J. Great Lakes Res. 20(1):179-195Internat. Assoc. Great Lakes Res., 1994

The Influence of Ice on Southern Lake Michigan Coastal Erosion

Peter W. Barnes

U.S. Geological Survey, MS 999345 Middlefield Road

Menlo Park, California 94025

Edward W. Kempema

School ojOceanography, WB-I0University ojWashington

Seattle, Washington 98195

Erk Reimnitz and Michael McCormick

U.S. Geological Survey, MS 999345 Middlefield Road

Menlo Park, California 94025

ABSTRACT. Coastal ice does not protect the coast but enhances erosion by displacing severe winterwave energy from the beach to the shoreface and by entraining and transporting sediment alongshoreand offshore. Three aspects of winter ice in Lake Michigan were studied over a 3-year period and foundto have an important influence on coastal sediment dynamics and the coastal sediment budget: (1) theinfluence of coastal ice on shoreface morphology, (2) the transport of littoral sediments by ice, and (3)the formation of anchor and underwater ice as a frequent and important event entraining and transport­ing sediment. Coastal lake ice includes a belt of mobile brash (ice blocks) and slush and a dynamicnearshore ice complex consisting of an icefoot, a lakeward sequence of wave-generated ice ridges, andintervening ice lagoons. Our studies indicate that the nearshore ice complex contains a sediment load(0.2 - 1.2 tim of coast) that is roughly equivalent to the average amount of sand eroded from the coastalbluffs and to the amount sand ice-rafted offshore to the deep lake basin each year. Up to 0.28 tim of coastcan be entrained by ice in a single anchor-ice event, and separate events occurred on 15 days in January1991. The brash/slush belt is the most important system component responsible for ice-induced sedimenttransport. Estimates of longshore ice drift, ice volume, and ice-borne sediment load suggest that 0.36 to4.14 x ]03 tid are transported alongshore.

INDEX WORDS: Ice rafting, beach profiles, sediment budget, icefoot.

INTRODUCTION

The Lake Michigan ice cover develops fromnorth to south (Assel et al. 1983, Bolsenga 1988),and from the more rapidly cooling shallow edges todeeper parts of the lake (Saulesleja 1986). Ice ispresent along the coast of southern Lake Michigan(Fig. I) for 2 to 4 months each winter (Assel et al.1983), initially forming in December and persistingintermittently until March. The central part of thelake is generally ice free throughout the winter (Fig.1). The period of ice cover corresponds to periods

179

of high wave energy from winter storms (Saulesleja1986); the coast is thus exceptionally vulnerable tochange and to the influence of ice.

Prevailing northerly and westerly winter windsdrive the ice east and south across the lake where itaccumulates against the coast. On the southernperimeter of the lake, where wind and surface cur­rent patterns concentrate ice along the shore, icemay extend as far as 24 km from the coast(Reimnitz et al. 1991). Storm winds are signifi­cantly stronger in the winter than they are duringthe summer and, at the southern end of the lake,

180 Barnes et al.

WAVE CLIMATEDominant directions

NW N S S S NW

6 12Month

o

Lake

Michigan.RierSt.(tJM"~).. ·< ·.M:I

•.<...< ""___-""""Il.;:::;} .....................•.....·.·n.

··WestBEJC1PO(VVI3A..1)

~ :E20 >3

'WI..:::tiIiiIi, :........"__ •

IL·

••.. . .••

I 100 km I ••••••••••••...

••••....

TENTHS ICE COVER ANDWATER TEMPERATURE

(February)o Open waterm 1-6110ths_ 7-9/10ths-2- Mean temp. (OC)

o

FIG. 1. Index maps showing study sites and location names along with ice and wave climates in southernLake Michigan (modified/rom Saulesleja 1986).

Coastal Erosion by Ice 181

storm winds normally have velocities of 22 to 35m/s which cause significant wave heights of 1 to 2m and extreme heights of more than 3 m (Fig. 1)(Saulesleja 1986, Wood et ai. 1988). The strongestwinds and largest storm waves come from thenorthern quadrant and result in net southerly littoraltransport (Allender 1977). This trend is shown bycoastal landforms that indicate net southerly littoraldrift along both the western and the eastern coastsof the lake (Hough 1935, Chrzastowski 1990).

Recent literature on Great Lakes coastal ice islargely qualitative and has focused on its role inbeach protection (O'Hara and Ayers 1972; Dionneand Laverdiere 1972; Marsh et ai. 1973, 1976;Davis 1973; Davis et ai. 1976; Evenson and Cohn1979), and erosion (Bajorunas and Duane 1967,Miner and Powell 1991) and on ice morphology inrelation to lakebed morphology (Davis 1973, Seibel1986).

Fieldwork and laboratory work carried out from1989 to 1992, have been partially reported by Mc­Cormick et ai. (1990, 1991). Field observations in­cluded wading and diving traverses and videoobservations using a tethered, remotely operated ve­hicle. Descriptions of early phases of these studiesand the methods used have been reported byReimnitz et ai. (1991), Barnes et ai. (1992a, b),Kempema and Reimnitz (1991), and Kempema etai. (1992a). The geologic and sedimentologic settingof the coastal environment is discussed elsewhere inthis volume (Chrzastowski et ai., Shabica and Pran­schke, Foster and Folger, and Colman et al.).

This paper reviews ice zonation and its influenceon coastal morphology, discusses the processes thatentrain sediment into coastal ice, and providesquantitative information on sediment content of icein southern Lake Michigan. These observations areused to discuss the role of ice in removing andtransporting sediment from the coast.

