Climatology of Sediment Transport on Indiana Shoals, Lake Michigan

J. Great Lakes Res. 15(3):486-497 Internal. Assoc. Great Lakes Res., 1989

CLIMATOLOGY OF SEDIMENT TRANSPORT ON INDIANA SHOALS, LAKE MICHIGAN

Barry M. Lesht

Atmospheric Physics and Chemistry Section Center for Environmental Research Biological, Environmental, and Medical Research Division Argonne National Laboratory Argonne, Illinois 60439 ABSTRACT. Seven time-lapse films of the lake bottom covering 2,445 hours of observation were made over a period of 3 years during the months of July through November on Indiana Shoals in southwestern Lake Michigan (water depth 10 m). These films show 25 distinct sediment transport events, consisting ofperiods of bedform migration or sediment resuspension and occupying approximately 25 % of the total record. Simultaneous observations of the surface winds showed that most of the sediment transport occurred during periods of northerly winds, implying that surface waves were the predominant mechanism for sediment remobilization at this location. A simple, empirical, sediment transport forecast model was used to determine threshold criteria for the initiation of bedload and resuspension of the local sediments (fine-medium sand). The model correctly predicted 85 % of the total record when forced with estimates of the near-bottom wave orbital velocity calculated from the wind measurements qnd a parametric dynamical wave model. The critical wave orbital velocity for resuspension was found to be 17.8 cm/s. Estimates of the probability of sediment transport occurring on Indiana Shoals, determined as a function of wind speed and direction, were combined with climatological observations of wind conditions to estimate monthly probabilities of sediment transport. These probabilities are in agreement with previous forecasts based on historical observations of the Great Lakes wave climate. ADDITIONAL INDEX WORDS: Bottom sediments, suspended sediments, photography, wave action, mathematical models.

of material put into suspension were higher than the observations but of the correct order of magnitude. No data were available, however, with which to evaluate the estimates of the expected number of days of resuspension. The purpose of this paper is to present some direct observations of sediment transport made in a coastal region of the Great Lakes (Indiana Shoals, Lake Michigan) in a context similar to that presented by Chesters and Delfino. Based principally on time-lapse photography of the lake bottom and simultaneous measurements of surface wind velocity, these observations were used to determine both the frequency with which sediment transport occurs and the critical wave orbital velocity for the onset of resuspension of sandy sediments. Our results generally support the assumptions made by Chesters and Delfino. We find that the critical wave orbital velocity for resuspension

INTRODUCTION

In an effort to provide climatological estimates of the frequency and extent of sediment resuspension in the Great Lakes, Chesters and Delfino (1978) conducted an extensive literature and data review in which they related the occurrence of sediment transport to observations of Great Lakes wave conditions and the properties of nearshore surficial sediments. By making a series of assumptions based on linear wave theory and by using laboratory estimates of critical wave orbital velocities and sediment entrainment rates they were able to calculate expected values of both the number of days that sediment would be resuspended and the amount of material that would be put into suspension in the coastal waters of the Great Lakes. Comparisons with the very few field data that were then available showed that the estimates of the amount 486

SEDIMENT TRANSPORT ON INDIANA SHOALS of sands on Indiana Shoals is close to 18 cm/s and that resuspension occurs about 13% of the time; both estimates in accordance with Chesters and Delfino's predictions for southern Lake Michigan. We also find that the frequency of occurrence of sediment transport on Indiana Shoals can be parameterized in terms of wind speed and direction. The results should be applicable to other Great Lakes coastal regions where surface waves dominate the nearshore flow field. STUDY AREA AND DATA Argonne National Laboratory maintained a meteorological research tower on the northern edge of Indiana Shoals during the ice-free months of 1978-1981. Indiana Shoals is a region of shallow water (minimum depth 10 m) approximately 90 km 2 in area, originating near Indiana Harbor at the southwestern corner of Lake Michigan and extending about 12 km into the lake (Fig. 1). The tower, which was anchored in water 10 m deep, extended 10 m above the surface and was equipped with a variety of instruments for several different experiments. Mean wind speed and direction were measured routinely, as were air and water temperature at several levels (Williams et af. 1979). The sediments at the tower location were dominated by well sorted fine to medium sands (predominant grain sizes - 2cP-3cP) , although some gravel was mixed with the sand and also was found in the troughs of large sand waves on the flanks of the shoal. Very little geological information about Indiana Shoals is available. In particular, we were unable to find any literature discussing either the detailed morphology or the origin of this feature. However, exploratory surveys of the shoals done for the purpose of locating the meteorological tower found both small- and large-scale bedforms, suggesting that the sediments on the shoals were disturbed periodically. Consequently, instrumentation intended to measure variables relating to sediment transport was occasionally installed at the tower location, both for field testing and for investigation of the local conditions. These instruments included an electromagnetic current meter and several optical transmissometers mounted on the tower itself (Lesht et af. 1981) and a downwardlooking time-lapse camera system mounted on a separate tripod (Lesht and White 1980), which was deployed within a few meters of the tower. Most of the data used in this study come from

