Ice Scour Research Articles

Numerical modeling of seabed ice scour

Abstract In coastal regions throughout arctic oceans the seabed is frequently scoured or gouged by moving bodies of ice such as icebergs and sea ice keels associated with pressure ridges. Ice scouring may present a severe environmental hazard pertaining to marine pipelines and other seabed installations. Lateral loading due to excessive movement of sub-soil may lead to failure of the buried pipelines. To facilitate a rational design of marine pipelines, a model is therefore required to evaluate the subscour soil deformation. This paper describes a numerical approach for analysing the seabed response during ice scour. A finite element model, based on large displacement and plastic formulation, was adopted. Results from 2-D finite element analyses compared reasonably well with experimental data obtained from geotechnical centrifuge tests. The subscour soil movements with a buried pipeline were investigated by 3-D finite element analyses. The results show that in certain cases, a pipeline may not have significant influence on the subscour soil deformation.

Geotechnical evidence of ice scour as a guide to pipeline burial depth

Cette note suggere que la profondeur maximale de l'affouillement de la glace dans les fonds marins arctiques est indiquee par un bris dans les proprietes geotechniques. Ceci nous donne un moyen alternatif economique de trouver la profondeur d'enfouissement requise pour proteger un pipeline en utilisant des donnees qui sont de toute facon requises pour d'autres fins.

Habitat Persistence Underlies Intraspecific Variation in the Dispersal Strategies of Planthoppers

Dispersal is considered a vital life history characteristic for insects exploiting temporary habitats, and life history theorists have often hypothesized an inverse relationship between dispersal capability and habitat persistence. Most often, this hypothesis has been tested using interspecific comparisons of dispersal capability and qualitative estimates of habitat persistence. Consequently, most assessments have failed to control for possible phylogenetic nonindependence and they also lack quantitative rigor. We capitalized on existing intraspecific variation in the dispersal capability of Prokelisia planthoppers to examine the relationship between habitat persistence and dispersal, thereby minimizing possible phylogenetic effects. Two congeneric species (Prokelisia marginata and P. dolus) occur in the intertidal marshes of North America, where they feed exclusively on cordgrasses (Spartina). Because these planthoppers exhibit wing dimorphism, flight—capable adults (macropters with fully developed wings) are easily differentiated from flightless adults (brachypters with reduced wings). Thus, dispersal capability can be readily estimated by the percentage of macropters in a population. At a regional spatial scale, we found a highly significant negative relationship between dispersal capability (percent macroptery) and habitat persistence. In this system, habitat persistence is influenced by a combination of marsh elevation, winter severity, and tidal range, which interact to determine the ability of planthoppers to endure through winter in their primary habitat for development. P. marginata develops primarily in low—marsh habitats during summer, habitats that can be subjected to pronounced winter disturbance due to ice scouring and/or extensive tidal inundation. Levels of winter disturbance of the low marsh are extreme along the Atlantic coast, intermediate along the Pacific, and low along the Gulf. Both the failure of P. marginata populations to remain through winter in this habitat, and the dispersal ability of these populations (92%, 29%, and 17% macroptery, respectively), are correlated with levels of disturbance. Thus, in regions where winter disturbance is high, levels of dispersal are correspondingly high to allow for recolonization of extirpated habitats from overwintering sites on the high marsh. Unlike P. marginata, P. dolus develops primarily in high—marsh habitats, which are much less disturbed on all coasts during winter. Consequently, this species remains year—round in its primary habitat for development, and most populations exhibit relatively low levels of macroptery (<10%). When raised under common garden conditions, many more macropters of both species were produced from Atlantic compared to Gulf populations. Thus the proportion of macropters produced from the populations used in this experiment paralleled the incidence of macroptery measured in the field, providing evidence that the geographic variation in dispersal capability in both species has in part a genetic basis. The results of this study provide strong intraspecific evidence for an inverse relationship between the dispersal capability of insects and the persistence of their habitats.