TYPES OF LAKE ICE

A number of classification schemes (of increas­ingly complex detail) can be used to describe lacus­trine ice types. In the broadest sense, ice can bedivided into two categories on the basis of how itformed (Foulds and Wigle 1977): (1) static iceforms on the surface of small lakes and ponds, and(2) dynamic ice forms in running water or on thesurface of large bodies of water agitated by winds.Static ice is relatively unimportant as a contributorto coastal erosion and sediment transport because itforms in calm conditions and contains very little

sediment. Also, as Lake Michigan does not com­pletely freeze over during the winter, large areas ofopen water combined with winter storms result inpredominately dynamic ice formation in the lake.

Dynamic ice in the Great Lakes can be dividedinto three subcategories: (1) a mobile band of brashand slush, (2) the nearshore ice complex, and (3)anchor ice. The development and morphology ofice in these three subcategories is described fromthe literature and our own field observations.

Mobile Brash/Slush Zone

A common feature of the winter coastal zone is ahighly mobile belt of unconsolidated ice. This beltis made up of a combination of brash (solidlyfrozen fragments of ice less than 2 m in diameter)and slush (a water-saturated, internally mobile ac­cumulation of ice). Brash is formed by mechanicaldestruction of other ice types, most notably portionsof the nearshore ice complex. When a large blockof brash is entrained into the mobile ice zone, it iscommonly broken down by wave action and byabrasion against other pieces of ice. In the extremecase, a piece of brash can be broken down into afield of slush-sized particles. In addition to abradedbrash, this zone can contain snow, frazil crystals(ice crystals that form in turbulent, supercooledwater), and remnants of released anchor ice.

The brash/slush zone is unconsolidated and mo­bile, so the zone tends to be highly variable in timeand space. When the brash/slush zone is present, itswidth can vary from a few meters up to 17 km(O'Hara and Ayers 1972) or more. The zone tendsto thin lakeward but has been observed to be incontact with the lakebed at depths of a meter and issuspected of being in contact at twice that depth.The brash/slush zone reacts quickly to changes inwind and incident wave directions. Often, a band ofbrash/slush will be advected onshore from over thehorizon shortly after a change to an onshore wind.Conversely, a change to offshore wind can advectcoastal brash/slush out to the center of the lake. Theice found in the brash/slush zone is extremely im­portant because it is the building material for othertypes of dynamic ice.

The Nearshore Ice Complex

In this report, the nearshore ice complex (NIC) isa zone of relatively static, solidly frozen ice foundin the coastal zone. Barnes et ai. (1992a) have re­viewed the distinctive morphology of this complex

182 Barnes et aI.

based on literature and field observations. The mor­phology usually consists of an icefoot and a lake­ward sequence of ice ridges and ice volcanoesseparated by intervening shore-parallel ice lagoons(Fig. 2). Formation of a complete NIC requires acritical combination of events (Bryan and Marcus1972), including (1) freezing air and water tempera­tures, (2) large bodies of open water, (3) onshorewind and storm waves, and, we add, (4) a supply ofbrash/slush. All of these conditions are usually metrepeatedly during the winter months in southernLake Michigan (O'Hara and Ayers 1972, Miner andPowell 1991). The associated processes of sedimententrainment into ice, rafting, and profile adjustmentoccur to varying degrees each time these conditionsare met.

A frozen beach is the first coastal ice feature toform, usually a week or more before lake-watertemperatures drop to freezing. Thin slabs of ice-ce­mented sand are dislodged by undercutting and dis­tributed along the beach at the shoreline (O'Haraand Ayers 1972, Davis 1973). When coastal lakewaters reach the freezing point, slush forms and canbe readily driven against the coast to build an ice­foot several meters wide. The icefoot usually ismarked at its outer edge by a vertical scarp a meteror more high (Davis 1973). Once established, theicefoot locks beach-face sediment in place while fo­cusing wave energy at its outer edge. As lake condi­tions change, the icefoot can be destroyed andrebuilt several times throughout the winter (Minerand Powell 1991, Davis 1973).

Continued sub-freezing temperatures, ice growth,and onshore winds will cause a thick, wide mass ofmobile brash/slush to accumulate. For a given set ofwave conditions, an optimum mobile ice-bandthickness and width will absorb all incident waveenergy, and the inner edge of the mobil ice band nolonger moves, freezing into a solid mass. As newbrash/slush is added to the outer edge of the mobileice band, the inner edge of the band is cut off fromthe wave action and a zone of congealed ice thathas low-relief undulations that are roughly parallelto the beach. We call this zone of ice the ice lagoon(Figs. 2, 3).

Normally, a grounded ice ridge forms at the 1ake­ward edge of the ice lagoon where wave energy ishigh, such as the breakpoint, which is often locatednear offshore bars. Ice ridges also form when in­creased wave energy results in overwash of theouter edge of the ice lagoon and the subsequent pil­ing of spray, slush, brash, and sediment into a ridge.Ice ridges are shore-parallel features often com-

posed of coalesced ice volcanoes (Figs. 2 and 3;also see Seibel et ai. 1976, Fahnestock et ai. 1973,and Dozier et ai. 1976). Ice ridges commonly rise 1to 2 m above water level, but ridge thicknesses ofup to 11 m, and heights up to 7 m above water levelhave been reported (Marsh et ai. 1973). The aver­age widths of the NIC is about 25 m but reached 50m at Gillson Beach (Fig. 1). Ridges are more or lessfrozen together, having porosities that range from22 to 57 percent (Miner and Powell 1991). Sub­merged parts are less indurated than subaerial por­tions, except for random included blocks of brash(O'Hara and Ayers 1972). Multiple ridges often de­velop, resulting in alternating bands of ice lagoonsand ridges in the NIC (Fig. 2). The degree of NICdevelopment varies from location to location. Thebroadest and highest NIC's form off sand beachesexposed to waves (Barnes et ai. 1992a). Where theshoreline is protected by engineered structures(revetments, seawalls, and timber or sheetpile bulk­heads), the NIC is poorly developed or absent.

