ENERGY FLOW TO ARCTIC TUNDRA SURFACE AND EXPERIENCES OF INDUCED DISTURBANCES

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1 Bengt Erik RydŁn Department of Geosciences University of Upsala Sweden XX Polar Symposium Lublin, 1993 ENERGY FLOW TO ARCTIC TUNDRA SURFACE AND EXPERIENCES OF INDUCED DISTURBANCES ABSTRACT In cold regions the mass exchange balances on low precipitation and limited evaporation. Energy balance show apparent low values; although great income in summer and large heat losses in winter, the annual temperature variation is considerably reduced by the latent heat consumption, and release, respectively, taking place in phase changes. A disturbance in the exchange of thermal energy is thus easily created in arctic and polar climates, either by man or by nature itself. INTRODUCTION Examples of disturbances are such application as pipelines, where man induced changes in energy flow of small amonts may cause strong effects because of the long time available for reactions in tundra areas, where maintenance is rare. This applies to the transport of oil over permafrost surface, since the acceptable temperature for this liquid to flow is well above permafrost temperatures. The opposite situation is the gas transport, since an economic transport requires reducing of the volume, and needs the preparation of chilling the gas. The technique results in a cooling of the environment to temperatures below freezing. Effects are several. Sublimation of vapour from the air as ice on the surface causes accumulation of ice. Moreover, moisture transport within the soil is possible because of the temperature gradient developed towards the cooler pipe. Because of the long time period available during the lifetime of a pipeline, the accumulated transport will be considerable. Next phenomenon is the palsa, for which growth the inhomogeneities in the soil, of porosity, water content, density, among others, play the important role of creating small places where an interior ice body can be created. This system of processes is made possible by the unique thermal properties of peat. 429

2 MAN INDUCED DISTURBANCES ON PERMAFROST The case of oil t r a n s p o r t in pipes Enormous resources of oil is located in the North, i.e. the along the poleward coasts of North America, Russia and Sibiria. The oil has to be transported south to densely populated regions. The Arctic Ocean leaves, however, too small a time period of the year free from ice. Thus tankers have to be replaced by land based pipelines. The 1300 km Alaska pipeline, known as the Alaska pipeline, was built in the 1970s. Its thermodynamics is an example of large environmental disturbances, mostly irreversible, and the technique to solve these problems. Firstly, a transport at the air temperatures of the actual environment is hardly possible, or at least economically unacceptable. The oil would become viscous and waxy. Through heating at the well to 80 C the viscosity of the oil reduced to a value roughly 200 times smaller. The 120 cm diameter steel pipe constituting Alaska pipeline has, with heat losses, a content at a temperature of about C. Passing permafrost areas with air temperatures well below zero, the result is a flow which is times faster than it would be if allowed to cool to temperatures of air and ground. Thus a pipeline creates a steady heat flow. Thawing around a buried, warm pipeline would progress downward, although at a decreasing rate, for many years. Near the source the permafrost is some 600 m thick. Three-quarters of the proposed route overlay permafrost, and at least half of this was estimated to contain ice, which on melting would cause settlement. The effect of the warm pipeline was such that up to 10 m of soil would be expected to thaw in thefirst year. The amount would vary, of course, depending on the pre-existing ground temperatures and the type of soil. The possible settlement, danger of thawing, and the steps in the technical development have been outlined by Williams (1979). The degree of settlement, or subsidence, would depend on the amount of ice in the thawed layer, but for quite long distances there would be several metres displacement, Obviously, such effects left unchecked would cause great disruption of the pipeline. The maximum settlement would appear at places with presence of excess ice. Excess ice", or ground ice, refers to ice in excess of the normal volume of soil pores. If this ice thaws there must be settlement of the ground. The necessity of preventing the thawing of ground ice by heat from the pipeline is obvious. T e c h n i q u e to p r e v e n t t h a v i n g The conclusions led gradually to the decision to build more and more of the pipeline above ground elevated on pile supports, rather than buried in the ground (Williams, 1979). In a strongly windy environment the air surrounding an 430

