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A study of artificial destratification of fresh water lakes Ritchie, John Clarke Weldon 1970

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A STUDY OF ARTIFICIAL DESTRATIFICATION OF FRESH WATER LAKES by JOHN CLARKE WELDON RITCHIE B.A.Sc., Uni v e r s i t y of B r i t i s h Columbia, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of C i v i l Engineering We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1970 In p r e s e n t i n g 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 o f the r e q u i r e m e n t s f o r an advanced deg ree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e 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 r e f e r e n c e and s t u d y . I f u r t h e r ag ree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t he Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depar tment o f /^.s / - eer! The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, Canada Date <^e<r. /ffj /.9"7G A B S T R A C T The water q u a l i t y c h a r a c t e r i s t i c s of various lake c l a s s i f i c a t i o n s are discussed and are rel a t e d to the natural p h y s i c a l and b i o l o g i c a l de-velopment of lakes. The impact of human a c t i v i t y on th i s natural trend i s considered. The r e s u l t s of an extensive l i t e r a t u r e search are pre-sented, discussing seasonal v a r i a t i o n s i n water q u a l i t y i n fresh water lakes. Methods of a l t e r i n g water q u a l i t y are out l i n e d , with p a r t i c u l a r emphasis on the a r t i f i c i a l e limination of summer thermal s t r a t i f i c a t i o n . Results of observations made of Osoyoos Lake during the summers of 1969 and 1970 are presented. Tests of an aeration i n s t a l l a t i o n i n the lake are reported and discussed. Laboratory investigations into the mechanisms of d e s t r a t i f i c a t i o n are described. Simulation of thermal s t r a t i f i c a t i o n was achieved through the use of a fresh water-salt water system. Three d e s t r a t i f i c a t i o n de-vices were tested under various s t r a t i f i c a t i o n conditions and t h e i r energy requirements are compared. Discussion of the r e s u l t s includes some comments on the r e l i a b i l i t y of s t a b i l i t y and energy c r i t e r i a f o r the evaluation of such devices. i i TABLE OF CONTENTS LIST OF TABLES . . . v LIST OF FIGURES v i LIST OF PLATES . . . . . . . . . . . . . . . . . . . v i i INTRODUCTION 1 CHAPTER I WATER QUALITY IN FRESH WATER LAKES . . . . . . . . . . 2 1.1 Trophic Levels i n Fresh Water Lakes . . . 2 1.2 The Influence of Algae on Water Quality 4 1.3 The Annual Cycle 6 1.4 Need For and Attainment of Good Quality Water 10 1.5 Summary 15 CHAPTER II OSOYOOS LAKE STUDIES 17 2.1 C h a r a c t e r i s t i c s of the Study Area .17 2.2 Climate 20 2.3 Quality Studies of Osoyoos Lake . . . 20 2.4 F i e l d Observations of Study Area 1969-70 22 2.5 The Water Quality Improvement Program . . 23 2.6 Test Results 28 2.7 Reasons f o r the System's Performance 34 2.8 Conclusions 36 CHAPTER I I I LABORATORY STUDIES 37 3.1 Objectives 37 3.2 Theory of A i r D i f f users 37 3.3 Theory of Mechanical Pumping 41 3.4 Apparatus and Instrumentation 43 3.5 Discussion of Results . 51 i i i XV Page SUMMARY OF CONCLUSIONS 61 BIBLIOGRAPHY . . .'-,67 APPENDIX A LIST OF SYMBOLS . . 73 APPENDIX B FIELD DATA FROM OKANAGAN LAKE 75 APPENDIX C LABORATORY DATA 81 APPENDIX D DESIGN OF A DESTRATIFICATION SYSTEM . . . . . . . . 95 LIST OF TABLES Table Page 1 Summary of Physical Characteristics of Middle Basin, Osoyoos Lake, B.C ' 19 2 Summary of Laboratory Test Results (Aug.-Sept., 1970) . 50 3 Performance Data for Field Installations as Reported in the Literature 59 v LIST OF FIGURES Figure Page 1 Trend of Trophic Level i n Lakes When Subjected to Natural and C u l t u r a l Enrichment . . . . . . . . 4 2 Temperature vs. Density f o r Fresh Water . . 7 3 I l l u s t r a t i o n of Wind on Temperature P r o f i l e i n a Lake . . 8 4 I l l u s t r a t i o n of E f f e c t of Mechanical Pumping on Temper-ature and DO P r o f i l e s . . . . . . . 14 5 Depth Contour Map of Middle Basin, Osoyoos Lake, B.C. •. . 18 6 Volume vs. Depth Curve for Middle Basin, Osoyoos Lake, BC 19 7 Plan of Aeration I n s t a l l a t i o n , Osoyoos Lake, B.C 27 8 Plot of Isopleths vs. Depth, Osoyoos Lake, B.C. July 14, 1969 . 