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Contemporary geomorphic processes and change on Lake Michigan coastal dunes: an example from Hoffmaster State Park, Michigan.

ABSTRACT

Geomorphic changes to Lake Michigan coastal dunes are strongly influenced by seasonal wind, temperature, and precipitation patterns. Between September 2000 and April 2003, geomorphic processes were studied in a coastal-dune system in P.J. Hoffmaster State Park, located south of Muskegon, Michigan. From October through April each year, sand transport and dune surface changes were measured with erosion pins and sand traps. During the same time periods, surface conditions and microclimate were measured at the study area. During the summer months, dune conditions were observed and photographed at irregular intervals. Measurements and observations focused on the active foredune and established foredune ridge located between the lake and the large parabolic dunes common to Lake Michigan's east coast. Results show a distinctly seasonal pattern to Lake Michigan coastal dune change. Aeolian processes are most effective during the late fall and winter when winds are strong and vegetation cover is at a minimum. Other variables that influence sand transport patterns are wind direction, surface moisture, ground-freezing, snow, ice and beach width. Despite considerable constraints, foredune growth was measured at 1.9-5 [m.sup.3] [m.sup.-1] yea[r.sup.-1], with yearly deposition at some foredune locations exceeding 30 cm. All wind-blown sand from the beach was captured by the foredune, but local redistribution of sand by wind occurs in active blowouts on the established foredune ridge. The measured dune activity takes place in the context of low lake levels; rates and types of coastal dune activity are expected to change with rising lake levels.

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INTRODUCTION

Lake Michigan coastal dunes are some of the most dynamic landforms in Michigan, undergoing significant changes over relatively short periods of time. The coastal dunes grow, change shape, and are destroyed as local winds move sand from one location to another. Processes of sediment entrainment, transport and deposition by wind are affected by surface moisture, vegetation, snow, ice, ground-freezing, and lake level fluctuations--variables that are typical of the moist, temperate dunes of the Great Lakes region.

As an important Michigan resource, the coastal dunes are valued for ecological, recreational, economic, and aesthetic reasons. Ongoing management and conservation activities require an accurate understanding of dune properties and behavior. However, little information is available on contemporary Lake Michigan coastal dune processes. The research summary below shows that a current momentum towards understanding coastal dune history has not been matched by studies on contemporary activity. The objective of this paper is to describe contemporary processes and change on Lake Michigan coastal dunes using the example of activity in P.J. Hoffmaster State Park near Muskegon, Michigan.

RESEARCH CONTEXT

The last decade has been an exciting time for those piecing together the history of coastal dunes along Lake Michigan's east coast. As some researchers have been constructing a Holocene record of lake levels based on cores from beach-ridges around the Great Lakes (Thompson and Baedke 1995, 1997; Baedke and Thompson 2000), other researchers have been identifying the sequence of coastal dune development using buried soils as indicators of dune stability followed by dune activity (Arbogast and Loope 1999; Loope and Arbogast 2000; Van Oort et al. 2001; Arbogast et al. 2002). The picture that has emerged refutes previously held assumptions that the large coastal dunes were products of rapid dune growth during the Nipissing high lake stage (approximately 5,500 years ago) followed by thousands of years in which the dunes remained essentially unchanged (for example, Dorr and Eschman 1970). Instead, Loope and Arbogast (2000) discovered that dunes on the northeast coast of Lake Michigan formed within the last 1,500 years and experienced numerous periods of activity that correspond to lake high stands recurring at ~150-year intervals. From Manistee, Michigan, south to Indiana Dunes National Lakeshore, buried soils point to an initial period of dune building corresponding to the Nipissing high stage or later at some locations (Arbogast and Loope 1999; Van Oort et al. 2001; Arbogast et al. 2002). The buried soils records indicate a long period of dune growth, probably lasting several thousand years, punctuated by brief periods of stability. Dune activity was followed by a period of stability, perhaps lasting as long as 1,500 years, before the current period of dune activity lasting 300-500 years (Van Oort et al. 2001; Arbogast et al. 2002). Studies are underway to add more dates and locations to the story of Lake Michigan coastal dune history.

In contrast to the research momentum generated by new information about coastal dune history, knowledge about contemporary Lake Michigan dune processes relies on classic studies with only a few recent quantitative measurements. Lake Michigan dunes are internationally known for the pioneering work by Cowles (1899) on ecological succession and Olson (1958a, b, and c) on the foredunes along the south shore of Lake Michigan. Olson examined relationships between wind-velocity profiles and sediment movement over dunes (Olsen 1958a), vegetation and dune formation (Olson 1958b), and changing lake levels and dune distribution and form (Olson 1958c). He concluded that active beach-margin dune ridges on the south shore of Lake Michigan maintain average deposition rates of 30 cm per year. Working in Indiana Dunes National Lakeshore thirty years after Olson, Olyphant and Bennett (1994) used wind data and a computer simulation to calculate that 3.8-7.3 [m.sup.3] [m.sup.-1] of sand are transported inland from the beach annually. Olyphant and Bennett (1994) note their calculated sand flux value is high compared to historical rates of 0.8 [m.sup.3] [m.sup.-1] [yr.sup.-1] between 3.2 and 0.5 ka, calculated from beach-ridge dune topography. On the Indiana dunes, wind and sand transport patterns are temporally and spatially variable because antecedent soil moisture, rainfall, snow cover and ground freezing affect transport rates (Bennett and Olyphant 1998). The topography of large blowouts affects wind flow patterns and resulting sand transport rates (Fraser et al. 1998), and different coastal dune subenvironments--such as a foredune compared to a parabolic dune--may have sand transport rates with orders of magnitude differences (Bennett and Olyphant 1998).