Centrifuge Modeling and Numerical Simulation of Ice Scour

In coastal region throughout arctic oceans the seabed is frequently scoured or gouged by moving bodies of ice such as icebergs and sea ice keels associated with pressure ridges. Ice scouring may present a severe environmental hazard posed to marine pipelines and other seabed structures. To investigate the ice scour events, both centrifuge model tests and numerical simulations were conducted. Attention was focused on the subscour soil deformation. The centrifuge model tests yielded extensive data which allowed to clarify the scour mechanism and to verify the numerical analysis technique. A finite element model, based on large displacement and plastic formulation, was adopted to analyze the ice scour events. The results from finite element analyses were examined and compared with the experimental data from the geotechnical centrifuge tests.

Mechanical Model of Ice Gouging and Its Apphcability

The coastal area facing the sea of Okhotsk in Hokkaido is annually covered with drifting ice. When theseice floes with a deep draft move toward the shore, they gouge sandy sea bottoms.Due to this ice gouging, marine resources along the shallow sandy coast are damaged. For example, sea shells, buriedstructures, pipelines.In this study, the Authors understand the mechanism of ice gouging and obtain basicknowledge necessary to take preventive measures in future against damage by ice floes.

Nunataks of the last ice sheet in northwest Scotland

High‐level weathering limits separating ice‐scoured topography from an upper zone of frost‐weathered detritus were identified on 17 mountains in NW Scotland at altitudes of <600 m to< 900 m, and a further 6 peaks were found to support evidence of ice scouring to summit level. Weathering limits are most clearly defined on Torridon Sandstone, which is resistant to frost shattering, but can also be mapped on Cambrian Quartzite, Lewisian Gneiss and Moine Schist. Contrasts in degree of rock surface weathering above and below the weathering limits were evaluated using measurements of joint depth and rock surface hardness, and through X‐ray diffraction analyses of clay mineral assemblages. The results indicate significantly more advanced rock and soil weathering above the weathering limits. Widespread persistence of gibbsite above the weathering limits suggests that they represent the upper limit of Late Devensian glacial erosion, and the regularity of the decline in weathering limit altitude along former flowlines eliminates the possibility that it represents a former thermal boundary between protective cold‐based and erosive warm‐based ice. The weathering limits are therefore interpreted as periglacial trimlines defining the maximum surface altitude of the last ice sheet around former nunataks. Calculated basal shear stresses of 50–78 kPa are consistent with this interpretation. The altitude of the trimlines implies that the former ice shed lay at 900–930 m in the Fannich Mountains and descended gently northwards, and that the ice surface descended NW from the ice shed to >500 m over the extreme NW tip of Scotland and to 700–730 m at the head of Little Loch Broom.

Sea ice scouring on the inner shelf of the southeastern Canadian Beaufort Sea

About 2200 ice scours were observed and analyzed over a distance of approximately 500 km on the inner shelf of the southeastern Canadian Beaufort Sea. Ice scours were divided into two types based on their morphology: multiple scours consisting of a series of parallel scours and ridges, and single scours. Single scours are the dominant type representing more than 85% of the total observations. The mean scour depth and width are 0.3 m and 11 m respectively, but scour depths of more than 2 m and a scour width up to 345 m were documented. The magnitude of ice scouring processes increases with water depth. In water depths less than 10 m, less than 25% of the seafloor surface is reworked by ice scours. This percentage increases significantly seaward of the 10 m isobath, being more than 75% in water depths in excess of 12 m. A break in the seabed slope at about 10 to 12 m water depth marks a boundary between a nearshore zone moderately influenced by ice processes and an outer zone affected by intense ice scouring. This morphological boundary could be due to intense erosion by the keels of pressure ice ridges at the inner edge of a zone of grounded ice ridges. Most of the observed ice scours appear to be reworked, especially in water depths of less than 10 m, and represent small-scale sediment sinks. Inshore of the 10 m isobath, scour reworking is believed to be mainly due to frequent bottom disturbance by wave orbital currents and mean near-bottom flows during the open water season. Scour orientations show that the dominant motion of ice during scouring events is east or west, which is subparallel to the bathymetric contours and coastline. Scour terminal push mounds, however, suggest a dominant east to southeast movement that may contribute to onshore sediment transport during ice push events.

