t 1

I'institut oceanographique de Bedford Dartmouth/Nova Scotia/Canada

Statistics of Currents for Navigation and Dispersion in Canso Strait and Come By Chance Bay

D. J. Lawrence, L. A. Foster, and R. H. Loucks

Report Spries BI-R-73-6 / May 1973

•

• May 1973

BEDFORD INSTITUTE OF OCEANOGRAPHY

Dartmouth, Nova Scotia Canada

STATISTICS OF CURRENTS FOR NAVIGATION

AND DISPERSION

IN CANSO STRAIT AND COME BY CHANCE BAY

by

D.J. Lawrence, L.A. Foster, and R.H. Loucks

Atlantic Oceanographic Laboratory Marine Sciences Directorate

Department of the Environment

This is an internal technical report which has receiv~d only limited circulation. On citing this report the reference should be followed by the words 'UNPUBLISHED MANUSCRIPT'.

REPORT SERIES BI-R-73-6

ABSTRACT

This report arose in response to requests for information that

would be of use to those using the supertanker ports located in the Strait

of Canso, Nova Scotia, and Come By Chance Bay, Newfoundland. It is some­

what unconventional in that the currents have been treated in statistical

w~s, rather than in the time sequence form in which the data were collected.

Specifically, the observed data have been displayed in the form of frequency

of occurrence of speed in each 30-degree direction segment, and they have

been analyzed to predict the width a pollutant plume might have at given

times after discharge. With the help of mathematical models, the data have

been extended to predict the extremes of current that might be expected to

occur once in any seven-summer period. This report contains the first

practical application of new techniques for estimation of plume width and

extremes.

(ii)

ACKNOWLEDGEMENTS

It is a pleasure to acknowledge the assistance or Des Dobson,

Bernadette Flemming,l Diane Ingraham,2 Ruth Sinclair,l and Bill Warshick

in the preparation or this report.

1. Supported by the Winter Works Program.

2. Supported by the Summer Student Program.

LIST OF CONTENTS

Summary ............................................•.......

Acknowledgement s ................................................................................... ..

List of Figures .........•.....•.....................••...••

Introduction •..•...............••..............•.•......•..

Data Collection ........•...•..........•...•••.........•.••.

Data Presentation .......•.•.••...............•.......•.....

Results

1.

2.

Strait of Canso, Nova Scotia

1.1 1.2 1.3 1.4 1.5

Physical Description and Industries .••...•. Observed Currents ...................•..•.•. Extremes Dispersion ................................................................ .. Internal Waves .....••...••.............•.••

Come By Chance, Newfoundland

2.1 2.2 2.3 2.4

Physical Description and Industry ......... . Observed Currents .............•....•....... Extremes Di spersion •..........................•...•.

Conclusion .....................•............•.......•......

Bi bli ography .......•.....•...•.••...•......................

Table 1: M2 Tidal Constants Determined for Canso Current Meter Stations ................. .

Table 2: Index of Current Meter Data, Come By Chance, Newfoundland •.........•.................

(ii)

(iv)

1

1

1

2

2

2 2 3 4 5

6

6 6 8 8

8

9

4

6

(iv)

•

LIST OF FIGURES

1. Map of Canso

2. Frequency .distribution of observed currents, middle of Strait of Canso

3. Frequency distribution of observed currents, mouth of Strait of Canso

4. Frequency distribution of observed currents, Chedabucto Bay

5. Predicted extreme currents, Canso

6. Dispersion scales vs. time, Canso

7. Time series of resolved currents from Station 43, Strait of Canso

8. Vertical profile of current at Station 4A, Strait of Canso

9. Station location map, Come By Chance, Newfoundland.

10. Frequency distribution of observed currents, Come By Chance, 3 metres, July 1968

11 . Frequency distribution of observed currents, Come By Chance, 5-6 metres, January 1972

12. Frequency distribution of observed currents, Come By Chance, 20-21 metres, January 1972

13. Frequency distribution of observed currents, Come By Chance, 5-6 metres, July 1972

14. Frequency distribution of observed currents, Come By Chance, 30-31 metres, July 1972

15. Predicted extreme currents, 5 metres depth, summer, for Come By Chance, Newfoundland

16. Predicted extreme currents, 5 metres depth, winter, for Come By Chance, Newfoundland

17. Dispersion scales vs. time, 5-6 metres depth, Come By Chance, Newfoundland

INTRODUCTION

The development of Come By Chance Bay, Newfoundland, and Canso Strait, Nova Scotia, as supertanker ports has generated a need for predic­tions of water currents insofar as they affect the handling of very large crude carriers (VLCC) and for the rate of dispersion of pollutants in the event of an oil spill.

The task of predicting water currents is complicated because only the tidal generating forces are regular in their variation. In general, tidal currents are small near the heads of bays, while other factors such as the effects of wind and land run-off in the presence of various water density gradients are large and not predictable. Our approach with respect to the questions of currents as they affect navigation and ship handling is to treat the tidal components, which are predictable, separately from the remainder or residual and to treat the residual statistically. In the event that the tidal constituents are too small to be significant, the total current is treated statistically. This statistical treatment is presented pictorially~ both as a display of the observed data showing the frequency of occurrence of the observed current for each 30 degrees of direction, and as an estimate of the extreme current as described in Loucks et ala (1973).

With respect to questions concerning the dispersion of pollutants, the approach is again statistical. The technique described by Lawrence and Loucks (1973) has been applied to estimate the width of a pollutant plume given the time elapsed since discharge. The plume width estimate is obtained by the analysis of the observed data in nonoverlapping blocks of a few days duration.

