Experts warn of high tidal wave in Hunza: How long can Attabad landslide dam last?

PESHAWAR: The natural lake formed by the blockage of the Hunza River from January 4, 2010 Attaabad landslide has grown to a volume of over 132 million cubic meters.

Glacial melting, avalanches and outburst of glacial lakes in the catchment since the beginning of May 2010 has substantially increased the rate of inflow, bringing the surface level of the lake 4m short of the bottom of the spillway created for the outflow of the impounded water across the landslide dam on May 20, 2010. An enigma for authorities and public has been the lack of reliable forecasting of scenarios when overflow starts in the next couple of days. There are more questions than answers. Will the dam withstand overtopping through the recently constructed spillway?

Pakistan has successfully experimented this with the Hattian landslide dam formed by 2005 Kashmir earthquake. But what if, unlike the Hattian situation, the dam fails as the overtopping begins? According to a 2003 survey by Italian scientists, most natural dam collapses coincide with the first dam overtopping. This leads to the next most crucial question that determines the downstream impact of the dam failure. Will the dam fail catastrophically and send a sediment-laden flood wave (as high as 60 meters) downstream inundating overbank habitats and seriously damaging infrastructure of roads, bridges and other communication means.

Alternatively, the dam may breach slowly over a time span ranging from a few days to a few weeks. This will, obviously be the most favourable situation. It will not only reduce the intensity of the debris-laden flood, avoiding serious inundations of overbank populace and damage to the KKH and bridges.

One would expect that experts, including geotechnical engineers and geologists, would have ready answers to these questions. The reality is that under natural environments there are so many variables involved that technical experts cannot make guaranteed predictions about the dam stability or the nature of the breach. At the most, different situation scenarios are developed. In the present case, governmental agencies have modelled flood heights and arrival time at various points in downstream sections of the Hunza, Gilgit and Indus rivers, assuming different durations of breach. In the case of catastrophic failure, their models predict a flood level of 36m that reduces to 7m in case of a breach spread over 24 hours.

In order to address these critical questions, a different approach adopted here involves comparison of the current situation with existing or past documented landslide dams. Italian scientists (2003) developed a database of world-wide landslide dams amounting to more than 500. According to these data, landslide dams widely vary in geographic distribution, physiographic settings, dimensions and stability. The world’s largest landslide dam is located in Tajikistan which formed in 1911 on Murghab River as a consequence of a strong earthquake in the Pamir Mountains. The 550 m high dam has survived to date and has never been overtopped as the inflow in the lake is balanced by outflow through seepage. World’s worst landslide dam outburst disaster is recorded in Sichuan province, China. The resulting flood extended 1400 km downstream drowning over 100,000 people.

Pakistan has a history of formation and catastrophic failure of landslide dams. Ken Hewit, a Canadian geomorphologist, has documented more than 35 natural dams formed in Karakoram-Himlayas of northern Pakistan in the past 500 years. Whereas a great majority of these formed in response to glacier surges, half a dozen of the natural dams formed in response to landslides. Amongst the historic landslide dams, an event at Raikot-Astor (Nanga Parbat) blocked the Indus River for more than six months with a catastrophic breach in June 1841 causing 25 m high devastating floods all the way to Attock. A Sikh army camped downstream from Tarbela was caught unaware resulting in causality of 500 soldiers. Another massive landslide dam and outburst occurred on Hunza River (some 4 km downstream from the site of the present-day Attaabad landslide dam) in 1858 when exceptionally high floods were again recorded at Attock. The same location was site of another landslide dam and outburst flood in 1962 but with lower intensity levels.

Pakistan is probably the only country in the world having suffered two major landslide-dam forming events in last five years. The Hattian dam formed on Oct 8, 2005 when a landslide was triggered by the magnitude 7.6 Kashmir quake. It survived to-date without failure. This well-documented landslide dam can be taken as an example for comparison with the January 4, 2010 Attaabad landslide dam, providing insight into the stability the latter. Both are formed by gigantic rock avalanches blocking the respective rivers, but have some critical differences.

