Distributions of crystals and gas bubbles in reservoir ice during growth period

Water Science and Engineening, 2011, 4(2): 204-211doi:103882jisn167423702011.02008http://www.waterjournal.cnemailwse2008@vip.163.comDistributions of crystals and gas bubbles inreservoir ice during growth periodZhi-jun LI*, Wen-feng HUANG, Qing JIA" 2, Matti LEPPARANTAI. State Key laboratory of Coastal and Offshore Engineering, Dalian University of TechnologyDalian 116024. P. R. china2. College of Water Conservancy and Hydropower, Heilongjiang University Harbin 150086, P.R. China3. Department of Physics, University of Helsinki, Helsinki F1-00014, FinlandAbstract: In order to understand the dominant factors of the physical properties of ice in icehemodynamics and mechanics, in-situ observations of ice growth and decay processes werecarried out. Two samplings were conducted in the fast and steady ice growth stages. Ice pieces wereused to observe ice crystals and gas bubbles in ice, and to measure the ice density. vertical profilesof the type and size of ice crystals, shape and size of gas bubbles, and gas bubble content, as well asthe ice density, were obtained. The resuits show that the upper layer of the ice pieces is granular iceand the lower layer is columnar ice; the average crystal size increases with the ice depth and remainssteady in the fast and steady ice growth stages; the shape of gas bubbles in the upper layer of icepieces is spherical with higher total content, and the shape in the middle and lower layers is cylinderwith lower total content; the gas bubble size and content vary with the ice growth stage; and the icedensity decreases with the increase of the gas bubble contentKey words: ice crystal; gas bubble; crystal structure; porosity: ice density1 IntroductionIce crystals and gas bubbles control ice electronics(Li et al. 2010a), optics Light et al2003), and mechanics(Timco and Weeks 2010)to a large degree, and also play a key role inice remote sensing(Shokr and Sinha 1994). The ice fabric determines the diffusion of lightthat have impacts on living conditions of under-ice creatures and sea ice ecology(belzile et al2001). A large number of studies and observations have been carried out on the relationshipbetween ice properties and density or porosity that can shed light on gas bubbles in ice(Timcoand Frederking 1996). Recent publications address the relationships between the density andporosity(Consolmagno et al. 2008), between the uniaxial compressive strength and porosity(Moslet 2007), between the coefficient of thermal properties of ice and porosity (Usowicz et alThis work was supported by the National Natural Science Fe中国煤化工50879008,theOpen Fund of State Key Laboratory of Frozen Soil Engine904), the VilhoYrjo and Kalle vaisala Fund of the Finnish Academy of SYHCNMH Gegian researchCouncil Project AMORA( Grant No. 193592/S30)Correspondingauthor(e-mail:lizhjun@dlut.edu.cn)Received Dec 23, 2010; accepted May 11, 20112008; Shi et al. 2009), and between the thermal conductivity of snow-covering ice and density( Sturm et al. 2002). The history of inland fresh water ice studies in China began with river icedams(Mao et al. 2002; Wang et al. 2005; Wang et al. 2007)and freeze-thaw of reservoir ice(Xiao et aL. 2004), but a systemic study on the basic properties of fresh water ice has beenlacking up to now.Studies on ice fabrics and gas bubbles are important for understanding the basicproperties of ice. Parameters of ice fabrics involve the type and size of crystals. Parameters ofgas bubbles include the shape and size of bubbles as well as relative gas bubble content.To study the basic physical properties of fresh water ice, field observations wereconducted in the Hongqipao reservoir in Heilongjiang Province, from December 18, 2008 toApril 8, 2009. The growth of fresh water ice is mainly controlled by thermodyprocesses. Two ice pieces were taken from a site of the reservoir, which was about 150 m fromthe dam, on December 19, 2008 and February 9, 2009, corresponding to the fast growth andsteady growth periods, respectively. Both pieces were typical reservoir ice and cut from thesurface to bottom, and were used to analyze the type and size of ice crystals, thearrangement, shape, size and content of gas bubbles, and ice density. this paper provides theresults of the investigations.2 Ice sampling and preparation of ice sectionsThe thicknesses of the ice pieces taken on December 19, 2008 and February 9, 2009 were50 cm and 85 cm, respectively. Complete ice pieces were cut out using ice augers and ice sawswith a mark of the north direction. After the ice pieces were cut off from the reservoir ice sheet,they were transported to a cold room for observation. Because the air temperature was lowerthan-15C, the ice fabric in ice pieces kept their original form Four samples with a cross-sectionof 10 cm x 10 cm and thickness of 50 cm or 85 cm were vertically cut off from the intact icepiece directly, as shown in Fig. 1. Sample l was sectioned vertically for macro observations ofice stratigraphy and gas bubble distribution, and for micro observations of ice crystals. Sample 2was sectioned horizontally for observations of crystals and fabrics as well as image analysis ofgas bubbles in thin sections. Sample 3 was used to measure the ice density, and Sample 4 wasstored as a backup.Granular iceSample 1中国煤化工Fig. 