Jökull


Jökull - 01.06.2000, Blaðsíða 23

Jökull - 01.06.2000, Blaðsíða 23
An ice-dammed lake in Jökulsárgil process is only effective in situations where water pressure is less than the ice pressure in the tunnel, and is generally regarded as a minor contributory factor in tunnel closure. Ice collapse has been cited as a factor contribut- ing to tunnel closure following the drainage of ice- dammed lakes where little or no water is available to melt the blocks or to carry them through the tunnel (e.g. Kerr, 1934; Ricker, 1962). This process is un- likely in channels deep within ice where compressive stresses operate, as ice blocks are seldom generated from the glacier in this type of environment, but wh- ere extending forces dominate, crevassing or calving may result in the disintegration of parts of the glacier and ice blocks may find their way into tunnel systems. Most analyses of tunnel closure assume that the longitudinal compressive strain rate normal to the ax- is of the tunnel is negligible and that the primary closure process is that of deformation under the over- burden pressure of ice. However, Jones et al. (1985) assert that where an ice conduit is orientated across a glacier, compression of the tunnel caused by glacier movement may contribute to tunnel closure. They describe a situation, analogous to that at Sólheima- jökull, in which a glacier lies across a valley with forward movement across the longitudinal axis of the tunnel limited by a valley wall. Compression of the glacier, under these circumstances would increase the rate of tunnel closure over that existing solely due to overburden pressure of ice. However, it must be possible for the tunnel to move as a whole along the bedrock through the sliding of the glacier. Thus glacier sliding rates would not necessarily correspond to lateral compressive closure rates and it would be difficult to ascertain what proportion of the glacier sliding velocity contributes towards closure by lateral compression. This conduit closure mechanism is not widely discussed elsewhere in literature dealing with ice tunnel dynamics. THE TUNNEL AT SÓLHEIMAJÖKULL Water from Jökulsárgil enters Sólheimajökull via an arcuate glacier portal on the northern side of the glacier, and travels through the glacier by means of a tunnel for c. 1 km, in an approximately straight line (Figure 2). The ice above the tunnel is compressed by glacier flow abutting against a bedrock obstacle to the west, and exhibits evidence of compressive flow through the presence of surface water and the lack of crevassing over most of the tunnel length. Where crevasses do exist, they are dominantly water-íilled, again supporting the idea of compressive flow. At the downstream portal the emerging water sometimes undercuts the glacier, and ice blocks frequently cal- ve into the water around the portal. The tunnel has a diameter of approximately 5 m. APPLICATION OL MODELS OL ICE CONDUIT DYNAMICS TO THE TUNNEL AT SÓLHEIMAJÖKULL In order to identify the conditions under which tunn- el closure, and hence ice-dammed lake formation, would occur at Sólheimajökull it is necessary to apply models of conduit dynamics to the tunnel at the site. Nye’s (1953) model of tunnel closure due to over- burden pressure and Hooke’s (1984) model of melt- widening were used to ascertain the conditions under which the tunnel would close. It is assumed that the water flow in the tunnel is at atmospheric pressure and that the conduction of heat from the flowing water is efficient enough to keep the water at the melting po- int during the flow. The effect of ice pressure on the melting point of the ice surrounding the tunnel is neg- lected, as suggested by Hooke (1984). Measurements were taken at Sólheimajökull dur- ing 1989, 1990 and 1991 for comparison with the models described above. Although in many cases, measurements taken identified a range of values, the maximum and minimum of the data are used here as it is the likely threshold values that define the conditions required for the closure of the tunnel. A) TUNNEL CLOSURE DUE TO OYERBURD- EN PRESSURE Tunnel closure due to ice deformation was calculated using Nye’s (1953) theory of closure by deformation. It should be noted that Nye’s theory was developed for use with cylindrical tunnels fully enclosed in ice, and the need to rely on this theory to calculate closure rates for tunnels that deviate from this form is a JÖKULLNo. 48 21
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