DATA COLLECTION

Surveys were carried out in Canso Strait by the Bedford Institute of Oceanography in 1968-1970 to determine patterns of water movement and to monitor water quality. The temperature, salinity, and water quality data have been reported (Lawrence, 1972). Currents were measured primarily by the use of moored meters of the shallow self-buoyant German Hydrowerkstaaten type. These record on film an instantaneous direction every 5 (or 10) min­utes and an average speed over the same interval, for periods up to 28 (or 56) days.

In Come By Chance, surveys were done in 1968 and 1972 using the same type of moored current meter.

DATA PRESENTATION

As previously mentioned in the Introduction, the current meter data are presented pictorially in the form of a histogram for each 30 degrees of direction ('dialplot'). This type of presentation (much like a wind rose but avoiding its inherent area distortion) was chosen to emphasize the statistical

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nature of the currents, the speed and persistence of which are important to the navigator. It was derived from the work of Neu (1971) on waves.

By referring to Figure 2 an example of a dialplot can be seen. At the centre of the plot is shown all the important mooring information -station number, depth of mooring, starting date, length of record, and speed scale. Also stated as a percentage of the whole record is the number of near zero current observations. Around the legend are displayed histograms representing for each increment of 0.1 knot the percentage of all 'observa­tions recorded in each 30-degree sector. In Figure 2, station 42, 10 metres depth, it can be seen that predominant current was in a direction 300 degrees with a speed range of 0 to 0.4 knot, most readings being about 0.2 knot. The maximum observed speed in each sector is indicated as a small vertical arrow - in a direction OOT, the maximum speed recorded was 0.2 knot.

RESULTS

1. Strait of Canso, Nova Scotia

1.1 Physical Description and Industries

The Strait of Canso separates Cape Breton Island from main­land Nova Scotia (Fig. 1). Since 1955 a causeway has linked the two land masses and has stopped the flow of water and ice. As a result, the strong currents due to the differing tidal regimes of the Atlantic Ocean and Gulf of St. Lawrence have been reduced to nearly negligible amplitudes and the Atlantic side now remains ice free. This part of the Strait, from the Causeway south to Eddy Point, is 20 km long, 1500 metres wide, and has a minimum depth of about 30 metres, making it one of the finest and deepest year-round ports on the North American continent. The entrance is well protected from ocean waves by several islands. The mean tidal range is 1.4 metres. Industrializa­tion has come in several thrusts. In 1962, a bleached sulfite pulp mill and gypsum loading facility were completed at Point Tupper. In 1969 the pulp mill was expanded to produce newsprint, and three interdependent industries were started in the Madden Cove area: an oil refinery, a thermal power plant, and a heavy water plant. Finally, in late 1972, announcement was made of a second oil refinery and common usage dock facility for the Mulgrave area. Both oil refineries are to be supplied by supertankers in the 200-300,000 ton class.

1.2 Observed Currents

The data in the form of 'dialplots' have been grouped according to location: vicinity of the dock serving the Gulf Oil refinery, the entrance to the Strait, and Chedabucto Bay. Specific details can be found in the individual legends. Characteristics of the current are as follows:

a. Strait of Canso, near oil dock (Fig. 2): Here the currents were very strongly confined in direction to the axis of the Strait. In the upper layer (10-12 metres), the inner stations (42 and 43)

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had velocities generally less than 0.3 knot, with the preferred flow direction being inward (NW). The outer station (77) had velocities generally less than 0.4 knot, with neither flow direction dominating. In the lower layer (32-33 metres), velo­cities were generally less than 0.2 knot, with Station 42 showing an outward flow dominance. Velocities were not obtained in the lower layer at the outer station.

b. Entrance to Strait of Canso (Melford Point, Fig. 3): The only measurements were 12 metres depth. They showed the directions restricted somewhat "to the axis of the Strait , with southeast or outgoing flow being slightly favoured. Velocities were usually less than 0.3 knot.

c. Chedabucto Bay (Fig. 4): In the centre of the bay (Station 2, depth 3 metres) the predominant directions were north-south with south the more frequent. With increasing depth, the directions became more uniformly distributed. The velocities were generally less than 0.3 knot. Near Cerberus Rock (Station 85, 3 metres) the most favoured quadrant was southeast, with velocities generally less than 0.5 knot. However, there were an appreciab1enumber of readings up to 0.8 knot in the east-northeast sector.

1.3 Extremes

From Figures 2 to 4 the observed extremes of current in the various directions can be seen. However, the observed records are limited in length to several weeks. To make predictions for much longer periods, one needs a model to provide the relationship between the magnitude of extremes and the frequency of occurrence or return time. Such relationships have been observed or derived or postulated in several fields: e.g. winds (Draper and Wu, 1969), waves (Draper, 1963; Neu, 1971), meteorological vari­ates in general (Gringorten, 1966, 1968) and hydrology (Mandelbrot and Wallis, 1968). The above papers all deal with scalar quantities. In order to deal with vectors, i.e. rate and direction, some adaptations have been made (Loucks et al., 1973). Basically the extreme current E along a given coordinate axis is given by:

E = M ±SY

where M is the over-all mean current,

S is the average standard deviation of the current,

Y is a scaling factor for the extremes (Gringorten, 1968).

By splitting each record into block~ of several days, estimates of the errors were obtained for M, Sand Y. These error estimates and S itself were increased to allow for the increased variance to be expected when predicting for periods longer than one block (Loucks et al., 1973). The technique is new, this being its first practical application.

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Using a data block size of 4 to 6 days, predictions haye been made of the magnitude of the current that might be expected to be exceeded once in every three-summer period (500 days), and its standard error. These have been depicted in Figure 5. Note that this figure also depicts the tidal signal, which is seen to be relatively small.

