Ice Scour Research Articles

In-situ ice ridge scour tests: experimental set up and basic results

An in-situ program was carried out in the Van Mijen fjord on Spitsbergen during the 2002 and 2003 winter seasons to investigate the ice ridge scour process. Two ice scour tests and one shear off test were performed. Ice ridges with dimensions of up to 4.0×4.0×2.45 m were artificially made by cutting the level ice into pieces to produce ice rubble. The ridges were left to consolidate for up to 4 weeks and then pulled towards the beach scouring the seabed that consisted of clayey sediments (ice scour tests). The ice ridge for the shear off test was built up in a pre-excavated trench in hard moraine. During pulling the keel of the ice ridge was sheared off by the sidewall of the trench. The pulling force, displacements, failure of the keel and resulting plough marks were measured. This paper provides a description of the test arrangement and the basic experimental results.

Temporal variation in community interfaces: kelp-bed boundary dynamics adjacent to persistent urchin barrens

We examined, over 2 years, factors affecting the temporal stability of the lower limit of kelp beds (Alaria esculenta) at five subtidal sites in the Mingan Islands, northern Gulf of St. Lawrence. The position of the lower limit of the beds varied markedly among sites and over time and was largely controlled by the green sea urchin, Strongylocentrotus droebachiensis, which formed dense (up to 500 individuals m−2) feeding fronts at the lower edge of the beds. These aggregations advanced over the kelp most rapidly during the summer (at rates as high as 2.5 m month−1), and there appeared to be a threshold urchin biomass of ~5 kg m−2 below which the fronts could not substantially reduce the limit of the beds. The fronts consisted mainly of large individuals, whereas smaller urchins predominated in the barrens zone below the kelp beds. At one site, we recorded large seasonal shifts in overall urchin densities, with large increases and decreases during the summer and winter, respectively. An urchin exclusion experiment indicated that algal recruitment in the barrens was two orders of magnitude greater in the absence than in the presence of urchins. The kelp Agarum cribrosum greatly restricted urchin movements, and the greater temporal stability of the kelp bed at one site appeared related to the gradual replacement of Alaria esculenta in the lower kelp bed by a large stand of Agarum cribrosum. We propose that perturbations by abiotic factors (e.g., ice scouring and water motion) trigger important but localized changes in urchin densities that, in turn, largely determine the limits of kelp bed distribution in this region of the Atlantic where urchin barrens are a persistent community state.

Ｉｃｅ Ｓｃｏｕｒ Ｅｖｅｎｔにともなう地盤内の変形・発生応力に関する基礎的研究

Ｉｃｅ Ｓｃｏｕｒ Ｅｖｅｎｔにともなう地盤内の変形・発生応力に関する基礎的研究

Effects of small-scale disturbances of canopy and grazing on intertidal assemblages on the Swedish west coast

The effects of small-scale disturbances (80×30-cm plots) of canopy and grazers on intertidal assemblages were investigated in this 4-year experiment on sheltered rocky shores on the Swedish west coast. Canopy disturbances due to ice scouring were mimicked by removal of adult plants of the seaweed Ascophyllum nodosum (L.) Le Joli. Density of the main epilithic grazing gastropods, Littorina spp., was lowered by exclosure and handpicking. Based on earlier experiments in other areas, the general hypothesis was that canopy removal and grazer exclosure, alone or in combination, should increase the recruitment of A. nodosum or other fucoid juveniles, and change the structure of the understorey assemblage. There was an effect of canopy removal on the development of this assemblage, lasting for more than 31 months. Both increased and decreased abundances of species were found as short-term effects, but there was also a longer-term effect with increased abundance. Grazer exclosure was only effective in combination with canopy removal, causing a short-term increase in ephemeral green algae. Short-term effects of canopy removal were also the increase in recruitment of Semibalanus balanoides (Linnaeus) and the decrease of the red alga Hildenbrandia rubra (Sommerfelt) Meneghini. Fast recruitment and growth of fucoid species ( Fucus serratus L. and F. vesiculosus L.) restored the canopy and conditions of the understorey within 18 months. Thus, the canopy removal changed the physical conditions for the understorey, making it possible for other species to coexist in this community. Surprisingly, no effect of canopy removal or grazer exclusion was found on the recruitment of juvenile A. nodosum, neither by canopy removal nor grazer exclosure. The lack of such effects might be due to the early mortality caused by other grazers (small, mobile crustaceans), or to the low density of periwinkles on these shores. However, despite the patchy and generally low recruitment of A. nodosum juveniles, observations suggested that the cover of A. nodosum in manipulated patches would return to initial levels, either by recruitment or regrowth of small holdfasts and from growth of edge plants.