487

several different periods of observation at the tower during 1979 and 1980. All of the data were collected by using a microprocessor-based data acquisition system (Williams et af. 1983) with which the sensors could be sampled at a high frequency and the samples accumulated and averaged before being recorded. In general, the data collection methods were similar across the several observation periods. There were, however, slight differences in detail, either in the selection of instruments or in the frequency of sampling and length of the averaging periods. The biggest difference is that the averaging period used for the meteorological data was changed from 12 min to 15 min late in 1979. This change was of no consequence in our analysis of these data. The typical interval between frames in the time-lapse films was 15 min, although in one film (designated 80-4) the interval was 5 min. Nearly 4,500 hours of meterological data were collected at the tower. Seven time-lapse films of the lake bottom, covering 2,445 hours, were made. Four of the seven films, spanning more than 1,420 hours, were fully coincident with meteorological observations at the tower (Table 1). In this study, all seven films are used to describe the sediment transport environment on Indiana Shoals. The four coincident sets of film and meteorological observations are then used to relate the observed sediment transport to the local winds and modeled surface wave conditions. Finally, all of the meteorological data collected at the tower are used to produce a climatological description of sediment transport on Indiana Shoals. RESULTS AND DISCUSSION Observations of Sediment Transport Time-lapse photography has been used as a tool for monitoring sediment transport for many years. In quantitative studies (Rukavina 1978, Wimbush and Lesht 1979), films are analyzed frame by frame and occurrences of sediment motion are identified by changes in the appearance of the bot~ tom in successive frames. Two modes of sediment transport are readily discernible, especially when the bottom sediments are noncohesive: formation and migration of sediment bedforms (when most of the transport takes place close to the bed), and resuspension (when bottom material is carried up away from the bed). Film observations of sediment transport were

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489

SEDIMENT TRANSPORT ON INDIANA SHOALS TABLE 1. Summary of coincident film and meteorological observations made on Indiana Shoals, Lake Michigan (N = number of observations). Meteorological Tower

Time-Lapse Camera System Local Time (CST)

Local Time (CST) Year

Tape

Start

Stop

N

Hours

Film

Start

Stop

N

Hours

1979

58

396.1

79-4

912.0

80-1

1,248

312.0

1980

36

858

214.5

80-4

9 Sept 2245 24 July 1215 10 Oct 1120

226.0

3,648

2,580

215.0

1980

37

569

142.2

80-5

31 Aug 1245 II July 1215 1 Oct 1220 10 Oct 1245

904

26

1980

38

567

141.8

1980

39

769

192.2

1980

40

17 Sept 0031 7 Aug 1232 10 Oct 1036 16 Oct 1051 22 Oct 0936 30 Oct 1136 17 Nov 1006

2,038

1980

31 Aug 1226 30 June 1247 1 Oct 1206 10 Oct 1236 16 Oct 1151 22 Oct 1121 30 Oct 1351

1,713

428.2

2,687

671.8

represented as time series in this study by assigning an arbitrary numerical value to each frame on the basis of the mode of transport observed in that frame. Frames that show no sediment motion were assigned a value of 0, frames showing migrating bedforms (primarily ripple marks) were assigned a value of 1, and those showing sediment in suspension were assigned a value of 2. Although we do not require that the two modes of transport are mutually exclusive, this assignment scheme does assume a hierarchy of occurrence. That is, we assume that conditions resulting in sediment resuspension also result in some form of bedform migration. This assumption is reasonable, although (by definition) sediment resuspension so obscures the photographic image of the bottom that it is difficult to see any bedforms. Other assignment schemes that could be used would depend only the analyst's ability to discern and identify distinct phenomena. It might be possible, for example, to quantify the rate of bedform migration (Kachel and Sternberg 1971) and/or the intensity of resuspension (Rukavina 1978, Butman et al. 1979) in addition to simply identifying the occurrence of these two modes of transport as we have done here. Analysis of the seven films showed that the sediments on Indiana Shoals were in motion about 25070 of the time. Episodes during which bedform

