Edward W. Kempema

Anchor-Ice Formation and Ice Rafting in Southwestern Lake Michigan, U.S.A.

ABSTRACT Anchor ice, i.e., ice attached to the bed in a lake, stream, or ocean, is common in the nearshore zone of southwestern Lake Michigan during winter. Lacustrine anchor ice has at least four distinct morphologies and was observed on sand, pebble, and boulder substrates in water depths to 4 m, the limit of diving traverses. The maximum depth of anchor ice formation may be much greater. Anchor ice is released from the lake bed on mornings following formation events. Released, floating anchor ice carries sediment to the water surface. This sediment is ice-rafted along shore and offshore under the influence of prevailing winds. We estimate that 0.85 m3 of sand per meter of beach is being removed from the nearshore zone of southwestern Lake Michigan by anchor ice annually. Melting ice drops this sand in deep water far from shore. This is a significant loss of sand from the sediment-starved nearshore zone of southwestern Lake Michigan.

Dynamic Ice-Wallow Relief of Northern Alaska's Nearshore

ABSTRACT Contour maps with 0.5-m depth interval were prepared for a small area seaward of Reindeer Island, a barrier island in the Beaufort Sea, Alaska, by repeated surveys with very accurate navigation and very close trackline spacing. The maps reveal numerous closed depressions and mounds about 50 to 100 m in diameter and 2 to 3 m in relief, presumably related to grounded ice floes common in the area year round. Some of the features were obliterated over the course of three seasons while new ones formed. Although the depressions resemble kettles, they are formed by very different mechanisms. We believe that these bedforms represent erosion and deposition caused by: a) intensified flow around stationary ice floes serving as obstacles and b) pulsating currents generated by vertical oscillation or rocking motions of grounded floes in a seaway. Because sediment transport occurs around the ice, not where it directly touches the sea floor, the depressions are much larger than the base of the acting floes. Ice-wallow bedforms, although not found everywhere, are characteristic of arctic nearshore regions with non-cohesive sediments, and most likely occur in other ice-stressed coastal environments in differing degrees. The bedforms studied here are highly active and must be considered in planning nearshore construction activities.

Anchor ice in polar oceans

One feature of high-latitude areas is the formation of ice clusters attached to the beds of rivers, lakes and the sea. This anchor ice, as it is widely known, plays an important role in mobilizing bed sediments, as well as ecological roles as a food source, habitat and potentially fatal environment. Much work has been devoted to fluvial anchor ice in the Northern Hemisphere, yet comparatively little work has described anchor ice in polar marine environments, despite its description by Antarctic expedition scientists over a century ago. In this paper, we review the current understanding of anchor ice formation in polar marine environments. Supercooled water is a necessity for anchor ice to form and frazil adhesion is the most likely common mechanism for initial anchor ice growth. Strong biological zonation has led some authors to suggest that anchor ice does not form to depths of greater than 33 m, yet in Antarctica there appear to be no physical reasons for such a limit given the production of supercooled water to substantial depths associated with ice shelves. Future work should focus on the potential extent of anchor ice production and identify the key oceanographic, glaciological and meteorological conditions conducive to its formation.

Effects of advancing freeze fronts on distributions of fine-grained sediment particles in seawater- and freshwater-slush ice slurries

ABSTRACT Experiments were performed to evaluate the effects of advancing freeze fronts on movements of fine-grained sediment particles in seawater- and freshwater-slush ice slurries. In the freshwater experiment, the horizontal migrations of clay-sized particles in response to a laterally advancing freeze front were documented photographically. Differential migration of particles as a function of their size was also noted. In the seawater-slush ice experiment involving downward movement of a freeze front through a sediment-laden slush ice layer, time-series measurements were taken for 1) accumulations of sediment released from the ice into the underlying water column, 2) decreases in temperature in the ice matrix, and 3) increases in salinity in the water beneath the ice. Results of the latter experiment support the contention that a downward congealing slush ice matrix can purge itself of at least a portion of an entrained sediment load during an evolving freezing process. However, most of the sediment load initially carried by the slush ice remained at the conclusion of the experiment. This observation has implications for the importance of spatial transport and deposition of ice-rafted sediments to overall sediment budgets in regions affected by sea ice. In summary, migration of sediment particles in congealing slush ice matrices due to freeze front advances can be important to geological, physical and biological processes as well as commercial activities in arctic marine and nearshore regions.