As expected, the predicted extremes tend to have much the same features as the observed values: as one progresses landward, the magni­tudes decrease and the flow becomes more channelled. The largest extremes were at station 85, Cerberus Rock, 3-metres depth, 1.27 ±0.25 knots in direction 135°T. (This value includes a tidal component of 0.12 knot, and the uncertainty or confidence limit is due to the combination of the errors in all the terms of the extreme equation.)

The semidiurnal tide (M2) was removed from the current meter data before the analysis for extremes was done. The tidal amplitude and phase lag values are given in Table 1.

TABLE 1

M2 Tidal Constants Determined for Canso Current Meter Stations

Major Component Minor Component Station Depth Start Axis Ampli- Phase Axis Ampli- Phase

(see Direc- tude Lag Direc- tude Lag Fig. tion (knots) (AST) tion (knots) (AST) 2-4) (OT) (OT)

42 10

32 10/07170 3150 0.05 1350 45 0 0.007 3090

43 11

33

77 12 16/07/69 3300 0.08 1910 60 0 0.004 3380

78 12 16/07/69 2900 0.10 1660 20 0 0.005 1310

40 12 09/07170 3050 0.08 119 0 35 0 0.007 3290

85 3 07/06/68 270 0 0.11 l30 0 00 0.06 1560

1.4 Dispersion

In the first practical application of a new technique. the dispersion scales S(t) were estimated from the current meter data in the minor direction (Lawrence and Loucks, 1973.) This direction was chosen to be at right angles to the mean current (mean over the full length of each record). Thus it is the cross plume direction. The data were analyzed in nonoverlapping blocks of 4 to 6 days. The variations

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between blocks were small compared to the variation from one station to another, and showed no correlation with wind conditions or the depth of the instruments. The magnitude of the dispersion scales increased with distance seaward from the Causeway (Fig. 6). Note that the slopes of the dispersion curves decrease at scales comparable with distances to the nearest shoreline, due possibly to boundary effects.

A further use for the dispersion data is in the estimation of the time-dependent lateral diffusivity, K(t), which is useful in hydro­dynamic calculations. It can be defined as:

where

K(t) = 1/2 ~ (02 ) dt

0 2 = variance of the distance between water parcels.

If we identify our dispersion scales S(t) with 0 (see Lawrence and Loucks, 1973, for discussion), then the diffusivity can be eValuated.

1.5 Internal Waves

In tracking parachute drogues in the Strait, it was found that their motions usually did not agree well with the ebb and flood predictions based on tidal heights. Examination of the current meter records showed that the tidal signal was not the only one present (Fig. 7 for typical example). The interfering signal seemed to be regular, with a period of several days. This fact is brought out more clearly when the data are low­pass filtered to remove the tides (Fig. 7). Nearly all of the records avail­able showed such periodicities, for part if not all of their length. At Stations 2, 42, and 43 (1970) where there were observations at more than one depth, the long period signal tended to be out of phase at the two depths (10 and 32 metres).

Such behaviour suggests the motion is in the form of an internal wave. In its simplest form, the water moves inwards in one layer and out in another, while the surface level remains undisturbed. To examine this possibility, we have used data from a current meter that was lowered in one-metre steps from a vessel anchored at various points in the Strait, for periods of up to 13 hours. Typical data (Fig. 8) show a three-layer structure. The motion in the top layer (0 to 8 metres) was found to be rather variable, and somewhat dependent on the local wind. The depth of the top layer corres­ponded roughly to the depth of the thermocline (Lawrence, 1972). The internal wave took place in the lower region where there was a more uniform density gradient.

The exact mechanism for the generation of these internal waves is not known. Heath (1973) has studied the phenomenon in several inlets along the Atlantic coast, and has found it is related to the passage of storms. It has not been established whether it is due to the change in atmospperic pressure, or to the associated winds. Thus the frequency of occurrence of the storms largely determines the period of the internal waves. They are a forced rather than a free resonance type of phenomenon.

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Since storms are present at all seasons of the year, the internal waves would also be expected to be present although their ampli­tude and the relative thickness of the two layers might vary as the density gradient varied. Since storms also cause changes in the sea level, the 1971 tide gauge data from Point Tupper was examined after low pass filtering. Oscillations with periods of about 4 days were indeed present at all seasons. Seibert (1968) found that in this frequency range, the sea level changes were definitely related to atmospheric pressure changes. Since the occur­rence and amplitude of these waves ' is impossible to predict, another reason is provided for choosing to treat the currents in a statistical fashion.

2 . Come By Chance, Newfoundland

2.1 Physical Description and Industry

Come By Chance Bay is located at the northern end of Placentia Bay, Newfoundland (Fig. 9). Physically, it is 3.3 km wide at the entrance and 6.5 km long with a navigable depth of 40 metres. It has a mean tide range of 1.6 metres and is a secondary port of Argentia (near Placentia) at which place a permanent tide gauge is maintained. For this study, current meter data were collected in Come By Chance as shown in Table 2 with the station positions detailed in Figure 9. With the exception of station 95-68, all stations were located in the main shipping channel.

A 100,000 barrel a day refinery is being built at Come By Chance and is scheduled to be operational in the fall of 1973. In February 1973, an announcement was made that an additional 300,000 barrel a day refinery was to be built.

Station No.