7 Nov 1230

migration alone was observed occupied 12070 of the film record, while resuspension episodes occupied an additional 13 070. If we consider a sediment transport event to be a continuous period during which sediment was observed in motion regardless of the mode of transport, a total of 25 events was recorded, spanning 617 of the 2,445 hours of film. Histograms of the durations of the 25 events and of the intervals between successive events on the same film are shown in Figure 2. Although the average event duration was slightly over 24 hours, the most common duration was less than 12 hours. As might be expected, the shorter events tended to occur during the summer months. Similarly, the interval between events, which average over 63 hours, tended to be longer during the summer. The relationship between the occurrence of sediment transport and the time of year is discussed in more detail below. Relationship Between Local Conditions and Sediment Transport Figure 3 shows time series of wind speed (10 m above the surface), 15-min-average, near-bottom current speed measured 1 m above the bottom, the alongshore (roughly north-south) component of the wind velocity, and sediment transport observed during film 80-5: the only film for which coincident current meter observations are available.

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INTERVAL BETWEEN EVENTS (HOURS)

FIG. 2. Distribution of sediment transport event durations (a) and intervals between successive sediment transport events (b).

Although the mean current speed is qualitatively correlated with the wind speed, the observed sediment motion does not seem to be consistently related to either. A more consistent relationship is seen between the observed sediment transport and the alongshore component of the wind velocity; in general, sediment transport occurs when the magnitude of the north component of the wind (Le., the negative alongshore wind) exceeds about 9 m/ s. The sediment transport events of 23 and 24 October, which were associated with southerly winds, occurred during the highest observed average current speeds and probably were associated with the mean flow. Because Lake Michigan is oriented north-south and is over 500 km long, strong winds from the north generate large waves in the southern areas of the lake. The correspondence between the observed sediment transport and strong north winds suggests that surface waves are responsible for the water motions that transport the bottom sediments. Unfortunately, no direct measurements of surface waves were made at the tower, and we must rely on calculated estimates of surface wave conditions in the discussion that follows. It should be noted that, because of the 15-min averaging time, the currents recorded during film 80-5 did not contain any direct information about wave orbital speeds.

A simple numerical wave prediction model (Schwab et al. 1984) was used with the meteorological data collected at the tower to calculate time series of wave height, period, and direction at Indiana Shoals coincident with the time-lapse camera observations. Estimates of near-bottom wave orbital velocity were then made with linear wave theory, and these estimates were compared to the observations of sediment transport. Although the wave model, which predicts the wave field over the entire lake, was run by using only winds measured at the tower, the correlation between modeled wave orbital velocity and the occurrence of sediment transport on Indiana Shoals is quite striking (Fig. 4). Of the 18 transport events observed in the four films, 13 are clearly identified with periods of relatively high (> 10 cm/s) average calculated nearbottom wave orbital velocity, and only four of the events seem totally unrelated to wave activity, occurring when the calculated wave orbital velocities were below 2 cm/s. The qualitative correlation between the observed sediment transport and the calculated orbital velocities suggests that a quantitative relationship might be found as well. Classic models for sediment transport (Miller et al. 1977) relate the initial motion of the sediments (usually assuming uniform grain size) to a critical value of the bottom shear stress resulting from the flow. Because shear stress is very difficult to measure in the field and because natural sediments are rarely uniform, empirical methods have been used to determine threshold criteria in the field from simpler features of the flow such as speed (Wimbush and Lesht 1979, Lesht and Hawley 1987) and wave orbital velocity (Lesht et al. 1980). In this approach, empirical threshold criteria are determined in such a way as to minimize the differences between predictions and observations of sediment transport. We used the following method to determine empirical critical values of the calculated wave orbital velocity for the initiation of the two modes of sediment transport. First, we produced predicted sediment transport time series by using a simple model in which it was assumed that bedform migration occurs when the orbital velocity exceeds some threshold value Urn and that sediment resuspension occurs when the orbital velocity exceeds another threshold Us (Us > Urn)' These predicted series then were compared to the observed record and optimum values of Urn and Us (Le., those that minimize the prediction error) were

