Abstract: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 m 3 /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 m 3 of sediment was deposited about 14 km downstream from the lake mouth. The calculated peak of the water and sediment mixture at 14Ð4 km was found to be 30 000 m 3 /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.

PurposeThis paper aims to describe the major causes of massive destruction due to floods in developing countries and to elaborate the usefulness of flood hazard maps under the framework of community‐based flood management.Design/methodology/approachThis paper elaborates the usefulness of flood hazard maps and their application.FindingsIt is a clear perception that flood risk management cannot be treated in isolation – rather it should be a part of community development. In this context, it is essential to build a community's capacity to understand their vulnerabilities, strategies, activities and the role they could play in managing flood risks without relying on external entities. Therefore the proposed community‐based flood hazard‐mapping technique can be a good solution for addressing current issues. The approach will not only focus on the effective development and application of FHM but also it will correct the defects of the top‐down approach in disaster planning and also encourage all stakeholders' participation in an integrated and sustainable manner.Practical implicationsBased on the findings, it is strongly recommended that agencies should adhere and incorporate the idea while developing programs and projects for communities. In addition, It is simple to understand and easy to implement by the community.Originality/valueIt is hoped that the idea will be beneficial and a catalyst to promote a community's response for flood disaster management in developing countries, thereby helping agencies to develop an operational strategy in advance.

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 m 3 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.

The megacities around the world have been located in fertile river basins since the beginning of civilization. In recent years, population has been increasing drastically; as a result, people migrate to cities and live in vulnerable and marginal areas, thus giving rise to slums. Therefore, we would like to point out why and how megacities in Asia are vulnerable to flood. The vulnerability of the cities, especially the slums, is shaped by forces such as global changes, internal migration patterns, development practices and political instability, which constantly reshape the risk of floods, which may be frequent and disastrous. Hence, development choices with a strong scientific background, especially in the cities prone to monsoon, are necessary to protect development, and not only floods but all disaster risks should be regulated and mainstreamed into the development planning of cities. Figure 7 The proportional inundation area of each city represented by the three-dimensional circles and the population change ratio in inundation areas.

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