95-68

96-68

92,93,94,95,96

92,93,95,97,98

TABLE 2

Index of Current Meter Data Come By Chance, Newfoundland

Depth Starting Date

3 m July 14, 1968

3 m July 14, 1968

5,6,20,21 m Jan. 8, 1972

5,6,30,31 m July 12, 1972

2.2 Observed Currents

Length of Record

15 days

28 days

27 days

23 days

In the 1972 survey, current meters were duplicated at each depth (i.e. 5 and 6 metres) for comparison purposes and for a back-up in

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case one meter failed. In the discussions and diagrams that follow, the data from only one of the two meters at each depth are presented. In most cases, the duplicate meters had similar records. In cases of very weak currents, the record with the fewest number of zero readings was selected. All the records were analyzed for tidal constituents and it was found that in every case, the tidal signal was too small to be significant (less than 0.05 knot). These results are similar to those of Long Harbour (Trites, 1969) and St. Georges Bay (Seibert, 1972) for which the conclusions were that the wind was the dominant factor in water movement, with tide and fresh water influence being secondary. As discussed in the Introduction, the data from the current meters are displayed in the form of 'dialplots' (Fig. 10-14). The plots have been grouped by season and depth, but will be discussed by location:

(a) Head of Bay. off oil dock: At Station 92 currents at all levels were weak both in summer and winter.

(b) Middle of Bay: In the January 1972 records, the currents at 5 metres from Stations 93 to 96 all indicated a southeasterly flow with the current becoming stronger outside the bay mouth - the general speed range being 0.2 to 0.6 knot. In the July 1972 records this same trend was found at 5 metres at Station 95; however, at Station 93 (5 metres) the trend was to the south­west but weaker .. The July 1968 record from Station 95-68 showed a north­easterly current at about 0.1 to 0.3 knot. At 20 to 30 metres in depth the currents were very weak and variable at stations 92 to 96 though a south­easterly trend was noted at 95 and 96 in January 1972. This trend is similar to that at 5 metres at the same locations.

(c) Outside the Bay: These stations (97, 98 and 96-68) exhibited diverse patterns. The July 1968 record at Station 96-68 indicated an essentially constant direction of southwest with speeds of 0.2 to 0.5 knot. It should be noted that this location is somewhat off the shipping track. At Station 97, occupied only in July 1972, the stron~est current of all the moorings was found. At 5 metres the current was due west at the start of the mooring period with speed up to 1 knot. The direction slowly changed in a counterclockwise (backing) manner throughout the month until at the end of the mooring period the current was flowing in a northerly direction. The back-up meter at 6 metres confirmed this pattern. At 31 metres the current was generally to the northwest with speeds of 0.2 to 0.4 knot. The situation was completely different at nearby Station 98 during the same period where this circular current was not evident and the speeds were much smaller both at 5 and 30 metres. The difference was unexpected since both Stations 97 and 98 were in the shipping channel and only 2 km apart.

Wind data from Arnold's Cove (47°45'N, 54°00'W) was made available by the Atmospheric Environment Service for the periods of mooring in January and July 1972. It was possible to see some subjective correla­tions between winds and currents (i.e. with small wind speeds or variable direction, the currents were reduced) but no detailed correlations could be seen over the mooring periods.

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2.3 Extremes

Using a statistical model as outlined in Loucks et al. (1973) and reviewed in Section 1.3, an estimate was made of the extreme current likely to be exceeded only once in every three-summer period (500 days) for various current meter sites - the results are shown in Figures 15 and 16.

As in the case of the observed currents, the most significant predicted extreme current is found at Station 97 at 5 metres - here the extreme is estimated to be about 1.6 knots but with an uncertainty of ±1.3 knots. The large uncertainty at this station is a result of the large, slowly varying direction changes found in the original record. No other stations on the shipping channel showed such a large predicted extreme and, in general, the magnitude of the extremes decreased towards the head of the b~.

2.4 Dispersion

The 5-metre current meter records were analyzed in nonover­lapping 4- to 6-day blocks covering different wind conditions. It was found that the predicted width of pollutant plume at a given elapsed time since discharge, became greater as the distance increased from the head of the b~ (Fig. 17). It was also noted that the variation of plume width estimate with wind conditions was less significant than the variation with distance from the head of the b~. As in the case of Canso (Section 1.4), it was found that the slope of the dispersion curve decreased at scales comparable with distance to the nearest shoreline.

CONCLUSION

To describe the climate of the current regime, all available data has been presented in three statistical formats: the frequency of occurrence of each vector, the extreme current to be expected, and the probable width of a plume of pollutant at a given, time after discharge.

In the Strait of Canso, the most significant feature was the presence of internal waves. These had periods of 3 to 4 days, the motion was usually in opposite directions in upper and lower water layers, and the amplitudes were up to 0.15 knot. Thus they were of comparable or greater magnitude than the tides. However, unlike the tides, these waves are not predictable well in advance as they are believed to be related in a complicated fashion to the passage of weather systems along the Atlantic coast.

In Come By Chance, the most significant feature was the presence at Station 97 of a strong, slowly backing current. This feature was not present at Station 98, only 2 km aw~. The amplitudes were up to 1 knot, with the direction taking 23 days to complete 270 degrees. No explanation has been found for such behaviour, but it would appear to warrant further investigation.

In both areas the only predictable part of the current signal - the tides - were found to be so small as to be insignificant compared with

•

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currents due to other causes (winds, internal waves, etc.). Therefore, if simultaneous information is needed to ensure safe docking of VLCCs, it is recommended that current measurements be taken from or near the dock before and during the docking operation.

BIBLIOGRAPHY

DRAPER, L. 1963. Derivation of a 'Design Wave' from Instrumental Records of Sea Waves. Proc. Inst. Civil Engineers (London) 26: 291-304.

DRAPER, L., and H.J. WU. 1969. Extreme Mean Wind Speeds over Canada. Canadian Dept. of Public Works, Marine Engineering Division, Design Branch, Report No. 14.

GODIN, G. 1972. The Analysis of Tides. The University of Toronto press.

GRINGORTEN, 1.1. 1966. A Stochastic Model of the Frequency and Duration of Weather Events. J. Appl. Meteor., 5: 606-624.