491

SEDIMENT TRANSPORT ON INDIANA SHOALS WIND SPEED Z = 10 m

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OCTOBER

1

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5

6

7

8

NOVEMBER

1980

FIG. 3. Time series of wind speed, near-bottom current speed, north-south component of wind velocity, and sediment transport observed during film 80-5.

found by repeated examination of different combinations of the two parameters. Although the procedure generally will result in a fairly well-defined set of parameter values, there is no guarantee that these values will be globally optimum. In practice, however, the initial parameter values are chosen so

as to span a wide range that includes all reasonable values. We applied this method to each of the films individually, to the combination of films 80-4 and 80S, and to the combination of all four of the films. Results of these analyses were entered into contin-

492

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1980 FIG. 4. Modeled wave-orbital velocity and observed sediment transport for films 79-4, SO-I, S0-4, and S0-5.

gency tables (e.g., Table 2) and the prediction errors were quantified by using a prediction skill score (Panofsky and Brier 1965) based on the frame-by-frame comparison of the observed and predicted sediment transport series. The skill score was defined

randomly. The value of E is calculated under the assumption that there is no relationship between the predictions and observations and is given by

55 = (P-E)/(T-E),

where R and Ci are the total number of observations in the i-th row and column of the contingency table. Expected values for each cell of the contingency table were calculated under the random (noskill) hypothesis and the chi-square statistic was used to test the significance of the difference

in which P is the number of correctly predicted frames, T is the total number of frames, and E is the number of frames that would be expected to be predicted correctly even if the prediction was done

j

493

SEDIMENT TRANSPORT ON INDIANA SHOALS TABLE 1. Results of forecast model with threshold parameters Urn = 8.4 cm/s and Us = 17.8 cm/s applied to films 79-4, 80-1, 80-4, and 80-5. The number of observations in each category that would be expected from a random ''no-skill'' forecast is given in parentheses. Frames Observed No Motion Migration Suspension

Frames Predicted No Bedform Total Motion Migration Suspension Observed 4,019 (3,216) 258 (580) 86 (566)

146 (374) 260 ( 67) 101 ( 66)

36 (611) 240 (Ill) 553 (108)

4,201

OBSERVED

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Total Predicted

4,363

507

829

5,699

1 OCT 1980

6

9

13

17

21

26

29

2 NOV

e

10

FIG. 5. Observed and predicted sediment transport for films 80-4/80-5 using modeled wave orbital velocity and threshold criteria Urn = 8.4 cm/s and Us = 17.8 cm/s.

between the actual prediction and the no-skill prediction. The results of the prediction model based on the combined data from all four of the films is shown in Table 2. The optimum values of Urn = 8.4 cm/s and Us = 17.8 cm/s resulted in a highly significant (ex < 0.001) skill score (SS = 0.624). Skill scores for nearby (± 0.5 cm/s) parameter combinations also were high and were slightly more sensitive to changes in Us than in Urn' Outside of a wider parameter range (± 2.0 cm/s), however, the prediction errors increased rapidly. Figure 5 shows the sediment transport time series predicted using these threshold values with the wave orbital velocities calculated for films 80-4 and 80-5 along with the observed time series. The agreement between the two is very good. Despite the success of the simple model, the assumption of constant threshold speeds for sediment motion under waves is not strictly valid. Komar and Miller (1975) showed that the critical wave orbital speed for wave-induced sediment motion will be a function of wave period, with shorter period waves having lower threshold speeds than longer waves. Madsen and Grant (1975) pointed out that the threshold criterion for sediment motion under waves can be expressed in terms of bottom shear by using a so-called friction factor which is a function of both the waves and a parameter related to the roughness of the bottom. Although for a planar bed this roughness parameter usually is taken to be the mean grain size, it is