The role of Langmuir circulation in suspension freezing

Abstract In November 2004 we used an acoustic Doppler velocimeter (ADV) to measure the three components of velocity in Langmuir circulation (LC) cells when frazil was forming in a lake. LC circulation was indicated by windrows of slush ice on the water surface. We also collected samples of lake water, ice interstitial water and ice to determine their sediment concentrations. The ADV record showed rotating currents in the cross-wind/vertical plane indicative of LC. Downward-directed velocities were large enough to entrain frazil into the water column for 27% of the 97 min observation time at the ADV location. Sediment concentrations in the ice and interstitial water samples were greater than concentrations in water-column samples collected in upwelling zones of LC cells. We conclude that suspension freezing was forming particle-laden ice as frazil trapped below LC convergence zones grew in downwelled, supercooled water. The rotating roll vortices of Langmuir circulation play an important role in forming sediment-laden ice.

Geotechnical considerations in hydraulic modeling of bridge abutment scour

Scour-related failure of earthen abutments in bridge waterways involves interacting geotechnical and hydraulic processes. Current bridge design guides, however, inadequately account for geotechnical factors influencing scour depth. Our paper presents results of laboratory experiments investigating how soil strength affects scour depth at spill-through abutments. We focus on several difficulties faced when conducting flume experiments on scour at abutments formed of erodible soil. The difficulties include attaining scale-reduced shear strengths, controlling soil compaction, and quantifying the shear strength of model soil. We indicate the similitude considerations involved, and describe a process of soil tests relating model soil strength to soil compaction that uses hand-held devices to determine soil strengths. The correlations developed between soil compaction and shear strength helped to work around some of the difficulties in flume experiments. We suggest practical solutions to issues pertaining to simulating the shear strength of compacted soils used in flume studies of abutment scour.

Failure of Spill-Through Bridge Abutments during Scour: Flume and Field Observations

AbstractThis paper presents early findings from laboratory tests and field observations on the failure of spill-through abutments subject to abutment scour. These findings show that geotechnical and hydraulic processes interact to erode embankment soil during abutment scour, producing lesser scour depths than predicted using leading abutment scour equations. A major failure location is the flow waterline beginning at an abutment’s upstream corner where soil is exposed to the highest values of flow velocity and turbulence. Undercutting and toppling of soil blocks occurs sequentially along the face of the spill slope, eroding it back and eventually exposing the abutment column. Further erosion then may breach the embankment. The laboratory findings, based on uniform sand compacted to varying densities and thereby shear strengths, show that soil strength influences scour depth.

Fish, Ice, and Wedge-Wire Screen Water Intakes

AbstractThe U.S. Clean Water Act requires that the Environmental Protection Agency ensures that the location, design, construction, and capacity of water intakes for thermal power plants reflect the best technology available to reduce entrainment mortalities of fish and invertebrates during their early stages of life. Cylindrical wedge-wire screens are an efficient technology for protecting fish and invertebrates at water intakes, as they draw water at low flow velocities through a fine mesh. Although substantially reducing fish and invertebrate mortality, wedge-wire screens run significant risks of blockage by ice. This paper discusses how the characteristics of wedge-wire screens that enhance their ability to reduce mortalities also make them susceptible to blockage by frazil and anchor ice, and presents observations and field data regarding ice formation on a segment of wedge-wire screen placed in a small river. Ice formation on this screen involved initial frazil adhesion with subsequent metamorphism ...

Winter discharge of coalbed methane water in the Powder River Basin, Wyoming

ABSTRACT The heat transported with coalbed methane (CBM) water is shown to influence the thermal, flow, and ice characteristics of the Powder River, the principal river draining the Powder River Basin, Wyoming, during winter. The writers suggest how management of CBM water discharge from numerous tributary streams will minimize potential adverse influences on river channel stability and ecology. Their study, conducted over two winters (2009–2010 and 2010–2011), indicates that besides augmenting the rivers natural flow, a substantial winter influence of CBM water discharge was the formation of open-water leads typically extending up to several kilometres along the river. The leads result from warm coherent plumes of CBM water discharge that gradually cool and dissipate when exposed to frigid air. Lead formation alters flow thalweg alignment, concentrating flow along the lead, and plays a potentially significant role for biota along the river. The leads were observed to attract fish, birds, and other wildlife during winter.