The
basic materials required for this research are satellite images, large-scale
topographic maps and the inventory of glacier and glacial lakes. The TM
image acquired in the 1990 and the ETM image obtained in 2000 were used to
study the activity of glaciers and for the identification of potentially
dangerous glacial lakes. The topographic maps as reference used to help
interpreting image and obtaining some attribute of glaciers such as
elevation, orientation.
Topographic Maps
Satellite Images Fig4.2: Index Map of Landsat ETM images
Inventory of glaciers
and glacial lakes The glacier margins The glacier margins on each topographic map are delineated and compared with aerial photographs, and the exact boundaries between glaciers and seasonal snow cover are determined. The coding system is based on the subordinate relation and direction of river progression according to the World Glacier Inventory ( WGI). The descriptions of attributes for the inventory of glaciers are given below.Numbering of glaciers The lettering and numbering start from the mouth of the major stream and proceed clockwise round the basin. For convenience, the major river systems are further divided into five levels sub-basins. For example, "5" indicate Asia, " Z " indicate the inner-land water system of Qingzang plateau (level 1 basin),"2"indicate the selincuo lake basin(level 2 basin). The coding of level 3 and level 4 basin is in Arabic numerals. As for the level 5 basin, the English letter is used. "5Z211F1" indicates Asia, inner-land water system of Qingzang plateau (level 1 basin), Selincuo lake basin (level 2 basin), Chibuzhang lake (level 3 basin), Jinxiwulan lake (level 4 basin), Xianche river (level 5 basin), and the last number indicate the glacial number in the last level basin. Registration of snow and ice masses All perennial snow and ice masses are registered in the inventory. Measurements of glacier dimensions are made with respect to the carefully delineated drainage area for each ‘ice stream’. Tributaries are included in main streams when they are not differentiated from one another. If no flow takes place between separate parts of a continuous ice mass, they are treated as separate units. Delineation of visible ice, firn, and snow from rock and debris surfaces for an individual glacier does affect various inventory measurements. Marginal and terminal moraines are also included if they contain ice. The ‘inactive’ ice apron, which is frequently found above the head of the valley glacier, is regarded as part of the valley glacier. Perennial snow patches of large enough size are also included in the inventory. Rock glaciers are included if there is evidence of large ice content. Snow line In the present study, the snow line specially refers to the firn line of a glacier, not the equilibrium line. The elevation of the firn line of most glaciers was not measured directly but estimated by indirect methods. For the regular valley and cirque glaciers from topographical maps, Hoss’s method (i.e. studying changes in the shape of the contour lines from convex in the ablation area to concave in the accumulation area) was used to assess the snow line. Accuracy rating table The accuracy rating table proposed by Muller et al. (1977) on the basis of actual measurements is used in the present study. For the snow line an error range of 50-100m in altitude is entered as an accuracy rating of ‘3’. Table 4.1 Accuracy rating adopted from Muller et al. (1977)
Mean glacier thickness and ice reserves According to Muller et al. (1977), mean depth can be estimated with the appropriate model developed for each area by local investigators. For example, the following model was used for the Swiss Alps where h is the mean depth (m), F is the total surface area (km2), and a and b are arbitrary parameters that are empirically determined. There are no measurements of glacial ice thickness for the Himalaya-China regions. Measurements of glacial ice thickness in the Tianshan Mountains, China, show that the glacial thickness increases with the increase of its area (LIGG/WECS/NEA 1988). The relationship between ice thickness (H) and glacial area (F) was obtained there as = –11.32 + 53.21 F0.3 if F>=0.03km2 This formula has been used to estimate the mean ice thickness in the glacier inventory. The same method is also used here to find the ice thickness. The ice reserves are estimated by mean ice thickness multiplied by the glacial area. Area of the glacier The area of the glacier is divided into accumulation area and ablation area (the area below the firn line). The area is given in square kilometres. The delineated glacier area is measured by the digital planimeter and checked repeatedly. But in this study, we digitized the glaciers with Arcview software and automatically re-calculated the area and ice reserve. Length of the glacier The length of the glacier is divided into three columns: total length, length of ablation and the mean length. The total (maximum) length refers to the longest distance of the glacier along the centerline. The mean value of maximum lengths of glacier tributaries (or firn basins) is the mean length. Mean width The mean width is calculated by dividing the total area (km2) by the mean length (km). Orientation of the glacier The orientation of accumulation and ablation areas is represented in eight cardinal directions (N, NE, E, SE, S, SW, W, and NW). Some of the glaciers are capping just in the form of an apron on the peak, which is inert and sloping in all directions, is represented as ‘360’. The orientations of both the areas (accumulation and ablation) are the same for most of the glaciers. Elevation of the glacier Glacier elevation is divided into highest elevation (the highest elevation of the crown of the glacier), mean elevation (the arithmetic mean value of the highest glacier elevation and the lowest glacier elevation) and lowest elevation. Morphological classification The morphological matrix-type classification and description is used in the inventory, which was proposed by Muller et al. (1977) for the TTS to the WGI. Each glacier is coded as a six-digit number, the six digits being the vertical columns of Table 4.2. The individual numbers for each digit (horizontal row numbers) must be read on the left-hand side. This scheme is a simple key for the classification of all types of glaciers all over the world.Each glacier can be written as a six-digit number following Table 4.2. For example, ‘520110’ represents ‘5’ for a valley glacier in the primary classification, ‘2’ for compound basins in Digit 2, ‘0’ for normal or miscellaneous in frontal characteristics in Digit 3, ‘1’ for even or regular in longitudinal profile in Digit 4, ‘1’ for snow and/or drift snow in the major source of nourishment in Digit 5, and 0 for uncertain tongue activity in Digit 6.The details for the glacier morphological code values according to TTS are explained below. Digit 1 Primary classification
Digit 2 Form : Two or more tributaries of a valley glacier, coalescing (Figure 4.4a). 2 Compound basin: Two or more accumulation basins feeding one glacier ( Figure 4.4b).3 Simple basin: Single accumulation area ( Figure 4.4c).4 Cirque: Occupies a separate, rounded, steep-walled recess on a mountain ( Figure 4.4d).5 Niche: Small glacier formed in initially a V-shaped gully or depression on a mountain slope ( Figure 4.4e).6 Crater: Occurring in and /or on a volcanic crater 7 Ice apron: An irregular, usually thin ice mass plastered along a mountain slope. 8 Group: A number of similar ice masses occurring in close proximity and too small to be assessed individually. 9 Remnant: An inactive, usually small ice mass left by a receding glacier.
