IntroductionA certain combination of hydroclimatic( 水文氣候 )

and geotechnical conditions and reservoir operation may induce the occurrence of large-scale landslides in natural river channels.

某種特定的水文氣候與地質條件與水庫可能會導致天然河道中發生大型的崩塌。

When landslides take place, landslides may block the river channel and form landslide dams. Subsequently, both inundation upstream and failure of landslide dams are inevitable( 不可避免 ).

當崩塌發生時，土體可能會阻塞河道形成堰塞湖，隨後不可避免的在上游蓄積水體和壩體潰決可能發生。

Experimental Facilities and Procedure

Bottom Slope ： 1/1000Manning roughness ： 0.012

Results and analysis

3.1 Summary of experiments

動床

定床

Tb ： Time of river-blockage before dam failureTf ： Time of dam failureHr ： The depth of initial sediment layer on river bed

Sand Sample

Hcs: 壩高Wcs: 壩寬

Generally, after landslide dam has formed and flow overtopping has occurred, full processes of landslide dam failure can be observed and the characteristics are also similar regardless of the fixed-bed and the erodible bed.不論是動床或定床，在壩體形成至潰壩後的過程大致上大同小異。

• When the upstream water level approached the height of the dam, overtopping occurred as the water flow gradually eroded the sediment of the top of the dam (see Figs.3(a) and 3(b)). In a later time, the dam began to fail (Fig.3(c)).當水位上升至壩頂時，溢頂流隨即發生並快速沖刷壩頂，一段時間後壩體開始潰決。

3.2 Overview of landslide dam-break processes

In the experiments, a hydraulic jump sometimes was observed near the dam toe. Especially, the phenomenon was obviously observed for the dam formed by the landslide of large volume. 實驗中常有水躍在下游坡趾處發生，原因則是由於大量體積的壩體造成。

3.2 Overview of landslide dam-break processes

• The flow was turbulent at the location of the hydraulic jump. The water and sediment were intensely mixed.

• Finally, when the water level difference between the upstream and downstream decreased to a certain value, the water flow became insufficient( 不足 ) to further erode the dam body and the course of dam failure was completed.

Comparing Fig.4 and Fig.6, Fig.5 and Fig.7, one may find that the differences of the water-level variations are not apparent( 明顯 ) at the two different river bed situations.

Water level variation 水位變化

Like the previous study on fixed-bed situation (Figs.6 and 7), the characteristics of water-level variations over the erodible bed are given in Figs.4 and 5, including the rising of water levels in the upstream and the falling in the downstream after the landslide dam formation and failure. Meanwhile, the hydraulic jumps are also depicted in CS5 of these figures, which is the nearest to landslide occurrence position at downstream.

By comparing Tb and Tf, the time of river-blockage is found to be also longer than that of the dam failure.

Water level variation 水位變化

Topographic change

In these figures, Dscs denotes the distance of cross section from initial cross section, Dmp denotes the distance of measuring point from initial point, and Hds denotes the height of sediment deposition.

For the cases of different inlet flow discharges, through comparing Fig.8 and Fig.10, or Fig.9 and Fig.11, it is found that the topographic changes of large inlet flow discharge are stronger than those of small inlet flow discharge after landslide dam failure.

For the cases of different sediment samples, through comparing Fig.8 and Fig.9, it is found that the topographic changes of Sediment Sample 1 with the finest sediment size are more complicated than those of Sediment Sample 2 with the coarse sediment size.

For the cases of different river bed situations, through comparing Fig.8 and Fig.12, or Fig.9 and Fig.13, it is found that the topographic changes over the fixed-bed mainly represent the sedimentation of downstream river bed after landslide dam failure, while the erodible bed situation, the sediment particles of landslide dam participate in the topographic charges.

It may be clear from the above analysis that the topographic change is affected by flow discharge, sediment particle size and initian river bed situation.

ConclusionsThe variation of water level over erodible bed is similar to that

over the fixed-bed after landslide dam failure. 潰壩時水位變化在動床條件下與定床相似。

For large landslides, hydraulic jumps are observed in the downstream during the process of dam failure. 水躍現象會出現在大型崩塌的堰塞壩下游的潰壩過程中。

The topographic change is influenced by the inlet flow discharge, the sediment particle size and the initial riverbed scenario.地形變化受到入流量、泥砂粒徑和河床初始情形影響。

As the inlet discharge increases, the landslide dam-break flood induces more substantial changes in bed topography, and vice versa.

隨著入流量增加，潰壩洪水會引起較大幅度的地形變化， ，反之亦然。