The geomorphic impact of glaciers as indicated by tors in North Sweden (Aurivaara, 68° N)

Geomorphological investigations carried out on 15 tor-like features located on the Aurivaara plateau (North Sweden, 68° N) provide new insights in the greatly debated age of these landforms. Erratics and till trapped deep in the tor joints support a pre-Weichselian age for tor formation. Moreover, the occurrence of various weathering stages in allochtonous material, the joint width up to 1.5 m (requiring long-term weathering), and the frequent association of tors with pediment-like forms, suggest pre-Quaternary tor formation. The juxtaposition of fresh erratics and in situ old weathering features (mushroom rocks, concentrically weathered well-rounded corestones, and grus) indicates a predominantly cold-based regime for the Scandinavian ice sheet, with erratics carried by the overlying moving ice being repeatedly deposited on tor summits during deglaciation phases. The relationships between tors and ice action indicated for the Aurivaara plateau result in the proposal of a morphodynamical succession of five tor subtypes ranging from the preservation of well-rounded corestones still embedded in grus (suggesting negligible glacial erosion) to the almost complete removal of tor features by ice scouring. A comparison with tors in similar geological and topographical contexts from the unglaciated Dartmoor area allows a tentative evaluation of an average overall glacial erosion of 0–10 m on the northern Sweden plateaus, in sharp contrast with the 190 m overdeepening of the nearby Torneträsk basin. Thus, this case study of Swedish tors provides additional support to the recent interpretations of relict landscapes in previously glaciated areas and is in accordance with the classical «model» of glacial selective erosion established in the Nordic and Arctic mountains.

Laboratory experiments of ice scour processes: buoyant ice model

A laboratory program was carried out to measure the loads and seabed response due to a buoyant ice block scouring sand and gravel representative of the Grand Banks seabed, offshore of eastern Newfoundland, Canada. The results complement previous experimental work carried out at the Canadian Hydraulics Centre (CHC), which used a rigidly affixed ice block to scour sediment material [Cold Reg. Sci. Technol. 35 (3)(2002a) 195]. The present study used the same sediment to create a model seabed; however, in this test series, the model was towed along an ice tank, free to move throughout the water column. Six tests were carried out. Freshwater ice, built up into a large block with an overall dimension of 0.76×0.76×0.71 m, was used to scour the seabed at depths up to 0.2 m. The scouring loads, displacement, angular movements and resulting trenches were measured. This paper provides a description of the test arrangement and the experimental results.

海氷と地盤との摩擦係数に与える因子の効果について

The ice ride-up and ice gouging processes are related to the interaction between ice and sand or gravel. The friction coefficients of ice with those materials are important factors in calculating loads during these processes. However, there is little information available on these friction coefficients. Therefore, tests on the friction between saline ice and sand were conducted in an indoor cold chamber to evaluate the static (μs) and kinetic (μk) coefficients of friction of ice on sand as functions of the normal stress, temperature and other variables. This paper examines the results of those tests.

Polarization of competition increases with latitude.