Research north of the Indiana border has not focused on small-scale processes. Lichter (1995) showed that beach-ridge dunes on the northeast shore of Lake Michigan record lake level fluctuations related to changes in late Holocene climate. Lichter's work focused on the timing of sequential dune development rather than rates of sand transport. In Ludington State Park, Brown and Arbogast (1999) explored digital photogrammetry as a method of monitoring coastal dune change. From sequential air photographs, they were able to discern directions of movement, identify areas of surface deposition or erosion, and estimate volumes of transport over a period of 22 years and at the scale of individual dunes. At smaller spatial and temporal scales, quantitative data on coastal dune activity on the east shore of Lake Michigan are not available.

Contemporary coastal dune studies have taken place at many locations besides Lake Michigan, including studies in the Great Lakes region (e.g., Davidson-Arnott and Law 1990; Law 1990), on the east and west coasts of North America (e.g., Gares 1990; Psuty 1990; Wiedemann and Pickart 1996; Giles and McCann 1997), and locations outside North America (e.g., Hesp 1984; Illenberger and Rust 1988; Sarre 1989; Carter and Wilson 1990; Arens and Wiersma 1994). A small number of the studies report annual volume changes to foredunes, with results ranging from 3 to 50 [m.sup.3] [m.sup.-1] [yr.sup.-1] (Illenberger and Rust 1988; Sarre 1989; Carter and Wilson 1990; Arens and Wiersma 1994). Actual sand transport rates compared to potential (calculated) rates may vary widely because vegetation, changing surface conditions, wind patterns, and variable sediment supply areas affect sand transport (Hesp 2002). As a result, a robust conceptual framework for understanding coastal dunes does not exist (Bauer and Sherman 1999), and studies on coastal dune evolution and variables governing coastal dune change are needed.

THE STUDY AREA

Study Location

P.J. Hoffmaster State Park is south of Muskegon, MI, on the east shore of Lake Michigan (Figure 1). The park has approximately 4 kilometers of Lake Michigan shoreline consisting of sandy beaches and coastal dunes. Coastal dunes range in size from relatively small, but dynamic, foredunes to large parabolic dunes which extend up to 0.5 km inland. East (inland) of the active dunes along the shoreline, the Hoffmaster dunes are stable and forested. Topographic maps and field observations indicate similar dune forms along the Lake Michigan shoreline south and north of the park.

Between September 2000 and April 2003, contemporary dune processes were studied in the south end of the park. Detailed measurements take place in a 30 X 90 m area that includes beach, foredune, and an active blowout on an established foredune ridge. The study area is at the southwest tip of a large parabolic dune. Outside the detailed study area, dune morphology, processes, and sites of activity have been more generally observed on the beach, foredune, dune ridge, parabolic dunes and blowouts of Hoffmaster State Park.

The Coastal Dune System

Dune morphology is determined by a number of factors, including sand availability, sediment characteristics, vegetation, topography, climate changes, and wind energy, velocity distribution, and directional variability (Trenhaile 1997). Unfortunately, no standard classification system for dunes exists, although numerous classification systems have been proposed. For clarity, the following paragraphs describe three types of coastal dunes found at Hoffmaster State Park: foredunes, the established foredune ridge, and blowouts. Alternate names for dune types and features are given in parentheses.

The foredune (beach-margin dune) is a shore-parallel dune ridge formed at the top of the backshore (upper beach) by aeolian sand deposition within vegetation (Hesp 2002). Active foredunes occupy the most lakeward position in a coastal dune system, and they are often the smallest and youngest dunes along the Lake Michigan coast. Dune shape varies from relatively flat terraces to convex ridges, and most foredunes lack distinct slip faces. Foredunes have been described as the only distinctive coastal dunes because they are integrally coupled to nearshore processes and their geomorphology is a function of coastal vegetation (Bauer and Sherman 1999). Lake levels and storm wave activity control foredune existence and size by influencing the distribution of sand between the beach and the dune (Psuty 1992). Low lake levels create wide beaches, and onshore winds can move sand inland to build incipient foredunes or increase the size of existing dunes. (Incipient foredunes are small, coalescing sand deposits within or behind vegetation that indicate the first stages of foredune growth.) High lake levels expose the foredune to wave erosion; the dune sand may be blown inland by wind or released by waves back to the nearshore system.