expected to be a function of the bed morphology when bedforms are present. Our empirical threshold value of 8.4 cm/s is below the 14 cm/s predicted by the method of Komar and Miller (1975) based on the mean sediment grain size (0.0175 cm) at the tower. If bedforms are considered, however, the bed roughness increases and the threshold speed predicted by the method of Grant and Madsen (1979) is much lower, approximately 5 cm/s. Chesters and Delfino (1978) assumed that fine sediments were put into motion by orbital velocities of 5 cm/s and that all particles, including sands, were resuspended by orbital velocities of 20 cm/s. These approximate values are confirmed by our observations. It must be recalled, however, that laboratory threshold criteria usually were based on the initial motion of uniform sediments under uniform flows (Miller et al. 1977) and comparison with our somewhat subjective identification of changes in bedform configuration of natural sediment may be inappropriate. Furthermore, we have based our empirical threshold criterion on wave orbital speed modeled from wind velocity, and would expect some errors, which are impossible for us to quantify, in terms of both magnitude and timing. Nevertheless, our results seem to indicate that nearshore sediment transport can be predicted using a relatively simple model.

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FIG. 6. Fraction of time sediment transport was observed versus average wind speed (a) and average atmospheric friction velocity (b) for films 79-4, SO-I, S0-4, and SO-5.

Climatological Correlations Because both waves and currents in Lake Michigan are directly related to the winds, the frequency of occurrence of sediment transport should depend on average wind conditions. Indeed, the fraction of time that sediment transport was observed in the four films with coincident meteorological observations increased with increasing average wind speed at the tower (Fig. 6a). Although the momentum transfer from the atmosphere to the water will also depend on atmospheric stability, this effect seems minor, as is illustrated by the relationship between the frequency of transport and the calculated atmospheric friction velocity (u.) at the tower (Fig. 6b). The large difference between the frequency of transport observed in films 79-4 and 80-1 is probably due to the directional distribution of the winds. Integrated wind roses showed that the winds were predominantly from the southwest or the direction of the shortest fetch during film 80-1, while the predominant winds were from the southeast and northeast during film 79-4. The influence of wind direction on sediment transport at Indiana Shoals may also be seen in Figure 7, which shows contour maps of average transport level and transport frequency as functions of wind speed and direction. These maps were constructed by assigning each film frame to a wind speed and direction bin and calculating averages for those bins that had at least five observations. The bin widths were 2 mls for wind speed and 22.5 deg for direction, and the 5-min interval

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WIND SPEED (m/s)

FIG. 7. Average sediment transport level (a) and frequency of occurrence of sediment transport (b) as functions of wind speed and direction for combined data from films 79-4, SO-I, S0-4, and S0-5. Wind speed bin width is 2 m/s, and wind direction bin width is 22.5 deg.

frames from film 80-4 were combined to correspond to the IS-min meteorological sampling interval for this analysis. On the average, sediment transport is clearly associated with moderate (5 ml s) winds from the north or northeast but not with much stronger (> 12 mls) winds from the west or southwest. This pattern is similar to that which would be expected if fetch-limited waves were the primary cause of sediment transport at this site. It is important to recognize, however, that the relationship illustrated in Figure 7 is a simple correlation that does not necessarily imply causation. We would expect, for example, that there might be some time lag between the onset of a strong wind and the response of the near-bottom flow. Simi-

SEDIMENT TRANSPORT ON INDIANA SHOALS larly, sediment put into motion may remain in motion because of the inertia of the bottom flow even after the wind has diminished. Figure 7b may be thought of as an approximate probability map for the occurrence of sediment transport on Indiana Shoals. That is, the fraction of time sediment transport was observed for each wind speed and direction combination is an empirical estimate of the probability that sediment transport will occur for that combination. The statistical accuracy of the estimate, of course, depends on the number of observations and the statistical properties of the process. The absolute values for each discrete wind speed and direction combination also depend on the analysis procedure used in the contouring to estimate values for those combinations for which there were no observations. Such an analysis procedure generally will smooth the fields and tend to reduce the highest probabilities somewhat because they are associated with relatively infrequent wind conditions. In the analysis shown in Figure 7, edge effects were reduced by overlapping the region of analysis to include 180 0 ± 15 0 • Thus, in effect, the contouring was done on the surface of a cylinder with a seam at 180 0 • The fraction of time that sediment transport would be expected to occur on Indiana Shoals may be calculated by using the approximate sediment transport probability map and the bivariate joint probability distribution function for wind speed and direction at the tower through the relationship F = ~~P(TIOj,U)·P(Oj'Uj), 1