Although the NIC is relatively stable, it changesas weather conditions and sea states vary. The NICis destroyed by wave action or by in situ melting.Often the decay process is the reverse of the growthprocess (O'Hara and Ayers 1972, Marsh et ai. 1973,Davis 1973, Evenson and Cohn 1979). Our obser­vations indicate that wave-induced NIC erosion oc­curs even when air temperatures are subfreezing if aprotecting mobile brash/slush zone is narrow or ab­sent. In these conditions, breakup of the NIC re­leases brash to form the mobile brash/slush zone.At one site the outer edge of the NIC was erodedshoreward by building a ridge that overstepped on­shore ice features. Miner and Powell (1991) esti­mated that over 90 percent of the NIC formedduring a single winter is destroyed by wave actionrather than by melting. All of this broken-up ice isreintroduced into the mobile brash/slush zone.

Anchor Ice

The most poorly understood form of dynamic icein southern Lake Michigan is anchor ice. Anchorice is ice that is attached or anchored to the bottom(WMO 1970). Most observations of anchor icecome from fluvial and marine settings, and rela­tively little is known about the extent of anchor-iceformation in lakes. However, anchor ice is knownto occur in shallow areas throughout the GreatLakes, mainly because of the problems it causes byplugging municipal and industrial water intakes to adepth of at least 7 m (City of Chicago Water De-

Coastal Erosion by Ice 183

FIG. 2. Diagram of extensively developed nearshore ice complex characterized by an icefoot, multiple lagoons andridges, ice volcanoes, and a zone ofattached ice and drifting brash and slush. Drawing by Tau Rho Alpha (U.S. Geo­logical Survey).

184 Barnes et al.

FIG. 3. Photograph of inshore part of a nearshore ice complex at GillsonBeach, Illinois, showing the sediment rich icefoot in the foreground, and an icelagoon inshore of an ice ridge 1.5 to 2 m high lakeward. Immediately offshorefrom the ice ridge is a brash/slush zone. Further offshore are patches andstreamers of ice. Survey panels in the foreground and on the icefoot are 40 cmon a side (0809 hrs., 26 January 1991).

partment). Anchor-ice formation to a depth of 15 mhas been reported in Lake Ontario (Foulds andWigle 1977).

Observations from fluvial and marine environ­ments show that anchor-ice formation is intimatelyrelated to frazil formation (Tsang 1982, Arden andWigle 1972, Benson and Osterkamp 1974, Os­terkamp and Gosink 1982, Reimnitz et ai. 1987,Kempema et ai. 1989). Frazi1 suspended in super­cooled water tends to be "active" (Carstens 1966),adhering to one another and to material suspendedin the flow and resting on the bottom. Anchor iceforms in rivers when frazil crystals suspended insupercooled, turbulent water strike and adhere tothe bottom. Once a frazil crystal adheres to the bot­tom, anchor-ice masses can grow rapidly by fraziladhesion and ice-crystal growth (Tsang 1982).

Our first observations of nearshore anchor ice inthe Great Lakes were made within a few meters ofan eroded NIC in water 75 cm deep, where sedi­ment-laden ice masses collapsed when disturbed by

divers. When the centimeter-sized platelets about 1mm thick were disturbed, the anchor ice (alongwith entrained sediment) rose slowly to the surfacein clumps 10 to 20 cm in diameter (e.g., Fig. 4). Atemperature profile through the water column, theanchor ice, and into the underlying lakebed at thislocation (Fig. 5) shows that only the anchor-icemass and the sediment 1 to 2 cm below were belowfreezing. This suggests a lakebed surface heat sink,perhaps relict from a time when cold ice wasgrounded in the area. In fact, the majority of the an­chor ice was seen adjacent to an eroding NIC. How­ever, conduction of heat from the lakebed through agrounded ice ridge to the atmospheric heat sink isslowed by the presence and circulation of above­freezing lake water in the subaqueous portions ofporous ice ridges. Thus, anchor ice resulting fromgrounded ice ridges is probably rare.

Additional anchor-ice observations were col­lected at Gillson Beach (Fig. 1). Evidence of anchorice formation was observed on 15 of the 34 days of

Coastal Erosion by Ice 185

FIG. 5. Temperature profile of water column andlakebed during the presence of anchor ice at Kohler­Andrae State Park, Wisconsin, 21 February 1990.