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Inland from the active foredune position, Hoffmaster State Park has a stabilized foredune ridge (established foredune) which exists along the entire shoreline of the park. This feature will be referred to as the dune ridge in this paper to distinguish it from the active foredune. The shore-parallel dune ridge reaches heights of 8-15 meters and possesses greater vegetative diversity than the active foredune. The complexity of the dune-ridge ecology relative to the foredune suggests that the dune ridge is both older and more stable than the foredune. In Hoffmaster State Park, the dune ridge anchors the arms of the large parabolic dunes. There is a visible difference between the intermediate species on the dune ridge inside the arms of a parabolic dune (Calamovilfa longifolia, Panicum virgatum, Schizachyrium scoparius, Elymus Canadensis) and the deciduous forest vegetation on the dune ridge between parabolic dunes (Quercus rubra, Hamamelis virginiana, Fagus grandifolia, Carex spp., Poa spp., Aquilegia canadensis, Pedicularis canadensis). The age and origin of the dune ridge is not known.

Blowouts are common features on the Hoffmaster dunes. A blowout is a depression or hollow formed by wind erosion on a pre-existing sand deposit (Hesp 1999). Blowouts form where openings in vegetation cover allow strong winds to interact with the exposed sand surface. Wind erosion enlarges the initial disturbance and as the blowout grows, it modifies wind flow patterns--often accelerating the flow and directing it along the blowout axis. Although blowouts take many forms, two main types have been identified. Saucer blowouts are "shallow, ovoid, dish-shaped hollows with a steep marginal rim and commonly a flat-to-convex downwind depositional lobe" (Carter et al. 1990, 234). Trough blowouts have "relatively deep, narrow, steep-sided topographies with more pronounced downwind depositional lobes, and marked deflation basins" (Carter et al. 1990, 234). On the dune ridge, blowouts are mostly trough blowouts that may funnel wind and wind-blown sand into the large parabolic dune systems.

Climate

P. J. Hoffmaster State Park has a warm, humid continental climate with warm summers, cold winters and precipitation throughout the year (Figure 3). The mean monthly temperature recorded north of the park at Muskegon ranges from -5[degrees]C (23[degrees]F) in January to 21[degrees]C (70[degrees]F) in July (National Climatic Data Center 2001). The average annual precipitation is 89 cm (33 inches) with an average of 290 cm (114 inches) of snow in the winter months (National Climatic Data Center 2001). The region experiences large day-to-day variability in weather conditions because of the frequent passages of low-pressure storm systems characterized by cloudy skies, windy conditions, and precipitation (Kunkel et al. 1993). Mean monthly wind speeds at Muskegon are between 4 and 6 m/s (9-12 mph) and peak gust speeds greater than 22 m/s (50 mph) have been recorded in every month of the year (National Climatic Data Center 1998). The northwesterly winter winds are generally stronger than the southwesterly summer winds. The climate produces seasonal variations in wind, vegetation cover, surface moisture, and ground freezing which affect the timing and amounts of sediment transport and dune change.

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Recent Lake Levels

Lake level fluctuations affect sediment availability to aeolian processes as the subaerial beach narrows or widens in response to rising or falling lake levels. The historic record for Lake Michigan-Huron shows approximately 1.5 meter variation over time (Figure 3) produced by variations in precipitation and evaporation within the lake basin and drainage area. From 1999-2003, Lake Michigan-Huron levels were at their lowest since the historic minimum in 1964, and the decrease in lake level by more than a meter since 1998 was the fastest drop ever recorded. During the 2000-2003 study period, there was a 0.6 m variation in the level of Lake Michigan (Figure 3, inset).

Study Methods

From October 2000 through April 2003, aeolian activity and geomorphic changes were recorded for the subaerial beach, active foredune, and foreduneridge blowout that comprise the study area in Hoffmaster State Park. Study methods included using erosion pins, sand traps, microclimate instruments, and field observations to record local conditions, sand transport, and topographic changes. This paper focuses on study results illustrating general patterns of change and seasonal influences, including examples of specific measurements to demonstrate the seasonal patterns. A detailed analysis of the sand transport and wind data will be presented elsewhere. All of the study methods are described below, although the bulk of the results and discussion are based on field observations and erosion pin measurements.

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Topographic changes to dune surfaces were measured with erosion pins installed in the study area between October and April each year. Elevation change was measured at more than 100 wooden pins (1.5 cm diameter) installed at 5m intervals in a grid that extended across the backshore, foredune and blowout on the dune ridge (Figure 4).

Sand transport was measured with Leatherman-style sand traps (Leatherman 1978) installed in sets at strategic locations in the coastal dune system (Figure 4). Sites were chosen to measure relative amounts and directions of sand transport at the lakeward boundary of the foredune, the foredune crest, the lakeward edge of the blowout on the dune ridge, and the dune ridge crest. Except for sand traps on the dune-ridge crest, each location had four traps with openings facing north, east, west, and south. The sand traps were emptied weekly; collected sand was taken back to the lab to be dried and weighed.

Microclimate conditions were monitored at the study area from October 2001 through April 2002 and from October 2002 through April 2003. An instrument tower with anemometers at 5 heights, a wind vane, a temperature and relative humidity probe, and soil temperature probes was located just north of the study area.

Field observations of surface conditions and geomorphic activity were made during regular visits to the study area, including weekly visits from October through April and monthly visits from May through September. Beach width (the distance between the lake edge and the foredune) was measured in meters and the presence of any coastal ice was recorded. Sand samples were collected from beach and dune surfaces and dried to determine surface moisture contents. The presence and location of wet sand, snow, ice, and frozen ground across the study area were recorded.

