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The role of tidal mixing in Rupert and Holberg Inlets Drinkwater, Kenneth Francis 1973

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Co THE POLE OF TIDAL MIXING IN RUPERT AND.HOLBERG INLETS by KENNETH FRANCIS DRINKKATER B.Sc, University of Calgary, 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE -in the Department of Physics and the I n s t i t u t e of Oceanography Vfe accept t h i s thesis as conforirdng t o the required standard. THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1973 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of fiWis\&s C Oc£ftMe>«-.s.ac»viM^ The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date Y[M \ yq-*3 i i ABSTRACT Analysis of monthly observations of temperature, s a l i n i t y and dissolved oxygen content i n the basin formed by Rupert and Holberg I n l e t s reveals a greater degree of mixing than that found i n most B r i t i s h Colum-b i a I n l e t s . Although r e l a t i v e l y uniform water properties are constantly found, there are large monthly variations of the actual values. The water temperature correlates with the so l a r r a d i a t i o n while the s a l i n i t y changes follow the r i v e r runoff which i s i n turn con-t r o l l e d by p r e c i p i t a t i o n . The v a r i a t i o n i n dissolved oxygen content appears due to a combination of b i o l o g i c a l influences and i n f l u x of P a c i f i c Oceanic water. A model has been developed which ascribes the monthly f l u c -tuations and v e r t i c a l homogeneity t o an accumulation of i r r e g u l a r mixing events associated with the t i d a l flow through Quatsino Narrows, a shallow connecting channel. Thermal microstructure measurements disclose a region of deep turbulent mixing near the narrows and provide evidence of an up-inlet flow beneath the tiiermocline i n Rupert and Holberg I n l e t s . TABLE OF CONTENTS Page ABSTRACT . . . . . i i LIST OF TABLES ......... V LIST OF FIGURES v i ACKCWLEDGEMENTS v i i i Chapter I. INTRODUCTION . 1 1.1 General Intrcduc±ion 1 1.2 Geographical D e s c r i p t i o n . . . 1 1.3 Oceanographic Data 3 I I . CHARACTERISTICS OF THE RUPERT-HOLBERG BASIN WATERS . 6 II.1 Monthly V a r i a b i l i t y i n the P h y s i c a l P r operties 6 I I . 2 River Runoff and P r e c i p i t a t i o n 11 11.3 T i d a l and Fresh Water Inflow Volumes 11 11.4 Discussion of Data 16 I I I . ENERGY CONSIDERATIONS 24 111.1 Energy Input . 24 111.2 Energy Required f o r Mixing 27 111.3 Comparison of Energy Input to Energy Required f o r Mixing 29 IV. DESCRIPTIVE MODEL . 30 V IV. 1 A Model 30 IV. 2 Discussion 30 i v Page V. THERMAL MICROSTRUCTURE MEASUREMENTS . 34 V . l I n t r o d u c t i o n 34 V.2 General D i s c u s s i o n o f Thermal M i c r o s t r u c t u r e . 38 V.3 M i c r o s t r u c t u r e i n the Rupert-Holberg B a s i n . . 45 VT. COMPARISON WITH OTHER SHALLOW SILLED INLETS . . . . 52 VTI. CONCLUSION . . . 55 REFERENCES . 57 V LIST OF TABLES TABLE Page I. Comparison of the volumes of the Poipert-Holberg basin t o the estimated t i d a l prism and f r e s h water inflow i 5 I I . Energy required f o r mixing o f water from Quatsino Narrows i n t o the Rupert-Holberg basin during the times of the mine conducted surveys 28 v i LIST OF FIGURES FIGURE Page 1. Map of Rupert and Holberg Inl e t s 2 2. Quatsino Narrows . . . „ 4 3. Monthly values of temperature, s a l i n i t y and d i s -solved oxygen content at Station B, i n Holberg I n l e t . 7 4. Monthly values of temperature, s a l i n i t y and d i s -solved oxygen content at Station D, i n Quatsino Sound 9 5. Depth p r o f i l e s of temperature, s a l i n i t y and dis-x " solved oxygen content f o r Station B, i n Holberg I n l e t and Station D, i n Quatsino Sound during September 1971 and February 1972 10 6. Daily p r e c i p i t a t i o n recorded at the mine s i t e and , and the d a i l y discharge frcm the Marble River. . . . . 12 7. Weekly and monthly means of p r e c i p i t a t i o n record-ded and the mine s i t e 13 8 . Weekly and monthly means of the Marble River out-flow. . 1 3 9. Comparison of a i r temperatures with sea surface temperatures 17 10. Chlorophll "a" measurements at Station A-in Rupert I n l e t • •. • 2 0 11. P l o t s of constant 0*4 surfaces f o r UBC and POG data 23 12. Schematic diagram showing flow conditions between Quatsino Sound and the Rupert-Holberg basin during flood and ebb t i d e conditions 25 13. Diagram explaining several of the constituents found i n equation (1) . • • 31 14. Map showing location of microstructure measurements wi t h i n Rupert and Holberg Inl e t s 35 15. T i d a l record at Coal Harbour on Holberg I n l e t show-ing the time at which the microstructure measurements were taken 36 v i i FIGURE Page 16. A r e g i o n o f s t i r r i n g observed d u r i n g drop number 14 39 17. Temperature g r a d i e n t a s s o c i a t e d w i t h thermal i n t e r f a c e on drop number 21 40 18. "Older" i n t e r f a c e s observed i n Rupert I n l e t d u r i n g drop number 16 41 19. A t u r b u l e n t i n t r u s i o n d u r i n g drop number 23 43 20. M i c r o s t r u c t u r e a s s o c i a t e d w i t h the mixed l a y e r and the thermocline i n Holberg I n l e t 44 21. Comparison o f the c e n t r a l and outboard t b e r m i s -t o r s from t r a c e s taken d u r i n g drop number 22 46 22. Deep s t i r r i n g found o p p o s i t e Quatsino Narrows on drop number 22 . . . . . . 48 23. Deep s t i r r i n g i n Rupert I n l e t d u r i n g drop number 18 • 48 "24. Map o f Fortune Channel 5 4 v i i i I am s i n c e r e l y g r a t e f u l t o a l l who have a s s i s t e d i n the p r e p a r a t i o n o f t h i s t h e s i s . I would e s p e c i a l l y l i k e t o thank my s u p e r v i s o r , Dr. T. R. Osbom f o r the guidance and encouragement he has g i v e n me. A l s o my thanks t o Dr. G. L. P i c k a r d f o r h i s many h e l p f u l comments and h i s c r i t i c i s m o f t h i s manuscript. P r o f e s s o r J . B. Evans was most h e l p -f u l ' i n making a v a i l a b l e the data c o l l e c t e d by Utah Mines. The h e l p o f Mr. C. P e l l e t i e r o f Utah Mines has been much ap p r e c i a t e d . I am a l s o g r a t e f u l f o r the co-operation from a l l the members o f the I n s t i t u t e o f Oceanography o f UBC. F i n a l l y , I w i sh t o thank the N a t i o n a l Research C o u n c i l o f Canada who have supported me p e r s o n a l l y d u r i n g the l a s t tavo years o f t h i s study. CHAPTER I -INrRODUCTION 1.1 General Introduction I n d u s t r i a l , s c i e n t i f i c and p u b l i c i n t e r e s t i n Rupert and Holberg I n l e t s was generated during 1970 with the a p p l i c a t i o n and subsequent granting of a permit t o discharge wastes from a copper-molybdenum mine i n t o the bottom of these i n l e t s . To determine the environmental e f f e c t s of t h i s d e c i s i o n , those processes which c o n t r o l the n a t u r a l phenomena wi t h i n t h i s region must be understood. Although s e v e r a l published data reports e x i s t f o r the Rupert-Holberg system, d i s c u s s i o n of i t s p h y s i c a l oceanography has been l i m i t e d t o b r i e f comments i n a general d e s c r i p t i o n of Vancouver Island i n l e t s by Pickard, (1963). The infl u e n c e o f f r e s h water inflow, the uniformity of the water prop e r t i e s and the high oxygen values i n the deep water were mentioned. I t i s i n these respects that Rupert and Holberg d i f f e r most s i g n i f i c a n t l y from other i n l e t s i n B r i t i s h Columbia. This paper attempts t o explain the unusual uniformity of the water prop e r t i e s and t h e i r monthly changes. 