научная статья по теме NEW APPROACHES ON THE RECONSTRUCTION OF ICE-DAMMED LAKE OUTBURST FLOODS IN THE ALTAI MOUNTAINS (SIBERIA) Геофизика

Текст научной статьи на тему «NEW APPROACHES ON THE RECONSTRUCTION OF ICE-DAMMED LAKE OUTBURST FLOODS IN THE ALTAI MOUNTAINS (SIBERIA)»



New approaches on the reconstruction of ice-dammed lake outburst floods in the Altai

Mountains (Siberia)

J. Herget

University of Bonn, Germany

Представлены результаты исследований седиментологических и геоморфологических следов прорывов ледниково-подпрудных озер в горах Алтая.

Area of investigation and evidence of floods

The headwaters of the River Ob which form the centre of this study have their origin close to the border to Mongolia in the Altai-Mountains, located in south-western Siberia. During the Pleistocene, several mountain glaciers reached the valley bottom of the River Chuja, the main tributary of the River Katun which is the major source of the River Ob. Near the village of Aktash the extended glaciers blocked the course of the River Chuja and formed an ice-dammed lake, which attained a volume of 607 km3. The water surface of the lake reached a maximum altitude of 2100 m which is indicated by validated strandlines and ice-rafted debris in the intramoun-tainous basins of Kuray and Chuja. At times of ice-dam failure, ensuing giant floods passed through the valleys of Chuja and Katun River. The collapse was probably caused by overtopping and rapid incision into the glaciers. The flow transported gravel in suspension [5, 12] and deposited this gravel in giant bars at local valley expansions and tributary mouths along the flood route. At the mouths of some tributaries along the flood route the giant bars blocked valleys and generated secondary lakes which

Fig. 1. Longitudinal profile along Katun and Chuja rivers downstream of the former ice-dam with locations and elevations of giant bars and related levels: 1 — run-up sediments, 2 — giant bars and levels, 3 — deepest point of visible susp. Gravel, 4 — bedload terrace level, 5 — mean recent water level Рис. 1. Продольный профиль вдоль рек Катунь и Чуя (вниз по течению) с положением древних ледниковых подпруд и высотами гигантских валов и соответствующих им уровней

lasted over different periods of time. These lakes are indicated by lacustrine deposits, such as at Injushka reaching Katun valley at Inja from the east, where the lacustrine sediments are interbedded with suspension gravel indicating repeated outburst floods from the ice-dammed lake upstream. Other flood related features are gravel dunes and deposits of erratic boulders. In combination with exposures of characteristic suspension gravels at the current bedrock valley bottom, the surfaces of the giant bars reveal depths of flow of up to 400 m in Chuja valley close to the downstream extent of the ice-dam. In the wider valley of Katun River depth of flow reached about 250 m. At several locations, drawdown levels along the slopes of the giant bars indicate phases of temporary stable water surface levels during waning flow. Run-up sediments are deposited in front of local valley obstructions as thin layer of suspension gravel on more or less steep bedrock surfaces and can be used to estimate local flow velocity. They indicated a rising water surface level when flow velocity abruptly decreased at the valley obstruction caused by energy conservation (Fig. 1).

This review focuses on selected approaches to reconstruct the ice-dammed lake outburst flood, while several previous publications present general reviews based on the state of knowledge available at the time of publication [2, 7, 12, 21—23]. Other studies focus on special aspects, e.g. by Parnachov [5] on the geology of giant bars, Carling [10, 11] on gravel dunes or Borodavko [1] on the ice-dammed lake itself.

Different approaches of reconstruction

Previous estimations of the discharge of the outburst floods were carried out by different researchers. Butvilovski ([2] and written communication) applied a number of different empirical equations to the estimate mean velocity of flow based on geometrical parameters of valleys and basins. Discharges are calculated for cross-section areas consisting of the entire basins resulting in estimated peak discharges of up to 50 mln m3/s. Baker et al. [8] applied the water level calculation software HEC-2 to estimate discharge based on the elevation of flood related features. With this attempt assumed values for the discharge are iteratively optimised until the model calculation of the water level fits the altitude of water level indicators in the field. They chose a location within the extension of the former ice dam and estimated a peak discharge of 18 mln m3/s based on a hydraulic jump located in the modelled reach of the valley. Carling [10] investigated the hydraulic implications of the

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gravel dunes in Kuray Basin, which was inundated by the ice-dammed lake. He found a discharge in the magnitude of 750,000 m3/s for the flow draining Kuray Basin when the ice dam has failed. This estimated discharge cannot be related to peak discharge of the outburst flood downstream of the ice dam as the dune field is located upstream of the valley blockage. In opposite to Butvilovski ([2] and written comm.) Carling's approach to estimate discharge of an unspecific stage of the decreasing flood also located in Kuray Basin appears more plausible.