RESULTS: CONTEMPORARY PROCESSES AND CHANGE IN THE HOFFMASTER COASTAL DUNES

Foredune Changes

From 2000-2003, the active foredune grew in size as wind moved sand from the beach to the foredune. During each of the three fall-winter measurement periods, the average sand deposition at erosion pins along the foredune crest was 0.2-0.3 m (Figure 5). The total volume of sand added to the foredune (calculated from deposition at erosion pins) was 4.5 [m.sup.3] [m.sup.-1] in 2000-01, 5.0 [m.sup.3] [m.sup.-1] in 2001-02, and 1.9 [m.sup.3] [m.sup.-1] in 2002-03. By April 2003, the foredune rose up to 2.2 m above the beach and contained an estimated 28 [m.sup.3] sand per meter of shoreline. Approximately 40% of the foredune height and volume had been added during the 2000-2003 study (approximately 0.9 m increase in height and 11.4 [m.sup.3] [m.sup.-1] increase in volume). Although the rate of foredune growth was similar in the first two years of the study, the volume of foredune deposition in 2002-03 was less than half the amount of each of the previous two years.

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Dune-Ridge Activity

The dune ridge received almost no inputs of sand from the beach and foredune, but erosion and deposition occurred where dune-ridge sediments were reworked by wind in the blowout. Sand traps installed at the lakeward edge of the blowout recorded negligible amounts of sand, indicating that wind-blown sand from the beach was deposited on the foredune before it could reach the dune ridge. Very little recorded change at erosion pins outside the blowout and noticeably less vigorous American beachgrass on the dune ridge compared to foredune beachgrass also indicated dune ridge stability. Significant changes occurred at the dune-ridge blowout, with pin measurements recording erosion in a deflation area on the windward slope of the dune ridge and deposition on the dune-ridge crest and lee-slope (Figures 5 and 6). Sand traps on the duneridge crest were often filled as winds moved sand from the deflation area to the depositional lobe. Erosion rates within the blowout were the highest measured within the study area; for example, up to 40 cm of erosion was measured at pin locations in the blowout between October 2001 and April 2002. Approximately 40% of the erosion took place during a single October storm, and the remainder of the erosion occurred throughout the fall and winter. Measured deposition rates on the crest and lee slope of the dune ridge were much less than deposition rates on the foredune (Figure 5).

Local Conditions

The measured changes to the Hoffmaster dunes took place under variable weather conditions that are described here in very general terms for the three October-April measurement periods. All three years had passing weather systems during the fall which included strong wind events (some lasting for several days), frequent periods of precipitation, and strong storms that included high winds and sometimes intense precipitation. The largest storm, occurring in October 2001, pushed waves onto the beach up to the vegetation marking the lakeward edge of the foredune. The 2000-2001 winter started with large amounts of snow throughout December which immobilized the study area well into January; the rest of the winter had near-average temperatures and precipitation. The 2001-2002 winter was very warm (monthly temperatures around 5C[degrees] above average) with less than a week of coastal ice formation along the Hoffmaster beach. 2002-2003 was a cold winter with near-average snowfall and substantial coastal ice present from the first week of January through the end of March.

Measured beach widths varied during the study period with changing lake levels and storm activity. In 2001-2002, average beach width was 35 meters, compared to 15-23 meters during the 2001-2002measurement period and 15 meters during 2002-2003. Waves only reached the edge of the foredune during the strongest storms during the fall of 2001. As a result, foredune vegetation and sediments were not eroded by waves during the three-year study. In contrast, accumulations of wind-blown sand around discrete clumps of vegetation between the active foredune and the lake in 2001 were completely destroyed by wave activity when lake levels rose slightly in 2002.

Beach and dune surface conditions varied widely during the study. Surface moisture measurements ranged from dry sand (0% gravimetric water content) to saturated sand (>24% gravimetric water content). Results depended on whether the sampling location had recently experienced precipitation, melting snow or ice, or wave activity. Without recent inputs of moisture, the general surface pattern was moist-to-saturated sands on the beach face (5-25% water content), dry-to-damp sands on the beach and west slope of the foredune (0-7%), moist sands on the lee slope of the foredune (3-17%), and drier sands in the dune-ridge blowout (0-5%). Other surface conditions, such as snowcovered areas, surface ice, frozen ground, and niveo-aeolian deposits are described by season in the following section.

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Seasonal Patterns of Coastal Dune Change

Changes to the Hoffmaster coastal dunes exhibited a seasonal pattern related to local winds, temperatures, and precipitation along with changing surface conditions. Factors that promoted sand movement were stronger winds, drier surface conditions, larger sediment supply areas, and decreases in surface roughness such as reduced vegetation cover during the cold season or burial of vegetation by sand and/or snow. Factors that inhibited sand movement were weaker winds, wetter surface conditions, snow or ice cover, ground freezing, and high or dense vegetation. The specific combinations of these factors at different times of the year produced a seasonal pattern of coastal dune activity.