1.2 Geographical Des c r i p t i o n Rupert and Holberg I n l e t s are located near the northern end of Vancouver Island (Fig. 1) and possess a shape t y p i c a l o f B r i t i s h Columbia f j o r d s (Pickard, 1956) being elongated, narrow bodies o f water having roughly p a r a l l e l s i d e s . Rupert I n l e t i s 10 kilometers long, 1.8 k i l o -meters wide and has a mean mid-channel depth of 110 meters. Holberg I n l e t i s 34 kilometers long, 1.3 kilometers wide and has a mean mid-3 channel depth of 80 meters (Pickard, 1963). Together they form a basin of 170 meters maximum depth which i s separated frcm Quatsino Sound and Neroutsos I n l e t by Quatsino Narrows.' This r e s t r i c t i o n i s a long, narrow channel with a sharp bend at i t s southern extreme. I t s depth decreases northward, culminating i n the s i l l or minimum depth of 18 meters west of Makwazniht Island (Fig. 2). The p r i n i c p a l r i v e r a f f e c t i n g the water properties of the Rupert-Holberg basin i s the Marble Paver. I t has a drainage area of 512 k i l o -2 meters, flowing from A l i c e Lake and entering Rupert I n l e t near the junc-t i o n with Holberg and Quatsino Narrows. This discharge location presents a d i f f e r e n t s i t u a t i o n to that of the t y p i c a l i n l e t i n which the fresh water enters near the head. 1.3 Oceanographic Data The oceanographic data used i n t h i s study have been co l l e c t e d by three agencies. The I n s t i t u t e of Oceanography at the University of B r i t i s h Columbia has conducted three cruises i n t o the Rupert-Holberg area. A survey i n -cluding Quatsino Sound-Neroutsos I n l e t was undertaken i n May of 1959. Two cruises investigating only Rupert and Holberg were conducted during 1971, one i n March, at which time thermal microstructure measurements were taken, and the other i n A p r i l . During the years 1965-67, the P a c i f i c Oceanographic Group of the Fisheries. Research Board of Canada, Nanaimo, B r i t i s h Columbia (Waldichuk et a l . , 1968) conducted an extensive survey of several i n l e t s along the west coast of Vancouver Island which included the Quatsino Sound Region. Surveys during August 1957, November 1962 and August 1966, included 4 Figure 2: Quatsino Narrows s t a t i o n s i n Rupert arid Holberg I n l e t s . A programme t o determine the e f f e c t s o f the copper-molybdenum mine was begun i n the s p r i n g o f 1971 and i s p r e s e n t l y scheduled t o run u n t i l September 1976. Monthly surveys, encompassing s i x s t a t i o n s ( F i g . 1) , i n c l u d e d observations o f temperature, s a l i n i t y and d i s s o l v e d oxygen. A f t e r June 1972, s a l i n i t y and d i s s o l v e d oxygen were measured once every t h r e e months. Temperature was recorded u s i n g an A p p l i e d Research A u s t i n Model ET 100 Marine Thermometer u n t i l March 1972, a f t e r which r e v e r s i n g thermometers were used. S a l i n i t y and d i s s o l v e d oxygen were determined by t i t r a t i o n u s i n g the methods d e s c r i b e d by S t r i c k l a n d and Parsons (1968). A l l d a t a from the mine monit o r i n g surveys were made a v a i l a b l e through P r o f e s s o r J . B. Evans, Chairman o f the M i n e r a l E n g i n e e r i n g Department o f The U n i v e r s i t y o f B r i t i s h Columbia. CHAPTER I I CHARACTERISTICS OF THE RUPERT-HOLBERG BASIN WATERS I I . 1 Monthly V a r i a b i l i t y i n the Physical Properties Plots of the monthly values, of temperature, s a l i n i t y and dissolved oxygen content f o r several depths at Station B are shown i n F i g . 3. The data collected during the surveys conducted by the mine show uniform d i s t r i b u t i o n of these properties w i t h i n Rupert and Holberg I n l e t s and hence t h i s s t a t i o n i s representative of the e n t i r e basin. The temperature throughout the water column changes s t e a d i l y with a minimum i n early spring and a maximum i n l a t e summer or e a r l y autumn. Within Holberg the maximum temperature i n the deep water occurs during early October whereas i n Rupert and at Station E the maximum i s found at"the beginning of 'September. Below 30 meters, the water i n Rupert I n l e t during September i s approximately 0.2°C higher than at an equiva-l e n t depth at Station B. During October i t i s 0.2° C lower. S a l i n i t y shows more v a r i a b i l i t y with a general trend of high i n mid-summer and l a t e winter and low i n early spring and l a t e autumn. Surface s a l i n i t i e s at Station B (not plotted) vary between 12%»and 31%o, while below 30 meters s a l i n i t i e s vary from 29%« to 32%o, . Below 30 meters, the difference between maximum and minimum sa l i n i t y - i s 3 , independent of depth. Monthly changes of the order of l%«.are not uncommon. As density (and C \ ) l s Primarily dependent upon s a l i n i t y , the decreases i n s a l i n i t y correspond to decreases i n i t s density. The dissolved oxygen content tends towards high values i n winter' and spring, with lower values during summer and autumn. S i g n i f i c a n t short 8 term increases i n oxygen appear during August 1971 and December 1971. Monthly data f o r Quatsino Sound, Station D, are displayed i n F i g . 4. The v a r i a b i l i t y df the upper layers, surface t o 60 meters, i s s i m i l a r t o that within Rupert and Holberg. The bottom water (60 to 116 meters) has di f f e r e n t c h a r a c t e r i s t i c s with a maximum i n January and a smaller tempera-ture range than the upper layers. Except during the autumn months, the s a l i n i t y trends at s t a t i o n D, fo r a l l depths are a l i k e and follow a pattern close t o that of the basin. The difference i n the s a l i n i t i e s between the top and bottom layers i s greater at st a t i o n D, than i n the deeper Rupert-Holberg basin. The mag-nitude of the s a l i n i t y fluctuations decrease with depth at s t a t i o n D. , Although a wider range of oxygen values e x i s t w i t h i n the water column at stat i o n D, the trends of the oxygen are s i m i l a r to those found i n the basin. A notable increase,especially i n the upper layers, occurs during August 1971. Oxygen values at the 30 and 46 meter depths are f r e -quently higher than those found at the 9 meter depth. Cause of t h i s un-usual. occurrence may r e s u l t from an i n f l u x of low oxygen values i n the surface waters of Neroutsos I n l e t due to the existence of a pulp and paper m i l l at Port A l i c e . The m i l l i s s t i l l i n operation. Figure 5 shows the temperature, s a l i n i t y and dissolved oxygen content p r o f i l e s at Stations B and D during surveys taken i n September 1971 and February 1972. These months are representative of summer and winter structure respectively. A l l stations within the Rupert-Holberg basin exhib i t s i m i l a r shaped p r o f i l e s to those of Station B, excepting Station E, where a s l i g h t l y greater degree to v e r t i c a l uniformity normally e x i s t s . These p r o f i l e s indicate that'below 30 meters a more homogeneous Water body i s found i n the basin, than i n Quatsino Sound. . STATION D 9 33 32 28 27 P. \ • A / * \ / V / o V A v 7 \ °/\ \ r * I I i i i l I I , I J 1 L M A M J J A S O N D J F M A M J 7* LU O >-X o o UJ >J o to to D J J I L_ J I I l_ J l_ M A . M J J A S O N D J F M A M J 1971 1972 Figure 4: tonthly values of temperature (a) s a l i n i t y (b) and d i s s o l v e d oxygen content (c) S t a t i o n D i n Quatsino Sound CL UJ Q SALINITY (%o) 30 31 200 L Figure 5: Depth profiles of temperature (a), s a l i n i t y (b), and dissolved oxygen content (c) for Station B i n Holberg Inlets and Station D i n Quatsino Sound during September 1971 and February. o Studies by Pickard (1961 , 1963) indicate the water properties i n most B r i t i s h Columbian i n l e t s are not as uniform as those found i n the Rupert-Holberg basin. 