29 9 Pl o t of Isopleths vs. Depth, Osoyoos Lake, B.C. July 16, 1969 . . . 30 10 P l o t of Isopleths vs. Depth, Osoyoos Lake, B.C. July 18, 1969 31 11 Plot of Isopleths vs. Depth, Osoyoos Lake, B.C. July 5, 1970 33 12 C a l i b r a t i o n Curves for Barnstead PM-70CB Conductivity Bridge 82 13 Sample Plot of Fluorescence vs. Time 83 14 Laboratory Test Density P r o f i l e (Test No. 4) .84 15 Laboratory Test Density P r o f i l e (Test No. 6) 85 16 . "Oxygenation Capacity" Design Curves for Central Basin, Osoyoos Lake .97 17 S t a b i l i t y vs. Time f o r 1970. Central Basin, Osoyoos Lake 100 v i LIST OF PLATES Plate Page 1 Gasoline Engine and Compressor, Osoyoos Lake, B.C 25 2 Boil at Water Surface Due to Aerator, Osoyoos Lake, B.C. . 26 3 View of Study Area, Osoyoos Lake, B.C. . . . . . . . . . . 26 4 Laboratory Tank, U.B.C 44 5 Stratified System Prior to Mixing . . . . . '. . . . . . .47 6 Model Helixor in Operation . . . 47 7 Bubble Column Breaking Surface . . 48 8 Discharge Jet From Mechanical Pump 48 9 Fresh-Salt Water Interface Illustrating Secondary Mixing . 52 10 Wave Travelling Left to Right Along Interface Towards Aerator . . . . 54 11 Wave Travelling Right to Left Along Interface 54 v i i A C K N O W L E D G E M E N T S The author wishes to thank his supervisor, Dr. M. C. Quick, for his encouragement and assistance throughout the course of this project. Sincere thanks are also due to Professor S. 0. Russell, whose suggestions were always valuable. The author also wishes to thank the B.C. Water Resources Service for funding the Osoyoos Lake aeration project. It would be impossible to l i s t a l l those who helped on this pro-ject. Special thanks are due, however, to S. B. Mould, Water Resources Engineer at Oliver, for his co-operation during the aeration studies. Two Water Resources Service employees, Messrs. Eric Cottrell and Larry Small, also deserve special thanks for their considerable efforts to collect accurate data from a sometimes unco-operative Osoyoos Lake. Vancouver, B.C. December, 1970 v i i i INTRODUCTION In recent years i t has become incr e a s i n g l y obvious that many fresh water lakes have been undergoing accelerated enrichment and conse-quent water q u a l i t y problems. This increased enrichment has arisen from domestic, i n d u s t r i a l and a g r i c u l t u r a l return flows. While a gradual buildup i n b i o l o g i c a l a c t i v i t y i s natural for most lakes, i t has been found that by accelerating t h i s trend, man has upset the symbiotic r e l a t i o n s h i p s necessary f o r a balanced food chain. This r e s u l t s i n dominance of a few species i n the lakes affected, pro-ducing wide seasonal f l u c t u a t i o n s i n water q u a l i t y . To achieve long term r e v e r s a l of t h i s "eutrophication" trend, man must sharply reduce h i s nutrient inputs to fresh water lakes. How-ever, i t may be possible to achieve short term improvements i n water q u a l i t y by a r t i f i c i a l l y breaking down the intense summer thermal s t r a t i -f i c t i o n which accompanies eutrophication and creates some of the water q u a l i t y problems. The purpose of th i s study was to investigate the e f f e c t s on water q u a l i t y of a r t i f i c i a l d e s t r a t i f i c a t i o n , and the various devices which have been used to achieve d e s t r a t i f i c a t i o n . This was accomplished through an extensive l i t e r a t u r e search. An aeration i n s t a l l a t i o n was studied at Osoyoos Lake, B.C., to examine i t s e f f i c i e n c y and to become f a m i l i a r with "the behaviour of the lake. Laboratory„tests were made to compare various mixing devices i n a model of a thermally s t r a t i f i e d lake, and to examine the mixing mechanisms involved. 