Summer: During the summer, winds were generally weak, vegetation was thicker and higher, and little geomorphic change was observed on the foredune and dune-ridge. On average, summer winds are southwesterly and weaker than winds during the rest of the year. Moving sand or evidence of sand movement (ripples, recently deposited sand) was only infrequently observed during the summer. During summer site visits, beach and dune surfaces were dry, except following precipitation events, because warmer temperatures promoted evaporation. On the foredune, A. breviligulata reached maximum heights and density during the summer months. Because the beachgrass was effective in reducing surface windspeeds, little or no wind erosion occurred in vegetated areas, and deposits of wind-blown sand were limited to the lakeward edge of the foredune. Where beachgrass from the foredune was colonizing the upper beach, the deposited sand caused the foredune to grow towards the lake. The summer dunescape of Figure 7 is the one most recreational visitors equate with the Lake Michigan shoreline: dry sand beaches and vegetated hummocky dunes.

Autumn: Strong winds and cool temperatures in the autumn increased the amount of aeolian activity. The autumn increase in average wind speeds reflects the high-energy weather systems that track through the area, often bringing precipitation along with strong winds that may be sustained for several days (Kunkel et al. 1993). In autumn, the leaves of A. breviligulata dried out and became less rigid, thereby decreasing their ability to slow down the wind near the dune surface. As the effective density of the beach grass decreased (the plants remained rooted in the dune, but their senescent leaves covered less of the dune surface), wind scour around plants on the lakeward slope of the foredune was observed.

Sand trap results, observations, and anemometer data showed that the effect of autumn aeolian activity depended on the wind direction. Some of the autumn winds transported sand along the beach towards the north or south, bypassing the foredune system entirely. Winds with a westerly component (SW, W, or NW winds) moved sand onto the foredune from the beach and windward dune slope. Even with the decreased autumn sand-trapping potential of the A. breviligulata, the beachgrass slowed the wind as it moved over the foredune, and sand deposition frequently occurred upwind of the foredune crest. Some A. breviligulata were left perched on islands of sand held in place by roots as wind scoured the lakeward edge of the dune; as wind flow accelerated around the obstacles, wind and sand were moved further up the slope. Deposition on top of the dune reduced surface roughness by partially burying some plants, with the effect of allowing further transport during the next wind event. In December 2001, the inland boundary of sand transport and deposition was marked by a 15 cm scarp on top of the foredune (Figure 8).

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Surface moisture from precipitation, reduced evaporation, and waves on the beach influenced autumn aeolian activity. Variable surface moisture contents, such as saturated sands near the lake and drier sands near the foredune, produced visible differences in sand entrainment as the drier sands moved while wetter sands remained in place (Figure 9).

Winter: During the winter months, strong winds continued to move sand from the beach to the foredune and from the blowout over the dune ridge, but aeolian activity was complicated by the winter conditions. The strong winter winds are predominantly northwesterly. Sand trap results (Figure 10) from the cold 2002-2003 winter--only three days in January 2003 had average temperatures above 0[degrees]C--show that sand movement generally occurred through the entire winter and was not halted by the colder weather. However, snow, ice, and ground freezing substantially reduced transport amounts compared to autumn conditions. The cold winter conditions were interspersed with short warm periods during which melting snow and ice, thawing ground, and sometimes rain kept beach and dune sands damp or wet.

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Certain winter conditions, such as persistent snow cover, halted activity for days to weeks at a time. The snow cover protected beach and dune sands from wind erosion. The protection was often short-lived as strong winds removed the snow and then eroded exposed beach and dune surfaces. Snow deposition occurred in the same areas as sand deposition, that is, on the leeward slopes of the foredune and dune ridge. The deposits were niveo-aeolian: interlayered or mixed sand and snow deposits that revealed the recent aeolian history when examined (Figure 11). The niveo-aeolian deposits buried lee-slope vegetation, eliminating their role in trapping sand and smoothing the dune surface so that subsequent sand transport moved further into the dune system (Figure 12). On the foredune, the deposits "grew" in the landward direction through the winter; by March strong winds were moving sand 20 meters further than similar strong winds in December.

Snow and ice limited winter wind erosion to specific locations in the study area (Figure 13). Surface ice formed from waves, melting snow/ice, and rain freezing onto beach and dune surfaces. Ice offered more persistent protection than snow because the ice was generally too cohesive to be transported inland by wind. Over time, sublimation and melting followed by evaporation could reduce the ice. Melting without evaporation was less effective because the meltwater could not drain through the underlying frozen sand--instead the water collected in depressions where it froze again when temperatures fell. The extent of ice- and snow- free areas varied greatly through the winter, ranging from 0% of the study area immediately after snowfall to 40% when the beach and windward slopes of the dunes were clear. With the exception of the warm 2001-2002 winter, midwinter warm periods were not long enough to remove all the snow and ice as well as evaporate excess water; as a result the entire study area did not return to a snow-and-ice-free condition until spring.

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From early January through mid/late March, coastal ice along the shoreline maintained a constant beach width (Figure 13). The icefoot protected the beach from wave activity, halting onshore, offshore, and nearshore sediment transport that might supply or remove sediments from aeolian activity. The coastal ice created roughness elements in the nearshore that might have decreased wind speed or enhanced turbulence in the winds reaching the beach. The published literature records no studies of the effects these disturbances have on beach-dune processes.