Digit 3 Frontal characteristics
Digit 4 Longitudinal profile
Digit 5 Major source of nourishment The sources of nourishment could be uncertain or miscellaneous (0), snow and/or drift snow (1), avalanche and/or snow (2), or superimposed ice (3) as indicated in Digit 6 Activity of tongue A simple-point qualitative statement regarding advance or retreat of the glacier tongue in recent years, if made for all glaciers on Earth, would provide the most useful information. The assessment of an individual glacier (strongly or slightly advancing or retreating etc) should be made in terms of the world picture and not just that of the local area; however, it seems very difficult to establish the quantitative basis for the assessment of the tongue activity. A change of frontal position of up to 20m per year might be classed as ‘slight’ advance or retreat. If the frontal change takes place at a greater rate it would be called ‘marked’. Very strong advances or surges might shift the glacier front by more than 500m per year. Digit 6 expresses qualitatively the annual tongue activity. If observations are not available on an annual basis then an average annual activity is given. Moraines: Two digits to be given. Digit 1: moraines in contact with present-day glacier. Digit 2: moraines further downstream.
Remarks: The remarks can, for instance, consist of the following information.
The inventory database form (see Annex I and II ) used for compilation of the inventory of glaciers includes basin numbers, map/satellite codes and year, as well as the glacier parameters described above.The glacial lakes on each image are delineated by the help of DEM. The descriptions of attributes for the glacial lakes’ inventory based on LIGG, WECS and NEA (1988) are given below. Numbering of glacial lakes The numbering of the lakes starts from the outlet of the major stream and proceeds clockwise rounding the basin. Longitude and latitude Reference longitude and latitude are designated for the approximate centre of the glacial lake. Area The area of the glacial lake is determined from the digital database after digitization of the lake from the topographic maps. Length The length is measured along the long axis of the lake, and estimated to one decimal place in km units (0.1 km). Width The width is normally calculated by dividing the area by the length of the lake, down to one decimal place in km units (0.1 km). Depth The depth is measured along the axis of the cross section of the lake. On the basis of the depth along the cross section the average depth and maximum depth are estimated. The data are collected from the literature. Orientation The drainage direction of the glacial lake is specified as one of eight cardinal directions (N, NE, E, SE, S, SW, W, and NW). For a closed glacial lake, the orientation is specified according to the direction of its longer axis. Altitude The altitude is registered by the water surface level of the lake in masl. Classification of lakes Genetically glacial lakes can be divided into the following.
In the glacial lake inventory, end moraine-dammed lakes, lateral moraine lakes, trough valley lakes, glacial erosion lakes, and cirque lakes are represented by the letters M, L, V, E, and C respectively; B represents blocking lakes. Activity According to their stability, the glacial lakes are divided into three types: stable, potential danger, and outburst (when there have been previous bursts). The letters S, D, and O represent these types respectively. Types of water drainage Glacial lakes are divided into drainage lakes and closed lakes according to the drainage pattern. The former refers to lakes from which water flows to the river and joins the river system. In the latter, water does not flow into the river. Ds and Cs represent those two kinds of glacial lakes respectively. Chemical properties This attribute is represented by the degree of mineralisation of the water, mg l–1. Other indices One important index for evaluating the stability of a glacial lake is its contact relation with the glacier. So an item of distance from the upper edge of the lake to the terminus of the glacier has been added and the code of the corresponding glacier registered. Since an end moraine-dammed lake is related to its originating glacier, this index is only referred to end moraine-dammed lakes. As not enough field data exist, the average depth of glacial lakes is difficult to establish in most cases. Based on field data, and as an indication only, the average depth of a glacial lake formed by different causes can be roughly estimated as follows: cirque lake, 10m; lateral moraine lake, 30m; trough valley lake, 25m; blocking lake and glacier erosion lake, 40m; lateral moraine lake, 20m. The water reserves of different types of glacial lakes can be obtained by multiplying their average depth by their area (LIGG/WECS/NEA 1988). The inventory database form (see Annex II) used for compilation of the inventory of glacial lakes includes basin numbers, map/satellite image codes and year, as well as the lake parameters (attributes) described above. |