1 CHAPTER I WATER QUALITY IN FRESH WATER LAKES 1.1. TROPHIC LEVELS IN FRESH WATER LAKES: When f r e s h water lake basins are f i r s t formed by g l a c i a l , v o l c anic or i s o s t a t i c action, the water which f i l l s them i s usually b i o l o g i c a l l y barren. Soon, how-ever, lower forms of l i f e become established i n the lake, helping the processes of erosion to e s t a b l i s h a nutrient supply and food chain. As time goes on and the nutrient supply gradually builds up as inflows exceed outflows, the species inhabitating the lake become more diverse. A f t e r a c e r t a i n l e v e l of a c t i v i t y has been reached, b i o l o g i c a l growth occupies a progressively larger proportion of the lake volume, u n t i l the lake becomes a swamp, then a bog. The terms used to define the "trophic" or " b i o l o g i c a l a c t i v i t y " l e v e l of a lake are overlapping ones. They are d e s c r i p t i v e rather than d e f i n i t i v e terms. Some b i o l o g i s t s have attempted to sharpen the boundar-i e s , p r i m a r i l y by enumerating c h a r a c t e r i s t i c organisms for each category. Generally speaking, an " o l i g o t r o p h i c " lake i s one i n the early stages of development. I t i s r e l a t i v e l y c l e a r , and b i o l o g i c a l a c t i v i t y i s at a low l e v e l . Phytoplankton are not present i n high concentrations, and the f i s h population i s usually dominated by the "sport" v a r i e t y . This type of f i s h i s an active one, requiring high dissolved oxygen con-centrations f o r adequate r e s p i r a t i o n . A large part of i t s food supply i s external to the lake, i n the form of insects which a l i g h t on the 2 3 water surface. A "eutrophic" lake, on the other hand, i s i n the l a t e r stages of development. It i s characterized by heavy summer algae "blooms," shallow water weed growth and a f i s h population r e s t r i c t e d to scavengers such as carp or suckers. These are sluggish f i s h , having low oxygen r e -quirements and feeding on a food supply i n t e r n a l to the lake. A "meso-trop h i c " lake f a l l s between the two aforementioned extremes. While ex-periencing occasional algae blooms and weed problems, i t s t i l l r e tains some of i t s o l i g o t r o p h i c c h a r a c t e r i s t i c s . The nutrient inputs to a lake come from natural and c u l t u r a l or man made sources. Nutrients are;required i n varying proportions by aquatic organisms. Those required i n the r e l a t i v e l y large amounts are known as "macronutrients." They include nitrogen, phosphorus, potassium and carbon. "Micronutrients," required i n smaller q u a n t i t i e s , include vitamins and trace metals such as molybdenum, boron and i r o n . Some micronutrients are s p e c i f i c a l l y required by some aquatic species, and not by others. Natural sources of micro and maCronutrients can be p r e c i p i t a -t i o n , organic decomposition, erosion, s o i l which i s prone to leaching, or the o r i g i n a l bottom sediments of the lake. C u l t u r a l sources are p r i -marily sewage o u t f a l l s and a g r i c u l t u r a l runoff carrying excess f e r t i l i -zer. Figure 1 shows how c u l t u r a l nutrient addition accelerates the natural aging process. 4 A Time Figure 1 Trend of Trophic Level i n Lakes When Subjected to Natural and C u l t u r a l Enrichment 1.2. THE INFLUENCE OF ALGAE ON WATER QUALITY: Since many of the "nuisance" symptoms of advancing eutrophication stem from explosive growths or "blooms" of algae, a b r i e f summary of t h e i r c h a r a c t e r i s t i c s and requirements f o r growth follows. Algae are microscopic, s i n g l e c e l l e d phytoplankton, c l a s s i f i e d as plants due to the presence of c h l o r o p h y l l , which i s e s s e n t i a l f o r photosynthesis. They l i v e e i t h e r as independent units or joined together with other c e l l s i n c o l o n i a l structures. Photosynthesis removes carbon dioxide (CO2) from s o l u t i o n , y i e l d -ing c e l l u l a r protoplasm and oxygen. The oxygen produced from a l g a l photo-synthesis, p a r t i c u a r l y i n the ocean, replenishes a large part of the earth's oxygen supply. The photosynthetic reaction s h i f t s the equilibrium 2 HC03~ 2 C0 2 + C0 3 = + H20 (1) to the r i g h t , producing a buildup i n CO-}', a f f e c t i n g the secondary equilibrium 5 H 20 + C0 3 Z HC0 3 + OH . The increase i n concentration of hydroxide i r o n r e s u l t s i n a r i s e i n pH. Since photosynthesis takes place only i n the presence of sunlight, at night the algae " r e s p i r e , " or absorb oxygen, r e s u l t i n g i n a r e v e r s a l of the above reactions and a drop i n pH. The s e v e r i t y of these e f f e c t s depend p