Where beach and dune surfaces were exposed, ground-freezing affected sediment entrainment and transport by wind. Frozen pore moisture immobilized the sand grains, creating a solid surface which resisted grain entrainment by wind as well as researchers' attempts to remove samples for water content analysis. (Chipping and scraping with a rock hammer was ultimately successful.) The frozen sand grains were not permanently immobilized. Pore-ice sublimation desiccated the frozen sand and occasionally several millimeters of loose grains were observed at the surface under calm conditions. More frequently, however, the released grains were quickly removed by the wind, leaving a wind-swept frozen surface and evidence of sand transport in the form of measurable erosion at erosion pins, sand in sand traps, ripples or small dunes of loose sand on downwind surfaces, and visible sand deposits on snow.

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Winter variability makes it hard to define a typical winter appearance of the Lake Michigan coastal dunes, but there are some general patterns. Wind eroded sand from frozen exposed surfaces on the beach and windward slopes of the dunes. The erosion steepened the windward dune slopes, isolating some A. breviligulata plants on the foredune and scouring out the upper slopes of the blowout on the dune ridge. Niveo-aeolian deposition occurred on the dune crests and lee slopes. On the foredune, niveo-aeolian deposits extended the dune crest and created a gentler profile which allowed wind-blown sand to travel further across the dune. On the higher dune ridge, niveo-aeolian deposition also extended the top of the dune, but the result was an oversteepening of the slip face. With large niveo-aeolian deposits, slope angles became nearly vertical at times. Wind-blown sand was part of these structures, forming internal layers in the niveo-aeolian deposit or blanketing the steepened structure of the snow. The snow provided the frozen moisture that welded the deposit together. During warmer winter periods or the spring thaw, flows and slumps occurred when uncemented sand could not support the same steep slope angles.

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Spring: A decrease in westerly winds in the spring decreased the amount of sand transport onto the foredune but thawing caused substantial readjustments to dune morphology. Spring winds were variable in strength and included frequent winds from the east. The strength of easterly winds was reduced by the topography and vegetation of inland dunes before the winds reach the foredune and the beach. Some movement of sand from the foredune to the beach and even onto the icefoot was observed at Hoffmaster, as was alongshore transport on the beach bypassing the foredune. Sand transport was constrained by surface sands wet from spring melting and precipitation. Sand trap measurements show less transport than in the fall and winter (Figure 10), but infrequent large storms did move significant amounts of sand in short time periods.

Most changes to dune morphology resulted from thawing pore ice, surface ice, and snow. Wet sand flows, slides, and slumps occurred on the windward slope of the foredune and both slopes of the dune ridge when the thawing sediments could not maintain steep winter slope angles. Underlying frozen layers contributed to the wetness and instability of the surface materials by not permitting water to drain through them. As the snow and ice in niveo-aeolian deposits melted out, the sand in the deposits became concentrated and was lowered onto the former dune surface below. The composition of the niveo-aeolian deposit affected the melt-rate: thin or patchy sand on top of snow accelerated melting locally as the sand absorbed solar radiation, but thicker layers of sand acted as insulation to slow melting. As buried snow melted, dampness was visible on the dune surface, and cracks and wrinkles appeared when the deposit dried and settled (Figure 14). The sedimentary structures were short-lived, lasting only until the next strong wind smoothed the surface.

The transition from spring into summer was marked by decreased wind activity and new vegetation growth. In early spring, the recently deposited sand was visible on dune crests and lee slopes, completely obscuring the vegetation under the thickest deposits (Figure 15). Stimulated by sand deposition, A. breviligulata sent vertical rhizomes to the surface and new shoots emerged through the sand deposits in April. On the lakeward slope of the foredune, horizontal rhizomes from A. breviligulata extended the vegetated area lakeward. As the beachgrass grew, beach and dune surfaces were drying out and the sand became more susceptible to movement by wind. At the same time, wind speeds decreased and very little sand moved onto the foredune.

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DISCUSSION

Aeolian processes on Hoffmaster coastal dunes

The study of Hoffmaster coastal dunes reveals a dynamic aeolian environment, both in terms of sand transport amounts and the variables that interact with transport and depositional processes. Sand moves from the beach to the foredune, and from the blowout to the leeward slope of the dune ridge, under the influence of strong onshore winds during the fall, winter, and early spring. In coastal dune environments, wind characteristics are not the only factors determining the timing, amounts, and directions of sand transport. The effects of changing beach widths, topography, and dune vegetation have been addressed in studies by Davidson-Arnott and Law (1990), Sherman and Lyons (1994), Arens (1996), Bauer and Davidson-Arnott (2002), and Hesp (2002). Temperate and high-latitude coastal dunes also face constraints of wet, frozen, snow-covered, or ice-covered surfaces; at Hoffmaster these conditions occur during the fall and winter when much of the sand transport takes place.

