Hummock Fields Research Articles

Satellite-Based Identification and Characterization of Extreme Ice Features: Hummocks and Ice Islands

The satellite-based techniques for the monitoring of extreme ice features (EIFs) in the Canadian Arctic were investigated and demonstrated using synthetic aperture radar (SAR) and electro-optical data sources. The main EIF types include large ice islands and ice-island fragments, multiyear hummock fields (MYHF) and other EIFs, such as fragments of MYHF and large, newly formed hummock fields. The main objectives for the paper included demonstration of various satellite capabilities over specific regions in the Canadian Arctic to assess their utility to detect and characterize EIFs. Stereo pairs of very-high-resolution (VHR) imagery provided detailed measurements of sea ice topography and were used as validation information for evaluation of the applied techniques. Single-pass interferometric SAR (InSAR) data were used to extract ice topography including hummocks and ice islands. Shape from shading and height from shadow techniques enable us to extract ice topography relying on a single image. A new method for identification of EIFs in sea ice based on the thermal infrared band of Landsat 8 was introduced. The performance of the methods for ice feature height estimation was evaluated by comparing with a stereo or InSAR digital elevation models (DEMs). Full polarimetric RADARSAT-2 data were demonstrated to be useful for identification of ice islands.

Morphometric and sedimentological characteristics of Late Holocene earth hummocks in the Zackenberg Valley (NE Greenland)

A multi-approach characterization of three earth hummock fields has been conducted to understand the morphometrical characteristics and distribution pattern of these periglacial features in the Zackenberg Valley, NE Greenland. Earth hummocks develop in poorly-drained areas affected by intense cryogenic conditions. An accurate analysis of the morphometrical properties of hundreds of earth hummocks distributed between different Early Holocene moraine systems of the eastern slope of the Zackenberg Valley reveals an important control of microtopography on their distribution. Sedimentological analysis of selected earth hummocks shows evidence of alternating organic-rich layers and mineral units. Radiocarbon dates of the basal organic layers in contact with the permafrost table yielded ages 615 ± 25 and 1755 ± 60 cal yr BP, with lower sedimentation rates over the last centuries when soil formation prevailed. Geochemical analysis of the soils (Glacic Reductaquic Cryosols) showed also significant differences in the properties and composition among the soils of the different fields of hummocks.

Enhanced landslide mobility by basal liquefaction: The 2014 State Route 530 (Oso), Washington, landslide

AbstractLandslide mobility can vastly amplify the consequences of slope failure. As a compelling example, the 22 March 2014 landslide near Oso, Washington (USA), was particularly devastating, traveling across a 1-km+-wide river valley, killing 43 people, destroying dozens of homes, and temporarily closing a well-traveled highway. To resolve causes for the landslide’s behavior and mobility, we conducted detailed postevent field investigations and material testing. Geologic and structure mapping revealed a progression of geomorphological structures ranging from debris-flow lobes at the distal end through hummock fields, laterally continuous landslide blocks, back-rotated blocks, and finally colluvial slides and falls at the landslide headscarp. Primary structures, as well as stratigraphic and vegetation patterns, in the landslide deposit indicated rapid extensional motion of the approximately 9 × 106 m3 source volume in a closely timed sequence of events. We identified hundreds of transient sand boils in the landslide runout zone, representing evidence of widespread elevated pore-water pressures with consequent shear-strength reduction at the base of the slide. During the event, underlying wet alluvium liquefied and allowed quasi-intact slide hummocks to extend and translate long distances across the flat valley. Most of the slide material itself did not liquefy. Using geotechnical testing and numerical modeling, we examined rapid undrained loading, shear and collapse of loose saturated alluvium, and strong ground shaking as potential liquefaction mechanisms. Our analyses show that some layers in the alluvium can liquefy when sheared, as could occur with rapid undrained loading. Simultaneous ground shaking could have contributed to pore-pressure generation as well. Two key elements, a large and rapid failure overriding wet liquefiable sediments, enabled the landslide’s high mobility. Basal liquefaction may enhance mobility of other landslides in similar settings.