GRINGORTEN, 1.1. 1968. Probabilities of Moving Time Averages of a Meteorological Variate. Tellus, XX (3): 461-472.

HEATH, R.A. 1973. Flushing of Coastal Embayments by Changes in Wind Associated with Atmospheric Pressure Changes. Submitted to Limnology and Ooeanography.

LAWRENCE, D.J. 1972. Oceanographic and Water Quality Parameters in the Strait of Canso, 1968-1970. Atlantic Oceanographic Laboratory (Bedford Institute of Oceanography) Data Series BI-D-72-10.

LAWRENCE, D.J., and R.H. LOUCKS. 1973. Dispersion in the Ocean Estimated from Current Meter Data - to be published.

LOUCKS, R.H., D.J. LAWRENCE, D. INGRAHAM, and B. FLEMMING. 1973. A Technique for Estimating Extremes of Ocean Current Vectors. Atlantic Oceanographic Laboratory (Bedford Institute of Oceanography), Report Series BI-R-73- / D I a...,.......~ 13- cf\- ') 5 (p

NEU, H.J.A. 1971. Wave Climate of the Canadian Atlantic Coast and Contin­ental Shelf - 1970. Atlantic Oceanographic Laboratory (Bedford Institute) Report Series 1971-10.

SEIBERT, G.H. 1968. Mean Sea Level Fluctuations in the Gulf of St. Lawrence. M.Sc. Thesis, McGill Univ.

SEIBERT, G.H. 1972. Physical Oceanographic Study of St. George's Bay, Newfoundland. Atlantic Oceanographic Laboratory (Bedford Institute of Oceanography) Report Series BI-R-72-2.

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TRITES, R.W. 1969. Capacity of an Estuary to Accept Pollutant&. Pxoc. Conf. Poll., Chem. Inst. Can., Halifax, N.S., August 24-26, 1969, pp. 148-160. (Also reprinted in Effects of Elemental Phosphorus on Marine Life. Fisheries Res. Bd. Canada, Circular No.2. Jangaard, P.M. ed. (1972).)

FIGURE 1: Chart showing the Strait of Canso area and the associated industries. The same chart, but without nam~s, is used in Figures 2 to 4.

"'1 ..... \Q ~ Ii (\)

I-'

•

45° 35'

30'

25 '

20 '

Causeway

25' 20'

15 ' 6 1°10 '

CERBERUS ROCK +

CH£OABUCTO BAY

15' 61°10'

OS'

OS'

45° 35'

30'

25'

FIGURE 2: Frequency distribution (dialplot) of observed currents near the middle of the Strait of Canso. See Figure 1 for the key to the chart.

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Figure 2

FIGURE 3: Frequency distribution (dialplot) of observed currents at the mouth of the. Strait of Canso. See Figure 1 for the key to the chart.

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z ~ 10 stAr UN '10 0[P1H 12"

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.. ' ". : .... :,; .. :/ .. : .... .' .. :.: . ...::.>.:.:~

Figure 3

FIGURE 4: Frequency distribution (dialplot) of observed currents in Chedabucto Bay. ' See Figure lfor the key to the chart.

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..... 1. OF' All OBSERYATIONS ARE lESS THAN .0" KNOTS

Figure 4

FIGURE 5: Predicted extreme currents for Canso, Nova Scotia, for the follow­ing depths: Station 43 at 11 and 32 metres, Stations 77 and 78 at 12 metres, and Station 85 at 3 metres. The large ellipses represent the random signal, the dashed lines are the estimated plus and minus errors. The small solid line or ellipse in the centre is the tidal part of the signal. In using the plots, distances from the centre of the speed scale to the large solid­line ellipses represent speed in a chosen direction. In any chosen direction the total predicted extreme would be the random part plus the tidal part. The prediction is based on current meter data, and it is the extreme that is estimated to be exceeded only once in every three-summer period (500 days) (Loucks et al., 1973). If all the speeds are increased by 20%, then these will represent values that have only a 2% probability of being exceeded once in every three-summer period.

In making these calculations, specific valuees used were: autocorrelation p = 0.94, Gringorten's scaling factor y = 3.7 ±0.2, Hurst factor H = 0.97, block size. 6-7 days, record length = 3 weeks(5), time interval =; 5 minutes (10). Bracketed values apply to Station 77 only. Since doing the production runs on the data, several small discrepancies have come to light. The y estimated from the graph (Loucks et al., 1973) should be higher by about 10%. However, to match the low frequency end of the composite spectrum, one should perhaps use a succession of Markov models having increasing values for p. This would give a series of lower values for y. Thus these two changes tend to cancel out. Finally, the value for the Hurst factor was estimated from the literature, but when a program was available to calculate it directly from the current meter data, an average value of 0.87 was found for the Strait of Canso stations. The net effect would be to decrease the augment factor for Stations 43, 78, and 85 from 2.19 to 1.62. Similar effects might be expected for station 77. This would reduce the large extreme current ellipses by approxi­mately 10%, but they stand now as conservative (large) estimates. The necessity for these corrections betrays the newness of the technique which was developed in the course of this project.

I •

I I

I,

I

I.

'I

?

1.0

r-\ " \ '- .... ,- 0.5 ,

1.0

0.5

, , , \

.. .... : . . :" .85

1.0

,-----.- 0.5

I \ ,---\ ( ---" \ '--::-'-, '\ " " , , " , , "

" '........ 0.5 " "" '_' KNOTS\

',1.0 \

"-':. .. ) -"'. -----,1 I ~,~.-~

---._/

--------_/ 1.0 ---,

"

\ \ ,.

