Ninth International Conference on Permafrost ... - IARC Research

Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tm depth) is the zone where ice impact on the sea floor is thestrongest. Here, hummock ridges and barriers reaching thefloor develop during the whole winter.Within a beyond-fast-ice polynia up to 19–20 m depthssea ice influence on the bottom is also high, since hummocksfrozen into the ice fields drift here during the winter as singlelarge bergs which have high kinetic energy due to their mass.As a result, the largest ice gouges can reach 1.8 m deep, 10–40 m wide, and several km long (Fig. 4). They are orientedconformably with the direction of tidal currents—lengthwisethe Baidaratskaya Bay.The main method of indirect estimation of ice gougingintensity is the estimation of density and deepness (Fig. 5) ofice gouges. Meanwhile, the lifetime of such forms essentiallycan vary according to sea depth, types of sediment, andduration of the dynamically-active period; so the questionabout sea ice gouging intensity of the coasts and bottom isdirectly connected with the problem of safety of ice gougingforms.Investigations on the Baidaratskaya Bay, Kara Sea,show that the frequency of occurrence and the density ofice-gouging forms is the largest at 17–19 m depth; but thatdoes not mean that the intensity of ice gouging is lower inshallow depths with rarer occurrence and smaller depths ofice gouges.The effect of coastal hummock ridges and barriers onbeaches and in shallow areas (down to a depth of 7–10 m)can be traced only immediately after fast ice is destroyed. Thelife period of ice-gouged forms developed on sand beachesand shallow areas is very short—until the first summerstorm. These forms, whose depth is mainly < 0.5 m here,commonly disappear with first strong waves in summer andautumn. Therefore, as a rule, the age of ice gouges in shallowareas does not exceed one year. Due to high hydrodynamicactivity here, the gouges quickly smooth over, and the gougedensity in shallow areas is lower than in the zone of the fastice edge.At depths of 14–16 m, the occurrence and density ofgouges turn out to be lower than at higher depths, thoughice gouging is the most intense (most of the mobile systemsof hummocks and grounded hummocks are formed here).The above situation is due to more active hydrodynamicsat smaller depths, where the effect of waves still manifestsitself and the velocities of tidal currents are higher. Due tothis, the gouges can exist over several years here (as distinctfrom their short existence in shallow areas), graduallysmoothing over and vanishing. The lifetime of ice gouges issubstantially determined by proportion between the freezingperiod and the dynamically-active period, and hence differsin diverse seas. For example, in the Pechora Sea, with adynamically-active period two times longer in comparisonwith the Kara Sea, it is quite difficult to find ice gouges atthe same depth. Another significant factor for the lifetimeof a gouge is wave action. While comparing ice gouges atBaidaratskaya Bay at 12–14 m depth, the fewer number ofgouges in the western part as compared with the eastern oneis obvious. Indeed, the frequency of strong waves higherFigure 4. Sonar record, 19 m depth: Largest and deepest icegouges.Figure 5. Winter underwater photo at a depth of 7 m: ice gougedeepness measurement. Stamukhi generated holes and gouges willbecome level by the first strong summer storm.than 3 m, which can transfer sediments at stated depths, is afew times higher on the western coast as compared with theeastern one.Deeper than 19 m, the ice gouges occur rather frequently.However, ice formations, particularly grounded hummocks,rarely occur at these depths. This situation is caused by lowhydrodynamic activity and low sedimentation rates. Undersuch conditions, the gouges, especially large ones, can existon the floor surface over decades. Thus, low intensity of icegouging is compensated by a long life of gouge forms. This“accumulation” effect gives a wrong idea of high intensityice gouging here.The depth of ice-gouging forms depends not only on seadepth, ice thickness, and intensity of sheer stress, but also oncomposition and the state of bottom deposits. The safety of icegouging forms probably depends also on plasticity, mobility,and granulometric composition of sediments. Attemptsto find regularity between field data about occurrence anddensity of gouges on the one hand and the type of sedimentson the other hand were not successful, except single examplesof correspondence. Observations show that ice gouges,especially large ones, have considerable lengths, sometimesup to several kilometers; that is, hummocks can constantlygouge the bottom for a long time due to their large kineticenergy. Therefore hummocks can gouge bottom sectionswith rather different characteristics of the sediments.232