A field study of grounded ice features and associated seabed gouging in the Canadian Beaufort Sea

During the late summer of 1983, the southern Canadian Beaufort Sea was invaded by vast areas of thick sea ice features called hummock fields. Thousands of these features drifted into shallow water (10–27m water depth), became grounded and remained in place throughout the following winter. During the grounding event, it was assumed that the deeper ice ridge keels would have gouged the seabed. In April 1984, eight features with gouging potential were visited and investigated from the ice surface. Conditions allowed for measurement by sonar of the geometry of seabed gouges at two locations, named M1 and M2, as well as the geometry of the ice keels that caused the gouges. Volume and mass of the ice features were estimated from stereo air photography gathered earlier in the winter, from which elevation profiles were obtained. On-site elevation profiles confirmed elevations. The hummock field at site M1, estimated at 277,000tonnes, produced a gouge 2m in depth and a front mound about 3m high ahead of its keel. This feature had also been uplifted 2m at its leading edge. The deepest gouge was 3.5m, caused by hummock field M2, with an estimated mass of 1.16milliontonnes, which fragmented during the grounding process. A seabed gouging survey conducted in August 1984, in open water conditions, confirmed gouge depth at M2. Data on seabed soils were obtained at the M2 site one year later, in September 1985, as part of a geotechnical field survey. These cases of seabed gouging by massive sea ice features are unique in that they were found and documented with the ice features that made them still in place. All gouge, ice and soils data relevant to these two events are summarized herein, as a full-scale data set intended to increase understanding of seabed gouging and allow validation of models.

The volcanic debris avalanche on the SE submarine slope of Nisyros volcano, Greece: geophysical exploration and implications for subaerial eruption history

A spectacular hummocky topography was discovered offshore of the south-eastern slope of the Nisyros island volcano in the eastern sector of the Aegean volcanic arc in 2000–2001, using multibeam bathymetric mapping and seismic profiling, and interpreted as part of a volcanic debris avalanche originating onland. During E/V Nautilus cruise NA011 in 2010, a detailed side-scan sonar and ROV exploration aimed at evaluating the surface morphology of this avalanche field. Combining the new data with selected older datasets reveals that the debris avalanche is characterized by numerous (at least 78) variously sized and shaped hummocks. Some of these are distinctly round, either scattered or aligned in groups, whereas others are elongated in the form of ridges. This is consistent with existing models accounting for variations in the longitudinal and lateral velocity ratio of landslides. Maximum dimensions reach 60 m in height above the sea bottom, 220 m in length and 230 m in width. The structures outline a large tongue-shaped, submarine hummock field of about 22.2 km2, approx. 4.8 km wide and 4.6 km long and with an estimated volume of 0.277 km3. Due to its characteristic shape, the collapsed volcanic flank is interpreted to represent a singular failing event, involving a rapid and virtually instantaneous downslope movement of the slide mass into the sea. Indeed, the H/L (height of 280 m vs. run-out of 7 km) ratio for the Nisyros slide is 0.04; plotted against volume, this falls within the theoretical bounds as well as measured values typical of submarine landslides. The timing of the event is probably related to the extrusion of Nikia lavas and their subsequent failure and formation of a main scarp observed at about 120 m depth on an 8-km-long seismic profile and a map of slope angle distribution, at the depth where the palaeo-coastline was located 40 ka ago. An inferred age of ca. 40 ka for the avalanche awaits confirmation based on dating of core material.

