INITIAL CURRENT STUDIES IN LAKE MICHIGAN James L. Verber U.S. Public Health Service, Chicago, Illinois

Transcription

1 INITIAL CURRENT STUDIES IN LAKE MICHIGAN James L. Verber U.S. Public Health Service, Chicago, Illinois ABSTFlAcT Initial current studies in Lake Michigan indicate that the circulation of the southern basin varies according to the general wind field over the lake. Studies from May to July 1962 show a counterclockwise circulation, whereas studies from December 1962 to April 1963 show a clockwise circulation. Previous studies have implied that the general circulation of southern Lake Michigan was counterclockwise. Winter studies, including periods under an ice cover, indicate that northwest winds during the fall and winter period are responsible for a clockwise circulation during this period of the year. A new mooring system for current meters is also proposed. IN l33oduction A study of currents in Lake Michigan is part of the overall program of investigations undertaken by the Great Lakes-Illinois River Basins Project, U.S. Public Health Service. The ultimate objectives of the studies are: to assist in predicting the fate of pollutants introduced into the lake; to assess the effects of waste inputs on lake water quality-effects both local and areawide, both for the present and in the future; and to aid in making decisions for protecting this valuable water resource. The principal purpose of the current study is to determine the net circulation in the lake and the factors that influence the movements of water and to gain an insight on how and where chemical and biological constituents are moved and dispersed. Three studies on the currents of Lake Michigan have been reported. In the years from 1892 through 1894, a drift bottle study was made by Harrington ( 1895). More than 1,500 drift bottles were released in the lake, of which only 203 were recovered during the &year period. These returns formed the basis of the first map of the surface currents. The map showed a counterclockwise circulation in the southern basin. In , the U.S. Bureau of Fisheries completed an intensive study of surface currents using drift bottles and drift envelopes (Johnson 1960). Out of 6,260 releases in the lake, 2,871 were ~CCOVered. The 1954 results indicated that no stable pattern of surface currents existed during the period of study. In general, the drift was from west to cast, and the northsouth movements were about equal. A similar pattern existed in 1955, except that the drift in the eastern areas was mainly from the north. Ayers et al. ( 1958) published the results of a series of synoptic surveys in which they used drift bottles and applied a modification of the dynamic heights method to determine subsurface flow (Ayers 1956). The currents deduced from the drift bottle data were in good agreement with the counterclockwise patterns shown by Harrington ( 1895). As pointed out by Ayers et al. ( 1958)) the winds were westerly and what he considered as normal summer conditions. None of the above-mentioned studies reported on winter conditions. METHODS Of the various methods for the investigation of currents, a number of fixed stations with automatic recording meters was selected as most applicable to the Public Health Service comprehensive study of lake water quality. The Public Health Service adopted the current meter developed at the Woods Hole Oceanographic Institution ( Richardson 1962). The meter automatically records direction and speed for up to 6 months when operating for a l-min period every half hour. The meter has the following characteristics ( Richardson 1962 ) : 1) Speed measurement from the Savonius rotor. 426

2 INITIAL CUlUiENT STUDIES IN LAKE MICHIGAN 427 a) Minimum starting speed: 0.05 cm/set. b) Maximum speed: 256 cm/set. c) Approximate rotation rate: 100 rev/min. 2) Compass. a) Within 10 at 0.05 cm/set. b) Recording resolution: 2.5. The current speed and direction are recorded on 30-m rolls of 16-mm film and transferred electronically to magnetic tape. The tapes are analyzed on a CDC 1604 computer. The selection of station locations was based on the procedure of equal area and depth coverage where possible. Approximatcly 40 stations, reprcscnting an average area (1,300 km2) were established in Lake Michigan (Fig. 1). The number of meters per station posed a problem because of the variability of the thermocline. It was established that the meters should be set at fixed depths and more closely spaced in the upper layers. The depths selected were 10, 15, 22, 30 m, and each succeeding 30-m level. No instrument was set less than 10 m below the surface because of problems resulting from the orbital motion surface waves. The data may be recorded continuously or on a mode of 50 set of continuous recording once every 20 min (once every 30 min during the winter period ). The longer period was adopted for winter operation to provide a margin of safety against exhaustion of the power supply batteries inside each unit. During severe winter weather, it is impractical to visit the stations for retrieving data, replacing batteries, and other servicing. The original mooring system for current mctcrs used at Woods Hole in the Atlantic Ocean was modified for use in Lake Michigan (U.S. Public Health Service 1963). The oceanic installation used a synthet,ic line anchored to the bottom and held at the surface by a large float. Ice and severe winter conditions prohibited the use of this slack line mooring system in Lake Michigan. The adopted mooring system is FIG. 1. MICHIGAN Sites for current station locations. a two-line taut wire system with a subsurface float that has a positive buoyancy of 250 kg, which leaves the meter line almost completely free of external stress ( Fig. 2). A braided two-in-one nylon lint, or similar type, is used between the meters to prevent the individual meters from rotating on the taut line. The two-line system appears to work satisfactorily. Fins are now used as stabilizers for meters as an added precaution; however, line rotation has never been a problem. Our study began on 15 May The first station (number 4) was about 15 miles ( 24 km) northeast of Chicago. Additional stations were set in Dcccmber