J

where F is the expected frequency of sediment transport, P(TIOj,U j) is the probability of sediment transport for wind direction class OJ and wind speed class Uj' and P(O,Uj) is the probability of OJ and Uj occurring. Monthly estimates of P(Oi'Uj) were made with all of the tower meteorological data. These estimates of P(Oj'Uj) were combined with the estimate of p(TIOj, Uj) determined above to calculate the expected monthly frequency of sediment transport on Indiana Shoals (Fig. 8). On the basis of these observations, the expected frequency of sediment transport ranges from a minimum of 15070 of the time in August to over 22% of the time in October. Although the existence of a summer minimum is to be expected, both the location of the minimum in August and the narrow range of the expected

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transport frequency are somewhat surpnsmg. A monthly analysis of the film data (Table 3) shows a much wider range in observed transport frequency with the minimum in July, when the average wind speed is lowest. To a certain extent, the range of expected transport frequency will be reduced by the smoothing inherent in the estimation of p(TIOj,U)' More significant, perhaps, is the limited extent of the tower observations from which the monthly estimates of P(Oj,Uj) were made. In contrast, calculation of the expected monthly frequency of sediment transport based on estimates of P(Oj,U j) made from 5 years (1981-1985) of wind data collected at National Data Buoy Center buoy 45007 in the southern basin of Lake Michigan shows a similar range of frequencies but a minimum in June (Fig. 8). Unfortunately, the tower and buoy observations did not overlap, so it is impossible to assess in detail the velocity correlation between the wind measurements made at the tower and those made at the buoy. For compari-

TABLE 3. Monthly distribution of observed sediment transport. (TT = total hours of transport, FT = total hours of film) July August September October November TT FT Transport (0J0)

32.5 82.8 587.8 552.8 5.5

15.0

206.5 418.2

233.6 730.2

61.5 156.5

49.4

32.0

39.3

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MONTHLY AVERAGE WIND SPEED (m/s)

FIG. 9. Observed monthly frequency of sediment transport on Indiana Shoals versus average monthly wind speed measured at buoy 45007. Open circle is the observed monthly frequency of sediment transport for September, excluding data from film 81-2.

son, Chesters and Delfino (1978) estimated that the nearshore zone of southwestern Lake Michigan would experience between 51 and 122 days (14070-33070) of resuspension per year. A somewhat different picture emerges when the monthly probability of sediment transport obtained from the film record (Table 3) is plotted against the monthly average wind speed measured by buoy 45007 (Fig. 9). The relationship appears to be nearly linear with the exception of the datum for September, for which the sediment transport value is dominated by two long (147 hours total) events that occurred during the last 204 hours of film 81-2. Although attractive, considerable uncertainty must be associated with this relationship because the estimates of monthly sediment transport frequency are based on relatively few observations and the monthly wind speed averages come from a completely separate, albeit extensive, data set. CONCLUSIONS Direct observations using a time-lapse camera system showed that the sediments on Indiana Shoals are often in motion, either as migrating bedforms in which most transport takes place close to th; bed, or as sediment resuspension, in which material is moved at least several meters into the water