Vegetation on and around Periglacial Hummocks in the Nemuro Peninsula, Northern Japan

Hummocky microforms (non-sorted patterned ground) occur extensively under seasonal frost environments, including Japanese high latitude and altitude regions. This paper focuses on vegetation contrasts between hummock apexes and interhummock depressions, and on its contribution to hummock formation in a representative hummock field in Japan. Vegetation cover in the study area serves to classify the hummocks into three types: covered with mire herbs, pasture weeds and forest-related species. The first type has a remarkable contrast in vegetation, characterized by entirely vegetated (sedged) apexes and bared inter-hummock depressions. Such uneven vegetation is likely to generate and/or develop these sedged hummocks associated with differential frost penetration (cryoturbation). The last two types lack sedges, but permit weed invasion on their inter-hummock depressions as well as hummock apexes. The last type, covered with a bamboo grass, is considered to be relict forms which indicate changes in hydrological environments induced by deepening groundwater.

Ice loads on the caisson structures in the Canadian Beaufort Sea

This paper presents a comprehensive overview of the characteristics, instrumentation and measured ice loads on the caisson structures that were used for exploratory drilling in the Canadian Beaufort Sea in the 1970s and 1980s. Details are presented on the Tarsiut Caisson, the Single-Steel Drilling Caisson (SSDC), the Caisson-Retained Island (CRI), and the Mobile Arctic Caisson (MAC) Molikpaq. The global loads on the structures are presented as a Line Load (Global Load per unit width of the structure) and the Global Pressure (Line Load per unit ice thickness). Over 170 loading events are documented. There is excellent agreement amongst the measured loads on all of the structures if factors such as ice rubble and ice thickness are considered. Global loads are shown to be a function of the ice macrostructure (level first-year sea ice, multi-year ice, first-year ridges, hummock fields, isolated floes) and failure mode of the ice (bending, creep, mixed mode, crushing). The analysis shows that there is a general increase in the Line Load with increasing ice thickness. Empirical equations are presented to predict the global load in terms of the ice thickness and structure width for different ice failure modes. The most significant result of the analysis shows that the maximum Global Pressure measured for all types of ice loading events never exceeded 2 MN/m 2, with the vast majority less than 1.5 MN/m 2.

Quantifying the thermal dynamics of a permafrost site near Ny‐Ålesund, Svalbard

The dynamics of permafrost soils is manifest in the soil temperature which can be measured with high accuracy and high temporal resolution. Using continuous data over a period of 778 days from a mineral hummock field at the Bayelva site, Svalbard, we deduce and quantify the processes which constitute the dynamics. In particular, conductive heat flux, generation of heat from phase transitions, and migration of water vapor are analyzed. Support for the interpretation of the data comes from high‐resolution time domain reflectometry measurements of the liquid water content.

Hummock fields in the Norway Basin and Eastern Iceland Plateau: Rayleigh-Taylor instabilities?

Side-scan imagery and 3.5 kHz profiles in the Norway Basin reveal ∼3–4 × 10 4 km 2 of sea floor covered by regularly spaced (500–2000 m), low-relief hummocks previously attributed to bottom currents. Extrapolating from nearby borehole data, I attribute these mound fields to gravitational (Rayleigh-Taylor) instabilities resulting from superposition of 50–100 m of thick Pliocene-Pleistocene, glacigenic (glacier-derived) sandy clays (density 1800 kg/m 3 ) on a several-hundred-metre thickness of Miocene biosiliceous oozes having a density of 1300 kg/m 3 . This hypothesis predicts or explains similar hummock fields in many oceanic regions where such density inversions exist.

Major challenges in platform design for the Russian continental shelf

There are fascinating economical and technical challenges facing field developers in the Russian continental shelf. As far as production platforms and offshore terminal structures are concerned one has to find out how to build them so that attractive investment economics can be achieved. Controlling project risks and reducing key uncertainties are found to have significance in this respect. Different ice resistant offshore structures and their construction methods are illustrated in this paper. The cost structure and technical features of the production platform types are also discussed with special attention given to the Russian construction facilities and equipment. Extreme ice loads and ice induced vibrations still remain areas of some uncertainty in offshore platforms. An approach to solve complicated interaction cases involving extreme first year ridges and hummock fields is outlined. Means to control ice loads and vibrations are discussed.