,I

/,,/

// / ---- -" "" I // - '

I / - ' I I 0.5 " "-

/ I ' f / '\ \ I / \ \ I I ,\ I f \ \ I I \ \ I I O~S_----~~--~\~~~----j\-\ \ 0.5 I \ \ KNOTS! 1.0 1.51

\ \ I I '\ \ I I

\ \. I I " I I " / / , " / /

, ' / I

'" ----------// // -, / " ///

--- .--// ----------

Figure 5

FIGURE 6: Dispersion scale~ vs. elapsed time for Canso, based on data from moored current meters reso~ved along coordinate axes in the minor or cross plume direction. Station locations can be found in Figures 2 to 4. All instrument depths were covered (3 to 44 metres). The curves give estimates of the plume width after a given travel time from the source. The values are those that would enclose 96% of all the possible plume shapes (Lawrence and Loucks, 1973). For stations in the Strait the minor direction was at right angles to the axis of the Strait, for Chedabucto Bay the minor direction was 90oT. The results from the analyses from all the 4 to 6 day nonoverlapping blocks from stations as noted were combined to form three composite curves consisting of two straight line segments each.

(/) Q) ~

10000

~ 1000

W ...J i= z w u ffi 100 a.. w (1')

10 100 1000 TIME (MINUTES)

Figure 6

FIGURE 7: Time series plots of data from moored current meters at Station 43. See Figure 1 for location chart. Data have been resolved along the major or axial coordinate direction (315°T). The time axis starts at 00 hours (AST) 11 July 1970. The upper plot is of raw data. In the lower two plots the data were filtered using successive unweighted running averages of 25, 25, and 24 hours (Godin, 1972). This removed all frequencies higher than about 0.8 cycles/day, thus effectively cutting out the tidal signal. The resultant data shows motion that has a period of several days, and is out of phase between the two depths. This has been attri­buted to an internal wave. The filter response factor is 0.000 at 12 and 24 hours, 0.248 at 2 days, 0.724 at 4 days, and 0.868 at 6 days.

0.3 DEPTH 11m

0.2

Cf) 0.1 t-o Z ~ 0

-0.1

-0.2

0.2 DEPTH 11m (filtered)

0.1 Cf) t-o

0 z ~

-0.1

0.2

0.2

DEPTH 33m Cf) 0.1 (filtered) t-o Z ~ 0

- 0.1

, , , 5 10 15

DAYS

Figure 7

FIGURE 8: Vertical profile of current in the Strait of Canso, anchored-ship Station 4A (0.33 nautical miles, Il00T from Station 42, see Figure 2). The instrument was a Bendix, Model Q9, current meter that uses a Savonius rotor and small directional vane.

-en w a:: r w ~ -I r a....

o

10

~O

w 30 o

40

50

RATE (KNOTS) O.~ 0.1 0 0.1 O.~

TOWARD OCEAN TOWARD CAUSEWAY

STATION 4A (8) 26 JULY 70

1846 - 1949 ADT

. Figure 8

•

FIGURE 9: Station location map for Come By Chance, Newfoundland. The same map is used at the centres of the dialplots (Figures 10 to 14).

•

I'Ij .... ~ Ii (1)

~

•

47· 49'

COME BY CHANCE

6 2 km

47· 45 '

97.

98·

[r 54· 5'

'4.~' ."-S8

" . 96.

98-S8

.:.-::'::"::,":' '.'

NEWFOUNDLAND

~d o

56· I

•

54· I

49·

48·

ST· JOHN'S

47°

FIGURE 10: Frequency distribution (dialplot) of observed currents for Come By Chance, 3 metres depth, July 1968.

•

~ z

N 4'

LECEND

~ lD STAT leN 9S D(P T" )"

~ ~~ 1 o ----~ .. ,

S'U' T ,,,[ 1'4' 7i 68

LENCTH !!If RHtlAO 28 OATS

RAJ[ IN " .. tITS

3 . 7 1. Of ALL OBSERVATIONS RR( LESS THAN . 0" !l;N(lrs

COME BY CHANCE

O~' --...... -~2 km

•

•

••

•

•

• .. ' . ....

" , .

z

c C

L D

N 4'

LEGEND

ID STAT tON 95 DEPTH )"

START l["E 1'4 ' 1168

UNorM Of AHORD 15 OATS

I.' RATE IN KN~rs

21.81. or ALL OBSERVRT(O"lS AR( LESS THRN . 0 .. KNOTS

Figure 10

..

FIGURE 11: Frequency distribution (dia1p1ot) of observed currents for Come By Chance, 5 to 6 metres depth, January 1972.

L~ N 4'

SUit ,1ft[ " 1112

l[ ..... Ir 1(('10 27 01'"

... Un 'M .. ITS

n.o1. Dr _Ll .ISERVAfleNS ME L(" nUIiN .Olt I(NIUS

-.

N 4'

:: II . -~ J> v J_

'" ~ - . ~~ -LDD

LEI£IIID

sra"... II DE"" III

STAU IItil 1/ 1/12

UN"" If .lUIO 21 aI's

O ..... -~--, •• • aU '" ••• "

1ft ... l er,," .I[~"rrawl Mr l.ISS r .... . 0. IeMDrI

L

I

I L=-.....~_ ~~ .-~.

LEeEND

w

~ ID STATU'N 911 DEPTH Sit

" '" ~ J > U JW C~

'" W ~~a LDD

UNaTH elF .£CaID 21 OAf'S

o ..... ~~---... RATE 1M "Mens

]5.37. elF qU 8aS[lVATU!NS ARE LESS THAN .Olt KN~rs

• •

•

"0 ' •

:: " .. ' .. : . . . ' .....