column. Surface waves were the primary physical determinant of sediment transport on Indiana Shoals, the observed mode of transport roughly corresponding to the magnitude of the surface waves. Conditions resulting in sediment transport occurred, on average, between 15 and 25 percent of the time and may be parameterized in terms of wind speed and direction. A simple forecast relationship accounting for both modes of transport can be constructed by using near-bottom wave orbital velocities calculated from a parametric wave model. Although sediment transport can be related to climatological conditions, more observations would be necessary to refine and verify the relationships. Direct measurements of wave conditions and near-bottom current velocity made in conjunction with time-lapse photographs, or some other measure of sediment transport would be especially valuable for coupling predictions of sediment transport with meteorology-based wave models. ACKNOWLEDGMENTS Data collection at the Indiana Shoals tower was initiated by R. M. Williams (formerly of Argonne National Laboratory) and supported by the U.S. Environmental Protection Agency's Office of Research and Development from 1979 to 1981. Subsequent support of analysis of the data came primarily from NOAA's Great Lakes Environmental Research Laboratory. Ronald Dana, Richard Ely, and Robert V. White (all formerly of Argonne National Laboratory) were responsible for the design and construction of much of the tower system. Nathan Hawley and David Schwab of the Great Lakes Environmental Research Laboratory kindly provided access to the GLERL wave model and calculated the wave-orbital velocity time series. Jack Shannon of Argonne National Laboratory suggested the use of skill scores for model evaluation. I also thank Dr. Norman Rukavina and an anonymous reviewer for several suggestions that significantly improved this manuscript. REFERENCES Butman, B., Noble, M., and Folger, D. W. 1979. Longterm observations of bottom current and bottom sediment movement on the mid-Atlantic continental shelf. J. Geophys. Res. 84(C3):1l87-1205. Chesters, C., and Delfino, J. J. 1978. Frequency and extent of wind-induced resuspension ofbottom material in the U.S. Great Lakes nearshore waters. Report on Great Lakes Basin Commission contract 77D1.

SEDIMENT TRANSPORT ON INDIANA SHOALS Water Resources Center, University of Wisconsin, Madison, WI. Grant, W. D., and Madsen, O. S. 1979. Combined wave and current interaction with a rough bottom. H. Geophys. Res. 87:1797-1808. Kachel, N. B., and Sternberg, R. W. 1971. Transport of bed load as ripples during an ebb current. Mar. Geol. 10:229-244. Komar, P. D., and Miller, M. C. 1975. On the comparison between the threshold of sediment motion under waves and unidirectional currents with a discussion of the practical evaluation of the threshold. J. Sed. Pet. 45:362-367. Lesht, B. M., and Hawley, N. 1987. Near-bottom currents and suspended sediment concentration in southeastern Lake Michigan. J. Great Lakes Res. 13:375-386. ____ , and White, R. V. 1980. Development of a time-lapse camera for benthic research. Radiological and Environmental Research Division Annual Report, January-December 1979, Argonne National Laboratory, ANL-79-65, Part III, pp. 68-69. ____ , Clarke, T. L., Young, R. A., Swift, D. J. P., and Freeland, G. L. 1980. An empirical relationship between the concentration of resuspended sediment and near-bottom wave-orbital velocity. Geophys. Res. Let. 7:1049-1052. ____ , Williams, R. M., and White, R. V. 1981. Sediment resuspension processes in the Great Lakes. Radiological and Environmental Research Division Annual Report, January-December 1980, Argonne National Laboratory, ANL-80-115, Part III, pp. 50-52.

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Madsen, O. S., and Grant, W. D. 1975. The threshold of movement under oscillatory waves: A discussion. J. Sed. Pet. 45:360-361. Miller, M. C., McCave, I. N., and Komar, P. D. 1977. Threshold of sediment motion under unidirectional currents. Sedimentology 24:507-527. Panofsky, H. A., and Brier, G. W. 1965. Some Applications ofStatistics to Meteorology. University Park, PA: The Pennsylvania State University. Rukavina, N. A. 1978. Time-lapse studies of nearshore sediment transport. In Proceedings, Second Workshop on Great Lakes Coastal Erosion and Sedimentation (N. A. Rukavina, editor), pp. 21-24. Canada Centre for Inland Waters, Burlington, Ontario. Schwab, D. J., Bennett, J. R., and Liu, P. C. 1984. Application of a simple numerical wave prediction model to Lake Erie. J. Geophys. Res. 89(C3): 3586-3592. Williams, R. M., Wesely, M. L., and Hicks, B. B. 1979. Preliminary eddy-correlation measurements of momentum, heat, and particle fluxes to Lake Michigan. Radiological and Environmental Research Division Annual Report, January-December 1978, Argonne National Laboratory, ANL-78-65, Part III, pp.82-87. ____ , Haumann, J. R., and White, R. V. 1983. A battery-operated data-acquisition system. IEEE Trans. Instr. and Measurement IM-32(2):356-360. Wimbush, M., and Lesht, B. M. 1979. Current-induced sediment movement in the deep Florida Straits: Critical parameters. J. Geophys. Res. 84(C5):2495-2502.