Arctic continental shelf morphology related to sea-ice zonation, Beaufort Sea, Alaska

Landsat-1 and NOAA satellite imagery for the winter 1972–1973, and a variety of ice and sea-floor data were used to study sea-ice zonation and dynamics and their relation to bottom morphology and geology on the Beaufort Sea continental shelf of arctic Alaska. In early winter the location of the boundary between undeformed fast ice and westward-drifting pack ice of the Pacific Gyre is controlled by major coastal promontories. Pronounced linear pressure- and shear-ridges, as well as hummock fields, form along this boundary and are stabilized by grounding, generally between the 10- and 20-m isobaths. Slippage along this boundary occurs intermittently at or seaward of the grounded ridges, forming new grounded ridges in a widening zone, the stamukhi zone, which by late winter extends out to the 40-m isobath. Between intermittent events along the stamukhi zone, pack-ice drift and slippage is continuous along the shelf edge, at average rates of 3–10 km/day. Whether slippage occurs along the stamukhi zone or along the shelf edge, it is restricted to a zone several hundred meters wide, and ice seaward of the slip face moves at uniform rates without discernible drag effects. A causal relationship is seen between the spatial distribution of major ice-ridge systems and offshore shoals downdrift of major coastal promontories. The shoals appear to have migrated shoreward under the influence of ice up to 400 m in the last 25 years. The sea floor seaward of these shoals within the stamukhi zone shows high ice-gouge density, large incision depths, and a high degree of disruption of internal sedimentary structures. The concentration of large ice ridges and our sea floor data in the stamukhi zone indicate that much of the available marine energy is expended here, while the inner shelf and coast, where the relatively undeformed fast ice grows, are sheltered. There is evidence that anomalies in the overall arctic shelf profile are related to sea-ice zonation, ice dynamics, and bottom processes. A proposed ice zonation, including zones of (1) bottom-fast ice, (2) floating fast ice, (3) stamukhi, and (4) seasonal pack ice, emphasizes ice interaction with the shelf surface and differs from previous zonation. Certain aspects of the results reported here are directly applicable to planned offshore developments in the Prudhoe Bay oil field. Properly placed artificial structures similar to offshore shoals should be able to withstand the forces of the ice, serve to modify the observed ice zonation, and might be used to make the environment less hostile to human activities.

Sidescan Sonar Imagery of Sea Ice in the Arctic Ocean

SUMMARYA sidescan sonar system was fitted to the nuclear submarine HMS SOVEREIGN for a voyage beneath the sea ice of the Arctic Ocean in October 1976. Despite problems with the transducers, a successful profile was obtained along 40 km of track to a lateral range of 150 m. The location of the profile was about 83 °N, 15°W. Representative samples of the record are presented and interpreted with reference to simultaneous narrow-beam upward-looking sonar profiles. Features that can be seen clearly on the under-ice mapping include open water, thin ice, pressure ridges, hummock fields, first-year floes and multi-year ice. This was the first application of sidescan sonar profiling to sea ice, and a discussion is given of the potential value of this technique.