r i n c i p a l l y upon algae concentrations, l i g h t i n t e n s i t y , and depth of water. Dirunal dissolved oxygen v a r i a t i o n s of up to 8 mg/1 have been recorded [1]. The p h y s i c a l signs of algae growth, then, can be summarized as follows: d i u r n a l v a r i a t i o n i n dissolved oxygen (DO), and pH; increased t u r b i d i t y i n the water due to the a l g a l mass i t s e l f , and a change i n colour of the water, depending on the dominant species pre-sent . As well as the previously mentioned photosynthetic require-ments, algae have macro and micronutrient requirements s i m i l a r to other forms of l i f e . These nutrients are used i n the b u i l d i n g of c e l l s t r u c -tures . We have discussed the basic requirements for and benefits of algae growth. The following quotation puts the question of nuisance algae growth into perspective. "It must be remembered that algae provide an important l i n k i n the food chain. D i f f i c u l t i e s a r i s e only when these organisms become extremely out of balance and a bloom occurs." [2]. 6 Sawyer [3] has found i n his research on mid-latitude lakes that algae blooms can be expected to be a problem i f c e r t a i n c r i t i c a l average nitrogen and phosphorus concentrations are exceeded at the time of complete mixing i n the spring. He has found these c r i t i c a l concen-tr a t i o n s to be .01 mg/1 f o r inorganic phosphorus, and .30 mg/1 for i n -organic nitrogen. Other writers have pointed out that i n c e r t a i n i n s t a n -ces, one or other of the macronutrients need not be present i n such large concentrations. This i s e s p e c i a l l y true f o r nitrogen, which the "blue-green" algae are able to obtain from the atmosphere. When a bloom dies due to cutting o f f i t s sunlight or using up the av a i l a b l e supply of nut r i e n t s , the dead c e l l s u s ually sink to the lake bottom to decompose. The oxygen demand exerted i n th i s stage of the l i f e cycle balances the oxygen added to the water while the algae l i v e d . The nutrients with which t h e i r c e l l structures were b u i l t do not disappear, but l i e on the bottom to be put back into suspension when the lake c i r c u l a t e s or to be adsorbed by the bottom sediments. Since n u t r i e n t outflows and quantities adsorbed are usually smaller than c u l t u r a l and natural inflows, a net increase i n nutrient quanti-ty occurs from year to year. 1.3. THE ANNUAL CYCLE: Water q u a l i t y i n mesotrophic and eutrophic lakes varies seasonally due pr i m a r i l y to meteorological i n -fluences. The basic reason for t h i s i s the unique r e l a t i o n s h i p between density and temperature for water, as shown i n Figure 2. Figure 2 Temperature vs. Density for Fresh Water. 8 Intense s o l a r r a d i a t i o n of the surface waters during the summer r e s u l t s i n a temperature and consequent density gradient with depth. Since the surface water i s les s dense, i t tends to remain there, absorbing more heat and becoming s t i l l more s t a b l e . This layer of water i s known as the epilimnion. It i s mixed by wind ac t i o n to a depth which depends on wind i n t e n s i t y , duration and frequency. Thack-ston [4], i n h i s discussion of one of Symons1 papers, gives a good des-c r i p t i o n of the influence of wind on the temperature p r o f i l e . In F i g -ure 3, the dashed l i n e represents a temperature p r o f i l e i n the absence of any mixing. Under these conditions, the water body's center of gra v i t y would be at CG^. Under the wind's mixing influence, the pro-f i l e takes the shape of the s o l i d l i n e , r a i s i n g the center of gravity to CG^. This represents an increase i n p o t e n t i a l energy, brought about by the energy input from the wind. Lake Surface T Y Thermocline »  O CG Depth Without Wind Mixing With Wind Mixing Lake Bottom ^» Temperature Figure 3 I l l u s t r a t i o n o f E f f e c t of Wind on Temperature P r o f i l e i n a Lake [4] 9 The region denoted "thermocline" in Figure 3 i s usually be-tween five and 20 feet thick. It i s arbitrarily defined as the zone in which the thermal gradient i s greater than l°C/m, or 0.55°F/ft. It can be thought of as an interface between two fluids, in this case two layers of water having different