Based on own investigations of flood related features along Chuja and Katun valley seven independent attempts to gain information about the discharge of the outburst floods are used. The paper presents a review on the studies, however additional details are intensively discussed elsewhere [14, 15].

Conveyance — slope method. Based on the continuity equation and the empirical formula for the estimation of mean flow velocity in channels by Manning, the discharge Q of the outburst flood can be expressed as Q=An_1R2/3S1/2, where A is cross-section area, n is roughness coefficient, R is hydraulic radius and S is slope of the energy line. Data for the geometric parameters including water level surface are given by surveys of the giant bars along the flood's pathway or are derived from detailed topographic maps (for cross-section area and hydraulic radius by topographic maps of scale 1:50,000). It is assumed that the slope S of the energy line is more or less similar to the slope of the water surface along the direction of flow (indicated by the slope between the giant bar surfaces along the valleys: 6.0%c for Chuja valley, 4.3%c for Katun valley). The roughness coefficient n is an estimation. Previous studies on floods of comparable magnitude like the drainage of Pleistocene Lake Bonneville [19] and the outburst flood of Lake Missoula [20], the most plausible values for the roughness coefficient were determined in the range of 0.04<n<0.07. Hence, the calculation for the outburst floods in the Altai Mountains considered both values as extremes.

Based on investigations of hydraulic conditions of flow in steep gradient river flow [18] it is assumed that subcritical flow conditions are dominant along the outburst flood flow. Also for the Pleistocene Lake Missoula and Lake Bonneville Floods subcritical flow conditions are found to be predominant [19, 20]. Therefore, locations and parameter combinations indicating flow conditions of Fr > 1 are neglected for further interpretation. Based on this assumption, a peak discharge can be estimated to be in the range of 10 mln to 14 mln m3/s.

Run-up sediments indicating velocity head of flow. Run-up sediments are forming a relatively thin layer of suspension gravel deposited in front of local valley obstructions and are the uppermost deposits related to the outburst flood. They can be seen as indicator of locally risen water surfaces due to the physical law of energy conservation. Based on the energy equation after Bernoulli, total energy H is constant along the channel [13]. As pressure energy is not of relevance for free surface flow, the energy equation can be expressed as

H = (y+z) + (v2/2g) ,

where (y+z) is potential energy, y depth of flow, z elevation above datum, (v2/2g) is kinetic energy, v is velocity of flow, g is acceleration of gravity.

While total energy H, elevation above datum z and acceleration of gravity g remain constant an abrupt decreased of flow velocity leads to an increased depth of flow, hence the water level rises in front to an obstruction. This phenomenon can be observed e.g. at river flow against bridge piers where the risen water surface in front of the piers is obvious. The deposited suspension load indicates the risen water surface level in front of the obstruction, hence gives evidence on the energy head of peak discharge of the flood by comparison with giant bar surfaces nearby (Fig. 2).

Due to non-uniform distribution of velocity within open channel flow, an energy coefficient a is introduced to calculated the change of height of the water level from total loss of kinetic energy [13]. Except for the case of uniform flow the energy coefficient a is always larger than unity and increases with steepness of channel. The equation for the mean velocity head vh in a channel can be written as

Vh = a (v2/2 g) .

Depending on the amount of irregularities in channels, values for a vary between 1 and 2, rarely reaching values of up to 4.7 with a mean average value of 1.3 found in the literature. Assuming that run-up sediments and uppermost giant bar surfaces nearby are generated during the same stage of the flood, flow velocity v can be determined by taking velocity vh as difference in elevation of run-up sediments and giant bar surfaces.

While the risen water level in front of the obstructions is indicated by suspension gravel being deposited in

Fig. 2. Run-up sediments in lower Chuja valley (flow towards background). The difference of elevation between the giant bar surface in front and the uppermost run-up sediments is 133 m Рис. 2. Отложения выноса в низовьях долины Чуи (направление течения — от зрителя). Разница высот между поверхностью гигантского вала на переднем плане и самыми высокими отложениями выноса составляет 133 м

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