MODIFICATION OF COASTALPROFILE BY ICE

Although acknowledging that wave energy wasdisplaced by the NIC, neither Marsh et ai. (1973)nor Evenson and Cohn (1979) reported significanterosion associated with the lakeward edge of theNIC. As a result, they attributed a protective role tothe NIC. However, earlier studies by Zumberge andWilson (1953) suggested that the ice cover, al­though "mantling the beach," affects the coastalprofile offshore. Bajorunas and Duane (1967),Nielsen (1988), and our own studies using repeti­tive surveys document winter erosion lakeward ofthe NIC in water as deep as 6 m. They attribute theerosion to wave energy expended lakeward of thebeach against grounded ice ridges (Fig. 6). The re­peated and systematic offshore development of suc­cessive NIC ice ridges and associated lakewarddisplacement of wave energy may relate to the win-

fieldwork. This evidence consisted of direct under­water observations of anchor ice on the lakebed andon instruments mounted within I m of the lakebed,and on observations of anchor ice rising to the lakesurface on mornings following night-time, anchor­ice formation events. On mornings following for­mation, anchor ice was sporadically released fromthe bottom over a period of 3 to 5 hours. Anchor icewas observed on the lakebed on morning diving tra­verses in water 0.7 to 4 m deep. This anchor iceconsisted of randomly oriented plates that werefrom I to 40 cm in diameter and a few millimetersthick. These plates were either individually attachedto the lakebed or formed interfingering mats thatcovered as much as several square meters of thebottom. In total, anchor ice covered as much as 50percent of the bottom traversed, including sand,pebble, and boulder substrates. The concentrationof anchor ice was highest in water depths of 0.7 to1.5 m and decreased offshore.

Conditions for this type of anchor ice formationin southern Lake Michigan are similar to those nec­essary for anchor-ice formation in rivers (Wigle1970, Arden and Wigle 1972, Tsang 1982), exceptthat turbulence was exceptionally low in the lakesetting. Anchor ice usually formed on cold, clearnights when air temperatures fell below -6T. Winddirections were usually obliquely offshore, socoastal waters were relatively calm. These condi­tions suggest that the formation of anchor ice inlakes does not require vigorous water-column tur­bulence.

0.50

Anchor ice

Water

Air

a------

~nl

-5.]0

CDoas't:CD-cQ> 30Cti~"0CDeno-

Temperature (CO)-0.50 0.00

60 +-_-"-_--1__....._ ...-Eo'-"

FIG. 4. Photograph of a mass of anchor-ice plateletsand entrained sediment from the lake floor off GillsonBeach, Illinois, 1991.

186 Barnes et al.

FIG. 6. Photograph of a breaking wave that has beenreflected from an ice ridge "sea wall" at Gillson Beach,Illinois (Fig. 1). Note the presence of scattered brashand slush.

ter offshore displacement of bars along the Indianacoast noted by Wood and Weishar (1984).

Beach profiles show several ice and lakebed fea­tures that are associated with the NIC. Lakewardfrom the icefoot, comparisons of shore-normal pro­files (Barnes et ai. 1992a) show that ice ridges areoften associated with an underlying bar, as Seibel etai. (1976) noted. However, the association is notexclusive; commonly there are more ice ridges thanoffshore bars. Our profile data also show that asmall, ubiquitous erosional trough as much as 50cm deep and 2 to 3 m wide consistently developsadjacent to the icefoot and offshore of ice ridges,even with modest «0.5 m) wave action (Barnes etai. 1992a, Hayden et ai. 1992).

Underwater video and direct observations con­firm that a rough bottom often develops owing toice-keel gouging and current scour where ice keelsrest or wallow (Reimnitz and Kempema 1982) onthe lakebed. The roughened lakebed absorbs in­creased wave energy as the profile seeks to reestab­lish equilibrium in the spring. Wood and Weishar(1984) noted the presence of ephemeral nearshorebars at the end of the ice season in northern Indianathat might have been produced by scour underneathand around ice blocks. The development of the ero­sional trough indicates displacement of wave en­ergy along the outer margin of the NIC, whereas aroughened lakebed morphology suggests hydraulicscour and fill associated with grounded and neargrounded keels throughout the NIC.

PROCESSES OF SEDIMENTENTRAINMENT INTO ICE

The interaction between lake ice and sediment re­sults in ice-induced sediment entrainment, trans­port, and coastal erosion. Anchor ice, frazil ice,brash/slush ice, and NIC ice all interact with the hy­draulic regime and with both suspended andlakebed sediments along different sediment entrain­ment pathways (Fig. 7) capturing sediment. Thefollowing narrative, based primarily on our obser­vations and data, is a discussion of these processesand pathways.

Anchor Ice

Anchor ice entrains sediment by adfreezing tosediments on the lakebed. Sediment particles are en­trained when frazil laden turbulent water allows icecrystals to adhere to the bed for a short period oftime. In addition, diving traverses show that anchorice residing on the lakebed entrains sediment thatsettles from the water column or from drifting ice.

The release of sediment-laden ice from thelakebed allows anchor ice to be a major potentialsediment transport agent because of the high sedi­ment loads (Table 1). In fresh water, a neutrallybuoyant anchor-ice mass of 1 L volume would needto contain 122 g of sediment (Kempema et at.1986) to counteract the buoyancy of the ice. In ad­dition to its potential for entraining large quantitiesof sediment, anchor ice is capable of includinglarge clasts (Fahnestock et ai. 1973, Tsang 1982).

Frazil Ice

In Lake Michigan's supercooled, turbulent water,frazil crystals adhere to particles in suspension(Garrison et ai. 1983, Osterkamp and Gosink 1984).As these frazil crystals grow, form flocs, increase inbuoyancy, and rise to the surface, they carry thescavenged sediment to the brash/slush zone. In ad­dition, frazil crystals and flocs strike and roll alongthe lakebed, where they pick up fine-grained sedi­ment, as momentary anchor ice (Tsang 1982) beforebeing advected back into the water column. Frazilice probably entrains the most sediment into thebrash/slush zone.