densities. There is l i t t l e mixing of the two layers under normal c i r -cumstances, since "the existence of a stable density s t r a t i f i -cation inhibits large scale turbulent mixing and consequently greatly reduces the rate of natural reaeration of bottom water by the downward trans-port of oxygen from the air-water interface." [5] If there are any oxygen demanding materials present in this lower lay-er, known as the hypolimnion, the DO concentration w i l l start to de-cline. As f a l l approaches, the epilimnion begins losing heat to the atmosphere, i t s temperature and density decreases, and the ther-mocline drops lower with each successive wind mixing. Finally, the sta b i l i t y declines to a point at which complete mixing is achieved by a windstorm. This is known as the "great" or " f a l l " overturn. If the f a l l is a calm one, this overturn can take place when the surface lay-er loses sufficient heat to become more dense than the lower layer. This is an unstable situation, which is stabilized by a sudden physi-cal overturn of the lake waters. Through the winter, the lake remains more or less isother-mal at 4°C, with perhaps slight stratification due to the presence of less dense ice water at the surface. Wind mixing takes place in the 10 spring, but i s usually of le s s e r extent and importance, since spring winds are not normally as intense or as frequent as.those i n the f a l l . P h ysical overturn of the waters cannot take place, since the surface waters cannot become more dense than the bottom waters at 4°C. As spring progresses, heat inputs increase and summer thermal s t r a t i -f i c a t i o n sets i n again. Accompanying thermal s t r a t i f i c a t i o n i s chemical s t r a t i f i -cation, r e s u l t i n g p r i m a r i l y from the lack of reaeration of the bottom waters i n the summer. Its i n t e n s i t y i s d i r e c t l y proportional to the degree of eutrophication i n the lake. Among the q u a l i t y parameters showing v a r i a t i o n with depth are DO and algae concentrations, which decrease with depth; and dissolved i r o n , dissolved manganese, hydro-gen s u f l i d e , ammonia, and CO2 concentrations, which increase with depth. The presence of hydrogen s u l f i d e i s objectionable due to i t s t o x i c i t y to f i s h and i t s o l f a c t o r y offensiveness to man. Dissolved i r o n and manganese are present due to the reducing environment which characterizes anaerobic decomposition, allowing the reduction of these micronutrients to t h e i r soluble s t a t e s . It has also been shown [6] that phosphorus, which i s the macronutrient thought.'by some to be c r i -t i c a l i n l i m i t i n g algae growths [2], i s also held i n a soluble form i n the reducing environment. 1.4. THE NEED FOR AND ATTAINMENT OF GOOD QUALITY WATER: If a lake or re s e r v o i r i s to be used f o r r e c r e a t i o n a l p u r s u i t s , espe-c i a l l y those involving water contact, q u a l i t y requirements are usually 11 high. Algae blooms make a body of water generally uninviting for swim-ming, and large fluctuations in DO and hydrogen sulfide concentrations often cause f i s h k i l l s . Depletion of DO in the epilimnion of a drink-ing water supply reservoir reduces the quality below acceptable levels. Similarly, a reservoir which suffers from this problem loses i t s use-fulness for downstream low flow augmentation, since the water i t re-leases constitutes more of a pollutant than a good quality dilution water. Methods which have been used with some success to reverse or slow down the eutrophication trend in some lakes are based on an interruption of the nutrient cycle. This is accomplished by reducing cultural inputs through sewage treatment and agricultural runoff con-t r o l , or by some means of "harvesting" the nutrients already present, or a combination of these. Since such measures often require several years to take effect, some means of temporarily improving the water quality is de-sirable. One method used i s to "flush" the lake with low nutrient content inflows to temporarily reduce nutrient concentrations. Volumes required for dilution flushing depend on the nutrient contents of the flushing water, and the present and expected nutrient concentra-tions in the lake. The volume required has been calculated in