APPENDIX B 



ICEBERG DETERIORATION 



R Q. ROBE and D. C. MAIER, MST2, USCG 



U.S. Coast Guard Research and Development Center 



and 



R. C. KOLLMEYER, CAPT, USCG 



U.S. Coast Guard Academy 



A very large tabular iceberg was observed as 

 it drifted northeast of the Grand Banks of New- 

 foundland during May and June 1976. When 

 the iceberg was first sighted on 12 May 1976, it 

 showed only minor signs of deterioration. From 

 12 May until last sighted on 6 June, the iceberg 

 underwent a rapid reduction of the above water 

 surface area with the erosion largely confined to 

 the turbulent layer associated with gravity waves. 

 The erosion progressed along lines parallel to the 

 structure of the iceberg as indicated by the pro- 

 nounced ridge pair seen in the photographs 

 (Figure B-l), a trellis drainage pattern, and an 

 alternation of light and dark bands over the en- 

 tire surface of the iceberg. 



Photographs were taken of a large number of 

 icebergs during the 1976 flights of the Interna- 

 tional Ice Patrol. These photographs are to be 

 used for a study of iceberg populations off the 

 Grand Banks (Figure B-2). Black and white 

 photographs (9-inch format) were taken from 

 altitudes of between 300 and 3500 m. 



Among the photographs obtained are a unique 

 series of five taken of the same iceberg over a 

 period of 25 days (Figure B-l). It is highly 

 unusual for an iceberg to be relocated over such 

 an extended period when positive identification 

 is possible. Icebergs normally change their ap- 

 pearance so radically by a combination of calving, 

 melting and rolling, that it is impossible to posi- 

 tively identify them after only a few days. In 

 this case the unusually low profile, only 4-5m of 

 evaluation, and tabular shape maintained the ice- 

 berg in an extremely stable condition. 



From 12 May to 6 June 1976 the iceberg de- 

 creased in surface area from approximately 

 190,000m 2 to an are* of 109,000m 2 . The rate of 

 decrease in surface area was nearly linear (Fig- 

 ure B-3) and resulted from wave erosion, under- 

 cutting and minor calving. The surface water 

 temperature was 2-4°C. Subsurface tempera- 

 tures in this area typically decrease to a minimum 

 of less than -1°C at 75-100m depth. 



Wave erosion was concentrated at those points 

 which appear as slight irregularities in the 12 

 May 1976 photograph. The progressive enlarge- 

 ment of these embayments was the result of the 

 local concentration of wave, energy and continued 

 bathing of the ice by the turbulent water. The 

 embayments seemed to extend only several meters 

 below the water's surface and have an orientation 

 parallel to the structural features of the iceberg. 

 The underwater shape of the iceberg is not 

 known but it is suspected to have had a flat bot- 

 tom as inferred from the long-term maintenance 

 of its top parallel to the sea surface. 



The iceberg seemed to be of land ice origin. 

 Analysis of a piece of the iceberg, recovered by 

 the USCGC EVERGREEN, showed it to be 

 fresh water ice. The characteristic air bubble 

 bands of glacier ice were visible and surface melt 

 tests produced a pattern of hexagonal depressions 

 approximately 2cm across which typify the crys- 

 talline melt surface texture of glacier ice. The 

 two linear parallel ridges in the upper third of 

 each picture of Figure 1 are the most obvious 

 manifestations of the ice structure. In the origi- 

 nal photographic prints, a pattern of trellis 

 drainage can be seen which indicates structural 



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