The two essential parameters determining the stability of the landslide include volume of the landslide material (stabilizing factor) and volume of the impoundment/lake (destabilizing factor). The landslide volume in the case of Attaabad is lower by a factor of ~4, while the lake volume is 7 times higher than that of the Hattian landslide. The higher landslide volume in the case of Hattian landslide is due to its earthquake origin, in line with global data that suggest about three-times higher landslide volume for earthquake triggered landslides compared to those triggered by other mechanisms (e.g., rainfall or slope instability).

Likewise, the greater lake volume in case of Attaabad landslide dam is due to involvement of the high-energy Hunza River with a glacier-fed catchment region comprising over 12000 km2 area. In comparison, the Hattian landslide dam formed on a tributary stream rather than on the main Jhelum River, with a glacier-free catchment area of only 44 km2. Knowing that the Hattian landslide dam has stabilized (at least to-date, having survived more than four years), a simple stability factor can be derived based on lake volume/landslide volume ratio that amounts to 0.2. In comparison, this factor in the case of the Attaabad landslide dam amounts to 7, clearly demonstrating that the latter is substantially more unstable than the Hattian landslide dam. Historical data corroborates these observations. Of the 35 natural dams in the Karakoram-Himalayas of northern Pakistan formed in last 500, none has survived. This clearly shows that high-energy, glacier-fed rivers of this region wash out landslide debris dams, sometimes without leaving any remnants.

Once established that the Attaabad landslide dam is rated potentially unstable, the next question is when and how it is going to fail? Historic data from the Indus and Hunza rivers (1841 and 1858, respectively) suggest that landslide dams in these high-energy rivers have not survived more than 10 months at the maximum, depending upon the number of summer months involved. Glacial-melting induced water inflow in impounded lakes is so enormous that survival of any natural dam is questionable.

There is a general consensus the world over that overtopping is the most critical factor in dam failure. Overtopping commonly occurs when the reservoir is filled beyond its capacity. In some cases, overtopping can occur prior to complete filling in response to impact of avalanches, debris flow or landsliding in the newly formed lakes. In essence, it is the erosive action accompanying the overtopping that fails the dams.

Commonly, the slope failure on the downstream end of the landslide dam weakens the main dam to initiate breach. Sometimes, overtopping is accompanied by internal landsliding within the landslide material, resulting in lowering of the dam height. A third phenomenon is called piping, i.e., internal erosion through seepage channels. Surface erosion, landsliding and internal erosion may act simultaneously to bring about catastrophic failure of dam.

When Attaabad landslide dam is viewed in this perspective, it is clear that erosion accompanying the drainage through the newly constructed spillway will be a critical factor. Pre-existing active seepage through internal channels is expected to grow in volume and dimensions, enhancing internal erosion in the landslide body. Likewise, collapsing and internal landsliding within the landslide dam cannot be ruled out. Whereas failure is imminent, the time involved in complete failure is difficult to be ascertained. The presence of gigantic boulders in the landslide material is the only stabilizing factor capable of delaying catastrophic failure.

The historic data gives some useful clues about outburst floods following catastrophic breach of natural dams. The 1929 outburst flood from a natural dam on Shyok River was thoroughly documented and monitored up to some 1500 km downstream by Gunn (1930) and Mason (1932). According to these observations, the maximum flood rose to 13-26 m high in narrow gorges and 7-10 m in wide parts of the Indus River valley. Interestingly, flood levels rose to 7-8 m at Tarbela and Attock, respectively 1120 and 1194 km downstream from the outburst dam. This is almost identical to flood levels at Skardu at a distance of 500 km from the breached dam. This implies that flood levels rise 2-3 times higher in narrow gorges compared to places where valley floor is wide. Most importantly, Ken Hewitt (based on observations from Gunn, 1930 and Mason, 1932) concludes that “over much of their course in the mountains, the recorded (outburst) floods reach heights well above peak discharges from summer melting”.

In summary, whereas scientific and engineering forecasting about stability of landslide dams is marred by uncertainties, historical data both from the Karakoram region as well as from around the globe suggest that high-energy rivers (e.g., Indus, Hunza and Gilgit) marked by high lake volume of landslide material are unlikely to preserve such dams and, when the dams breach, flood levels reach heights far above the peak summer discharge in Karakoram-Himalaya rivers.The authors are on the faculty of the Department and National Centre of Excellence in Geology, University of Peshawar. Email: [email protected]

Source: The News, May 22, 2010

Comments

comments