one i n -stance [7] to be four, times the impoundment volume. "Slug" flushing requires lower volumes but high flows, and has undesirable side effects, such as sediment disturbance. Since the volumes and/or flows required for flushing are rarely available, considerable research has 12 recently focussed on an a l t e r n a t i v e method, a r t i f i c i a l e limination of summer thermal s t r a t i f i c a t i o n . Two basic methods of a r t i f i c i a l mixing have been i n v e s t i -gated. Both involve r a i s i n g the cold, usually low q u a l i t y hypolimnion water to the surface or near i t , to mix i t with the warm, better qual-i t y epilimnion water. This r e s u l t s i n a gradual lowering of the thermocline u n t i l the lake i s isothermal. "Mechanical pumping" i s a mixing method which most often employs a raft-mounted, low head-high discharge hydraulic pump, anchored over the deepest part of the lake. The suction l i n e extends to .within a few feet of the lake bottom, and the water i s discharged into the epilimnion at the surface, usually p a r a l l e l to i t . This transfers momentum to the upper l a y e r , s e t t i n g up a "return current" which induces some turbulent mixing across the thermocline. "Diffused a i r pumping" involves compressing a i r with a lake side compressor i n s t a l l a t i o n , and piping i t to a discharge point, located usually at the deepest point of the impoundment. The r i s i n g a i r bubbles induce upward flow i n the water column around them, r e -s u l t i n g i n v e r t i c a l transport of the cold bottom water to the surface. The turbulence of the bubble column assures s u f f i c i e n t mixing to pre-vent the water from sinking back below the thermocline. Instead, i t spreads out over the surface, inducing return currents which help the mixing process i n a manner s i m i l a r to that previously - described. The d i f f u s e d a i r system can assume numerons configurations. Individual point sources are usually combined with some stationary de-vi c e mounted above them to induce greater turbulence i n the air-water 13 mixture as i t r i s e s to the surface. The idea behind such devices i s to maximize oxygen transfer from the bubble to the water. The research from which they evolved was aimed at optimizing aeration i n small sewage lagoons. Bubble curtains, which are created by releasing a i r through long s l o t t e d pipes, can be set up i n any configuration. I f set up i n a c i r c u l a r or cross formation, a very large turbulent " b o i l " of water w i l l r e s u l t . "(The^object of such an i n s t a l l a t i o n i s to maximize the v e r t i c a l movement of the water f o r a given a i r flow. The e f f e c t s on water q u a l i t y of the various mixing methods are e s s e n t i a l l y i d e n t i c a l [8]. The primary e f f e c t i s on the temperature and therefore the density of the water. One mixing has been observed [9] to permanently r a i s e the bottom temperature, while temporarily lowering the surface temperature. As mentioned previously, the thermocline i s lowered by mixing u n t i l the lake becomes isothermal, i n d i c a t i n g that the cold water d i f f u s e s into the warmer water. An important side e f f e c t of" the lowering of the surface temperature i s a decrease i n evaporation losses. Koberg and Ford [10] calculated a reduction of s i x per cent i n evaporation losses during one test on a mid-latitude lake. Conditions w i l l vary from one l o c a t i o n and one season to the next; what should be remembered i s that the e f f e c t of d e s t r a t i f i c a t i o n on evaporation losses cannot help but be b e n e f i c i a l . The secondary e f f e c t of mixing, although i t s primary aim, i s the improved DO d i s t r i b u t i o n over depth i n the lake. Renewal of the air-water i n t e r f a c e has been shown [11, 12] to (be the most important 14 mechanism for the aeration of the bottom waters of shallow (maximum depth l e s s than 75 feet) lakes. T y p i c a l l y , the t o t a l dissolved oxygen content of a body of water remains unchanged during aeration, but a more uniform DO concentration p r o f i l e r e s u l t s . Figure 4 shows the e f f e c t of d e s t r a t i f i c a t i o n by mechanical pumping on temperature and DO p r o f i l e