Water as a risk factor – Soil sealing and glacial lake outburst flood

Floods and inundation are influenced not only by the intensity of the precipitation but also by the characteristics of the area on which the rain falls. Is the terrain fl at or sloping? Are there narrow or wide valleys in the region? What about the discharge capacity of river courses? How big are the retention and storage capacities? The degree of sealing determines how much water can seep into the ground. This involves not only artificial sealing, resulting from road-building and urban development, but also natural sealing, which depends on the type of soil and the amount of precipitation involved. In the case of prolonged and intensive rainfall, the ground becomes so saturated that sooner or later it cannot absorb any more water. Further precipitation then quickly leads to floods.

Artificial sealing, on the other hand, is usually a result of urbanisation. In many of India’s urban areas, upgrading of the sewage and drainage systems cannot keep up with the rapid pace of urban development. The systems are overloaded, there is backwater, and the ground is flooded.

Further perils emerge when glacial lakes burst and permafrost melts. The main factors for the advance or retreat of glaciers in the Himalayas are the volume and type of summer precipitation. In the wake of global warming, the zero-degree border rises, glaciers receive less snow and more liquid precipitation, and what were once giant masses of ice shrink. At the same time, the frozen slopes (permafrost) melt at an increasing rate at higher levels. Lakes that have been formed by glaciers and moraines burst, destabilising the sides of hills and mountains and dragging down loose material and debris fl ows. With the volumes of rain increasing, there is also a mounting danger of soil erosion. Moreover, debris, gravel, and sand add to the sediment carried in rivers, creating problems at reservoirs and hydropower stations.