11.2 River Runoff and P r e c i p i t a t i o n The close correspondence between p r e c i p i t a t i o n and the Marble River runoff i s shown i n Figures 6, 7 and 8. The d a i l y values of p r e c i p i t a t i o n and r i v e r discharge during the months of October and November,1971 are displayed i n F i g . 6. The p r e c i p i t a t i o n was recorded at the mine s i t e . The discharge data were supplied by J.S. Areneault of the Canadian Fisheries Service (personal communication). Generally, fluctuations i n p r e c i p i t a t i o n produce corresponding fluctuations i n the discharge. How-ever, the actual r e l a t i o n s h i p between these two features i s non-linear. The discharge data r e f l e c t the p o s s i b i l i t y Of a f i l t e r i n g e f f e c t on the p r e c i p i t a t i o n , f o r example, several of the low frequency fluctuations i n p r e c i p i t a t i o n are not evident i n the discharge data. A l a g of approxi-mately one to two days e x i s t s between peaks i n r a i n f a l l and r i v e r d i s ^ charge. Figures 7 and 8 show the weekly and monthly means of p r e c i p i t a t i o n and Marble River discharge f o r a l l the available data between March,1971 and June,1972. The sources of data were the same as f o r F i g . 6. The peak i n p r e c i p i t a t i o n during early September i s of questionable magnitude. As i n the d a i l y data, both the weekly and monthly means show a correspon-dence between p r e c i p i t a t i o n and r i v e r runoff. 11.3 T i d a l and Fresh Water Inflow Volumes A comparison of the basin volume to the t i d a l prism and fresh water 12 Figure 6: Daily p r e c i p i t a t i o n recorded at the mine s i t e and the d a i l y discharge from the Marble Paver. 150 o <0 E: iu 100 o tc < o to o E 50 > 0 5£ o WEEKLY M E A N o MONTHLY M E A N -1 1 1 ' ' I I L J 1- L M A M J J A S O ' N D J F M A M J 1971 1972 Figure 8: Weekly and monthly means of the Marble Pdver outflow. 14 inflow i s given i n Table 1. The volumes of Rupert and Holberg Inl e t s are calculated on the basis of a rectangular basin. The mean values previously quoted are used f o r the dimensions. The t i d a l prism i s de-fined as the surface area of the i n l e t s m u l t i p l i e d by the increase i n depth due t o the t i d e . The average t i d a l prism i s determined using a t i d e of 2.8 metres while an increase of 4.2 metres i s used f o r maximum conditions. These are the ranges f o r Goal Harbour (Canadian Tide and Current Tables(1971)). 2 A drainage area of approximately 580 km surrounds Rupert and Holberg 2 I n l e t s , excluding the Marble River drainage of 512 km . No major r i v e r system e x i s t s within the former area. Thus based.entirely on the r e l a t i v e sizes of the drainage areas, the t o t a l volume of the fresh water flow i n t o the basin i s assumed to be approximately twice that of the Marble River. The average discharge i s tabulated from the t o t a l of a l l d a i l y measurements divided by the number of days. The maximum inflow occurred November 11, 1971. 15 Table 1. (a) Volume of Rupert-Holberg Basin and Quatsino Narrows Volume (10 6 m3) Rupert I n l e t 2000 Holberg I n l e t 3400 Rupert-Holberg Basin 5400 Quatsino Narrows 30 (b) Volume of T i d a l Inflow (Tidal Prism) 6 3 Volume (10 m / t i d a l cycle) Average Conditions 170 Maximum Conditions 260 (c) Volume of Fresh Water Inflow 6 3 Volume (10 m /day) Average Conditions 12 Maximum Conditions 40 16 II.4 Discussion of Data The v e r t i c a l uniformity of temperature, s a l i n i t y and dissolved oxygen content, accompanied by t h e i r month to month v a r i a b i l i t y suggests a process operating on a time scale of less than a month which i s capable of increas-ing or decreasing the s a l i n i t y , temperature.and density of the e n t i r e basin water. Increasing deep s a l i n i t i e s and densities can be attri b u t e d t o i n -trusions i n t o the basin of denser water from Quatsino Sound. However the periods of decreasing bottom densities require an i n f l u x of energy to rai s e the po t e n t i a l energy of the water wi t h i n the basin. Comparison of the sea surface temperatures at s t a t i o n B with the monthly mean a i r temperatures and the mean a i r temperatures 3 days immed-i a t e l y preceding the sea measurements are shown i n F i g . 9 . A l l a i r temper-atures were recorded at-the mine s i t e . A closer resemblance between the sea surface temperatures and the 3 day average of the a i r temperatures implies the surface layers are controlled by short term changes of a i r temperature. Determination of the actual r e l a t i o n s h i p requires further data. I t seems safe t o assume however that the temperature of the sur-face layers are primarily controlled by solar influences. The s i m i l a r i t y i n temperature trends between the deep water and the surface layers (Fig. 3b) suggests these solar influences are f e l t throughout the water column. This further suggests the existence of a v e r t i c a l heat transfer mechanism within the basin. The likeness between the monthly fluctuations of surface s a l i n i t y and those of the deeper water wi t h i n the basin (Fig. 3b) i n f e r s the existence of a mixing process. These s a l i n i t y changes are inversely related tos fluctuations i n r i v e r discharge (Fig. 8) and hence also p r e c i p i t a t i o n (Fig. 7 ). • The time response of the basin water to discharge changes appears to be between one and four weeks. o — - o MONTHLY AIR TEMPERATURE MEAN AIR TEMPERATURE 1971 1972 Figure 9: Comparison of a i r temperatures with sea surface temperatures. 18 More frequent s a l i n i t y data are required to es t a b l i s h an accurate time response. Pickard (1961) has described seasonal variations i n Bute I n l e t and Indian Arm where s a l i n i t y fluctuations are observable to 30 and 50 meters respectively while temperature changes are apparent to 100 meters depth i n both. The extent of the seasonal v a r i a b i l i t y i n the Rupert-Holberg Basin thus indicates a d i f f e r e n t im-xing process than normally found i n B r i t i s h Columbian i n l e t s . Assuming s i m i l a r conditions of solar r a d i a t i o n and r a i n f a l l e x i s t on both sides of Quatsino Narrows, the temperature and s a l i n i t y d i s t r i b u -tions of the upper layers of Quatsino Sound and Rupert-Holberg I n l e t s would have marked resemblance. Thus mixing of surface water from e i t h e r side of the narrows i n t o the basin would produce the same r e s u l t . The question as to the r e l a t i v e amounts of water from either source that are mixed i n t o the Rupert-Holberg Basin can only be answered a f t e r determining the cause of the mixing process. The near uniformity of the oxygen content throughout the water column implies frequent mixing within the basin. The lower values during the summer and autumn may be attributed to an i n f l u x of low oxygenated water from the P a c i f i c Ocean into Quatsino Sound. Lane (1962) has shown upwelling to occur during surtmer along the coast of Vancouver Island (off Amphitrite Point) and then move in t o the shelf. Pickard (1963) reports evidence of upwelling o f f Quatsino Sound but with no d i s t i n c t seasonal va r i a t i o n s . The r i s e i n s a l i n i t y during June,1971 and the low terrperatures w i t h i n the deep water during the summer at Station D are addit i o n a l evidence f o r upwelling. Also during the summer, sinking organic matter, caused by an increase i n 19 b i o l o g i c a l activity^may use up oxygen i n the deeperiwaters. An increase of chlorophyll (Fig. 10) indicates the high August oxygen values to be associated with a plankton bloom. Investigation i n t o t i d a l e ffects leads to a possible explanation of the mixing. Assuming complete and instantaneous mixing with an average t i d a l volume per t i d a l cycle, then i n one month about 15% of the o r i g i n a l water w i l l remain i n the basin. The average monthly fresh water discharge i s 1/15 of the e n t i r e volume of the basin. During a high runoff month t h i s volume inflow doubles. Mixing the average fresh water inflow i n t o the basin throughout the water column would produce s a l i n i t y changes of approximately 2 Too . . Excellent conditions f o r mixing e x i s t w i t h i n Quatsino Narrows due t o i t s geographical configuration (Fig.