Many organisms overlap in their use of resources in space and time. Where and when resources are restricted, species must compete for them. Living space, often a critical resource controlling food and mate availability, is directly contested by organisms in most habitats. The ensuing animal interactions generally result in a winner gaining space and a loser, which may die. Contact matrices from studies of interference competition in encrusting marine Bryozoa (clonal and colonial animals), spanning at least 60 degrees latitude in both hemispheres, were analysed and subjected to a modern transitivity index. Only data for Bryozoa were used because (i) use of a single taxon with restricted ecology simplifies the scope for types of encounters, (and therefore) interpretation; and (ii) ecological bias is reduced because bryozoans are abundant at all latitudes. The analysis shows that assemblage competition is more hierarchical towards both poles. Thus, poorer competitors fail more frequently in interactions with increasing latitude. The cause of this trend is the simplification of overall outcomes between competitors, such as fewer ties, reversals in outcome or competitive loops (where low-ranking competitors beat those of higher ranking). The implication of such a trend is that the maintenance of biological diversity at high latitudes may principally be by physical rather than biological (competition) processes. Certainly, ocean surface energy increases with latitude through wind and wave action (and ice scour in polar regions).

Laboratory experiments of ice scour processes: rigid ice indentor

A laboratory program has been carried out to measure the loads and seabed response due to an ice block scouring a seabed. Fourteen tests (35 configurations) were performed with a variety of seabed types that are representative of the oil discovery areas within the Jeanne d'Arc Basin area of the northeastern Grand Banks. The tests showed that the scour profiles were generally very uniform along each section of the test channels. The ice did not fail in shear, but general erosion was observed. Scour depths measured in post test were smallest for scouring through a gravel seabed. Regression analysis yielded the following empirical relationships for the loads: P=−1.8v+10.3h+2.3B−0.8 for the gravel bed (r 2=0.93) P=8.5v+46.3h+28.5B−7.6 for the sand bed (r 2=0.90) P=53.4v+45.8h+13.5B−11.3 for the sand/gravel bed (r 2=0.96) P=13.2v+33.4h+11.6B−0.6S−2.4 for all seabeds (r 2=0.74) where v is velocity in metres/second; h is scour depth in metres; B is ice block width, also in metres; and S is the seabed type, represented by the D 50 value of the seabed material.

Intertidal zonation and seasonality of the marine hydroidDynamena pumila(Cnidaria: Hydrozoa)

The vertical zonation and temporal dynamics of the marine hydroid Dynamena pumila were assessed across a wave-exposure gradient on five rocky shores in the Bay of Fundy, Atlantic Canada. Hydroid abundance and occupancy (i.e., percentage occurrence in a quadrat) were measured in 0.25-m2quadrats at eight vertical elevations for each site over four consecutive seasons. Hydroid abundance and occupancy were highly correlated (R2= 0.877). Estimates of hydroid fertility (percentage of sexually reproductive colonies) and size (stem height and number of branches) were obtained in five quadrats at every elevation over the four seasons. Abundance peaked at about 37.5–62.5% of the mean tidal range, with maximum abundance at the site with moderate wave exposure. Abundance peaked during the summer and dropped dramatically over the winter, particularly at the more wave-exposed sites. Winter ice scour and unfavourable environmental conditions reduced hydroid abundance, fertility, and size during the winter. This study demonstrates dramatic shifts in the distribution, fertility, and size of a marine hydroid. The importance of wave action, ice scour, and seasonal changes in environmental conditions is highlighted to emphasize their roles in regulating intertidal hydroid communities on boreal rocky shores.

New perspectives on the dispersal mechanisms of the Antarctic brooding bivalve Mysella charcoti (Lamy, 1906)

Brooding is a widespread phenomenon among Antarctic bivalves. Although it should represent a handicap to dispersion, many brooding species have achieved a wide distribution in Antarctic and subantarctic waters, suggesting that they have alternative and effective methods of dispersal. Evidence of such an alternative method is presented here for the bivalve Mysella charcoti, unexpectedly found alive and healthy in feces expelled by Notothenia coriiceps (Nototheniidae: Pisces). The finding indicates that the snug-fitting shell of Mysella functions as a barrier to digestive enzymes. Withstanding passage through the digestive tract of fish allows Mysella to be passively dispersed (within the home range of the fish) and colonize new habitats or re-colonize shallow-water substrates severely impacted by ice scours.