Brash/Slush Ice

Brash and slush ice in the mobile ice zone formthe highway where most sediment is transportednearshore. Formed from primarily eroded bits of the

Coastal Erosion by Ice 187

SUSPENDEDSEDIMENT

IEOLIAN IIIII.I,,

::.t,#J-- ......, .., ...

I \, \

: ANCHOR i, ICE '\ ,

\ I... ,',,-----,'

FIG. 7. Conceptual model of sediment entrainment processes and sediment movement among the differenttypes ofcoastal ice.

TABLE 1. Sediment concentrations (in grams per liter) in ice from various ice types andlocations.

Miscellaneous SamplesOffshore ice 0.2 0.6Anchor ice 45.0 36.1Lake water 0.03 0.1Dirty nearshore brash 445.0 130.0 59.9

1989 1990 1991

Average concentration

Nearshore Ice Complex (NIC)39.2 53.5 34.010.7 1.8 9.011.2 15.8 13.121.3 19.5 14.8

5.8

3-yearAverage Range

40.6 0.4-87.67.6 0.1-31.2

12.9 1.0-44.017.7 0.1-87.65.8 0.3-30.3

13.1 0.01-97.0

0.4 0.003-5.737.5 0.8-410.7

0.1 0.01-0.48196.8 3.1-865.8

4.1Brash/Slush

22.110.8

IcefootLagoonsRidgesWeighted averagePrevious studies1

Brash/slush ice

Location

lEvenson and Cohn (1979) and Miner and Powell (1991).

188 Barnes et aI.

NIC, and frazil and anchor ice, brash/slush ice arethe source of both ice and sediment that becomepart of the NIC, and the initial conduit throughwhich an eroding NIC delivers brash ice to along­shore and offshore rafting.

NIC Ice

Sediment can be entrained into the NIC by waveaction, eolian transport, and basal adfreezing (Fig.7) (Evenson and Cohn 1979). Miner and Powell(1991) evaluated the relative importance of theseprocesses as sediment incorporation mechanismsand saw evidence of basal freezing only once dur­ing the course of a year-long study. They concludedthat wave-induced suspension and overwash werethe most important processes incorporating sedi­ment into the NIC. Our studies show that sediment­rich ice must be present in the wave train forsuspension and overwash to cause significant sedi­ment enrichment. When both these conditionsoccur, sediment concentrations increase in ice-ridgesamples (Barnes et ai. 1992a). A processes wheresediment is filtered out as the pumping action ofwaves carries sediment-laden water into and out ofthe stationary NIC (Ackermann et ai. 1990) doesnot appear to be important in Lake Michigan NIC(Barnes et ai. 1992a).

Sediment Movement Among Ice Types

This discussion so far has focused on the ways inwhich sediment is directly incorporated into thevarious ice types. However, once sediment-ladenice forms, it can change its state to another ice typealong predictable pathways (Fig. 7). Frazil ice be­comes slush when turbulence decreases and crystalsrise to the surface. Frazil may also impact the bedand form anchor ice, which in turn can be released(most likely along with a heavy load of sediment)and incorporated into the mobile brash/slush zone.Ice may move from mobile brash/slush to the sta­tionary NIC several times during the winter as im­portant building materials for the NrC, whilewave-induced NIC breakup recycles NIC ice backinto the brash/slush zone. Basal freezing of the NrCleading to a frozen substrate and possible anchor­ice formation (Fig. 7) is a less significant change ofstate.

As ice changes from one state to another (i.e.,brash/slush to NIC), it carries its entrained sedimentalong with it. Thus, in addition to directly incorpo­rating the sediment types discussed above, ice in-

herits sediment from its precursor. These pathwaysare probably as important as the direct methods ofincorporating sediment into ice types. For anchorice and NIC ice to contribute to sediment transport,they must be incorporated into the zone of mobilebrash and slush. Therefore, the brash/slush zone isthe system component that actually leads to ice-in­duced sediment transport.

SEDIMENT CONCENTRATIONSAND VOLUMES

Many previous studies (Bryan and Marcus 1972;Marsh et ai. 1973, 1976; Evenson and Cohn 1979;Fahnestock et ai. 1973; Miner and Powell 1991)noted that sediment is common in the NIC. We alsofind sediment is ubiquitous in brash/slush and inanchor ice. Lake-water samples contained relativelylittle sediment «0.1 gIL). The sediment was pri­marily well-sorted, medium-grained (0.25 mm)sand, similar to the sediment in the underlyinglakebed (Barnes et ai. 1992a). However, sedimentconcentrations (grams per liter of melted ice) insouthern Lake Michigan are highly variable (Table1). This variability needs to be discussed before thepotential for erosion transport along with a sedi­ment budget are considered.

Vigorous sediment movement in the swash zoneleads to high sediment concentrations in the icefoot.Lower wave energies in ice lagoons lead to lowersediment concentrations (Table 1), while intensifi­cation of wave energy on offshore bars favors in­creased sediment resuspension and sedimentenrichment of ice ridges (Table 1). In addition, re­peated overwash of water and ice containing sedi­ment also concentrates sediment in ridges (O'Haraand Ayers 1972, Barnes et al. 1992a). Sediment isconcentrated as a 1- to 2-cm-thick layer underlainby sediment-poor ice marked a mottled band of sed­iment-rich ice at the base of the ice ridge. Experi­ments showed that sediment concentrations at thesurface doubled during an hour of active overwash(Barnes et ai. 1992a).