s . I t i s believed that the decline i n DO at the surface 10 -20 30 L r Before mixing / "After mixing 10 20 Temperature, °C 2 4 6 Dissolved Oxygen, mg/1 Figure 4 I l l u s t r a t i o n of E f f e c t of Mechanical Pumping on Temperature and DO P r o f i l e s [13] i s due to the v e r t i f c a l transport of oxygen demanding materials. Most of the oxygen transferred during mixing i s used i n s a t i s f y i n g inorganic oxygen demand. This i s exerted by reduced materials, p r i n c i p a l l y s u l -f i d e s , i r o n and manganese. Hence, the presence of higher oxygen con-15 centrations does more than help aquatic animals, principally fish, to respire. It also removes toxic elements such as the sulfides and a l -gal nutrients such as the iron, and provides an aerobic environment which has been shown [14, 15] to enhance biodegradation of synthetic organics. The term "synthetic organic" applies here to compounds such as the pesticide 2-4-D and the household detergent component linear alkylate sulfonate, both of which are troublesome compounds i f they resist degradation. Parameters unaffected by destratification include total dissolved nitrogen, "chemical oxygen demand," and turbidity. It was thought at f i r s t the anticipated increase in circulation of nutrients would raise the overall productivity of destratified lakes. Based on admittedly short-term observations, however, Symons et al [8, 9, 11, 12},16, 17] have concluded that no significant effect of this nature has occurred. In fact, they have observed drops in total standing crop of algae during mixing, with an increase afterwards to less than bloom levels. It also appears from their observations that the nui-sance "blue-green" type of algae is most sensitive to mixing, possibly due to an i n a b i l i t y to flourish in a rapidly changing environment. Long term research is needed in this area to evaluate the effects of a r t i f i c i a l destratification on the overall l i f e cycle in the aquatic environment. 1.5. SUMMARY: Current research has shown that a r t i f i c i a l destratification i s a useful method to improve water quality in meso-trophic or eutrophic fresh water lakes. The principal beneficial o 16 effect Is the presence at a l l depths of dissolved oxygen, which pre-vents the formation of toxic sulfides, stops.reducing reactions in the hypolimnion, and provides a more favourable environment for fish survival. No detrimental effects have been observed on algae popu-lations, and some researchers have noted declines in population levels, even though mixing was not intended to be an algae control technique. CHAPTER II OSOYOOS LAKE FIELD STUDIES 2.1. CHARACTERISTICS OF THE STUDY AREA: Trie Okanagan Va l l e y i s located i n the I n t e r i o r Plateau of B r i t i s h Columbia. I t was f i r s t formed by f o l d i n g and f a u l t i n g along i t s long axis during the Middle T e r t i a r y period. G l a c i a l a c t i v i t y during the Pleistocene period resulted i n the Valley's present U-shape and the c h a r a c t e r i s t i c v a l l e y side outwash terraces. Osoyoos Lake occupies a depression i n the Okanagan Val l e y r e s u l t i n g from the melting of a stagnant g l a c i a l i c e lobe, which at one time extended from what i s now O l i v e r to the 49th p a r a l l e l . I t i s believed that when water ceased to flow south from t h i s lobe, mater-i a l from a t r i b u t a r y stream was deposited to dam the lake,[18]. The study area for t h i s project consists of the middle of the three basins that Osoyoos Lake includes. This basin l i e s south of the Number Three Highway bridge and north of Haynes Point P r o v i n c i a l Park. The study ;area's p h y s i c a l c h a r a c t e r i s t i c s are summarized i n Figures 5 and 6, and i n Table 1. Figure 5 i s a depth contour map, drawn from surroundings made i n June 1969 with a Raytheon Dell9D depth sounder. Figure 6 i s a volume versus depth curve, derived from Figure 5. Table 1 summarizes numerically the p h y s i c a l c h a r a c t e r i s t i c s of the basin. 