Case study of a relict iceberg scour exposed at Scarborough Bluffs, Toronto, Ontario: implications for pipeline engineering

Ice scour is a common process on high latitude shelves and has been studied extensively though laboratory and numerical modelling. Direct observations of ice scours in outcrop are very rare. This paper describes a Late Pleistocene iceberg scour observable in soil cliff exposures at Cudia Park, Scarborough Bluffs, Toronto, Ontario. The structure is cut into a glaciolacustrine clayey silt, is about 10 m wide and 4 m deep, and is filled with sands deposited in shallow water. A description of the scour and a simplified finite element model of sub-ice scour soil deformation are presented. It is estimated that the Cudia scour was caused by a small iceberg with a mass of 0.04 Mt, in a water depth between 10 and 30 m, acting with a downward force of 5 MN, which is consistent with modern iceberg scours. The relevance of the Cudia scour to pipeline engineering (design burial depths) is demonstrated, with reference to the design methodology of the Centre for Cold Ocean Research and Engineering (C-CORE). The Cudia scour provides an additional outcrop example of a subscour bearing capacity-type failure and is the only outcrop example to date with lateral berm piles, which are a characteristic part of modern iceberg scours.Key words: iceberg scour, Scarborough Bluffs, pipeline engineering, finite element modelling, Pleistocene geology.

PROCESSES OF ICE SCOUR EVENT AND CONTROL METHODS OF TEST CONDITIONS

Ice Scour Event is a phenomenon that occurs when ice comes into contact with seabed. Ice scour has been reported to have caused damage to communication cables and water intake pipelines. Since oil and natural gas exploration projects in offshore areas of arctic seas are becoming popular, more care must betaken over the design and installation of oil pipelines in such areas in order to avoid accidents due to ice scour. It will be very important to acknowledge mechanisms (ice motions, sub-seabed conditionsongouge events). In addition to our previous tests including the medium-scale model test (Kioka et. al., 2000, 2001), we have performed many series of model tests under various conditions. We examined the main factors for Scour-process, and obtained many results needed for future work (numerical simulation of Ice scour event). We also examined the control methods of test conditions due to an excess pore water pressure that influence the scour process, including scale-effect.

EXPERIMENTAL STUDY ON THE MECHANICAL PROPERTIES OF UNCONSOLIDATED LAYER MODEL

Unconsolidated layer, which is one of the categories of ice ridge structures, is composed mostly of loosely accumulated broken ice pieces. Although its strength is low, it imposes large load on offshore structure when it forms in sufficient size. Also it effects on buried structure due to ice scour event. Since the unconsolidated layer is a threat to many structures in the actual cold sea, it is important to know strength of the unconsolidated layer.In this study, a simplified unconsolidated layer model was produced, and a series of preliminary experiments was carried out for a wide range for confining stresses acting on unconsolidated layer actually, and then the dependence of the strength of the unconsolidated layer model on the shear rate and the ice block size was investigated

PATCH DYNAMICS OF MUSSELS ON ROCKY SHORES: INTEGRATING PROCESS TO UNDERSTAND PATTERN

Within any particular landscape, patch structure and dynamics likely will differ among habitat types because of environmental differences influencing the intensity and outcome of biological interactions. We compared the dynamics of natural and experimentally constructed mussel patches (Mytilus trossulus and M. edulis) in two intertidal habitats, tidepools and emergent rock, over a time series (5, 10, 15 mo) and among seasons (three successive 5-mo intervals). In tidepools, mussels naturally occurred in small patches (median <25 cm2). In contrast, mussels on emergent rock formed extensive beds with decimeter-scale gaps, but these beds start as small patches following disturbance such as ice scour. For our experimental patches (15 cm2) in both habitats, we assessed the relative importance of physical (wave disturbance) and biological (predation, growth, recruitment, and immigration) processes in determining patch size and structure. Although individual experimental and natural patches varied greatly in size...