Sediment-laden brash/slush originating from ero­sion of NIC's, from frazil ice formation, and fromanchor ice (Fig. 7) forms a low-lying, often visuallyinconspicuous zone incorporated nearly as muchsediment (13.1 gIL) as was found in the NrC (Table1). Samples of floating anchor ice (Fig. 4) con­tained sediment concentrations ranging from 0.8 to96 gIL, including pebbles up to 7 mm in diameter.

Sediment concentrations in the NIC and adjacentbrash/slush (Table 1) were adjusted for ice density

Coastal Erosion by Ice 189

and ice volume (Barnes et ai. 1992a). Ice-volumeestimates for the different ice types are based onmeasured ice cross-section profiles combined withaerial observations to determine lateral applicabilityof profile data. On average, 87 m3 of ice per meterof coast (Table 2) represents a synoptic picture ofthe coastal ice volume for three different ice years.The static, synoptic view of ice volume variationscontrasts with time-series observations. At GillsonBeach, Miner and Powell (1991) reported NIC icevolumes an order of magnitude higher (548 m3/m)after summing the volume of repeatedly formedNIC's during the winter of 1989. We expect re­gional NIC ice volumes for an entire winter seasonto be higher than our synoptic values (Table 2) butconsiderably lower than the values for the GillsonBeach location, partly because this beach isbounded to the south by a jetty, which tends to im­pound ice.

Miner and Powell (1991) estimated that the an­nual amount of sediment in the NIC at GillsonBeach (Fig. 1) through an entire winter season was5.9 tIm of coast. In contrast, sediment concentra­tions (Table 1), coupled with 3 years of ice-volumeestimates (Table 2), suggest that a regional "aver­age" NIC contains only 0.42 t (0.16 - 0.92 t) andthat the "average" brash/slush zone contains 0.14 t(0.02 - 0.25 t) of sediment per meter of coast. More

than half the NIC sediment is contained in the iceridge and lagoon complex (Table 2), even thoughsediment concentrations are highest in the icefoot.

Offshore, sediment is carried in widely distrib­uted ice patches having high sediment concentra­tions. A series of representative (time/distanceselected) samples, 5 to 10 km off the Illinois coast,contained 0.03 to 2.0 kg of sediment per cubicmeter of ice (Barnes et ai. 1992b). In contrast anon-representative (visually selected) sample con­taining a sediment load an order of magnitudelarger suggested that the sediment load in offshoreice may have been undersampled and underesti­mated. Extrapolating the measured sediment load tothe estimated offshore ice-cover concentration andthickness suggested that 33 kg (0.03 t) of sedimentper meter of coast was being rafted in a 10 km bandof ice along the coast.

The volume of anchor ice trapped under a solidice sheet adjacent to the NIC in 1991 was measuredto give a minimum estimate of the amount of an­chor ice formed and its sediment load during a sin­gle anchor ice event. The determined value was0.28 tim of coast and is similar to the sediment loadmeasured in the NIC. The high value representedby this single event, combined with observations offrequent anchor-ice events over the course of a win-

TABLE 2. Coastal ice volumes and sediment concentrations.

Nearshore ice complex (NIC)

8.5 0.29 42 0.640.189

lcefoot Ridges & Lagoons

Width Thickness Width Thickness(m) (m) (m) (m)

Brash/slush Total

Brash & Slushvolume Sedimentof ice concentration

Width Thickness (m3/m (tim(m) (m) of coast) of coast)

85.1 0.3 55.80.137 0.326

0.18018.08.2 0.3

0.0220.780.27 18.4

0.1594.3

1989 NlC average l

Sediment. content (tim coast)

1990 NlC average1

Sediment content (tim coast)

1.12.52.71.7

1991 NlCGillson Beach, Ill.West Beach, Ind.Pier St., Lakeside, Mich.1991 NlC AverageIce volume (m3/m coast)Ice densitySediment concentration (kg/m3)2Sediment content (tim coast)

8.69.12.07.6

1.00.70.30.9

6.840.82 (3)

34.0

31.378.6

10853.3

90.610.68 (39)

11.80.918

221.3 0.2-200 0.5-260 0.85

224.8 0.489.92

0.68 (5)4.10.251

187.4

1.168

1Averages modified from Barnes et af. (1992)2From Table 1: gIL = kg/m3

Sediment content =ice volume x ice density x sediment concentration.

190 Barnes et at.

ter, suggests that anchor ice can transport a signifi­cant amount of sediment.

RAFTING OF SEDIMENT-LADEN ICEField observations provide the basis for a sce­

nario of ice entrainment and rafting. Sedimenttransport by ice occurs as the NIC is eroded or re­worked during winter and as it disintegrates duringspring-time melting and breakup. Throughout theice season, mobile brash/slush is present nearshorein southern Lake Michigan and, therefore, rafts sed­iment almost continuously. However large or smallthe sediment load, ice rafting does not require con­stant wave energy for sediment resuspension and

can circumvent coastal obstacles at very low driftvelocities. Once entrained, sediment can potentiallycross deep water in long trajectories (Kempema etal. 1992b). As a result, ice rafted sediment de­flected offshore by jetties and promontories(Reimnitz et al. 1991) may easily remove materialsfrom the littoral zone. Ice rafting is fundamentallydifferent from the classic littoral drift, primarily be­cause it is not dependent on wave energy to keepsediment in suspension and has the potential tomove the sediment to a far different milieu.