The Marine Ecology of the Laguna San Rafael (Southern Chile): Ice Scour and Opportunism

Surveys of the intertidal fauna and flora, the plankton, fish, birds and marine mammals of the Laguna San Rafael were carried out by a Raleigh International Expedition in January-February 1993. The Laguna is dominated by the effects of scouring, low temperature and low salinity produced by the calving, tide-water San Rafael glacier that discharges into the Laguna. The fauna and flora are simple and largely limited to a small sector of the Laguna, relatively unaffected by ice. There is a predominance of herbivorous fish, ducks, geese and swans, feeding mainly on macroalgae. Penguins, cormorants, sea lions and porpoises make up the top predators. The strandline is influenced by very heavy rainfall and supports a fauna of freshwater and terrestrial molluscs and earthworms, fed upon by birds and frogs. Large numbers of mussels are present in the north-eastern sector of the Laguna, but many are found in poor condition, high on the shore. It is suggested that poor condition and mortality are caused by large calving waves that dislodge mussels. Such waves are caused by occasional loss of massive quantities of ice from the glacier.

Weight of the Evidence is the Right Approach

Sampling and field experiments were conducted from 1975 to 1990 to test how the structure of marine benthic communities around McMurdo Station, Antarctica varied with levels of anthropogenic contaminants in marine sediments. The structure of communities (e.g., infauna density, species composition, and life history characteristics) in contaminated and uncontaminated areas were compared with the structure of communities influenced by two large‐scale natural disturbances, anchor ice formation and uplift or iceberg scour. Benthic communities changed radically along a steep spatial gradient of anthropogenic hydrocarbon, metal, and PCB contamination around McMurdo Station. The heavily contaminated end of the gradient, Winter Quarters Bay, was low in infaunal and epifaunal abundance and was dominated by a few opportunistic species of polychaete worms, especially Capitella spp., Ophryotrocha claperedii, and Gyptis sp. These are relatively small and motile species. Closely related species live in organically enriched and anthropogenically disturbed sediments throughout temperate latitudes. The edge of the heavily contaminated bay, the transition area, contained several motile polychaete species with less opportunistic life histories, especially Tharyx sp. and Haploscoloplos kerguelensis. Uncontaminated sedimentary habitats harbored dense tube mats of infaunal animals numerically dominated by populations of polychaete worms, crustaceans, and a large suspension feeding bivalve. These species are generally large and relatively sessile, except for several crustacean species living among the tubes. Community patterns along the anthropogenic disturbance gradient were similar to community changes along a natural gradient of anchor ice disturbance. The same motile and opportunistic polychaete worms were most abundant where anchor ice freezing and uplift were most severe. The motile polychaetes, Tharyx sp., Haploscoloplos kerguelensis, and Polygordius sp., were abundant at the edge of the anchor ice disturbance zone. The undisturbed bottom harbored the same dense tube mat community and large bivalve population. Community changes along the anthropogenic disturbance gradient were also similar to colonization patterns into bottom areas gouged by icebergs. The most recent ice gouges along the east side of McMurdo Sound contained large numbers of the motile opportunistic polychaete worms. Ice gouges along the western sound contained more motile species of infauna compared to adjacent undisturbed bottoms with large numbers of tube‐dwelling infauna. Motile species were also the first to colonize experimental sediments placed on the sea floor for 6 yr. Although the community patterns around anthropogenic and natural disturbances were similar, particularly the motile and opportunistic species at heavily disturbed and marginal areas, the natural disturbances cover much greater areas of the sea floor around the entire Antarctic continent. On the other hand, recovery from chemical contamination is likely to take many more decades than recovery from natural disturbances as contaminant degradation is a slow process. This process can be enhanced by ongoing pollution abatement and cleanup operations.