3 elevated line would dissipate most of the heat from the pipe. Thus the thawing of the permafrost would be greatly reduced. Construction focused on pile supports, or vertical support members" (VSMs). These could also be designed to permit lateral movements of the pipe caused by its own expansion or contraction due to temperature changes. However, the raising of the pipe above ground on the VSMs could not prevent all heat flow to ground. The final aim was to ensure no thawing of the ground. During construction disturbances could also initiate temperature changes in the soil which could result in thawing. T h e r m a l VSM and a n h y d r o u s a m m o n i a r e f r i g e r a n t The solution of the problem was the so-called thermal VSM, which were equipped with interior heat pipes, or sealed 5 cm diameter tubes. They extend below the surface and contain anhydrous ammonia refrigerant. Admittedly, its latent heat of evaporation is smaller than that of water; one liter of ammonia refrigerant consumes approximately 45% of the thermal energy required the same volume of water at 0 C. But its temperature of vaporization is -33,4 C which means that the refrigerant - in winter, when the ground is warmer than the air outside - evaporates from the lower end of the tube and condenses at the top. Heat exchanger device releases the condensation heat to the air. The evaporation cools the lower end of the pipe and the surrounding ground. The mean ground temperature falls. By cooling the permafrost in winter its temperature is sufficiently lowered to prevent thaw during the summer. Construction damage was also reduced because the heat pipes enabled much shorter legs" on the VSMs. Generally the VSMs were drilled 8 m into the ground, maximum depth more than 20 m. In the absence of heat pipes the legs of the VSMs had to be overdesigned to prevent the deepest possible thaw (Williams, 1979). The case of gas t r a n s p o r t t h r o u g h p e r m a f r o s t t e r r a i n The gas resources are mainly in the same North as the oil. The transport of gas to the South focuses two major requisites, i.e. security and economy. Although oil transport conditions were not different from these issues, the requisites - when encountering a different aggregation state, the gas - will face almost opposite problems compared to those of the oil transport. The fundamental difference is the increase of rate of flow by reducing the volume of the gas. Thus compressor stations are located at interval along the pipeline. Minor failures in construction, significant thaw around the pipe, or unforeseen disturbances in pipeline position could easily result in leaking, in worst cases in explosions. Thus, security demands require the gas pipeline to be buried (Williams, 1979). 431

4 Naturally, compressing increase the temperature, but if the gas is also chilled the pipe could be buried in permafrost without danger of thaw settlements. Thus, for technical reasons, the issue for gas transport includes high pressure (above 110 atm) and temperatures below freezing (-10 C, or lower). Accordingly, the effect of a cold gas pipeline on surrounding soil would be the opposite to that of the oil pipe. The gas pipeline is initially several degrees colder than the ground in which it is buried. The temperature of the ground around the pipe is gradually reduced. The ground may be permanently, or annually, frozen. After some time the frost body will expand towards the surface and the annual thaw, the active layer, will become thinner, and eventually cease M o v e m e n t of w a t e r and ice t h r o u g h pores Thermodynamically, two processes of ice accumulation are created. At the surface, during periods of favourable air temperatures, vapour of the lower atmosphere will sublimate. In the soil, all around the pipe, a temperature gradient is established. In frozen soils, above-70 C, a considerable portion of the water content exist in unfrozen water films around the grains. Since hydraulic conductivity of frozen soil is very low (10 to 10 ~ n m/s), but far from zero, the temperature gradient will tend to move the water towards the pipe (Ryden, 1986). In this context it is worth recalling what was proposed by Miller (1970) that in frozen soils, not only the water moves along the water films, but also the ice moves slovly through the pores. Along the gradient, accretion af molecules occurs on one side of the pore ice, and, on the opposite side, a loss of molecules by their transfer into water. The driving force is somewhat reduced by lowering the soil temperature, but still significant, and experimental findings demonstrate the effect of the process (Williams, 1979). Over a period of 30 years, so much water would have moved through the frozen ground towards the pipe, and accumulated under it as ice, that the pipe, at some places, would be lifted a meter or more. Such lifting would, of course, not be uniform over long distances, and a resulting bending of the pipe would be unacceptable. From the point of view of induced environmental effects, it is striking that the technical problems associated with the chilled gas pipeline are not yet fully solved. One problem is that of a long life insulation to prevent the surrounding ground to approach the low temperature of the gas. 432