Models by Belly (1964), McKenna Neuman and Maljaars Scott (1998), and others predict the effects of surface moisture on sediment entrainment by wind. All of the models indicate that the potential effects of surface moisture are large, although the models diverge considerably in their predictions (Namikas and Sherman 1995). Dry sand is more susceptible to movement by wind than damp sand, in which the surface tension of pore moisture raises the threshold wind velocity (Namikas and Sherman 1995). Strong winds can release individual grains by drying the surface moisture, and Anderson (1989) suggests that for wet sand, understanding how the thermodynamics of evaporation determine sediment supply to the wind may be more important than defining the dependence of mass flux on wind speed. At Hoffmaster, strong autumn winds are often associated with storms when rain wets the surface sands and high waves saturate much of the subaerial beach.

Few studies have examined the winter effects of frozen surfaces, snow, and ice on sand transport and deposition. The ground-freezing of moist sands immobilizes the sand grains, but pore-ice sublimation desiccates the surface, releasing grains that can be entrained by wind and may themselves release downwind grains through impacts in a "sand-blasting" effect. Strong winds, warmer temperatures, low humidity, and low initial surface moisture contents are the conditions that will produce the largest amounts of surface drying and sand movement (van Dijk and Law 2003). Saltating grains retain more of their kinetic energy on hard surfaces (McKenna Neuman 1989), causing the saltating grains to bounce higher and travel further on frozen surfaces than they could on loose sand. This contributes to the increasing landward penetration of wind-blown sand over the foredune during the winter. Niveo-aeolian deposits are known to cover larger areas than warm-season activity (Belanger and Filion 1991; Ruz and Allard 1995). The steepening of slopes cemented by niveoaeolian deposits was observed by Law (1990) on the north shore of Lake Ontario, along with the subsequent readjustments when the dunes thawed in the spring.

Cumulative Changes to the Hoffmaster Coastal Dunes

The results of the 2000-2003 study indicate a growing foredune and a dune ridge that was not receiving new sediments but was changing locally within the blowout. Foredune growth was made possible by wind-blown sediments from the subaerial beach widened by the low levels of Lake Michigan. The lake level record and anecdotal accounts suggest that the foredune is a relatively young feature which developed after the high lake levels of 1997. By the beginning of the study in fall 2000, the foredune was large enough to cut off the supply of beach sediments to the dune ridge. Foredune growth continues while the lake levels remain low, with rates of growth varying as the supply of wind-blown sand is affected by the size and surface conditions of the beach.

With its supply of wind-blown beach sand cut off by the foredune, the dune ridge is more appropriately called a "secondary dune" than a true coastal dune because it has been dissociated from the beach (Psuty 1989). Sand movement consists of the reworking of existing dune sediments, with local transport from the blowout over the crest of the dune ridge to the leeward depositional area. Dune ridge changes are indirectly linked to lake levels and may be longer-term than foredune growth. The thinner dune-ridge deposits compared to foredune deposits may reflect both a smaller sand supply area of the blowout compared to the beach and a larger deposition area as sand travels down the slip face of the dune ridge.

The cycles of lake level changes on Lake Michigan suggest that the lifespan of the active foredune may be short and the dune ridge may become an active coastal dune again. As long as the level of Lake Michigan remains at or below its 2000-2003 levels (up to 176.4 m asl), foredune growth will continue. With an extended period of low lake levels, the foredune could grow larger than has been seen along the shoreline during the period of historic records. It is more likely that lake levels will rise again and waves will destroy the foredune as they remobilize dune sediments. Some of the remobilized sand grains will make their way inland as sand inputs to the dune ridge or sand transport through duneridge blowouts into the parabolic dunes.

Seasonal activity will remain an important component of dune change as the foredunes are destroyed. Maximum wave activity occurs during November and December storms, but the greatest amounts of wind erosion of the disturbed areas and inland sand transport by wind are likely to occur later in the winter when coastal ice protects the shoreline from wave action. Vegetation growth that could protect the disturbed areas from the wind will not occur until several months after the autumn storms disturb the foredunes. The amount of sand that will move inland depends on the size of the foredune, the frequency and magnitude of wave disturbances, and the nature of autumn and winter conditions (temperatures, snowfall, etc.) following the disturbance of the foredune.

Comparison of Study Area with Other Coastal Dunes

The coastal dunes and processes of the studied area are similar to coastal dune activity at other locations on the east coast of Lake Michigan. Although the foredune was not continuous in size, shape, or presence along the entire shoreline of Hoffmaster State Park, as much as 90% of the Hoffmaster shoreline had a foredune averaging 2 meters in height (Bierma et al. 2003). At one location a low foredune (<1 meter high) developing between the main foredune and the lake was observed to be decreasing the sediment supply to the larger foredune. An estimated 85% of the dune ridge in Hoffmaster State Park received no inputs of sand from the beach or foredune in 2002 (Bierma et al. 2003). At locations where the foredune was absent, smaller or ramped up against the dune ridge, wind-blown sand did enter the dune ridge from the foredune and beach. The Hoffmaster foredunes and dune ridge, along with similar dunes south and north of the park, are subject to similar conditions and processes as those measured within the study area.