Patch Dynamics of Mussels on Rocky Shores: Integrating Process to Understand Pattern

Within any particular landscape, patch structure and dynamics likely will differ among habitat types because of environmental differences influencing the intensity and outcome of biological interactions. We compared the dynamics of natural and experimentally constructed mussel patches (Mytilus trossulus and M. edulis) in two intertidal habitats, tidepools and emergent rock, over a time series (5, 10, 15 mo) and among seasons (three successive 5-mo intervals). In tidepools, mussels naturally occurred in small patches (median <25 cm2). In contrast, mussels on emergent rock formed extensive beds with decimeter-scale gaps, but these beds start as small patches following disturbance such as ice scour. For our experimental patches (15 cm2) in both habitats, we assessed the relative importance of physical (wave disturbance) and biological (predation, growth, recruitment, and immigration) processes in determining patch size and structure. Although individual experimental and natural patches varied greatly in size over time, mean patch area remained relatively constant. In general, mean size of individuals in experimental patches decreased due to the loss of larger mussels while numbers increased due to recruitment. Wave disturbance, which removes and redistributes mussels, appeared to be more important than predation (by the whelk Nucella lapillus) in determining patch structure and dynamics on this shore, although losses due to either process did not differ consistently between habitats over time. Individual growth rates were low (shell length ≤0.4 mm/mo) but greater in tidepools than on emergent rock, whereas recruitment and immigration rates generally did not differ between habitats. Although each process contributed to changes in patch size and structure, collectively they did not result in a marked divergence in mean patch area or biomass between tidepools and emergent rock over the 15-mo experiment. We suggest that the broad patterns of distribution and abundance of mussels develop slowly on this shore largely because of low individual growth rates. Over time scales of years, however, positive relationships between individual growth and recruitment rates and patch size may accelerate the expansion of aggregations on emergent rock. Mussel assemblages at our site differ from those examined in many other studies in terms of the relative importance of wave dislodgment and predation as causes of loss and in the role of growth in offsetting these losses. This study shows how integrative approaches, with monitoring of patches and individuals within patches, can provide detailed mechanistic understanding of patch structure and dynamics.

Description and numerical model of Pleistocene iceberg scours and ice‐keel turbated facies at Toronto, Canada

Near Toronto, in Southern Ontario, Canada, glaciolacustrine muds and shoreface sands of Late Pleistocene age (Early Wisconsin glaciation; ≈45 ka) were deformed by icebergs ploughing the floor of an ancestral ice‐dammed Lake Ontario. Outcrop mapping and a ground‐penetrating radar survey identify ice scour structures up to 4 m deep, up to 10 m wide and at least several hundred metres long with high‐standing lateral margins (berms). Ploughing of lake‐floor sediment is also recorded by ice‐keel turbated facies in which storm‐deposited shoreface sands have been mechanically mixed with underlying muds. Palaeoenvironmental information for one prominent scour, such as water depths, estimates of the mass and draft of the ice mass and scouring forces, can be determined using a simple finite element numerical model. A mass of 0·04 million tonnes, a keel draft of between 10 and 30 m and a scouring force of 5 × 103 kN is consistent with scouring by a small iceberg (‘bergy bit’) or pressure‐ridged lake ice; the former is preferred because of the considerable depth of the scour and the overall glacial depositional setting. Estimates of ice‐mass dimensions agree with independent reconstructions of palaeobathymetry (no greater than 20 m) derived from storm‐deposited hummocky and swaley cross‐stratified facies that fill the scour. Data presented here aid the identification of ice‐scour structures and turbated sediment in the ancient record; despite being a common process on modern high‐latitude shelves, descriptions of ancient structures and facies are few.