5 NATURALLY INDUCED ENVIRONMENTAL DISTURBANCES Climate change Among naturally induced environmental disturbances in polar and arctic regions are those caused by climate change (Williams, 1979, Burns et al., 1982, and others). As hitherto observed, the maximum in temperature changes occurs over the northernmost parts of the continents, and thus, may show the earlier and greater effects in arctic and polar regions (Liljequist, 1970; Thomas, 1975). Still it is valid, as pointed out by Williams (1982), that the significance of the disturbance of the surface energy and mass balance induced by climate change, and the role of the surface and the earth material in causing, or modifying, climate change have received little attention. Since these disturbances may be the subject of other scientists, the present study turns to a phenomenon which, basicly, is independent of climate change. PALSA, A DISTURBANCE BY NATURAL INHOMOGENEITY The P a l s a A palsa is an elevated feature, found in the zone of permafrost, and common in mires, bogs, or lowland tundra areas. These small mounds are covered by peat and consists of one or several bodies (lenses) of perennial ice imbedded in peat. The interior is often gradually mixed with silt, and underlain by mineral soils. Palsas are found as plateaus, ridges, strings and mounds. Sizes varies from over 1000 m 2 (plateaus), m 2 (ridges), m 2 (strings) to the circular or elliptic cupoles (mounds) having a diameter of a few meters, whereas heights generally are 1 to 5 m, and limited to less than 10 m. The literature on palsas is summarized, and there thermodynamics analyzed by Ryden (1993). The main requisite is the extience of peat. Dry peat is an excellent insolator, whereas wet peat is a good conductor of heat. The early growth of palsas is characterized by a thin ice lens within the active layer. The critical factors of formation include a thawed, and later dried, peat surface, a thin snow cover, or none, and winter release of thermal energy from the frozen body. A palsa is subject to a cyclic life; a frozen core forms, grows, reaches a maximum, and collapses (Seppala, 1986). The cycle may be completed over a decade. The surface soil material is always peat, the thickness of which may vary between 10 and 200 cm. The horizon underneath is mostly a relatively finegrained composition of mineral soils, mainly silt, of moderate or small frost heaving capacity (Ahman, 1976). Depending on the depth of the surface peat stratum, a transition layer of gradually increasing silt content may occur in 433

6 deeper parts of the core of the palsa, and originating from an underlaying silt stratum (Ryden et al., 1980). Silt may appear scattered within the core, but never occupying the entire core of the palsa. Accordingly, the physical properties of interest for the formation of palsas are those of its frozen or unfrozen components, i.e. air, ice, peat, silt, snow, and water; i.e. the properties which govern the energy exchange. Thermal properties Unfrozen conditions. A comparison between thermal conductivity coefficients (X) of a number of common soils in unfrozen state shows a variation from 0.2 to 2.5 W/mK; moreover, the thermal conductivity of peat is the lowest among soils in nature (Saare and Wenner, 1957). The density (p) influence on thermal conductivity coefficient (X) is stronger than linear; empiric data express approximately a logarithmic increase. At, e.g., a constant 0 = 15% by volume, X increases from 0.05 to 0.15 W/mK when p increases from 50 to 600 kg/m 3. Since porosity of soils decreases with decomposition, the maximum porosity and the minimum soil density will be found in the recently deposited, and dry, litter. Thus, it is in the topmost layer the lowest possible thermal conductivity occurs, and thus, the strongest temperature gradients. For comparison, X of air equals W/mK, whereas that of water at 0 C is approximately 0.7 W/mK. In summer, the thermal conductivity of dry unfrozen surface peat of low decomposition degree is among the extremes for natural soils. The heat insulating effect is extreme, too. The conditions of surface peat obviously play an essential, and characteristic, role in preserving the frozen core of the palsa throughout the summer. Frozen conditions. The frozen soil conditions not only add a new component, ice, even dry soils change the thermal conductivity coefficient upon freezing. The thermal conductivity coefficients of liquid water and ice, respectively, are significantly different, and the same is true for unfrozen and frozen peat, respectively. Upon freezing, water abruptly changes its thermal conductivity from approximately 0.7 to 2.2 W/mK. Thus, at temperatures close to 0 C the magnitude of the thermal conductivity coefficient of ice is more than three times greater than that of liquid water. In addition, X increases with decreasing temperatures, approaching 2.5 W/mK at -20 C (Hobbs, 1974). When peat freezes, is also subject to an abrupt change which is between 4 and 10 times its value in the unfrozen state. Now, X is closely related to density. The difference in X between unfrozen and frozen state increases considerably with density. For low density peat (50 kg/m 3 ), which corresponds to surface peat of a small degree of decompostition, X changes from 0.18 to 0.47 W/mK, which is 434