„2). The majority of water flowing i n t o or out of Quatsino Sound must pass through a narrow channel north of Quattische Island as shallow banks p r o h i b i t the movement of large volumes of water south of the i s l a n d . On a flood t i d e , t h i s e f f e c t produces a narrow eastward flowing current which passes north of Quattische Island. A large d e f l e c t i o n i s needed to r e d i r e c t the current northward towards Rupert and Holberg. I t s momentum however, r e s i s t s any immediate r e d i r e c t i o n of flow and causes the current to continue i t s approach towards the eastern shore of Quatsino Narrows. Surface current data (Canadian Hydrographic Service, 1972) show the current to s p l i t near the shore, part moving south to create a large eddy south-east of Quattische Island and part hugging the east shoreline as i t heads north through the narrows. Several smaller eddies, produced by indentations along the eastern shoreline are also observ-ed further up the channel. 20 JUL AUG SEP 1971 Figure 10: Chlorophyll "a" measurements at Station A i n Rupert I n l e t . 21 Data co l l e c t e d i n August 1957 (Waldichuk et a l . , 1968) during high t i d e suggests that t h i s eddy s t i r s and mixes the water. Over the 20 meter depth surveyed, the density changed by only 0.27 of a u n i t and an i n s t a b i l i t y was present between 9 and 14 meters. The density of the upper 10 meters was greater than that observed at equivalent depths outside the narrows. At 20 meters depth however, the density was s l i g h t l y less i n the narrows than that observed i n Quatsino Sound. Mixing w i l l also be associated with eddies. During an ebb t i d e , s i m i l a r conditions t o that of the flood t i d e produce mixing within the same region. The upper layers of the Rupert-Holberg basin flow southward through the channel and beyond Ohlsen Point. The required r e d i r e c t i o n of the flow westward i n t o Quatsino Sound i s at f i r s t prohibited by the momentum of the flow. The current studies previously mentioned show the formation of a large anticlockwise eddy south-east of Quattische Island and a westward flow past the northern end of t h i s i s l a n d . Very turbulent water throughout the narrows has been personally observed during both flood and ebb ti d e s . The volume of Quatsino Narrows, between Makwazniht Island and Quattische Island (Table 1), i s approximately one s i x t h of the average t i d a l prism. As the tides are semi-diurnal the water wit h i n the t i d a l prism spends approximately 1 hour within the narrows. This allows s u f f i c i e n t time f o r the mixing, described above, to occur. Thus the narrows mixes the fresher and denser sections of the upper layer which enters the channel. This mixing r e s u l t s i n the density of the water wit h i n the narrows being greater than that of the surface l a y -ers outside. Upon entering the Rupert-Holberg Basin on a flood t i d e , t h i s water w i l l sink beneath the less dense, low s a l i n i t y layer formed by the 22 Marble River. Less energy i s then required to mix t h i s water i n t o the deeper water of the i n l e t as i t i s already below or at le a s t p a r t i a l l y through the pycnocline. Plots of isopleths of crt are shown i n F i g . 11. These enforce the concept of exchange as envisioned above. The bottom waters i n the basin are less or at l e a s t an equivalent density to the upper 40 meters of water i n Quatsino Sound, i n d i c a t i n g that changes i n the upper layers w i l l l i k e l y be r e f l e c t e d by changes i n the bottom waters. Although isopleths are drawn smooth through the narrows, the water i s probably more homogeneous than indicated for reasons previously discussed. Observations suggest that the mixing energy i s supplied by the t i d e and the process enhanced by the penetration of the water below the pycnocline. Figure 11: Plots of constant surfaces f o r (a) UBC data,(Pickard)data and (b) POG data (waldichuk, et al) CHAPTER I I I ENERGY CONSIDERATIONS I I I . l Energy Input The rate at which available energy flows i n t o the Rupert-Holberg basin i s calculated using the method of Taylor (1919). Consider a given volune of water which at time t i s enclosed by a surface area A (extending the entire depth under consideration). The rate at which energy (W) i s transported across surface A i s given by: ^ w (D+h) " ~ n - , " a£= /<pg S~-'(v 0 0 3 0 ) (D+h)}dS + fp (D+ h) ^ (v cose) dS 2 (J) +f £~ (^h) (v cose) dS \ where h i s the t i d a l height above mean sea l e v e l p i s the density of the f l u i d (assumed homogeneous) g i s the acceleration due to gravity t i s the time measured a f t e r an a r b i t r a r y turn t o flood D i s the depth of the sea f l o o r below mean sea l e v e l V i s the v e l o c i t y 6 i s the angle between the current d i r e c t i o n and the element dS and dS i s the element of length along A (Fig. 12) . The f i r s t term represents the rate a t which work i s done by the mean hydrostatic pressure on that portion of f l u i d o r i g i n a l l y w i t h i n A; the second term i s the rate of g r a v i t a t i o n a l p o t e n t i a l energy transport across A, with zero p o t e n t i a l at mean sea l e v e l ; and the l a s t term i s the rate at which k i n e t i c energy crosses A. Figure 12 : Diagram explaining several of the constituents found i n equation (1). Combining terms = pg / Dh v(cosG) dS +2. / v(cos9) (2gh2+ Dv2+ hv 2) dS (2) dt ' 4 Averaging over one t i d a l cycle t h i s equals the energy dissipated by f r i c t i o n and by r a i s i n g the g r a v i t a t i o n a l p o t e n t i a l energy by mixing. This assumes no net increase of k i n e t i c energy w i t h i n tine basin. For the Rupert-Holberg system the t i d a l height and v e l o c i t y t o the f i r s t approximation take the form 2-rrt v = -V s i n where H i s the maximum current v e l o c i t y , h = H C O S -=— ?Tri-f—-T \ V i s h a l f of the t i d a l range, T i s the t i d a l period, and T 0 i s the t i n e difference between high water i n Rupert I n l e t and turn t o ebb a t Quatsino Narrows. Averaging over one t i d a l c ycle, the l a s t three terms dissappear and the average rate of energy crossing the s i l l (U), assuming conditions are uniform across the channel, i s given by 1 PTT'T1 U = 2 (sin=^i) (cos9) DL ( 3 ) where L i s the length across the s i l l . The numerical values of the constituents are p = 1.023 gm/cms g = 9.8 m/sec2 H = 1.4 m V = 7.0 knots = 3 . 