Stamukhi Zone Processes: Implications for Developing the Arctic Offshore Area

Offshore islands and shoals in the Beaufort Sea interact with the westward pack-ice drift, causing the formation of major grounded-ridge systems of pack-ice drift, causing the formation of major grounded-ridge systems of the Stamukhi zone. These systems resist the forces of pack ice and protect the inner shelf. Fully exposed islands and shoals show little modification for the past 25 years, suggesting that similar artificial structures, properly placed, should also survive and might modify sea-ice zonation. properly placed, should also survive and might modify sea-ice zonation. Introduction Landsat-1 and National Oceanographic and Atmospheric Admin. satellite imagery for the winter of 1972-73 and a variety of ice and sea-bed data, including side-scan sonar, high-resolution (Uniboom) seismic reflection, and precisely controlled bathymetric surveys, were used to precisely controlled bathymetric surveys, were used to study sea-ice zonation and dynamics and their relation to bottom morphology and geologic processes on the Beaufort Sea continental shelf of the Alaskan arctic. During early winter, the location of the boundary between undeformed fast ice and the westward-drifting polar-pack ice is controlled by major coastal polar-pack ice is controlled by major coastal promontories. Pronounced linear pressure and shear ridges, and promontories. Pronounced linear pressure and shear ridges, and hummock fields form along the boundary and are stabilized by grounding, generally between the 10- to 20-m isobaths. Slippage along this boundary occurs intermittently at, or seaward of, the grounded ridges, forming new grounded-ridge systems in a widening zone (the Stamukhi zone) that by late winter may extend out to the 40-m isobath. The Stamukhi zone protects the inner shelf and coast from pack-ice forces. There is a causal relationship between the spatial distribution of major ice-ridge systems and offshore shoals, downdrift of major coastal promontories. Ice has forced the shoals to migrate shoreward for distances of up to 400 m in the last 25 years. The sea floor seaward of such shoals (within the Stamukhi zone) shows a high density of ice gouging and depth of incision, and a high degree of disruption of internal sedimentary structures. The concentration of large-scale ice deformation and intense sea-floor disruption in the Stamukhi zone indicate that much of the available marine energy is expended here during winter. The inner shelf and coast (where the relatively undeformed fast ice grows) is sheltered. Certain aspects of the results reported here are directly applicable to planned offshore development in the Beaufort Sea. Artificial structures similar to offshore shoals, properly placed, should be able to withstand the forces of the ice, should serve to modify the observed ice zonation, and might be used to make the inner shelf environment less hostile to man's endeavors. Stamukhi Zone Processes Studies by the USGS of the Beaufort Sea shelf region of northern Alaska, using remote-sensing techniques and sea-floor data, have found a correlation between shelf morphology and the yearly development of sea-ice zonation. This relationship may have critical implications for the economic development of the continental shelf in arctic Alaska. Offshore development within the seasonal fast-ice zone on the inner shelf, extending to the 10- to 20-m isobath, generally is considered economically and technically feasible. However, development of expected high oil and gas potential beyond the relatively protected fast-ice zone will involve untried technology and high risks. Here, where the polar pack of the Pacific Gyre rubs against the continental margin, ice dynamics and deformational processes have the highest intensities of anywhere in the Arctic Basin.

Islands of Grounded Ice

In August 1972, the U.S. Coast Guard icebreaker Burton Island confirmed the existence of a large formation located at about 160 km northwest of Point Barrow, which had first been observed by 1971 satellite imagery of the U.S. National Ocean and Atmospheric Administration (NOAA-1). Visual observations made from a distance indicated that the relief of the formation, which appeared to rise as much as nine meters above the sea, was highly irregular. Although it was at first described as a large hummock field of pressured sea ice, news quickly spread that it was a large piece of tabular shelf ice, i.e., an ice island. ... With the launching into orbit of the Earth Resources Technology Satellite (ERTS-1) in July 1972, there became available for the first time multi-spectral imagery of sufficiently high resolution to allow detailed sea studies to be performed. Through the sequential imagery provided by the satellite during its passage over the High Arctic, measurements could be made of the deformation and drift of the pack and a continuous observation kept on the formation and break up of fast along the Alaska coast. The discovery of the ice island, together with the availability of satellite imagery, thus provided a unique opportunity to locate and monitor the movement of a specific feature for an extended period of time. ... This report serve the demonstrate the usefulness of ERTS-1 imagery for the location of islands of grounded ice, and for observing the growth and decay of these features with time. Through the imagery there has also been revealed a possible error on bathymetric charts for the location of two shoals in the southern Chukchi Sea. ...