7 a little more than 2,5 times. For peat of moderate density (200 kg/m 3 ), which is that of subsurface peat of palsas, A. changes from 0.19 to 1.0 W/mK, or approximately 5 times the value at unfrozen state (Ryden, 1993). Conclusions on frozen conditions. Other variables being equal, the conduction of thermal energy through ice is at least three times greater compared to that through water. Since frozen surface layers are related to the season of cooling, the flux of releasing thermal energy from the frozen body dominates over the flux of gaining heat in summer. The dependency of temperature on A also is of interest for the solid ice body of many palsas, where the portion of organic material is negligible. It means a greater loss of heat energy at lower air temperatures. Thus, the colder the winter, and the longer the cold spell, the stronger the ability of deeper layers in releasing thermal energy from the system of ice and frozen soil of a palsa. In addition, a strong increase of X follows compaction, particularly shown in the frozen state. Thus, frozen peat in deeper layers release thermal energy easier than peat of surface layers. Thus the vertical growth, at the bottom of the frost body, is enhanced. These conditions are of importance to the understanding of palsas. One of the prerequisites for the palsa formation is the abrupt and considerable increase of A on freezing. This constitutes the ability of the frozen peat to loose heat easier than the unfrozen. Winter conditions have a major influence on the vertical growth of the palsa, summer conditions preserve the results for additional growth during the following winters. Once peat is frozen, it looses heat through transfer towards the surface where is released in long wave radiation into clear arctic air masses. This must be the case, in particular, during long lasting temperature inversions common in northern regions in winter. Snow is generally accumulated along the foot of the palsa cupoles. Thereby the insulation effect is present, and lateral expansion of a palsa is prevented. This expains the gradually steeper walls of growing cupoles, which is characteristic of well developed palsas (Seppala, 1986). As with thermal conductivity, porosity, density and moisture content affect the thermal diffusivity. The relationships are, however more complicated. At low moisture contents (<20%) the quotient A,/Cv increases rapidly with increasing moisture content because of the rapid rise in conductivity. At high moisture contents additional moisture increases heat capacity C v, while'k increases little and may decrease. There are thus no general rules. This is shown, in particular, by peat the diffusivity of which is approximately constant around 10 x 10 _ 8 W/mK, over a moisture range from a few per cent by volume up to 60%, or more (Williams, 1982). Examples of magnitudes of thermal diffusivities (a): peat ЮхЮ" 8, water 14 x 10 ~8, clay 20 x 10 ~8, silt with some organic material 30 x 10 ~8, wet sandy soil 100 x 10 " 8, ice (80-150) x 10 " 8 all in units ofm 2 /s (Hobbs, 1974, Rosenberg, 1983). 435

8 Seasonal microclimate components The incoming solar radiation meets, in summer, a shallow vegetation cover over the tundra surface, and a peat surface of high porosity and low thermal conductivity. Observations show temperatures reaching more than +30 C within the vegetation cover and at the peat surface (Ryden et al., 1980). The ability of the surface layer to transfer such great energy amounts is low, and a strong temperature gradient is created. A considerable portion of this heat energy is consumed in evaporating moisture in the surface layers. They become dryer, and thereby the thermoconductivity is further reduced. The frozen body underneath will be preserved all through the summer. Autumn precipitation is small over tundra areas, but when little evaporation there is an excess of water to be stored as soil moisture (Ryden, 1976). Autumn excess adds to the water available for next winter growth of the frost body of the palsa. The second, characteristic period is the mid-winter. Since snow is neglible, the outgoing long-wave radiation can be considerable, and surface temperatures down to -25 C, or lower, are observed (Ryden et al., 1980). Therm oconductivity of frozen peat, and of its ice content, are high, and thermal energy is released from the whole frost body. Thus the growth of the palsa body occurs both at the top, at refreezing of the active layer, and at the bottom of the frost body. The two main periods meet excellently the requisites of palsa formation, and explain some of the processes in the vertical growth that continues over a row of winters until the palsa collapses. P r o p e r t i e s and p r o c e s s e s The thermal properties of the palsa illustrates the ability of various layers to transfer and store thermal energy. The thermal conductivity, and its variation ( W/mK), indicate the conductive property. Further characterization is given by the diffusivity ([10-190] x 10" 8 ), also indicating the relative change of storage of thermal energy, or heat. Compared to other components of the palsa, it is obvious that the frost body, in winter, is capable of a considerable release or gain. The dominance of thermal diffusivity in winter a over that in summer a proofs the negative heat balance over time, which is the climatic basis for the repeated seasonal growth of the frost body (Ryden, 1993). The properties essential for palsa processes are those of peat, only. Palsa formation can not take place in inorganic soils. It is a combination of properties and processes unique to organic soils. Since peat is common in surface layers of tundra, palsas are of a special interest to studies of organic soils. 436