Shore-parallel drift of brash and slush was domi­nated by southerly transport at average rates of be­tween 19 and 35 cm/s over the 3 years of study(Table 3). On days characterized by offshore winds

TABLE 3. Rate and direction of winter ice drift in southern Lake Michigan.

Date Time Location Direction Rate (cmls) Number

19891

Feb. Various Various SE 35 9

19901

Feb. Various Various SE 19 19

19919 Jan a.m. Gillson Beach SE 4.2 211 Jan. a.m. Gillson Beach NW 39.8 812 Jan. 1000 Gillson Beach SE 22.2 813 Jan. 0830 Gillson Beach N-offshore "Slight"14 Jan. 0800 Gillson Beach NE-offshore

1030 Gillson Beach SE "Slight"15 Jan. 0800 Gillson Beach 0 116 Jan. 1100 Gillson Beach SE 27.6 517 Jan. 0925 Gillson Beach SE 17.6 1

1130 Gillson Beach N-offshore 17.9 118 Jan. a.m. Gillson Beach SE "Several"20 Jan. 1000 Gillson Beach SE 34.1 6

1000 Illinois Beach SE 31.9 321 Jan. a.m. Gillson Beach SE 60.4 422 Jan. 1430 Gillson Beach N-? "Slight"23 Jan. 0800 Gillson Beach SE 22.9 225 Jan. 0930 Gillson Beach SE 21.2 326 Jan. a.m. Gillson Beach 0 127 Jan. 0800 Gillson Beach NW "Very slow"28 Jan. 1.m. Gillson Beach N29 Jan. 0800 Gillson Beach 0 130 Jan. 0800 Gillson Beach SE -20 1

1400 Gillson Beach E-offshore31 Jan. 0800 Gillson Beach SE -10 11 Feb. a.m. Gillson Beach 0 1Average SE 19 48

3-year Average SE 24 76

IData modified from Barnes et al. (l992a).

Coastal Erosion by Ice 191

TABLE 4. Average sediment transport rate in thebrash/slush zone.

FATE OF SEDIMENT INCORPORATED INLAKE ICE (SEDIMENT BUDGET)

An estimate of the erosion potential along thesouthern Lake Michigan coast from ice-rafted sedi-

and anchor-ice formation, a distinct offshore drift ofice could be seen, but its velocity was not mea­sured. Variability in ice-drift velocity from the innerto the outer part of the brash/slush zone is related tothe concentration of ice; high concentrations im­peded movement where the zone abutted the shoreor an ice ridge. This variability has been more com­pletely studied by Kempema and Holman (this vol­ume).

Kempema and Reimnitz (1991) estimated a mini­mum daily transport by brash/slush of 0.18x 103 tfor a single event. Longer-term averaged sedimentcontent for brash and slush (Table 2) and averagedice drift rates (Table 3) suggest higher potential foralongshore transport by mobile ice ranging from ahigh of 4.14x103 tid in 1989 to a low of 0.36x103

tid in 1990 (Table 4). These rates do not considerthe direction of transport, although the bulk of themeasurements indicate southerly drift. Long-termlittoral transport (which includes ice, wave, andcurrent transport) noted by Chrzastowski (1990)suggests yearly rates of 125xl03 t (at 1.6 bulk den­sity) or an average daily transport of 0.34x103 t,which is of the same magnitude noted forbrash/slush transport.

Ice heavily laden with sediment is not visuallyconspicuous, is often submerged (or nearly so), ormay be rolling on the bed, therefore, a satisfactoryestimate of transport of this component cannot bemade. The high sediment concentrations in selectedsediment-laden ice samples (Table 1) suggest thatthis component may contain a major part of the sed­iment moving alongshore. Therefore, ice repre­sented in Table 1 almost certainly underestimatesthe amount of the sediment transported by ice.

Year

198919901991Average

Sedimentcontent

(kg/m of coast)

136.721.7

250.7136.4

Averagedrift rate(em/sec)

35191924

Transportrate

(x103 tid)

4.140.364.112.83

ment transport can be prepared in three ways: (1)by directly measuring the rate of ice drift and thesediment content of that ice (as above), (2) by mea­suring the sediment content of the NIC and estimat­ing the amount rafted away each time the NIC ispartially or fully destroyed, and (3) by extrapolatingfrom the ice-rafted component in cores from thecentral basin.

Ice-Rafted Sediment From Destruction of the NIC

In addition to our observations of the winter-longsediment ice rafting in the mobile brash/slush zone,others have reported that most sediment containedin the NIC is also rafted during breakup events inwinter and at final breakup in the spring (O'Haraand Ayers 1972, Marsh et ai. 1973, Miner and Pow­ell 1991). The NIC is destroyed at rates too rapid tobe attributed to melting.

Once initiated, ice rafting alongshore is muchmore common than rafting offshore (Table 3). How­ever, evidence for offshore ice transport is seen inlake-basin cores that contain ice-rafted sand and oc­casional pebbles in a fine-grained (hydraulically de­rived) matrix (Hough 1958, Lineback et ai. 1979,Colman and Foster this volume). Sediments canalso be rafted to nearby coastal sites or, as Minerand Powell (1991) and Reimnitz et ai. (1991) sug­gested, to sites predominantly on the southern andeastern shores of the lake.