3 428 JAMES L. VERBER Surface buoys ( one or the other ) )!$$sr Temperature recorders -Woods Hole Current meters - Woods Hole type +5/E braided nylon line I >Meters at each Succeeding 30-meter level wheel type anchor Lb4 The studies from a single station in the summer of 1962 show a predominant movement of water from the northwest (41%). This southward flow on the west shore of southern Lake Michigan tends to confirm the counterclockwise flow during the summer period shown by Harrington (1895) and Ayers et al. (1958). Stations 3, 4, 17, 18, 20, and 39 (Fig. 1) were operated between December 1962 and April For a period of time, late January and February, these stations were operated under a complete ice cover. Stations 3 and 4 showed a pronounced movcment from the south, 60 and 63%, respectively, during the period of study ( Fig. 4). [_ ,.5,-j e-e+ FIG. 2. Typical current station. 1962, and full-scale operations were begun in June 1966 for a l-year period. RESULTS The data reported herein are from the stations operated in the summer of 1962 and the following winter. It will be nearly a year before an analysis of the remaining data is complete. An examination of a portion of a typical current meter record (set at 36-m depth) at a 5-min interval (from a continuous rccord) shows that the speed ranges from 1 cm/set to 45 cm/set. A polar coordinate diagram of speed and direction, constructed from data taken at 18 m for the period 26 May to 26 July 1962 ( Fig. 3)) and a similar diagram made from the data for the 27-m level were examined. These meters were located at Station 4, which has a total depth of 47 m. The polar coordinate diagrams indicate a predominance of currents from the northwest during the summer. The maximum consecutive readings from the northwest varied from 52 hr to 104 hr, with an average movement of 5 or more km/day at the 18- and 27-m levels. No other quadrant showed a continuous movement of more than 36 hr from one direction. NOTE. Dnrectmn IS from the 3ector shaded, toward the center CURRENT 25 May to 26 July 1962 FIG. 3. Prevailing speed and direction, Station 4. Depth 18 m.

4 Station 17 showed a flow from the north in the shallow water (up to 20 m depth) for 60% of the time, and from the south 40% of the time. Station 18 showed a flow from the south nearly 60 and 70% of the time, for the lo- and 30-m depths. Station 20 showed the water movement to bc from the west in the upper levels ( above 22 m ) and shifting to the north at the 30- and 90-m level for the period of record. Winter observations at Station 4, December 1962 to March 1963, revealed quite a different type of situation than shown for May to July. Fig. 4 shows a prevailing flow from the south at the 10-m depth. The 15-m depth showed a similar pattern. The polar coordinate diagram shows the percentage of time in a given direction (in 20 sectors), and the histogram shows the speed vs. per cent of time for the period of observation. Calm conditions, current speeds less than 1 cm/set, occurred about 20% of the time for the period from Decembcr 1962 to April INITIAL CURRENT STUDIIB IN LAKE MICIIIGAN 429 ANALYSIS These preliminary studies showed marked differences in prevailing directions in the two seasons of observations. A flow from the northwest prevailed in late spring and early summer, while a flow from the south occurred during the winter period. The flow was from the north over the summer period and from the south in the winter, suggesting that a clockwise or counterclockwise circulation, according to the season, occurs in the southern basin of the lake. Wind movements during the winter, from the west and northwest, are apparently responsible for the northward flow of water near Chicago. Apparently, water pileup on the southeast shore of the lake drives the water southward along the eastern shore, around the end of the basin, and northward in the vicinity of Chicago. Data from Stations 3, 4, 18, and 20 support this idea. Wind profiles for the period of May to July 1962 showed no apparent relationship between wind direction and the deepwater currents. It is believed that the prevailing southwest winds during the CURRENT 6 January March 1963 FIG. 4. Yrcvailing speed and direction, Station 4. Depth 10 m. summer period are responsible for the southward-moving currents near Chicago. Th e polar diagrams of direction show that there is less variability in the winter, as compared to the summer period. The ice cover may be partly responsible for this difference. Perhaps the most unusual aspect is the speed records. The mean speed for all stations, winter and summer records, is about 8 cm/set. The summer average does not appear to be much different from the winter average, and average speeds at the 90-m depth are about 7 cm/set or 10% less than the speeds at the 10-m level. The speeds at the 96-m and at the 150-m depths do not vary appreciably (about 10