6 m/sec T = 12 hr>25 min = 745 min To = l l m i n sin-^°= s i n 5°18'* .09 0 =0° cos6= 1 D = 20 m I. = 250 m -Substituting i n t o equation (3), the average energy which must be dissipated w i t h i n the basin by f r i c t i o n and mixing i s found to be on the order of 10 ? joules/second. This value m u l t i p l i e d by the t i d a l period (T) gives 5 x 10'1 joules as the t o t a l energy to be dissipated during one t i d a l cycle. The average energy per u n i t volume of the incepting t i d a l water i s then the t o t a l energy divided by the t i d a l prism and has a value of 3 x 10 3 joul.es/meter? III.2 Energy Required f o r Mixing Energy to overcome the buoyancy force i s needed when mixing waters of unequal density. To mix a layer of thickness h^ and den-s i t y i n t o a thicker layer l ^ o f density ^ some mechanism must do an amount of work per un i t area of which reduces to (prp,) g ^ • To simulate flood conditions i n Rupert and Holberg I n l e t s , a 10 meter layer, whose density corresponds to the 9 meter depth at s t a t i o n D, over a 150 meter layer whose density equals that found at the bottom of the basin i s chosen. Table 2 shows the mixing energy per u n i t volume required assuming i t i s contained wi t h i n the upper layer. These values were obtained by d i v i d i n g the work done per unit area by the thickness of the upper layer. 28 Table 2 Energy Required f o r M i x i n g During Times o f Mine Conducted Surveys Month ENERGY March 8-10, 1971 1.02333 1.02344 80 A p r i l 5-6 1.02326 1.02353 198 June 14-18 1.02434 1.02488 397 J u l y 6-8 1.02332 1.02407 552 Aug 2-4 1.02387 1.02487 736 Sept 1-2 1.02319 1.02440 891 Oct 4-5 1.02306 1.02360 397 Dec 7-11 1.02223 1.02297 528 Jan 3-6, 1972 1.02319 1.02368 360 Feb 1-3 1.02389 1.02405 51 March 17-22 1.02200 1.02325 ; 920 A p r i l 5-7 1.02238 1.02306 501 May 1-3 1.02245 1.02257 88 June 5-7 1.02291 1.02359 501 29 II I . 3 Comparison of Energy Input To Energy Required For Mixing On an average, each cubic meter of water wit h i n the t i d a l prism contains excess energy of the order of 3000 joules. I f rnixing s i m i l a r to that assumed i n the model d i d occur, then between 50 and 1000 joules per cubic meter of incoming water would be needed. Much energy would be dissipated by molecular v i s c o s i t y . However i f only a small f r a c t i o n of the available energy per t i d a l cycle i s used i n r a i s i n g the p o t e n t i a l energy, i t s continual i n j e c t i o n i n t o the basin would s t i l l produce the required mixing. Energy between 250 and 3000 joules per cubic meter of inccmLng water would be needed to mix the surface water from Quatsino Sound in t o the basin. Thus the mixing wit h i n the narrows decreases the required energy f o r mixing i n the basin by approximately 2 to 3 times. CHAPTER IV < DESCRIPTIVE MODEL I V . l A Model A descriptive model i s developed to v i s u a l i z e c l e a r l y the processes which occur. Figure 13 shows a diagrammatic picture of flood and ebb t i d e conditions. A three layer system i s considered i n Quatsino Sound: A, an upper layer; B, the pycnocline region; and C, the bottom water. Layer D, represents the homogeneous water with i n Quatsino Narrows. Rupert and Holberg consist of a low s a l i n i t y layer, E, above a layer extending to the basin f l o o r , F. The order of increasing density i s E,A,D,F,B and C. S l i g h t modifications to t h i s order are discussed l a t e r . During a flood t i d e , layers A and B are forced i n t o Quatsino Narrows where they mix to homogenity, thus producing more of layer D. At the opposite end of the narrows, layer D enters i n t o the Rupert-Holberg Basin, sinking below the low s a l i n i t y layer,E, and mixing i n t o the region F. The degree of mixing depends upon the density difference between D and F and the available energy. On an ebb t i d e , layer E and part of layer F move i n t o the narrows to become w e l l mixed. This water eventually passes i n t o Quatsino Sound at a depth dependent upon the density difference between it,and layers A and B. TV.2 Discussion This model i l l u s t r a t e s how water from the narrows i s injected i n t o the basin. The momentum of the incoming t i d e pushes aside the low s a l i n i t y layers of the basin between the narrows and Hankin Figure 13: Schematic Diagram shewing flow conditions between Quatsino Sound and the Rupert-Holberg basin dur- v ing (a) flood tides and (b) ebb t i d e s . Point. V i s u a l observations show turbulent water wi t h i n t h i s area during a flood t i d e and contrasts with the apparent calm conditions surrounding i t . The density difference eventually causes the incoming water to sink below the fresher layer. Thermal microstructure measurements (Chapter V) suggest that the majority of the t i d a l inflow i s then s t i r r e d i n t o the basin j u s t beyond the s i l l . This causes the greater v e r t i c a l uniformity observed at Station E. Internal mixing could then proceed by pressure gradient currents due to the density difference between t h i s region and the r e s t of the i n l e t . During ebb conditions the fresh water layer, the majority of which originates from the Marble River, flows out toward the narrows entraining the deeper more saline water i n t o i t . A flow i s required to replace the sali n e water. During flood t i d e s , the i n -j e c t i o n of the water beneath the low s a l i n i t y layer j u s t beyond the narrows necessitates a subsurface flow t o spread the water.throughout the basin. I t i s noted that normal estuarine c i r c u l a t i o n requires a subsurface u p - i n l e t flow to replace the sali n e water entrained i n t o the continual outflowing surface layer (Tully, 1949). However, lack of such a regular outflow, due to the major fresh water inflow being concentrated near the mouth, changes the nature of the c i r c u l a t i o n from that of the t y p i c a l i n l e t . The depth and water properties of the layers described i n the model undergo continual change. River runoff, solar r a d i a t i o n , winds, p r e c i p i -t a t i o n and, i n the case of Quatsino Sound, outside influences from the P a c i f i c Ocean a l l combine to modify these layers. The model presents an i n f l u x of less dense water i n t o the basin. I t also allows f o r deep flushing and mixing. I f layer A i s very shallow or i s denser than the 33 basin water, the t i d a l inflow would plunge over the s i l l , dropping to the basin f l o o r . The observed increases i n the density of the deep water i n the basin are most l i k e l y to have occurred through gradual rather than large scale changes of t h i s nature. A continual increase i n the density of the water flowing through Quatsino Narrows, caused by decreasing runoff; would r e s u l t i n a corresponding increase i n the density of the deep water i n the basin. Sudden large scale decreases i n the density of the deep water would seldom be expected. However, upwelling o f f the coast and subsequent inflow i n t o Quatsino Sound might produce the necessary conditions f o r t h i s to happen. The ebbing waters from Quatsino Narrows are assumed to flow i n t o Quatsino Sound at a depth dependent upon the density difference between the inccming and e x i s t i n g waters. I f t h i s depth i s shallow, the water could re-enter the narrows on the following flood t i d e . E x i s t i n g c i r c u l a t i o n patterns may remove i t e n t i r e l y . Further data are required to determine the actual process which occurs. CHAPTER V THERMAL MICROSTRUCTURE MEASUREMENTS V.1 Introduction Thermal microstructure refers t o temperature and temperature grad-ient fluctuations on the scale of a few centimeters. Measurements of t h i s nature were taken i n Rupert and Holberg Inl e t s between March 6 - 1 0 , 1971. Twenty successful recordings were obtained at nine separate loca-tions ; throughout the basin (Fig. 14). Figure 15 shows the state of the ti d e at the time the measurements were taken. Four other recordings were unuseable due to technical problems or