Use of ice-scoured depressions by marsh-foraging snow geese (Chen caerulescens atlantica)

We describe the foraging behaviour of staging greater snow geese (Chen caerulescens atlantica) as they feed in depressions caused by ice scouring in tidal marshes along the St. Lawrence River. We compared feeding success during the fall and spring staging season as the birds extracted rhizomes of Scirpus americanus. In these marshes, every growth cycle (summer) is followed by two distinct episodes of intensive exploitation by geese (fall and the following spring). Two kinds of foraging bouts were considered: (1) successful (i.e, with one or more ingestions) and (2) unsuccessful (i.e, without any ingestion). Geese sampled fewer depressions in the fall than in the spring because the proportion of depressions sampled with success was greatest in this season. Geese spent less time per depression in the spring, whether they were successful or not, but sampled more depressions instead. There was a decrease in the number of depressions sampled over the spring period. It is concluded that geese adjust their use of ice-scoured depressions to both inter- and intra-seasonal food availability (depletion). Behavioural mechanisms they might use to do so are discussed.

Mechanisms of Ice Gouging

Sea ice is carried to the cost of Hokkaido by wind and water currents every year. In low pressure systems or when there is much sea ice, it drifts out toward the Pacific Ocean. When sea ice moves in shallow water areas, the sandy subgrade on the sea bottom is gouged by the sea ice. This phenomenon is generally called ``ice gouging``. Substantial damage to sea food i.e. (shellfish) and to structures embedded in the seabed is reported every year. However, the mechanisms and behavior of ice gouging is not known sufficiently enough for discussion. Therefore, the authors have conducted a suitable experiment to clarify this phenomenon and have suggested formulas to measure ice gouging.

Zebra Mussels in a Coastal Marsh: The Seasonal and Spatial Limits of Colonization

We investigated the seasonal cycles and spatial distribution of zebra mussels (Dreissena polymorpha) in a coastal marsh in Saginaw Bay, Lake Huron, from 1993 to 1995. This marsh was characterized by a sandy substrate and ice scour which removes emergent vegetation each winter. Due to this lack of permanent substrates, we expected that populations in the marsh would only be maintained during the ice free season, with recruitment from open waters responsible for recolonization each year. Instead, we found that substrate independent clusters over-wintered in the zone of emergent vegetation and were the source of recruitment to the marsh. Peak larval production occurred during July of both years with greatest densities between 300 and 350 m from shore. Settlement of these larvae onto Scirpus stems occurred by late August, accounting for the greatest numbers of mussels attached to stems. Mussel abundance was highest between 300 and 450 m from shore, but was very low inshore of 300 m. Losses of attached mussels from Scirpus stems reduced abundances by as much as 80% by the end of September 1994 despite the fact that the stems were still intact and healthy. The greatest losses occurred between 300 and 400 m from shore, while mussel abundance was increasing at the bayward edge of the emergent vegetation. The high abundance of larvae and attached juveniles near the center of the marsh, as well as the high loss of attached mussels in this area, may be related to a recently described form of horizontal stratification in coastal wetlands.

The Submersed Aquatic Plant Community in Jackson Lake, Grand Teton National Park, Wyoming

Submergent macrophyte distribution in lakes is usually related to depth. At lower depths (downslope on the littoral lake bed), macrophyte distribution and growth have been related to light (Spence 1982), substrate texture, nutrient status (Carpenter and Adams 1977), and lake morphometry (Duarte and Kalff 1986). Factors limiting distribution and growth at shallower depths (upslope on the littoral shore) are not as well understood. Meaningful descriptions of plant distributions in reservoirs are problematic because water levels vary through the course of a year (Brewer and Rerslett 1987). Water level fluctuations are a challenge to plant growth. A plant which begins the growing season at a depth of 3 m may be under 6 m of water after spring runoff fills the reservoir. Later in the summer, the same plant may be left at a depth of only 1 m as water is removed from the reservoir during summer drawdown. In reservoirs, where lake levels fluctuate substantially durin_g the growing season, the physical environment is characterized by increased spatial and temporal heterogeneity. Disturbances associated with changing water levels include ice scour during winter drawdown, abrasion due to increased erosion along the lake shore and wave action. Recent work suggests that the magnitude and timing of water level fluctuations may be the most important factor regulating macrophyte community processes at shallower depths in reservoirs (Gasith and Gafny 1990; Brewer and Parker 1990; Rorslett 1984). The status of the aquatic plant community in Jackson Lake was re-evaluated from June - August, 1995. During this time, we compiled a species list and mapped the distribution of macrophyte species.

