a wave that could overflow the CFRD. An overflow lasting for more than a few seconds could cause the CFRD to fail. Even if the dam didn’t break, there still were some concerns of flooding the small upstream village of Chungtangh.
The steepest slopes in the Teesta River Valley are located just upstream of the dam, which is the region where a landslide is most probable. The goal of this analysis was to simulate a landslide into the reservoir and determine if the resulting wave overflows the dam.
Moving Objects Model Used to Simulate Landslide
Tecsult choose FLOW-3D to simulate this scenario since it had been used successfully to model sediment deposition in the reservoir. The reservoir simulation was used as a starting point for this simulation. The Moving Objects model in FLOW-3D was used to simulate the landslide and the VOF model was used to simulate wave generation.
The village lies at the fork at the bottom of the image. The dam is at the top. The landslide is on the right side.
Various methods were considered to estimate the landslide-induced wave in the reservoir. Empirical methods are commonly used to evaluate landslide-induced waves but these methods fall short in a number of ways. These methods do not provide information in the near field or the splash zone. Knowing the splash zone was important because the dam is very close to the slide area. A CFRD cannot sustain overflow for more than a few seconds. FLOW-3D provides a method of simulating this scenario in three dimensions by computing a fully-coupled interaction between the sliding land mass and the water.
The capability of FLOW-3D to simulate this problem was verified by comparison with an experiment consisting of a simple, small-scale, free falling block into water. This case is shown in the movie below. The resulting wave heights were in good agreement with the experiment.
CFD simulation of a small block falling into water.
The resulting wave heights at various locations from the block downstream are shown above in Figure 4.
The model is used for design purposes and STL files are imported directly into FLOW-3D. The size of the expected landslide zone was determined based on available geological information and surrounding landslide observations. A 30,000 m³, 100 m high landslide was simulated with a mesh of 3.1 million cells. Uniform cells of 3 m sides, by 1 m high were used. Maximum sliding velocity reached 23 m/s at point of entry. The wave reached the dam with a height of 8 m and a velocity of 10 m/s and overtopped it during a few seconds. No flooding occurred at the upstream village.
The primary concern in this work was the overflow of the dam which would have led to the destruction of the dam and the village of Chungtangh. The simulation showed that the dam only overflowed briefly and the wave did not reach the village. Since the village of Chungthang is high enough above the river, a significant wave height would be required to flood it.