-

N 4'

~ II STaTteN 92 DEPTH '"

• C ~J> V J_

C ~ - . ~~ a L DD

LUll" or REC310 27 DAYS

O ..... ~~~~-, ••

RATE," "G"

l'LO 7. eF ALL aaSERVATU'HS ARE LESS THAN .:'Ilt I("!'ITS

L ___ N 4'

lEaUD

SUlIGIO n DE'TH S"

SUU liRE 1/ 1111

lUIlM sr IECSID I' D.tI

O ..... ~~---, •• UTE 1M "SIS

n.o % cr Al.L S8SERVAlICHS JW: LESS INIIOI .00 KlI8ra

Figure 11

FIGURE 12: Frequency distribution (dia1p1ot) of observed currents for Come By Chance, 20 to 21 metres depth, January 1972.

..

N ~

! It "au'" " Di,r. taft

" •• , tUll " lin

U."III ., I(C'IO " DI"

o .... ~-~~~, .• •• n ••••• n

15.07. eF aLl .SUVA .. "'" .. t LES' JNM .0. KNe"

1'1.1 1.

N ~

LUlIa

= ,I IUflltN " D(pfM 20"

IUU flllE '1 111'2

LE.,," IF llealD 27 DAYI

o ...... ---~\.. UTE ._ •• en

ar _ll "IU:VAftOHS M' LEII TN ..... 0'1 tcHars

~

N ~

LEDUD

~ 10 S'IUION" DE"" 21M

STAIT TInt 1/ tilt

LEMarM er REceRD 21 DAYS

o ...... ~--~, .• RATE III lUIafS

20.t 1. or ALL eeSERYArraNS ARE LESS THAN .Oll KNOTS

LEUUD

~ a = 11 STAUIN It DEPU 20N

a c ~~ ~ u ~« C~

« n ~~ m LO>O>

STRIT TUE " 111!

LENUTH eF RECelD 2} DAYS

O ...... -~~~, •• un III lUIOTS

1t.07. fir ALL OBSERYAtleNS ARE LESS THAN .0" "NOTS

N ~

LEUENO

,. STATleN 9) DE"H 210

a ~~ START TInE 1/ lI72 u ~

C c L 0>

LENHH eF RE:eRD 2} DR'S

0 , .. RATE III KNors

20 . 77. or ALL aBSERVAflONS ARE LESS TMAN .. 0'1 KNOTS

Figure 12

FIGURE 13: Frequency distribution (dialp1ot) of observed currents for Corne By Chance, 5 to 6 metres depth, July 1972.

o

..

N <t

LUUD

~ to IUrU!.. 15 DEPIH '"

&lAU Ill' III lin

LUll" SF UtSID 23 DAU

o , .. IAf( 'N USI6

1.7 Z eF ALL fIaSE'VAlUINS ARE LESS rHAN .0"1 KN!nS

..

N <t

LU'NO

~ Ii SfATHUi !11 DEPTH 6" ~ ~

: ~ ~ UhRT TIRE 121 1112 u~~ :: ~ :: LuaTH SF REtSIO 23 OATS Lan ... ~~---, ..

• ATE 1M .lIens

29.' X III" ALL aSSEtVAriSHS AIlE LE66 lHAII .0. lHSIS

~

N <t

LEIiIEND

~ \0 srATleN 92 OEPTH SiR

SlRtY T UE 121 7172

LEN9TM dF RECeRO 23 DAYS

, .. lATE IN KNtHS

88.7l ar ALL 8aSERYATU,NS ARE LESS TNAH .Olf KHeTS

..

N <t

~ 10 stlTUN I' GEPTH '"

nAU 1l1E Itl lin

LUIlH SF UtaRa 23 DAU

, .. RAIE IN lNal&

72.3 l eF ALL eaSERVATUINS ARE LES' THAN .0"1 KHIUS

~ z

N <t

::! 10 ~ ~

" '" ~~>

u~" "'~ " ~ ~~m

Laa

LunD

SIAriaN u aEPIH '"

SIAII liKE 121 lin

LENII" SF REtaRD 23 DAYS

O .... ~~---, •• RATE IN USIS

9&.0 7. er ALL fIaSERVAT UNS ARE LEGG THAN .0'1 KHaTS

FIGURE 14: Frequency distribution (dia1p1ot) of observed currents for Come By Chance, 30 to 31 metres depth, July 1972.

I £T; I

L ___ _ _ _ _ 1,--_ ------ l. , _____ _

. W J

c C

L n

i. <-I.

"

lfGOIC

STATtON 9S DEPTH

srA'" lIft(

L£H!HH er '£CaRO

I .• RAH IN IUtlHS

121

23

31"

lin

DRYS

7"1,9 7. or RU 09~friVIH ["!t<OS ARC L(SS THAN .011 KNrHS

w

N ~

LEGEND

~ ,a STAJlftH 91 DEPTH 31"

z c W J > UJC

C W C W W~ m Lan

l£N9TH fir RECORD 23 DAYS

I .•

RAH IN Kwors

76," 1. er RLL OBSERVATIONS ARE LESS THAN .011 KNOTS

L~

LEGEND

~ 10 STRTItlH 92 DEPTH 31"

srAIH "ME 121 7112

lEHGTH Of RECCHID 23 DAYS

I .• RATE IN KHOTS

97.11 i: or RLL OBSERVATIONS ARE L(SS THAN .0"1 KNOTS

I L_-l ~ ~~.~.

w

N ~

lU£NC

~ 10 STAUON 9J CEPTH )Oft

z c W J > U JC

C W ~ W

L aa

SJARf T 1"[ 121 il12

l(NUM Of R£CORO 23 CAYS

I .•

lATE IN KNOTS

91.81. !!IF ALL eSS(RVAf[ON5 ARE LESS TH!;N .011 KNe'rS

N ~

l(BEND

w z !:: to STATION 98 DEPT" 31"