5 430 JAMES L. VERBER MEAN 24-HOUR WIND VELOCI - TIES FOR INDICATED DATES AT MIDWAY traveled 30.1 km to the northwest from its point of origin. The vector averages provide the basis for tracing water movements. KILOMETERS b l/21/63 FIG. 5. Vector averages. and 20% less) from those of the upper layers. Wind stress was probably responsible for the current speed and direction, IIowcver, under such conditions the current speed should decrease with depth. Also, under the total ice cover, speeds did not appear to decrease markedly, It would seem that pressure effects play an important part in the movement of the layers below 30 m. Differential pressure on the ice cover by wind could be responsible for some of the water movement during the midwinter period. Speeds as high as 50 cm/see have been recorded at the 150-m level. Defant ( 1961 ), in a discussion of ocean currents, disregards currents in the deeper layers as being negligible. Ayers et al. ( 1958) indicate that the maximum speeds are up to 24 cm/set (by use of dynamic heights ) relative to the 70-decibar surface, and speeds (deduced from drift bottle data) at the surface are about the same. These speeds arc about onehalf the maximum speeds found in the present study. The data were also plotted by using the daily average movement ( Fig. 5). hssuming that the daily conditions are approximately the same in a given area, the path of the water movement can be shown. In a 4-day period, 21 to 24 January 1963 (Station 4, 10-m level), the water would have SUMMARY Studies using the Woods Hole current meter from May 1962 to March 1963 suggest the possibility of a prevailing counterclockwise circulation in early summer and a clockwise circulation during winter in the southern basin of Lake Michigan. The water movements in these periods appcared to be consistent with prevailing winds. The two-line mooring system works well, as it reduces the effect of external stress on the meter lint to a minimum. The system works both in summer and in winter, when an ice cover can be expected. A braided nylon or similar line is on the instrument line to prevent spinning. Speeds as high as 50 cm/set have been found at nearly all depths as well as under an ice cover. It is suggested that prcssurc effects play a far greater role than has been previously thought. REFERENCES AYEM, J. C A dynamic height method for the determination of currents in deep lakes. Limnol. Occanog., 1: , D. CIIANDLEH, G. LAUFF, C. POWERS, ANI) E. HENSON Currents and water masses of Lake Michigan. Great Lakes Res. Inst. Publ. No. 3. Univ. of Mich., Ann Arbor. 169 p. DJSFANT, A Physical oceanography, v. 1. Pergamon, New York. 729 p. IIARRINGTON, M. W Surface currents of the Great Lakes, as deduced from the movcmcnts of bottle papers during the seasons of 1892, 1893 and U.S. Dep. Agr. Wcathcr Bur. Bull. B, 14 p. JOIINSON, J. II Surface currents in Lake Michigan, 1954 and U.S. Fish Wildlift Serv. Spec. Sci. Rcpt. Fisheries No. 338, 120 p. RICXIARIXON, W. S Instruction manual for recording current meter. Woods Hole Occanog. Inst., Reference No. 6~2-6, 10 p. U.S. PUBLIC HEALTII SERVICE Lake currents at a single station. Spec. Rcpt. No. LM9. Div. WS&PC, Great Lakes-Illinois River Basins Project. 15 p,