1 Sketch map of ice samples andCNMHGZhi-jun LI et al. water Science and Engineering, Jun. 2011, Vol. 4, No. 2, 204-211205The surface of Sample I was smoothened along the vertical direction and photographedwith a dark background in order to obtain the macro features of the stratigraphy and gasbubbles in ice It was then cut into vertical sections with intervals between 8 cm and 10 cm.Concurrently, Sample 2 was cut into horizontal sections with intervals between 8 cm and 10 cmAll of these horizontal sections were labeled with the north direction, and then smoothenedand affixed to glass sheets with a temperature initially slightly above 0C. After they werere-frozen, they were cut to about l-mm thickness with planning knives. Six vertical and sixhorizontal sections were prepared from the piece from December 2008, and ll vertical and tenhetal sections were prepared from the piece from February 2009. According toLangway s method of the analysis of ice crystals(Langway 1958: Iliescu and Baker 2007; Liet al. 2010b ), these sections were placed on a rigsby universal stage, photographed betweencrossed polarizers to measure the size and shape of crystals, and the C-axis orientations ofcrystals were determined. The result was processed by computer programs and presented in aSchmidt equal-area net, i.e., a fabric graph( Langway 1958). These sections were thenphotographed under normal light to measure the micro-arrangement and size of gas bubbles inice. It was found that the section thickness had to be less than half of the smallest bubblediameter to get a clear image of gas bubbles.3 Ice crystals and fabricsGenerally, with the decrease of air temperature, the water surface temperature firstdecreases to oC and undergoes a slight supercooling. Frazil ice then forms and progressivelyjoins together to form an ice layer, corresponding to thee fast-growlanular crvsgrowth rate slows down after the granular ice layer is formed. Ice crystals have enough time togrow, but are confined by ambient ones, so they have to grow downward, forming columnarcrystals whose sizes increase with the decreasing growth rate and increasing thickness of ice.Figs. 2 and 3 are pictures of ice crystals in the vertical and horizontal sections from an icepiece from February 9, 2009. We can determine the number and total area of crystals from thehorizontal thin sections and then calculate the mean area and equivalent diameter which isconsidered the mean size of crystals in this section. Fig. 4 indicates that the mean crystal sizeincreases with ice depth. Both ice pieces, having similar statistical features, were taken outduring the ice growth period, when the ice crystal properties seldom change.In addition, according to a spatial or planar directional distribution of C-axes on thefabric graphs, fresh water ice can be identified as isotropic, planar isotropic, or anisotropicmaterial. According to the statistics of tilt directions of C-axes in the ice piece from February 9,2009, the C-axes of the upper layer of the ice piece were distributed randomly in thehorizontal plane, and those of the deeper layer were oriented along the NNw-sse direction(Fig. 5). If the number of grains in one horizontal section is less than 20. the adjacent sectionat the same depth should be analyzed to increase the中国煤化工CNMHG206LI et al. Water Science and Engineening, Jun. 2011, Vol 4, No 2, 204-211a)0-8 cm(b)8-16cm(c)16-24cm(d)24-315cm(c)32-395cm(1)40-47cm(g)4856cm(h)56-63cm(1)64-71cm0)725-79cmk)80.585cmFig. 