Multi-sectoral and multi-scale disasters

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Losses and damage from natural hazards are increasing. More than 70 people died during the Seti flash flood in 2012, 150 during the Jure landslide in 2014 and 20 during the Melamchi flood in 2021. Tens of thousands of people are displaced. Weather-related losses in Nepal are among the highest in the world. The impacts are multi-sectoral, threatening human settlements, critical infrastructure, the economy and the ecosystem.

Extreme weather, climate and water conditions are increasing in Nepal, according to a recent study published in the scientific journal Natural hazards. And extreme events are likely to become more frequent and intense as the climate changes. This means more record rainfall, floods, heat waves, droughts and wildfires are expected in the coming decades.

Cascading Hazards

Cascading hazards result from a series of hazardous events and their interactions. One hazard often triggers another hazard (event A is followed by event B, which is followed by event C and so on), and the cumulative impacts lead to massive disasters. For example, rainfall after wildfires can trigger massive landslides and floods. Cascading events can start at a specific location; but can intensify, spread and generalize to affect larger areas.

The upper Himalayan regions are more prone to snow and glacier avalanches. Global warming can cause snowmelt to start earlier, amplify the rate of glacier melt, and contribute to glacier retreat. The number and size of glacial lakes are proliferating. Many of these lakes have unstable moraines (dams). Impacts from rock/ice avalanches or landslides can destabilize the dam, increasing the risk of glacial lake flooding with dire consequences downstream.

The Middle Himalayas are mainly affected by landslides. Landslides often block the river and create landslide barrage lakes. In August 2014, prolonged rains caused a catastrophic landslide in Jure, Sindhupalchok. Debris from the landslide formed an earthen dam that blocked the Sunkoshi River. The landslide dam lake destroyed sections of the Araniko highway, disrupting cross-border trade. The cascading event interrupted the transmission of electricity from five hydroelectric projects and caused a deficit of 66.5 megawatts of electricity in the national grid.

The 2021 Melamchi disaster resulted from multiple hazardous events that spread from high altitudes to river valleys. Heavy rains in the upstream region triggered massive erosion of glacial deposits, the overflow of a glacial lake and the temporary damming of a river. The dam failure resulted in a large debris flow and sediment-laden flood. The continuous toe cutting and aggravation of the rivers by landslides has made the river system vulnerable to even moderate rainfall.

Seemingly normal weather conditions can trigger a dangerous chain of events with disastrous consequences. Warmer days in higher elevations increase the risk of glacial lake flooding. A series of average rainfall events in regions where land use and other human activities have changed increases the likelihood of landslide dam failure flooding. The interaction between glacial lake overflows and landslide dam overflows amplifies the risk of flooding in downstream settlements.

The southern plains are more susceptible to flooding and sediment deposition. Sediment eroded from the Himalayas is transported and deposited on the southern plains. These deposits can alter the river landscape and inundate floodplains. The August 2008 Koshi flood is a typical example where an embankment failure altered the course of the river. The flood deposited large amounts of sediment on agricultural fields, drastically reduced crop yields and affected millions of people.

Changes in land use and land cover can increase the risk of forest fires, landslides and floods. Wildfires can make the landscape vulnerable to landslides, even under normal rainfall conditions. Deforestation and unplanned construction of dirt roads increase the risk of landslides and floods, especially during the monsoon season.

Human activities most often cause forest fires. Wildfires and severe heat waves in the southern plains have occurred more frequently in recent decades. The dry wind exacerbates forest fires, amplifies air pollution and threatens public health. Rainfall over burned areas can cause flash flooding, increase soil erosion and significantly deteriorate water quality.

Cascading hazards lead to cascading failures in critical systems. For example, urban areas like Kathmandu and Bhaktapur are at high risk of heavy flooding due to unmanaged development patterns and encroaching rivers. Flooding overwhelms the drainage system, lowers water quality, floods streets and destroys the power grid. Critical infrastructure such as fire departments, schools, and hospitals lose access to power, transportation, water, and communications.

Managing Hazards

Investing in science is crucial to improving prediction of cascading hazards and risks. Further research is needed to understand what drives natural hazards, their interactions and failure modes, and how human and physical systems respond to risks and failures.

Better data and computer models are needed for effective risk prediction. For example, urban flood modeling requires a detailed representation of local precipitation, land surface features, surface and subsurface flows, drainage networks, buildings, roads, and hydraulic infrastructure (culverts and bridges). Predicting how risk changes over time requires a good understanding of future climatic conditions, development patterns, vegetation dynamics, demographic changes and coping strategies.

There are opportunities to improve cascading hazard predictions. Recent advances in modeling capabilities, emerging computing innovations (high performance computing, artificial intelligence, and machine learning), and the availability of high-resolution remote sensing and satellite information can help improve our understanding, modeling, and predictions. cascading hazards.

Reliable early warning systems are essential for disaster preparedness and management. However, current early warning systems focus on a single isolated event, mainly heavy rains and floods. We need to develop, expand and automate early warning systems to communicate the risk associated with potential hazards, their interactions and cascading phenomena.

Infrastructure design specifications must change with climate change. Engineers often rely on historical climate records to design water infrastructure, transportation systems, and commercial buildings. Scientists have produced many different projections of future climate. Engineers and policy makers need to assess the performance of design strategies across the full plausible range of future climates to identify robust design choices.

Adaptation and mitigation responses must be socially equitable, scientifically based and economically efficient. Such an approach helps build the resilience of diverse, vulnerable and underserved communities. Resilient risk management strategies could be costly and require strategic investment policies. Engineers, climatologists, risk analysts and policymakers must work closely together to manage risks and optimize costs.

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