Clonal reproduction of riparian cottonwoods in southern Alberta

From 1990 to 1993, 690 small cottonwood saplings were excavated along four rivers in the Oldman River Basin to determine their method of origin. Overall, 52% began as seedlings while 48% originated through clonal mechanisms, of which 30% (of the total) were root suckers, i.e., adventitious shoots from preexisting roots, and 18% were shoot suckers, i.e., new shoots originating from buried shoots. The shoot suckers arose primarily through (i) flood training, i.e., new shoot emergence from young shoots that had been toppled and buried during flood events, and (ii) shoot regrowth following decapitation by ice scouring or animal browsing. Only two saplings originated through cladoptosis, propagation from excised shoot fragments. The proportion of seedlings versus clonally established shoots varied somewhat across years and sites. Within a site, saplings near the river's edge were more commonly seedlings, whereas saplings further from the river were more commonly root suckers. Three cottonwood species occurred in the forests studied and their reproductive strategies varied somewhat. Unlike the balsam poplar (Populus balsamifera), narrowleaf cottonwood (Populus angustifolia), and interspecific hybrids, the prairie cottonwood (Populus deltoides) did not propagate through root suckers; all species produced seedlings and shoot suckers. This study demonstrates that clonal reproduction has the potential to contribute to the regeneration of riparian cottonwood forests in southern Alberta. The contribution through clonal propagation complicates the forest population structure and should be considered in programs to conserve the remaining riparian cottonwood forests in southern Alberta and elsewhere in North America. Key words: Populus, cottonwoods, hybrid poplar, reproduction, clones, suckering, seedlings.

Benthic associations of the shallow hard bottoms off Terra Nova Bay, Ross Sea: zonation, biomass and population structure

Quantitative and semi-quantitative samples of phyto- and zoobenthic organisms were collected by SCUBA diving at five stations along a depth transect from 0.5–16 m on the shallow hard bottoms off Terra Nova Bay, Ross Sea, Antarctica. The benthic associations were dominated by two macroalgal species (Iridaea cordata and Phyllophora antarctica) and by few animal taxa (mainly polychaetes, molluscs and peracarid crustaceans). Distribution at the community and species levels revealed a well-defined zonation pattern as a function of depth, governed mainly by sea ice scouring and melting. Zonation of vagile fauna was also affected by the effects of covering and architecture of the two dominant macroalgae. Species richness and diversity were higher in the Phyllophora-associated community, where habitat complexity and sheltering were higher. The highest faunal abundance (over 82 000 ind.m−2) and biomass (macroalgae and fauna wet weight 2392 g m−2) were recorded at 2 m depth in association with the Iridaea covering, where the harsher environmental conditions select a few taxa. The biomass values, even if underestimates of the whole community standing crop, are among the highest recorded in shallow austral biotopes. An autoecological and demographic analysis of the most abundant animal species revealed for some species (e.g. Laevilitorina antarctica and Paramoera walkeri) a quite complex population structure with up to three size classes, including juveniles. In some species, the cohort of juveniles showed a well-defined depth preference probably related to sheltering by the macroalgae. As a whole, the species analyzed showed various and contrasting reproductive strategies, despite the fact that the environmental conditions along the transect were relatively similar and quite selective.

Role of Ice Scouring and Goose Grubbing in Marsh Plant Dynamics

1 The effects of scouring by ice and of grazing by the greater snow goose (Chen caerulescens atlantica) on the substrate and vegetation of a brackish tidal marsh in the St Lawrence estuary were studied from 1985 to 1987. During spring and fall migratory stopovers, the birds fed mainly on Scirpus americanus rhizomes. 2 The presence of ice-made depressions was related mainly to the physical characteristics of Scirpus marshes. Goose grubbing has no significant effect on the annual dynamics of these depressions. 3 In exclosures, i.e. iii the absence of geese, ice drift has little influence on species abundance and net above-ground primary production of the marsh except at high levels of disturbance. 4 In ice-made depressions, goose grazing reduced net primary production by approximately 45% and facilitated colonization by competitive species such as Zizania aquatica, Sagittaria spp. & Scirpus torreyi. History or frequency of disturbance seems to play an important role in this process. 5 By exploiting ice-made depressions, snow geese contribute to an increase in the heterogeneity but a decrease in the primary production of the Scirpus marsh.