Amounts of sand transport from the beach to the foredune in Hoffmaster State Park are comparable to amounts recorded on the south shore of Lake Michigan and other coastal dune locations. Volume changes to the Hoffmaster foredunes represent minimum values for annual transport rates because changes were not measured for the entire year. Inputs of sand to the foredune in the range of 4.5-5 [m.sup.3] [m.sup.-1] [yr.sup.-1] recorded in 2000-2002, with greater than 30 cm of deposition at locations along the foredune crest, are well-matched to Olson's (1958b) record of 30-cm annual deposition and Olyphant and Bennett's (1994) calculation of 3.8-7.3 [m.sup.3] [m.sup.-1] [yr.sup.-1] for the Indiana Dunes on the south shore of Lake Michigan. The Hoffmaster measurements are comparable to sediment inputs of 3 [m.sup.3] [m.sup.-1] [yr.sup.-1] on the coast of the Netherlands (Arens and Wiersma 1994), 4.3 [m.sup.3] [m.sup.-1] [yr.sup.-1] in southwest England (Sarre 1989), and 6.3 [m.sup.3] [m.sup.-1] [yr.sup.-1] in northern Ireland (Carter and Wilson 1990). Sediment inputs to the Hoffmaster foredune are small compared to the 50 [m.sup.3] [m.sup.-1] [yr.sup.-1] entering a Dutch foredune on a severely eroding coast (Arens and Wiersma 1994) or the 15 to 30 [m.sup.3] [m.sup.-1] [yr.sup.-1] entering coastal dunes on the south African coast (Illenberger and Rust 1988). In comparison to dunes world wide, Lake Michigan foredune growth can be described as significant but not spectacular. When the constraints of the environment--moisture, freezing, snow and ice--are considered, the growth of the Hoffmaster foredune is impressive. Regret-tably, few quantitative measurements of any kind have been made in high-latitude or seasonally cold coastal regions for comparison.

CONCLUSIONS

Changes to Lake Michigan coastal dunes have distinct seasonal patterns, as seen in P. J. Hoffmaster State Park, Michigan. Aeolian processes are most effective during late autumn and winter when vegetative protection of dune surfaces is reduced and strong northwesterly winds transport sand inland. The strong winds erode sand from the beach and exposed windward slopes of the foredune and secondary dunes. Wind-blown sand and snow are deposited on dune crests and leeward slopes. On the foredune, sand transport distances increase through the winter as niveo-aeolian deposits cover vegetation and smooth the dune surface. On the higher inland dunes, the slip face can be considerably steepened by frozen niveo-aeolian deposits. In the spring, aeolian activity decreases as wind patterns change and melting snow or thawing ground keeps dune surfaces moist. Downward slope movements bring oversteepened dune slopes back to stable angles. Geomorphic activity is at a minimum during the summer when winds are weak and vegetation cover reaches its yearly maximum.

In Hoffmaster State Park, the cumulative effects of recent seasonal activities are to increase the size of the foredune and redistribute sediments on inland dunes. Foredune growth ranged from 1.9-5 [m.sup.3] [m.sup.-1] [yr.sup.-1] from 2000-2003, with annual deposition of more than 30 cm at some foredune locations. The foredune is large enough to capture all wind-blown sand from the beach, thereby cutting off the transport of beach sand to the higher dune ridge that parallels the shoreline and foredune. As a result, the dune ridge is stable except where sand is locally being redistributed by wind. Measurements in a duneridge blowout show that erosion can exceed 40 cm at some locations in some years. The wind transports the sand over the crest of the dune ridge onto a depositional area on the windward slope.

The coastal dune activity measured in Hoffmaster State Park is taking place in the context of low lake levels. When lake levels rise the foredune will likely be destroyed by wave activity with some foredune sand becoming part of the nearshore sediment sand budget again. Secondary dunes will also be affected by lake level changes as some of the foredune sands are blown inland. All of this activity is subject to the dynamic conditions of the Lake Michigan coast which will determine rates and amounts of transport. Studies of current dune behavior during low levels provide valuable information for understanding activity during higher lake levels.

We have just begun to understand how Lake Michigan coastal dunes behave, and more studies of contemporary processes are needed. Studies that quantify sand movement in areas where transport is complicated by surface moisture, vegetation, ground-freezing, snow, ice, and changing beach width--sometimes all on the same day!--are rare. The dynamic conditions challenge our ability to predict coastal dune changes using conventional aeolian models for sand transport. The dynamic conditions also make the Lake Michigan coastal dunes a fascinating geomorphic environment.

ACKNOWLEDGEMENTS

Funding for this study was provided by Calvin College Science Division Student Summer Research Awards, a Calvin College Alumni Association Faculty Grant, a Calvin College Research Fellowship, and the Department of Geology, Geography, and Environmental Studies at Calvin College. The Michigan Department of Natural Resources granted permission to make measurements inside the park. I am grateful to Elizabeth Brockwell-Tillman and other staff members of Hoffmaster State Park for their interest in this research. The study would not have been possible without the invaluable field assistance of Calvin undergraduate students Adam Benthem, Ryan Bierma, Mammie Hutchful, Annelia Tinklenberg, Kristen Van Kley, and many others. Thank you to Karen Havholm and an anonymous reviewer who made many helpful comments on an earlier draft of this paper.

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DEANNA VAN DIJK

Calvin College
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