instrument c a l i b r a t i o n . The instrument used contained two thermistors separated h o r i z o n t a l l y by one-hal f meter .and attached to the .bottom of a f r e e - f a l l i n g , r o t a t i n g i n s t r u -ment package (Qsborn, 1973). The f a l l speed was on the order of 20 cen-timeters per second. One thermistor mounted along the central axis of the instrument recorded the v e r t i c a l temperature p r o f i l e . The other ther-mistor recorded the temperature i n a h e l i c a l path about the c e n t r a l axis. Absolute temperatures are'found by comparison of the instrument'1-s temper-ature signal to the temperatures recorded previously by reversing thermo-meters. Thus small errors may e x i s t i n these absolute values, however the temperature differences are accurate. The temperature signals were d i f f e r e n t i a t e d and sent through a 25 hertz law pass f i l t e r to obtain the -3 temperature gradients. The noise l e v e l of the gradients i s 4.25 x 10 C° /cm. Since a general account of microstructure i n i n l e t s has not yet been published the feature of thermal microstructure i n i n l e t s are discussed. Figure 14: Map showing location of microstructure measurements within Rupert and Holberg Inlets HEIGHT ABOVE LOW WATER MARK (metres) o — ro OJ . • 37 Due to the i:rregular topography, those features observed w i t h i n the Rupert-Holberg Basin may not be t y p i c a l of B r i t i s h Columbia I n l e t s but subsequent measurements i n other i n l e t s have produced comparable r e s u l t s . The fined section.of t h i s chapter contains the information revealed by the micro-structure about the Rupert-Holberg System. 38 V.2 General Discussion of Thermal Microstructure D i s t i n c t i o n between mixing and s t i r r i n g i n the oceans and atmos-phere was f i r s t made by Eckart (1948). V e l o c i t y differences w i t h i n a f l u i d i n i t i a l l y cause a steepening of any e x i s t i n g gradients of tem-perature or concentration and an extension of the i n t e r f a c i a l area. This process i s c a l l e d s t i r r i n g . Mixing i s the reduction of these gradients i n temperature of^concentration by molecular d i f f u s i o n . The molecular f l u x i s dependent only upon the steepness of the gradient and the c o e f f i c i e n t s of molecular d i f f u s i v i t y , thus by increasing the grad-ients s t i r r i n g serves to hasten mixing. In addition to thereby increas-ing the f l u x , s t i r r i n g increases the amount of mixing by enlarging the area over which mixing takes place. Turbulence i s a major source of s t i r r i n g i n the ocean. Causes of turbulence below the upper layers are un-known, but shear i n s t a b i l i t y i s strongly suspected. Measurements of small scale thermal gradients disclose the location and the extent of s t i r r i n g . Regions of s t i r r i n g are indicated by numerous, cl o s e l y spaced ( i . e . high frequency) v e r t i c a l temperature gradients (Fig. 16). These gradients vary i n magnitude and fluctuate about a l i n e of zero gradient. Velocity microstructure measurements by Osborn (personal coramunication) reveal the existence of v e l o c i t y fluctuations i n regions of high frequency temper-ature gradients. The steepness and magnitude of the temperature gradients depend on the rate of s t i r r i n g and the molecular d i f f u s i o n . Sharp gradients confined t o one side of the zero gradient l i n e indicate an interface be-tween two temperature zones (Fig. 17). Heat d i f f u s i o n operates to t h i c k -en these interfaces and causes the gradient to widen. The width of the gradient therefore indicates the time since the formation of the interface (Osborn and Cox, 1971). A wider spread denotes an older interface (Fig. 18). i I ! L I ! I L 48 49 50 51 52 53 54 DEPTH (metres) Figure 16: A region of s t i r r i n g observed during drop number 14. 66 : 10 dz in mC°/cm + 10 X INTERFACE 22 23 24 25 26 27 28 29 30 31 32 DEPTH (metres) Figure 17: Tertparature gradient associated with thermal interface on drop number 21. O , in C z mC°/cm 10 + 10 H -+ H h 7 8 9 10 D E P T H ( m e t r e s ) 13 14 6 in °C 6.8 7.8 Figure 18: "Older" interfaces observed i n Rupert I n l e t during drop number 16. 42 Intrusions are detected by the i n t e r j e c t i o n of warmer or colder water in t o a homogeneous region (Fig. 19). The v e r t i c a l component of the temperature gradient varies consider-ably throughout the water column. S t i r r i n g i s normally observed j u s t below the sea surface. Daily variations i n the i n t e n s i t y of t h i s s t i r r i n g suggests control by c l i m a t i c influences such as wind. Below t h i s zone, i r r e g u l a r l y positioned regions of s t i r r i n g s are detected which vary from tens of centimeters to several meters thick. Sheets and layers as seen by Woods (1970) near G i b r a l t a r are not observed. ' - '' Drop number 11 (Fig. 20), taken o f f Straggling Islands i n Holberg I n l e t , i l l u s t r a t e s the microstructure associated with the "mixed" layer and the thermocline zone of c l a s s i c a l theory. The top 3.5 meters shows —2 o an o v e r a l l temperature spread of only 7.8 x 10 C° but with gradients as -2 o - i high as 3 x 10 C cm . As mentioned above, the s t i r r i n g i n t h i s layer-i s most probably due to the "weather". This zone corresponds to the "mixed" layer. The next seven meters i s the thermocline region with the temperature changing by 1.17 C° . This change i s concentrated i n three interfaces (Fig. 20). Limited s t i r r i n g occurs below the t h i r d interface. The thermocline observed elsewhere i n Rupert and Holberg I n l e t s e x h i b i t s s t i r r i n g throughout the e n t i r e region. The.deep region, extending from below the thermocline to the maximum depth attained, usually consists of small temperature changes and l i m i t e d regions of s t i r r i n g . On drop number eleven, the temperature spread between 10.5 and 70 meters i s approximately 0.2 C° with the gradients seldom reaching beyond the noise l e v e l of the instrument. S t i r r i n g found within t h i s deep region i n most other parts of the i n l e t i s much lower i n i n t e n s i t y dz •10 in 25 6\n °C 7.15. 7.35—f 7.55' ' 26 27 28 29 DEPTH (metres) 30 32 Figure 19: A turbulent i n t r u s i o n during drop number 23. 45 than the s t i r r i n g i n the upper layers. A comparison of the temperature gradients recorded by the two therm-i s t o r s on the instrument shews s i m i l a r i t y i n large scale properties, such as the positions of regions containing gradients, average magnitudes of the gradients and the existence of large interfaces. Smaller features do not match, lending further support to the idea that these regions are a c t i v e l y s t i r r i n g . At times one thermistor can be seen to precede the other through a s p e c i f i c gradient by 10 to 30 centimeters (Fig. 21). C a l i b r a -t i o n of the instrument shows the rocking motion to be too small to produce t h i s difference. However, a required t i l t of 30° to the h o r i z o n t a l by the layers presumably causing the gradient seems highly u n l i k e l y . This e f f e c t has been observed i n other i n l e t s with s i m i l a r instruments but i s not always present, (Osborn, .personal ccKntunication). The reason f o r t h i s observed difference requires further i n v e s t i g a t i o n . The largest difference i n structure between the two thermistors occurs during drop number 22 while transecting an i n t r u s i o n (Fig. 21). The outboard thermis-t o r recorded an intense region of s t i r r i n g while the c e n t r a l thermistor, one-half meter away, recorded hamogeneous conditions. The r o t a t i o n rate of one ninth of a revolution per second means the outboard thermistor rotated by more than 270 . Cause of t h i s phenomenon i s unknown. V. 3 Microstructure i n the l^upert-Holberg Basin Thermal microstructure measurements reveal several important oceano-graphic c h a r a c t e r i s t i c s of the Rupert-Holberg system. Opposite Quatsino Narrows, s t i r r i n g i s observed t o depths of 98 meters. The s i z e and quantity of s t i r r i n g i n a region j u s t below the thermocline decreases towards Ac? 0.2 °C d67 : -10 dz in o. +|0 L mC/cm CENTRAL THERMISTOR 59 60 61 62 63 64 DEPTH' (metres) 65 66 69 Ci9 HO in dz mC°/cm + I 0 L ^mmh ^ ^ ^ ^ OUTBOARD THERMISTOR Figure 21: Comparison of the central and outboard thermistors from traces taken during drop number 22. 