2 Example photos of crystals of fresh water ice in vertical sections of ice piece from February 9.2009(a)0.2 cm(b)8cm(c)16 cm(d)27 cm(f)47 cm(gI57 cmh)65cm中国煤化工Fig 3 Example photos of crystals of fresh water ice in horizonYHaCNMHGebruary 9, 2009Zhi-jun LI et al. Water Science and Engineering, Jun. 2011, Vol 4, No. 2, 204-211207Crystal diameter(mmFeb9.2009Fig. 4 Variations of crystal size along depth in ice pieces30--+(a)0.2 cm: Random distnbution(b)47 cm Onented distnbutionFig 5 Distribution of C-axes directions at different depths of ice piece from February 9, 20094 Gas bubbles and their relationship with densityBecause of the random distribution of gas bubbles in ice, when the thickness of ice sectionis larger than the gas bubble size, the gas bubbles will be overlaid over each other and appear asa mess. Fig. 6 shows the bubble pictures of the ice piece from February 9, 2009. The gasbubbles in the layer between 0 and 18 cm are spherical, the bubbles in the 35-37 cm layer arearrow-shaped, which is abnormal, and the bubbles in the 70-80 cm layer are cylindrical. Resultsof the image processing reveal that the spherical bubble diameter varies between 0.3 mmand 5 mm and the slenderness ratio varies within 10-700-5cm: Small (b)12-18 cm: Big spherical bubbles (c)35-37cm: Arrow-shaped (d)70-80 cm: Cylindrical bubblesphencal bubblesFig 6 Gas bubbles in ice piece from February 9, 2009When an ice section was cut to a thickness less than the bubble diameter, there would beonly one layer of bubbles in the section. Then the secti中国煤化工I staCNMHGZhi-jun LI et al. Water Science and Engineering, Jun 2011, Vol. 4. No. 2, 204-211photographed in the background of normal light( Fig. 7(a)). By image processing, the photoswere converted into black and white images, where the white points are gas bubbles and theblack background is ice(Fig. 7(b)). Then the pixel area and perimeter of every bubble weredetermined, and the true equivalent diameters as well as the areal fraction of gas bubbles in thesection were calculated. The results are shown in Fig 8fa) Gas bubbles under normal Ii(b) Gas bubbles after image processing ( white points)Fig. 7 Photos of gas bubbles in horizontal section of ice2.5m ProbabilityProbability densit0Mean bubble size(mm)Fig 8 Size distribution of gas bubbles in horizontal sectionThe equivalent diameter of gas bubbles first increased rapidly with depth and thenremained at a stable level in the ice(Fig. 9(a)). Also, as shown in Fig 9, the variation trend ofbubble sizes in both pieces indicates that the bubble size decreased with time. Although theconcentration of bubbles decreased with depth, the total content of bubbles increased withtime(Fig. 9(b)), indicating that the gas bubble content varied in the growth period of ice. Thisprovides a basis for understanding temporal variability in the thermodynamic, mechanical, andother kinds of properties of iceSample 3 was cut into cubes based on the horizontal levels. The ice density was measuredusing the mass-volume method. Variations of density with depth are shown in Fig. 10. Duringthe ice growth period from December 19, 2008 to Felnng twn vertical profiles ofice density overlapped each other, indicating that the中国煤菜化ge so muchCNMHGZhi-jun LI et al. Water Science and Engineering, Jun. 2011, Vol. 4, No. 2, 204-211during the ice growth period. In fact, the ice density was inversely proportional to the contentof bubbles. Fig. ll presents the result of the ice piece from February 9, 2009. The measuredcurve(solid line)is very close to the curve(dash line) calculated by the two-phase methodBubble diameter(mm)Gas bubble content (%)08Fig 9 Variations of gas bubble size and gas bubble content in ice pieces with depthDensity(kg/m)7508008509009501000Cauleuted by two-phase methodFeb9,2009Fig. 10 Variation of ice density with depthFig. 11 Relationship between ice density and gasbubble content of ice piece from February 9, 20095 ConclusionsField observations of ice conditions, including ice stratigraphy and crystals, the shape andsize of gas bubbles, and the density of ice, were carried out in the Hongqipao reservoir inHeilongjiang Province. The results are summarized as follows:(1)The top layer of the ice piece is granular ice, and the middle and the bottom layers arecolumnar ice. The average crystal size increases linearly with depth During the growth period,variations in ice stratigraphy were not detected. C-axes in the top layer are nearly horizontalwith random orientation in the horizontal plane. C-axes in the middle and bottom layers arealso nearly horizontal and have a preferential direction.(2)The equivalent diameter of gas bubbles first increases rapidly with depth and thenremains at a stable level. The concentration of gas bubbles decreases with depth, but the totalcontent of gas bubbles increases with time indicatin中国煤化工 ary during thegrowth periodCNMHG210Zhi-jun LI et al. Water Science and Engineering, Jun. 2011, Vol. 4, No. 2, 204-211)Ice haalmost constant density during theis inverselyproportional to the content of gas bubbles in ice.ReferencesBelzile, C, Vincent, W. F, Gibson, J. A. E, and van Hove, P. 2001. Bio-optical characteristics of the snow, ice,and water column of a perennially ice-covered lake in the High Arctic. Canadian Journal of Fisheriesand aquatic Sciences, 58(12), 2405-2418 [doi: 10. 1139/cjfas-58-12-24051Consolmagno, G., Britt, D. T, and Macke, R. J. 2008. 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