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  4. Exhausting Glacial Lakes of HKH before they explode
    By Muhammad Raza KhanChairman APM PakistanDec 2012
    Many disastrous floods have occurred in the...

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MONSOON DESK

Exhausting Glacial Lakes of HKH before they explode
By Muhammad Raza Khan
Chairman APM Pakistan
Dec 2012

Many disastrous floods have occurred in the HKH (Himalayas Krakoram & Hindukush) when a dam of moraine failed suddenly and released massive amounts of water that had been stored in a glacial lake. Such eruption floods are far more destructive than floods generated by rainfall or snowmelt.

As a way of reducing the risk of catastrophe from some of these lakes, a simple siphon technique has been proposed to reduce the water level. Although this approach appears to be relatively effective, practical, inexpensive and safe, there is substantial risk of seismic events, ice calving, rapid incision of the outlet channel, and piping through the terminal moraine. Artificial drainage of glacial lakes presents a rare opportunity to actively reduce the potential for a natural disaster caused by destructive water.

Glacial lakes are a common feature of the HKH. There are many types of these lakes, ranging from melt water ponds on the surface of glaciers to large lakes in side valleys dammed by a glacier in the main valley. Water trapped behind moraines forms a particularly hazardous type of glacial lake. When glaciers retreat up valley as their lower portions melt away, a topographic depression is often formed by the moraine at the maximum extent of the glacier. If this enclosure is water tight, melt waters will accumulate in the basin until seepage or overflow limits the lake level.

Warmer climates of the past 100 to 150 years have resulted in widespread glacial retreat and formation of moraine-dammed lakes in many mountain ranges (Evans & Clague, 1994).

Where the impoundment of these lakes is unstable, there are considerable risks of a sudden release of large quantities of stored water. A common cause of outburst floods are surge waves generated by snow, ice or rock avalanches into the lake that then topple over the dam and rapidly grind down the outflow channel, ultimately collapsing the moraine in the surrounding area of the channel. Other failure mechanisms include seepage and piping through the moraine, slow melting of an ice core within the moraine, earthquakes, and progressive thinning of the moraine by landslides.

Outburst floods can be several times greater than floods produced by even extreme rainfall.

The destructive potential of an outburst flood depends on: the total volume of water released; the magnitude and duration of release, which depend on the dam failure situation; river slope and width and variations in these characteristics along the channel; the amount and size of erodible material in the dam area and downstream; the resistance of the channel and valley sides to erosion and landslides; the location of people at the time of the event; and location of houses, structures, trails and bridges relative to the flood itself and resultant landslides (Yamada, 1993; Braun & Fiener, 1995). Attenuation of the flood wave also influences the extent of damage downstream and is affected by channel conditions, landslide activity, and entrainment of debris.

Since mid of 1970’s glacial lake outbursts in the Himalaya have received increasing outside attention as they have threatened more people and property. More than a dozen outburst events have been documented in Nepal between 1964 and 1991 (Yamada, 1993).

Glacial lakes are also recognized as a significant hazard in the eastern Himalaya of Bhutan (Gansser, 1970). However, the glacial lakes outburst record for HKH junction in Pakistan yet not been maintained. The inhabitants are the only source to verify the past incidents of such accidents. For instance, such a moraine of the Bagrote Glacier about 70 Kilometer away in the north of Gilgit was burst out in 2008 damaged local agriculture and infrastructure and it all was misunderstood by the media and PMD (Pakistan Meteorological Department). It scared the locals and they demanded an imidiate construction of dam then to save the population. (www.apmpk.com)

Receding_Bagrote_GlacierMoraineOffshoot_of_Rakaposhi_Massif

Receding Bagrote Glacier      Moraine                                Offshoot of Rakaposhi Massif

                                                                                                                     Photos by the author; July 9 2010

Minimizing the Hazard

The basic approaches to reducing the hazard of glacial lake outburst floods include

  1. i.getting out of the way while letting the natural event take its course,
  2. ii.strengthening the dam and providing a reinforced outlet control structure,
  3. iii.and artificially reducing the water level of the lake.

Siphons carry the least risk of inducing catastrophic failure and are appropriate for remote installations. Theoretically, lake levels could be lowered about 5 m with a simple siphon under Himalayan conditions (Grabs & Hanisch, 1993). Appropriate technology for such a siphon system could consist of a series of independent pipes that are small enough for individual sections to transported by porter (e.g. 6 m lengths of 15 cm diameter plastic pipe). A design with valves on both ends and the high point along with a portable pump for filling has been proposed by Grabs & Hanisch (1993). The number of siphons in operation would be determined by the desired rate of lowering, which must account for the rate of inflow during the summer monsoon and melt season. Excessively rapid lowering of the water level could induce failure of the lateral moraines or calving of any floating glacial ice at the upstream end of the lake (Grabs & Hanisch, 1993). Collapse of large volumes of moraine or ice into the lake could generate a surge wave leading to erosion of the impoundment. Siphons have been successfully installed in a lake in the Peruvian Andes near Huaraz where two siphons with a combined capacity of 0.5 mY1 lowered the water level by almost 1 m per month (Reynolds, 1992, 1994). However, this project was prematurely halted by political conflict.

Another possibility to lowering the lake level might be cutting sections of ice in the winter and hauling them over the moraine at a point where it had been excavated manually and with explosives. The valley floor beyond the glacier must be higher than the lake level.

Lake levels need only be lowered to a point where dam failure is unlikely or where such failure would not cause catastrophic damage downstream. Once the dam is lowered to the level of a reasonable volume of water and a relatively stable spillway exists, then the prospects for a severe flood have been minimized because possibility of refilling the lake has been eliminated.

 

 

References:

Reynolds, J. (1992) Geological hazards in the Cordillera Blanca, Peru. AGID News 61/62, 31-33.

Reynolds, J. (1994) The identification and mitigation of glacier-related hazards: examples from the Cordillera Blanca, Peru. In: Geohazards: Natural and Man-made (ed. by G. J. H. Hall, D. J. C. Laming & S. C. Scott), 143-157. Chapman & Hall, London.

Watanabe, T., Ives, J. D. & Hammond, J. E. (1994) Rapid growth of a glacial lake in Khumbu Himal, Himalaya:

prospects for a catastrophic flood. Mountain Res. & Devel. 14(4), 329-340.

Watanabe, T., Kameyama, S. & Sato, T. (1995) Imja glacier dead ice melt rates and changes in a supraglacial lake, 1989-1994, Khumbu Himal, Nepal: danger of lake drainage. Mountain Res. & Devel. 15(4), 293-300.

Watanabe, T. & Rothacher, D. (1996) The 1994 Lugge Tsho glacial lake outburst flood, Bhutan Himalaya. Mountain Res. & Devel. 16(1), 77-81.

Yamada, T. (1993) Glacier Lakes and their Outburst Floods in the Nepal Himalaya. Water and Energy Commission Secretariat, Kathmandu, Nepal.

Yamada, T., Kadota, T., Kita, T., Mool, P. K., Markey, P. R. & and Joshi, S. P. (1996). Physiographic characteristics and hydrological conditions of Tsho Rolpa glacier lake, Rowaling Valley, Nepal Himalaya. Paper presented at the International Conference on Ecohydrology of High-Mountain Areas (Kathmandu, Nepal, March 1996).

 

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