Natural Recover y and Restoration Potential of Severely Disturbed Talus Vegetation at Niagara Falls: Assessment Using a Reference System

AbstractThe talus slope adjacent to the Niagara River at Niagara Falls (Ontario, Canada) has been severely disturbed over the course of the past century as the area has been increasingly developed for tourism. In addition the lower talus slope is disturbed by periodic ice scour, most recently one year prior to this study. Historical evidence suggests that the original vegetation was similar to that currently found on talus slopes of the Niagara Escarpment, which could, therefore, be used as a reference system and restoration target. The current community structure and physical environment were sampled in 75 randomly placed 1‐×‐1–m quadrats. A total of 137 species were found, of which 62%were aliens. ANOVA showed that species richness, woody debris cover, litter cover, soil depth, and photosynthetically active radiation changed along a gradient perpendicular to the river. Three different sections of the study area whose last severe disturbance dated back 100, 30, and 4 years, respectively, were similar in their physical environment but were significantly different in species richness. Ordinations of species frequencies showed that light levels and distance from the river were the principal gradients controlling the vegetation structure. The species composition of the section last disturbed 100 years ago was different from that of the more recently disturbed sections. Ordination and cluster analysis of the pooled data showed large differences between the Niagara Falls site and reference escarpment talus slopes. Natural escarpment vegetation was much more homogeneous and had more species and fewer aliens. By comparing reference talus with sections of the study area at Niagara Falls that were last disturbed at different times, we conclude that the trajectory of natural succession at Niagara Falls is leading to an alternative state, an urban forest dominated by aliens, and that active restoration will be required to return the talus to its original state.

Competition, sub-lethal mortality and diversity on Southern Ocean coastal rock communities

Overgrowth competition, sub-lethal mortality (some zooids killed but colony survives) and subsequent growth from fragments, and diversity of communities encrusting rocks (pebble through to boulder size) were examined from five Antarctic localities, along a latitudinal gradient. There were distinct gradients in the ecology of both assemblages and individual species with latitude within the Southern Ocean. Compared with warm-water equivalents, the polar assemblages had many fewer species, considerably less variability in species richness, highly transitive interactions and lower incidences of interspecific encounters. There is no gradual transition but a dramatic alteration of assemblage and species level ecology around the margins of the Southern Ocean, which may primarily be due to ice scour. Sub-lethal mortality was common and peaked on different boulder sizes along the Antarctic Peninsula. This is indicative of the disturbance cline along the region.

SCALE-DEPENDENT RECRUITMENT AND DIVERGENCE OF INTERTIDAL COMMUNITIES

Sheltered rocky bays in the Gulf of Maine support communities usually dominated either by the fucoid rockweed Ascophyllum nodosum or the mussel Mytilus edulis. In order to test whether clearing size determines the subsequent pattern of recruitment by intertidal organisms, we examined four common species in clearings in stands of A. nodosum. Circular experimental clearings of 1, 2, 4, and 8 m in diameter, which mimicked severe ice scour, and uncleared controls were established at 12 sites spread across four bays. Recruitment of A. nodosum and of three species not common in A. nodosum stands (mussels, barnacles, and the rockweed Fucus vesiculosus) varied with clearing size and the patterns did not vary among bays. Recruitment of A. nodosum declined exponentially with clearing size. Fucus and barnacles recruited in greater densities to large clearings (≥4 m) compared to small clearings (<4 m). Recruitment of F. vesiculosus was also site specific, reflecting patterns of local abundance, but that of A. nodosum was not. Mussels recruited onto artificial substrates in fewer numbers in large clearings, but in marginally greater numbers to rock underneath these substrates in large clearings. Recruitment densities of mussels and barnacles were strongly site specific. These results suggest that (1) the rate of succession varies site specifically, and (2) the trajectory of succession, either convergence toward or divergence from the surrounding community, is contingent upon the interplay between species interactions and the size of the clearing.