9 The main prerequisite, however, is a disturbance", i.e. an inhomogeneity in the soil-water system, and thereby the heat balance is disturbed. The result is a release of thermal energy which is greater than the absorbed energy, thus a growing frost body. CONCLUSIONS Peat is a thermal regulator of greatest importance to the energy budgets of arctic and polar environments. The latent heat transfer reduces the annual variation of temperature. To both of these prerequisities for cold region climate small disturbances have considerable effects. Also moderate, the temperature difference between a pipe and the ground may have a dangerous effect. The results are generally of long term character and, in several cases, the processes are irreversible. Moreover, disturbances often end in irreparable damage. ACKNOWLEDGEMENTS The author with to express sincere thanks to Professor Matti Seppala, Helsinki, for sharing his great experience in fruitful discussions. REFERENCES Burn, C Ryden, В. E., Smith, M. W. and Williams, P. J., 1982: Study of Climatic Change and its Implications for Northern Pipelines Phase I. Final report to the Earth Physics Branch, Dept. of Energy, Mines and Resources, Canada. Geotechnical Sci. Lab., Carleton Univ., Ottawa. 86 pp. Hobbs, P. V., 1974: Ice Physics. Clarendon Press, Oxford, pp. Liljequist, G. H., 1970: Klimatologi. (In Swedish) GLA, Stockholm, 527 pp. Miller, R. D., 1970: Ice sandwich: functional semi-permeable membrane. Science, 169: Rosenberg, N. J., BladB. L., and Verma, S. В., 1983: Microclimate. The Biological Environment. 2nd. ed. Wiley & Sons. Ryden, B. E., 1976: Water availability to some arctic ecosystems. Nordic Hydrology, 7: Ryden, В. E., 1986: Winter soil moisture regime monitored by the Time Domain Reflectometry Technique (TDR). Geogr. Ann. 68A(3): Ryden, В. E., 1993: Seasonal characteristics of thermal processes in tundra soils.-proc. of 1st Int. Conf. on Cryopedology, Pushchino, Russia, Nov Ryden, В. E., Fors, L. and Rostov, L., 1980: Physical Properties of the Tundra Soil-Water System at Stordalen, Abisko. In: Sonesson, M. (ed.) Ecology of a Subarctric Mire, Ecol. Bull. (Stockholm) 30: Saare, E. and Wenner, C.-G., 1957: Thermal Conductivity Coefficients of Various Soils. Swed. State Build. Res., Trans. No pp. (In Swedish). Seppala, M 1986: The Origin of Palsas. Geogr. Ann. 68A(3): Thomas, M. K., 1975: Recent climatic fluctuations in Canada. Environment Canada, Toronto, 92 pp. 437

10 Williams, P. J., 1979: Pipelines and Permafrost. Physical Geography and development in the circumpolar North. Longmans, London. 98 pp. Williams, P. J., 1982: The Surface of the Earth. An introduction to geotechnical sciences. Longmans, London. 212 pp. Ahman, R., 1976: The structure and morphology of minerogenic palsas in northern Norway. Biuletyn Peryglacjalny 26: [Cited by Washburn (1985)]. Address of the author: prof, dr Bengt Erik Ryden, Department of Geosciences, University of Upsala, Box. 554, S Upsala, Sweden 438