Method to improve the mitigative effectiveness of a series of check dams against debris flows

The advance of technology has led to more competent countermeasures, but lives and properties still continue to suffer from water‐induced disasters, such as floods, landslides, and debris flows. To increase the effectiveness of counter systems, improved methods of planning and designing such systems are prerequisite. This paper describes briefly a methodological approach for predicting debris flow characteristics, and proposes techniques for evaluating and improving the mitigative effectiveness of check dams against debris flows in steep mountain torrents. Additionally, a non‐dimensional parameter, namely potential storage volume, is introduced to generalize the evaluation processes. As an example, the 1999 debris‐flow event in the San Julian River, Venezuela, is chosen for discussion. The paper also proposes a method of evaluating the control function of a series of check dams as well as the criteria for the selection of their sizes, numbers and locations. It is hoped that this work will help to determine which combinations of check dams will fit best together for the optimal control of debris flows and available resources in any river basin. Copyright © 2008 John Wiley & Sons, Ltd.     

Hydrodynamic characteristics of the Tam Pokhari glacial lake outburst flood in the Mt. Everest region,Nepal

The Tam Pokhari glacial lake outburst flood (GLOF), which occurred in 1998 in the Mt. Everest region of Nepal, was evaluated using hydrodynamic models to gain a better understanding of the flow behaviour. The flood wave was analysed separately under rigid and erodible boundary conditions. In both cases, the calculated dam‐breach hydrograph, which had a peak discharge of about 10 000 m³/s, was routed through the Inkhu River, which originates from the lake. The morphologic changes along the river were also analysed and the results were compared with satellite images, field observations and recorded data. In the case of rigid boundaries, the routing procedure gradually attenuated the peaks of the hydrographs to account for hydraulic pooling in narrow gorges and storage in the channel. In the case of erodible boundaries, such effects were minimized due to the increment in channel capacity associated with erosion by debris flow. The study revealed that the GLOF event produced a large‐scale debris flow. Additionally, the results revealed that erosion and deposition took place intermittently, but that approximately 440 000 m³ of sediment was deposited about 14 km downstream from the lake mouth. The calculated peak of the water and sediment mixture at 14·4 km was found to be 30 000 m³/s, which is almost 6 times as large as that observed when the rigid boundary conditions were used. Further, the increase in the peaks of the hydrographs due to sediment transport was the primary reason for the destruction associated with the GLOF. These findings suggest that the local sedimentology and topography, as well as other geo‐hazard conditions in the area, should be carefully evaluated before recommending any control measures against GLOFs in the Himalayan region. Copyright © 2009 John Wiley & Sons, Ltd.     

Causes of catastrophic failure of Tam Pokhari moraine dam in the Mt. Everest region

The moraine dam of the Tam Pokhari glacial lake breached on 3 September 1998 and caused a catastrophic flood in the downstream areas. To learn from the event, a field survey was conducted. The survey team found that a landslide, which is considered to be responsible for the outburst flood, occurred in the northeast-facing slope of the moraine dam. The dam internal structure played a crucial role in forming a landslide that triggered the excess overflow and finally the breach of the dam. The internal structure of the dam was made of alternating layers of finer and coarser sediments inclining at 30° downstream and layers are truncated in the upslope direction by a huge pile of unconsolidated and structureless moraine materials. Since the upstream slope angle of the dam i.e., 40° is larger than the angle of repose i.e. 35° of sediments, the increased pore water pressure in the dam triggered a landslide. The rainfall and seismological activities of that particular day, which hit the record high, were crucial in triggering the failure. It is estimated that the dam’s north and northeast-facing slopes completely slid involving about 30,000 m³ of sediment mass of unconsolidated moraine materials above the shear plane. A slope stability analysis was also performed. The calculated safety factor was 0.85, and the calculated slip circle agreed with the shear plane marked in the dam. About 18 million cubic metres of water was swiftly released due to the sudden breach of the moraine dam.     

Prediction and assessment of multiple glacial lake outburst floods scenario in Pho Chu River basin,Bhutan

In this study, the characteristic of multiple glacial lake outburst floods (GLOFs) in the Pho Chu River basin in Bhutanese Himalayas is evaluated to help assess the potential impact. Thorthormi Cho (TC) and Lugge Cho (LC) in the east branch and two unnamed lakes labelled A and B in the west branch of Pho Chu are chosen for the study. Numerical models were employed to simulate different involved processes. The results show that the peak sediment discharge in the east branch of the Pho Chu River by the TC dam breach reached about 5000 m³/s (during the first GLOF) at 4 km whereas by the LC dam breach is about 600 m³/s (second GLOF) at 6 km. However, the highest peak hydrographs (sediment and water mixture) calculated during the first and second GLOF are about 10 000 m³/s at the 18‐km section and about 23 000 m³/s at the 10‐km section, respectively. In the west branch of Pho Chu, erosion and depositions are the frequent intermittent local processes during the first GLOF event from Lake A. Because the first event stabilized the irregular river bed profile, there is not much sediment discharge developed during the second GLOF from Lake B. At the 17‐km section of the west branch, the peak hydrograph reached about 9000 m³/s during the first event against the peak of about 800 m³/s during the second event. The results suggest that even if multiple dam breaches occur simultaneously, GLOF surges pass through the main river channel at different times with very different flood characteristics. The differences in travel time and flood characteristics mostly depend on the distributions of bed slope and potential erosion depth along the reach. Further, the amount of sediment accumulated in and transported by each surge is reliant on the temporal geomorphologic setting of the river and therefore on the impact of the previous GLOF on riverbed profile and potential erosion depth. The robustness in peak GLOF hydrographs is associated with sediment flow dynamics. As a consequence, serious inundation of Punakha, Lobeysa and major portion of Wangdue Phodrang is anticipated. Copyright © 2011 John Wiley & Sons, Ltd.