47 the head of the i n l e t . Sporadic s t i r r i n g of unknown o r i g i n i s observed below 30 meters, except i n the v i c i n i t y of the narrows. Of note are the possible d a i l y variations i n the r e l a t i v e amounts of s t i r r i n g between the i n l e t s , although confirmation of t h i s phenomenon requires further inves-t i g a t i o n . Measurements taken d i r e c t l y across from Quatsino Narrows towards Hankin Point (drop numbers 14,21 and 22) reveal numerous s t i r r i n g regions throughout the water column. S t i r r i n g as intense as that usually found only i n the upper layers i s exhibited between 49 and 53 meters on drop number 14 (Fig. 16), between 43 and 48 meters on drop number 21 and between 84 and 90 meters on drop number 22 (Fig. 22). The maximum depths attained during these drops are 65, 60 and 98 meters respectively. A l l these drops were taken near times of maximum expected t i d a l currents i n the narrows (Fig. 15). . These measurements are i n agreement with the pre-dictions of the model as presented i n chapter IV. The observed s t i r r i n g originates from two sources. These are the inherent turbulence of the intruding water and the l o c a l l y induced s t i r r i n g caused by these i n t r u -sions. Both sources work to mix the water i n t h i s part of the i n l e t . D i s t i n c t i o n between these sources of s t i r r i n g i n a p a r t i c u l a r region can-not be determined with the present instrument. The depth to which s t i r -r i n g occurs during these drops i s not surprising upon investigation of the water properties i n Quatsino Sound. Bottle casts during the f i r s t days of the microstructure measurements reveal the density at 10 meters depth i n the sound to be greater than i n the top 75 meters of water i n the basin. The density at 15 meters i n Quatsino Sound i s larger than the maximum density found i n the basin. Gravitational effects alone seem capable of trans 1-porting the upper layers from the sound to the depths of "observed s t i r r i n g CO 83 84 85 86 87 88 89 90 DEPTH (metres) Figure 22: Deep s t i r r i n g found opposite Quatsino Narrows cn drop number 22. mC/cm -10 + I 0U 49 50 51 52 53 54 DEPTH (metres) 55 56 57 Figure 23: Deep s t i r r i n g i n Rupert I n l e t during drop number 18. 49 f o l i a t i n g passage through Quatsino Narrows. The pushing aside of the upper layers by the incoming t i d e i s d i r e c t l y observed during drop number 14. No thermocline e x i s t s as the temperature increases by less than 0.3°C over the en t i r e depth. The measurements taken at the entrance to Holberg I n l e t (drop num-bers 7, 13 and 20) a l l reveal s t i r r i n g between the bottom of the therm-ocline (at 7 meters) and approximately 30 meters. This zone contains the warmest water w i t h i n the water column. The three drops (numbers 11, 12 and 19) taken near the Straggling Islands a l l show a warm zone between the bottom of the theriixx:line (10 meters) and 28 meters. S t i r r i n g occurs only near the bottom of t h i s zone, with the temperature gradients of i t s i n -t e r i o r implying small but stable interfaces. The magnitude of the tem-perature gradients decreases frcm the narrows towards the head. Across -3 from Quatsino Narrows, gradients of 50 x 10 °C/cm. are not uncomriion -3 while at the entrance to Holberg they decrease to 20 x 10 °C/cm. and by -3 Straggling Islands diminish t o between 5-10 x 10 °C/cm. The quantity of s t i r r i n g a l s o decreases i n an up i n l e t d i r e c t i o n . These features can be explained by a flow up Holberg I n l e t , o r i g i n a t i n g from the v i c i n i t y of Quatsino Narrows and reaching beyond Straggling Islands. Generation of such a flow i s expected (see section IV. 2). The warm layer i n which the flow i s found may have arisen through .localized wamung and subsequent cooling of the surface layer or may possibly be the remainder of an e a r l i e r warm water intrusion o r i g i n a t i n g from Quatsino Sound. The b o t t l e stations reveal the intruding t i d a l water to be colder than t h i s warm zone. Hence as expected the maximum observed temperature increases towards the head of the i n l e t . Occasional s t i r r i n g below t h i s zone i s also detected, the largest region being between 48 and 55 meters during drop number 20. 50 Conditions within Rupert I n l e t suggest an up-inlet flow under the thermocline, reaching h a l f way up the i n l e t . The magnitude and amount of v i s i b l e s t i r r i n g decreases towards the head of the i n l e t . A warm zone between the bottom of the thermocline (at 5 t o 10 meters) and 30 meters ex i s t s i n Rupert. Arguments, s i m i l a r to those produced f o r Holberg I n l e t , suggest a relationship between t h i s current and the t i d e s . As i n Holberg, the furthest up i n l e t measurements reveal stable interfaces below the thermocline (Fig. 18). Deep s t i r r i n g occurs within Rupert on an infrequent basis. I t i s most evident during drop number 18 (Fig. 23) between 49 and 57" • meters where magnitudes of the gradients are as high as any below the thermocline. Also a section between 80 and 90 meters on t h i s drop shows signs of s t i r r i n g . A 2 meter region of s t i r r i n g centered at 57 meters i s seen during drop number 24. The amount of s t i r r i n g at one s t a t i o n varies hourly as w e l l as d a i l y . On torch 9, drops number 15 and 18 were taken on a flood and ebb t i d e respectively with the l a t t e r showing more evidence of s t i r r i n g . This day i s not as active as March 10 (drop 23) during a flood t i d e nor March 6 (drop 3) near low water slack. Winds of 2 t o 8 knots from varying directions during March 9 compare with winds of 10 to 20 knots blowing generally up Rupert I n l e t during the other two days. This may indicate a c o r r e l a t i o n between; speed and d i r e c t i o n of wind and the amount of s t i r r i n g . The r e l a t i v e amounts of s t i r r i n g between Rupert and Holberg appear to change d a i l y . More s t i r r i n g occurs up Holberg I n l e t on March 8, equal s t i r r i n g occurs on March 9 and more s t i r r i n g occurs up Rupert on March 10. The days p r i o r to March 8 do not contain enough data to determine a pref-erence. Due to the hourly variations at each sta t i o n more data are needed 51 t o actually confirm that changes i n the r e l a t i v e amounts of s t i r r i n g do occur. CHAPTER VT COMPARISON WITH OTHER SHALLOW SILTED INLETS Mixing wi t h i n an i n l e t i s revealed by near uniform conditions of temperature • and s a l i n i t y plus high oxygen values. The mixing may be con-t i n u a l i n nature or caused by a recent flushing of the i n l e t . Lade of un-iform conditions or the existence of low oxygen values eliminates the p o s s i b i l i t y of a continuous mixing process. Data co l l e c t e d by the Univer-s i t y of B