,. ~

W J > U J~ ~ W

~ W W ~ ~ ~ aa

START TIME 121 7112

LENGTH OF RECflRO 23 DAYS

1.0

RATE IN KNIHS

11.21. OF ALL OBSERVRTIONS ARE LESS THAN .0Ll KNOTS

L~

Figure 14

1

FIGURE 15: Predicted extreme currents at 5 metres depth for the summer season at Come By Chance, Newfoundland. The solid-line ellipses repre­sent the predicted value, the dashed lines are the estimated plus and minus errors. In using the plots, distances from the origin of the speed scale to the solid line represent speeds in any chosen direction. The prediction is based on current meter data, and it is the extreme that is estimated to be exceeded only once in every three-summer period (500 days) (Loucks et al., 1973). If all the speeds are increased by 20%, then these will represent values that have only a 2% probability of being exceeded once in every three­summer period.

In making these calculations, specific values used were: autocorrelation p = 0.94, Gringorten's scaling factor y = 3.7 ±0.2, Hurst factor H = 0.97, block size = 3 days, record length = 14-28 days, time interval = 5 minutes. Since doing the production runs on the data, several small discrepancies have come to light. The y estimated from the graph (Loucks et al., 1973) should be higher by about 10%. However, to match the low frequency end of the composite spectrum, one should perhaps use a succession of Markov models having increasing values for p. This would give a series of lower values for y. Thus these two changes tend to cancel out. The necessity for these corrections betrays the newness of the technique which was developed in the course of this project.

i I , I \

I

/

\ \ \ \

\

,

( I

'.

, , '-

'-'-

........ -

1.9 ... __ .., .... ,

// ~5 / / / I

I I

"

\ , ,

" , ....

"

.... -

----

-----..-..-

, , , \

\ \ \ \

I.Oi I ,

{

I I

I I

I ./

/ ./

./

- -1:5

/' ,.,---

/0.5 /

/ I

{

0 I \ , ,

, , , \ \

0.5

--- ----- ................. .....

1.0

"-

/ --------- -------------------

BY CHANCE

6 2 km

47- 415'

-t!

.... .... " " " " , ,

----

, \ \ I I

3.0

I I I I / I / I I I 1 \ \ \ ,

() \

1.0 ----- .... -/

/ .... , / " /

,

\ / /

/ I 0.5 \

I --- ...... \ , / " I / / , I , / I , / I , I / {

I I 0 {

I I 1.0 / \ I

I \ / , '- ----- /

1 / \ /

/ , / , /

./ , , ., ,

"- .... -._---

I.~ .... ----- .... _"

, , ./

, ./ , \

\ ./ \ ./ / 1.0

\ ./ \

./ \ I \ /

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

I ./ 0.5

\ I / \ I I I I \

I I I . / I / I I

I I I I I I {

I I 0 I 10.5 I.q

/ I KNOT I I I I I I I I I I , / I I / I 1 / I I \ / /

/ / "----,

/ /

/ /

./ /

/ / , ./

/

" /

" ..-, .... ....... _-----

Figure 15

FIGURE 16: Predicted extreme currents at 5 metres depth for the winter season at Come By Chance, Newfoundland. The solid-line ellipses repre­sent the predicted value, the dashed lines are the estimate~ plus and minus errors. In using the plots, distances from the origin of the speed scale to the solid line represent speeds in any chosen direction. The prediction is based on current meter data, and it is the extreme that is estimated to be exceeded only once in every three-winter period (500 days) (Loucks et al., 1973). If all speeds are increased by 20%, then these will represent values that have only a 2% probability of being exceeded once in every three­winter period.

In making these calculations, specific values used were: autocorrelation p = 0.94 (0.93), Gringorten's scaling factor y = 3.7 ±0.2 (3.6 ±0.2), Hurst factor H = 0.97, block size = 3 days, record length = 27 days, time interval = 5 (10) minutes. Bracketed values apply to Stations 92 and 93. Since doing the production runs on the data, several small discrepancies have come to light. The y estimated from the graph (Loucks et al., 1973) should be higher by about 10%. However, to match the low frequency end of the composite spectrum, one should perhaps use a succession of Markov models having increasing values for p. This would give a series of lower values for y. Thus these two changes tend to cancel out. The necessity for these corrections betrays the newness of the technique which was developed in the course of this project.

•

I I I I \ \ \

.,

\

/ /

\ \

I. / .... -----

I I

-/

\ , ,

0.5

o.

1.0

............... , , \

\

-'

I /

\ \ I I I

I

1.0

/ I

I

/

, /

_--0.5 .-­ --- ........ _-r-_ , /--

/ .... ,

\

" \

\ I I

I I \

O''----bl--I---,,., 1.0

\ \ ,~

\' ....... ~~

...._---

-------

Figure 16

FIGURE 17: Dispersion scales vs. elapsed time for Come By Chance, Newfoundland, based on data from moored current meters resolved along coordinate axes in the minor or cross-plume direction. Station locations can be found in Figure 9. The instrument depths were 5 to 6 metres. The curves give estimates of the plume width after a given travel time from the source. The values are Lhose that would enclose 96% of all the possible plume shapes (Lawrence and Loucks, 1973). The minor direction was chosen to be at right angles to the mean current direction for each 4 to 6 day block. The results from all the blocks from the stations noted have been averaged to form three composite curves consisting of several straight line segments.

1

'J

10000

If)

<1.l ~

) -EIOOO

I

1 I-a 3: w :::!: :::) ..J a.

W ..J I-Z W U a::

100 w a. <D en

10 100 1000 TIME (MINUTES)

Figure 17