Amphipod crustaceans and environmental disturbance: a review

This review of the literature shows that amphipod crustaceans create disturbance through predation on commercial harvests, attacks on macroalgae, introductions as exotic species, and interference competition. Amphipods are also affected by disturbance events, both man-made and natural. Anthropogenic disturbance includes environmental alteration by toxins, oil, organic enrichment, acidification, salinity alteration, and ships' wakes. Natural disturbances are space clearance by the feeding activities of other fauna, and physical disturbance by sediment erosion and ice scour.

Occurrence of Zebra Mussel (Dreissena polymorpha) in the intertidal Region of the St. Lawrence Estuary

Abstract In the fall of 1992 at an ice-disturbed freshwater site along the St. Lawrence estuary, we found zebra mussels (Dreissena polymorpha) on rocks exposed at low tide. The population size structure suggested that the majority of these mussels had settled in early spring of 1992. In May 1993, we revisited the site and found adult zebra mussels in the cracks and crevices of boulders exposed at low tide, but there was a significant reduction in their numbers. Zebra mussels now appear able to invade the intertidal region and to survive the perils of exposure and desiccation in the summer as well as the abrasive action of ice scour in the winter.

Ice in the shore zone and the geomorphology of cold coasts

Approximately 90% of Canada's ocean coastline is affected by seasonal or multiyear sea ice and winter ice develops on most lakes. Recent studies of ice effects in the shore zone have included investigations of ice-congested and protected shores in the north-west Canadian Arctic Archipelago, processes involved in the construction by ice of large shore ridges in the same region, direct ice scour and enhanced hydrodynamic scour in the presence of ice (strudel scour and ice wallow), particularly as potential hazards to buried pipelines in the Beaufort Sea, and the dynamics of boulder-strewn tidal flats and boulder barricades in eastern Canada. The extent and frequency of shore nourishment by ice and details of the processes involved, including the relative importance of ride-up versus pile-up, remain important research questions. Reports emphasizing the contribution of ice rafting to shoreface retreat along the Alaskan coast of the Beaufort Sea suggest the need for quantitative studies of this phenomenon in Canada, in particular with respect to prodelta sedimentation at the mouth of the Mackenzie River. The coastal zone in the Beaufort Sea is particularly sensitive to climate change through effects on thermokarst processes, rising sea level, the relation between ice cover and wave energy through fetch limitation, and potential changes involving ice dynamics and freeze-up processes.

Selection and population genetic structure of the barnacle Semibalanus balanoides in the northwest Atlantic and Gulf of St. Lawrence

It has been proposed that population genetic structure at the mannose-6-phosphate isomerase (Mpi) locus in the barnacle Semibalanus balanoides (L.) could be due to selection by ice scour acting through variation in larval settlement behavior. Homogeneity of Mpi allele frequencies for barnacles within the Gulf of St. Lawrence could be maintained by a combination of selection and limited gene flow from populations of S. balanoides on the Atlantic coast of Canada south of the Cabot Strait. We sampled S. balanoides from 19 sites in Greenland and Baffin Island, the Gulf of St. Lawrence, and on the Atlantic coasts of Nova Scotia and New Brunswick, Canada, in order to test these hypotheses. Presumptive genetic variation at 2 enzyme loci, Mpi and glucose-6-phosphate isomerase (Gpi ) , was quantified by isoelectric focusing. A11 populations were in Hardy-Weinberg equilibrium, and no Linkage disequilibrium between loci was observed. We found significant spatial variation in allele frequencies at both loci. In particular, S. balanoides populations within the Gulf of St. Lawrence were not genetically homogeneous at either locus. A discontinuity in allele frequencies at both the Gpj and Mpi loci occurred in the vicinity of the Mirarmchi estuary, in the southern Gulf. Comparisons of allele frequencies between barnacle recruits and adults suggested this population structure may have resulted from selection, in the case of Mpi, possibly associated with environmental temperature. Selection by ice scour does not appear to affect spatial variation in allele frequencies in the Gulf of St. Lawrence.