7.1 Risk of Earthquakes
The Project area is located in a part of Pakistan where earthquakes frequently occur, though usually these are not of an exceptional magnitude. Tarbela is situated in the foothills of the Himalaya and Karakorum mountains. These mountain ranges were uplifted through the collision of the Indian and Eurasian tectonic plates. The zone of the main trust between the plates is located northeast of the project site at a distance 100- 200 km in Kohistan and Kashmir. However, the influence of associated local tectonic fault breaks can continue until the project area as far as the Potwar (or Potohar) plateau, which is situated south of the project area. High incidence of seismic activity through tectonical movements of local faults can be responsible for rupture of ground surface, ground acceleration, failures of natural slopes, and ground liquefaction.
The largest recent earthquake in the area was the 2005 NWFP/Kashmir earthquake with a magnitude of 7.6 on the Richter scale. More than 73,000 people were killed through this earthquake and 450,000 people made homeless. The epicenter was located at a distance of about 100 km northeast from Tarbela. The major earthquake was followed by a large number of aftershocks. The magnitude of 7.6 is exceptional (calculated frequency of once every 330 years). Most earthquakes in the area have a magnitude up to 6.4. As long as the process of mountain uplifting continues in the northern areas the project should be spared from major earthquakes with magnitudes in excess of 7. However, there is no guarantee that a local fault does not break and causes an earthquake. There is an active tectonic fault (the Darband fault) at the site which may displace by about 1.2 m. This makes the risk of failure of slopes and liquefaction of near-surface soil quite high, unless appropriate engineering measures are implemented to reduce these risks.
7.2 Risk of Flooding
Since its creation, Pakistan has faced eight severe floods. The floods of 1950, 1988, 1992 and 1998 resulted in a large number of deaths and severe loss of property, while the July 2010 floods have been described as the worst in the last eighty years. In July and August 2010 heavy monsoon rainfall in the northwest of the country caused flash and heavy riverine floods. Starting in the valleys of the Swat and Kabul rivers the flood peak after flooding large areas in KP province joined the Indus waters at Attock and travelled downstream through the densely populated irrigation areas in Punjab and Sindh and flooding large areas with around 8,000 deaths and nearly 20 million people being significantly affected through loss of housing, property, crops, and income.
During these floods the Tarbela reservoir experienced a historically high peak discharge of the Indus of 23,650 m³/sec, but this was considerably below the design discharge of 42,400 m³/sec. Reservoir and dam could therefore relatively easily cope with these high floods. Through operation of the reservoir the peak outflow at Tarbela even could be reduced with some 28 percent. There was no damage at Tarbela or surrounding areas.
The conclusion is that although the risk of flooding in the Indus Basin might increase in the coming years due to rising air temperature, shift in rainfall pattern and increased melting of glaciers in the upstream regions (see Section 6.3) the risk of flooding and related damage in the Tarbela area is very low.
Floods in the northern areas of Pakistan, including the upper part of the Indus catchment are not only associated with extreme rainfall events, but they can also occur after landslides and creation of river-dams and subsequent flood waves. However these flooding events are usually restricted to tributary areas and may have impacts on the upper Indus valley, but not on Tarbela.
7.3 Climate Change
Climate change is being considered as a critical factor behind changing rainfall patterns, the visible increase in precipitation during monsoon seasons, and more frequent extremely dry periods. Also the influence of climate change on air temperature such as minimum and maximum averages and the frequency of heat waves is often mentioned.
More than 13 percent of the Upper Indus Basin consists of glaciers and the melting of ice caps and retreat of glaciers is attributed to climate change. All these results have a considerable influence on the hydrology of the Indus Basin, the water availability and on the occurrence of floods and droughts.
During the last decade a lot of research is carried out to study the effects of long-term climate change on precipitation, air temperatures, and droughts. Some of the main conclusions of these studies are the following:
between 1980 and 2005 the frequency of heat waves (T> 40º C) has been increased in north-western Pakistan. It is expected that there will be more frequent periods with extreme drought;
based on predictions of the International Panel on Climate Change (IPCC) scenario’s estimates have been made by the Pakistan Meteorological Service from the increase in maximum daily temperatures, which ranges from 2.8º C to 4.2º C in the year 2080 for northern Pakistan;
more heavy rainfall events during monsoon season will occur over north-western Pakistan instead of the north-east of the country. Some models calculate 25 percent more rainfall during monsoon. As a result, areas along the western rivers of the country (Indus and Kabul) will be more vulnerable to flood episodes similar to the one experienced during 2010;
water availability might increase considerably (during kharif) but not when it is required for agriculture (rabi season);
a shift has been observed in the rainfall pattern with monsoons starting 1-2 weeks earlier and winter rains confined towards February;
Recent studies have been concentrated on the effects of glacial melt. Major issues to be investigated are amongst others:
the importance of the contribution of snow and glacial melt on the hydrology of the Indus;
the observed changes in the extent of the glaciers;
the effects of climate changes on the amount of melt-water.
From these studies it has been concluded that glaciers in the Himalaya and Karakorum are receding faster than happens in any other part of the world. From digital terrain models and satellite observations it might be concluded that the reduction of the thickness of ice in the Western Himalayan glaciers ranges between 0.50 to 0.90 m per year,
although in some areas in the Karakorum an extension and increase of glaciers has also been reported. A recent study (Immerzeel et al, 2010) suggests that 60 percent of the discharge in the Indus catchment is fed by melting of glaciers and snow. This is a very high percentage as compared to other major rivers originating in the Himalayas, such as Brahmaputra, Ganges and Yellow River. In a likely scenario of global warming based on IPPC predictions the reduction of the share of melt-water in the Indus discharge has been estimated at 8.4 percent. However this could be (over)compensated by an expected increase of the precipitation during monsoon in the area of 25 percent.
The relation between climate change and hydrology is extremely complex. This is because the high variability in data on climate and hydrology, requiring long time series and proper monitoring. Moreover regional circumstances might vary considerably, especially in high mountain areas. This often leads to conflicting data. More studies and more reliable data should be collected in the coming years. In view of the importance of these data for developing reliable and accurate knowledge of the basin hydrology and on future water availability of the Indus River it is recommended that the current project contributes to these studies with a Glacial Monitoring Program (Component C4: US$ 6 million). In the ESMP more details of such a study are presented and a cost estimate is given. This program includes extensive glacial studies, including satellite monitoring and studies into the effects of glacial outbursts.