NIC's are dynamic, growing and shrinkingthroughout the winter. Therefore, we are conserva­tive in estimating the contribution from a singlesnapshot of NIC geometry and sediment content(Barnes et ai. 1992a, b). As a first approximation,we assume that all sediment entrained in the NIC israfted. This assumption suggests that between 0.18and 1.17 t could have been rafted from each meterof coast (Table 2). This material is carried along­shore or offshore, depending on wind and currentsat the time of NIC breakup. Subsequent meltingwill release entrained sediments further southalongshore or offshore in deeper parts of the lakebasin or even transport sediment entirely across thebasin to the eastern and southern shores (Kempemaet ai. 1992b).

Coastal bluffs are a primary source of coastalsediments. The potential rates of sediment removalby ice rafting from the NIC are the same magnitudeas the rate of supply from the bluffs. The Illinoisbluffs are eroding at an estimated rate of 5.1m3/yr/m of coast (Jibson et ai. this volume.) or 8.2tim/yr. Assuming an average 10 percent sand con-

192 Barnes et ale

tent for the bluffs (R.W. Jibson, oral commun.1990) gives a sand supply of 0.82 tim/yr. This valueis of the same order of magnitude as found in theNIC ~Table 2) and readily available for rafting.

Ice-Rafted Sediment Accumulatingin the Central Lake Basin

The amount of sediment rafted offshore by ice isestimated from the quantity of ice-rafted sand incentral lake basin cores obtained by Lineback et ai.(1979) and Colman and Foster (this volume). Thissand is texturally similar to coastal sand (Barnes etai. 1992a) and is attributed to ice rafting becauseother mechanisms of transport to the deep basin(turbidites, slumps, hydraulic events) are not evi­dent (Colman and Foster this volume). The amountof ice-rafted sand found in postglacial basin sedi­ments requires an annual ice-rafted contribution of250x103 t to the basin (Barnes et ai. I992a). Thiscontribution is equivalent to rafting 0.83 tlyr fromeach meter of coast in the region. Rafting away theaverage NIC and brash/slush (0.56 tim of coast)(Table 2) would supply most of this need while ad­ditional rafting from multiple NIC growth and

'::~:

decay episodes and on going rafting from thebrash/slush zone could easily supply the rest.

CONCLUSIONS

A distinctive nearshore ice complex develops re­peatedly along the coast of southern Lake Michiganbetween December and March (Fig. 8). The growthof this complex does not stop waves from influenc­ing the coast but does force them to expend theirenergy lakeward of the shoreline, where multipleice ridges act as ephemeral seawalls. The cut andfill associated with this changed wave regime causescour at the base of the ice ridge and involve sedi­ment mobilization and transport. The lakebed iseroded and modified, although the net effect on thelittoral sediment budget due simply to modificationof the wave regime remains a matter of speculation.

Once entrained in ice, greater quantities of sedi­ment are rafted alongshore than are rafted offshore.Conservative estimates suggest that coastal iceprocesses entrain sufficient coastal sand of theproper textural character to account for the coarse,sediment found in postglacial sediments in the deeplake basin. The quantities entrained are also similar

To deep basin0.83 tIm of coast/year

FIG. 8. Summary diagram ofthe nearshore showing components of the sediment budget influenced by iceduring winter. Drawing by Tau Rho Alpha (U.S. Geological Survey).

Coastal Erosion by Ice 193

to the amount supplied by bluff erosion in northernIllinois (Fig. 8).

Five factors relate directly to the transport of lit­toral sediments by ice: (l) the NIC can develop andbreak up several times in a single winter, (2) theNIC contains littoral sand suspended and thrownupon it by wave energy, (3) NIC breakup occursprimarily from mechanical wave energy (not melt­ing), so that sand-charged brash is released to long­shore and offshore ice rafting, (4) after sedimententrainment into ice has occurred, further resuspen­sion energy is not needed and long range transportis possible, and (5) jetties and groins do not stop lit­toral transport but rather enhance sediment loss todeep water by deflecting ice streams offshore. Thebrash/slush zone is a critical component that actu­ally leads to ice-induced sediment transport.

The commonly-held belief that ice protects thecoast from erosion is wrong. Ice-related erosion andtransport processes associated with the growth anddecay of the NIC affect the morphology and sedi­ment budget of the central shoreface. Sediment en­trainment and ice rafting remove sediments inquantities comparable to those removed by long­term coastal transport and sufficient to account forthe ice-rafted component in the deep lake basin.Therefore, ice is an important influence on thelong-term stability of beaches, bluffs, and coastalengineering structures. In southern Lake Michigan,ice will continue to cause coastal erosion of the thinband of sand along the coast (I) as anchor andfrazil ice erodes and scavenges nearshore sedi­ments, (2) as waves cause erosion at the base of iceridges, and (3) as NIC breakup rafts sediment to thesouth and east.

Ice has a important influence on coastalprocesses when the major portions of the lake re­main open and agitated for extended periods in win­ter but is perhaps less important for those lakes andat those times when a complete ice cover is present.

ACKNOWLEDGMENTS

The authors acknowledge the invaluable help ofthose scientists who contributed substantially togathering and interpreting the data in this report, in­cluding Michael Chrzastowski, Illinois State Geo­logical Survey; John Haines, Ralph Hunter, andThomas Reiss all from U.S. Geological Survey;Guy Meadows, University of Michigan; andWilliam Wood, Purdue University. Tau Rho Alphaproduced the interpretive diagrams used in Figures2 and 8. The reviews by Stan Bolsenga, James

Miner, John Schlee, and David Folger helped to im­prove the manuscript.

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Submitted: 20 May 1993Accepted: 19 August 1993