r i t i s h Columbia since 1951 i s examined t o determine the mixing .characteristics of i n l e t s with a s i m i l a r geographical configuration to the Rupert-Holberg basin. Establishment of the e s s e n t i a l factors nec-essary f o r continual mixing are sought. This investigation i s r e s t r i c t e d t o B r i t i s h Columbia i n l e t s l i s t e d by Pickard. (1961, 1963) whose s i l l depths are less than 30 meters. Also, only those i n l e t s connected by a lcng, narrow channel are considered. Indian Arm, Sechelt, B e l i z e , Seymour Inle t s and Work Channel a l l possess the necessary geographic features. They do however e x h i b i t non-uniform water properties. Indian Arm and Work Channel show a more gradual increase i n depth away from the s i l l than i s found i n the Rupert-Holbert basin. This "may explain the absence of thorough mixing w i t h i n these i n l e t s . In Sechelt, Belize and Seymour I n l e t s , the r a t i o of the t i d a l prism to the t o t a l volume i s less than one to one hundred. Such small t i d a l flow would produce cor-respondingly small e f f e c t s on the water properties. Drury I n l e t and Porcher I n l e t both display near uniform water proper-t i e s . Only one survey has been conducted i n each i n l e t , therefore further data are required to determine i f t i d a l l y induced v e r t i c a l mixing does occur cn a regular basis. 53 uniform conditions of temperatures, s a l i n i t y and oxygen e x i s t below 20 meters depth i n Fortune Channel, between Bedwell Sound and Tofino I n l e t on Vancouver Island (Fig. 24). Coote (1964) attributes these con-di t i o n s to t i d a l action. A s i l l of 28 meters i n Matlset Narrows, which separates Fortune Channel from Bedwell Sound, causes t i d a l currents of 3 - 4 knots. The inaximum depth i n Fortune Channel i s 140 meters and l i e s j u s t beyond the s i l l . Of possible importance i s the shore d i r e c t l y oppo-s i t e the narrows as the basin l i e s perpendicular to the narrows. This shoreline may act to force the oncoming t i d a l waters into the deeper sec-tions of the basin. Rupert-Holberg also contains a shore opposite i t s narrows. This factor may play a s i g n i f i c a n t r o l e i n the mixing process. The major fresh water inflow f o r Fortune Channel originates from the Kennedy River which enters h a l f way up Tofino I n l e t and flows through Dawley Pass. Thus a much less s a l i n e upper layer e x i s t s i n Fortune Channel than i n Rupert I n l e t . CHAPTER VTI CONCLUSION The study of the Rupert-Holberg system reveals a w e l l mixed water body which experiences large monthly variations i n density. Two processes account f o r these observations. Decreases i n density are due to mixing lower s a l i n i t y water i n t o the basin. Vigorous s t i r r i n g , , caused by t i d a l action, mixes the water j u s t beyond the narrows. This s t i r r i n g i s enhanced by the flow of the turbulent t i d a l intrusions beneath the e x i s t i n g low s a l i n i t y layer. Energy f o r t h i s process derives from the t i d e . The depth and degree of mixing changes with each t i d a l cycle. Increases are caused by g r a v i t a t i o n a l flow of dense water over the s i l l and onto the basin f l o o r . Both processes contribute to high oxygen values i n the basin. Continuous mixing e x i s t s due to one or other of these processes. Internal mixing within the r e s t of the basin i s presumed to proceed by pressure gradient currents. Further investigation i s required to determine a d e t a i l e d picture of the mixing away from the narrows. A study to reveal the time response of the i n l e t to changes i n r i v e r discharge i s also suggested. Thermal microstructure measurements reveal s t i r r i n g i n the v i c i n i t y of the narrows and the existence of an up i n l e t flow i n both Rupert and Holbert I n l e t s . Studies within the i n l e t s which combine the use of large and small scale measurements w i l l greatly extend the e x i s t i n g knowlege of these areas. This seems even more l i k e l y with the newly developed i n s t r u -ments which measure v e l o c i t y , s a l i n i t y and temperature microstructure, simultaneously. Comparison with B r i t i s h Columbia i n l e t s of s i m i l a r topography reveals 56 only Fortune Channel on Vancouver Island to e x h i b i t l i k e mixing conditions. This suggests that a shoreline opposite the narrows may be important to d e f l e c t onccming t i d a l water i n t o the basin. Porcher and Drury I n l e t s show signs of s i m i l a r mixing properties, however confirmation requires f u r -ther data. A c l a s s i f i c a t i o n of a l l i n l e t s as to t h e i r mixing properties would be highly advantageous. I t would a s s i s t i n locating future industries planning to deposit wastes i n i n l e t s . Depending upon the type of disposal system required, such as high oxygen demand, d i l u t i o n , e tc., the most"suit-able i n l e t could be chosen. I f an industry i s required to be located cn a s p e c i f i c i n l e t , knowledge of the nuxing properties of that i n l e t would help to determine where the waste disposal system should be located. 57 REFERENCES Canadian Hydrographic Service, 1971. Canadian Tide and Current Tables. Volume 6. Queen's Pri n t e r . 75 pp. Canadian Hydrographic Service, 1972. T i d a l Currents, Quatsino Narrows, B r i t i s h Columbia Queen's P r i n t e r , Ottaws, 10 pp. Coote, A.R. 1964. A Physical and Chemical Study of Tofino I n l e t , Vancouver Island, B r i t i s h Columbia. M.Sc. Thesis University of B r i t i s h Columbia, Vancouver, B.C. Eckart, C a r l . 1948. An Analysis of the S t i r r i n g and Mixing Processes i n Incompressible F l u i d s . Journal of Marine Research, 7, 265-275. Lane, R.K. 1962. A Review of Temperature and S a l i n i t y Structures i n the Approaches to Vancouver Island, B r i t i s h Columbia. Journal of Fisheries Board of Canada, 19, 45 - 91. Osborn, T. R. 1973. V e r t i c a l P r o f i l i n g of Velocity Microstructure. (In Progress) Osborn, T. R. and Cox, C.S. 1972. Oceanic Fine Structure, Geophysical F l u i d Dynamics, 3, 321-345. Pickard, G.L. 1956. Physical Features of B r i t i s h Columbia I n l e t s . Transactions of the Royal Society, of Canada, Series I I I , 50, 47 - 58. Pickard, G. L. 1961. Oceanographic Features of Inle t s i n the B r i t i s h Columbia Coast. .Journal of Fisheries Research Board of Canada, 18, 907 - 999. Pickard, G. L. 1963. Oceanographic Characteristics of Inle t s of Vancouver Island, B r i t i s h Columbia. Journal of Fisheries Research Board of Canada, 20, 1109 - 1144. Strickland, J.D.H. and Parsons, T. R. 1968. A P r a c t i c a l Handbook of Seawater Analysis Queen's P r i n t e r , Ottawa, 311 pp. Taylor, G. I. 1919. T i d a l F r i c t i o n i n the I r i s h Sea. Philosophical Transactions of the Royal Society (A), 220, 1 - 33. T a l l y , J . P. 1949. Oceanography and Prediction of Pulp M i l l P o l l u t i o n i n Alberni I n l e t . Fisher-ie s Research Board of Canada, Bu].letin No. 83, 169 pp. Waldichuk, M. 1958. Some Oceancgraphic Character-i s t i c s of a Polluted I n l e t i n B r i t i s h Columbia Journal of Marine Research, 17_, 536 - 551. Waldichuk, M., Markert, J.R., and Meikle, J.H. 1968. Physical and Chemical Oceancgraphic Data from From the West Coast of Vancouver Island and the Northern B r i t i s h Columbia Coast, 1957 - 1967. Fisheries Research Board of Canada, Manuscript Report Series No. 990, 161 pp. Woods, J . W. 1968. Wave-Induced Shear I n s t a b i l i t y i n the Summer Tliermocline. Journal of F l u i d Mechanics, 32, 791 - 800. 

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