17 00 19 Volume Below Depth Indicated acre feet Figure 6 Volume vs. Depth Curve f or Middle Basin, Osoyoos Lake, B.C. TABLE 1 SUMMARY OF PHYSICAL CHARACTERISTICS OF MIDDLE BASIN, OSOYOOS LAK#, B.C. Surface Area V o l u m e Mean Depth L e n g t h Mean Breadth Perimeter Elevation Above MSL 532 acres 21,150 acre-feet 39.8 feet 4875 feet 4749 feet 23,400 feet 911 feet 2.15 x 10 6 m2 25.1 x 10 6 m3 12.1 m. 1487 m. 1445 m. 7135 m. 278 m. 20 2.2. CLIMATE: The South Okanagan's climate i s c l a s s i f i e d as a r i d to semi-arid. Average annual p r e c i p i t a t i o n at O l i v e r i s 9.42 inches over a 23-year period [18]. Very l i t t l e of the p r e c i p i t a t i o n f a l l s i n the summer. Winters are generally mild, with p e r i o d i c cold s p e l l s , while summers are characterized by hot, dry days and warm nights. The normal range of temperatures over a summer day i s from 55°F to 85 or 90°F, commonly with 13 hours of bri g h t sunshine, 2.3. QUALITY STATUS OF OSOYOOS LAKE: Booth [18] has des-cribed Osoyoos Lake as "a mesotrophic lake having some remnants of ol i g o t r o p h i c c h a r a c t e r i s t i c s and some indi c a t i o n s of ensuing eutro-ph i c a t i o n . " He bases t h i s conclusion on Rawson's system of c l a s s i -f i c a t i o n by means of t y p i c a l organisms, combined with observations of algae blooms and thermal and chemical s t r a t i f i c a t i o n . He also noted considerable movement of phosphorus between the sediment and the water column, which i s also i n d i c a t i v e of advancing eutrophication. Natural nutrient inputs to the study area are small, due to the barren nature of the v i r g i n s o i l t r i b u t a r y to i t . Since i r r i -gation systems were introduced i n the 1920's, however, c u l t u r a l inputs have increased r a p i d l y . This has been due i n large measure to the ph y s i c a l nature of the s o i l . Osoyoos s o i l s are sandy loams under-l a i n by a coarse gravel [18] allowing rapid p e r c o l a t i o n of i r r i g a -t i o n water, with attendant f e r t i l i z e r losses. This provides a sub-s t a n t i a l f e r t i l i z e d groundwater flow which i s a major contributor; to the nutrient supply of the lake. Overland runoff, i n the form 21 of irrigation return flows, contributed large quantities of nutrients unt i l the mid-1960's, when ditch irrigation was phased out in the area. Other cultural inputs arise from fru i t and vegetable pack-ing houses, industrial plants and municipalities located upstream from the study area, and from septic tanks located on or near i t s lakeshore. During June and July of 1968, a major blue-green algae bloom was observed in the north and south basins of Osoyoos Lake. Although continuous observations were not made in the study area, i t can be assumed from periodic observations made there that the bloom in the middle basin was dominated by the same algae species, and that population densities were of the same order of magnitude. The dominant species during the major bloom was the blue-green algae "anabaena flos aquae"* [18]. This alga is characterized by i t s a b i l i t y to " f i x " nitrogen from the atmosphere and to u t i l i z e inorganic nutrients in small concentrations, especially when water temperatures are high. A bloom of this alga-will give the water a blue-green colour, make i t s consistency appear thicker and sometimes cause a skin condition known as "swimmer's itc h " among those who con-tact the algae. Since one of the major vacation pastimes of this region is water sports such as swimming and water skiing, the presence or pros-pect of the presence of such a bloom has caused a sharp decline in tourist t r a f f i c . The economy of the whole Okanagan Valley was s t i l l suffering the effects of the 1968 blooms in 1970, even though the a l -gae problems seemed to have reduced slightly since 1968. 22 The 1968 algae bloom-had another, perhaps more tangible, e f f e c t . When the bloom.died and the decaying organisms s e t t l e d to the lake bottom, they exerted an oxygen demand which severely depleted the DO supply i n the bottom waters [15]. At t h i s time, South Okanagan Lands Improvement D i s t r i c t pumphouse No. 9, which takes i r r i g a t i o n and domestic water from the study basin at a depth.of 25 feet , was observed to be pumping water unsuitable for domestic consumption. Its lack of p o t a b i l i t y was a d i r e c t r e s u l t of the DO depletion, which permitted anaerobic decay to set i n , with i t s objectionable odours.and side e f f e c t s . 2.4. FIELD OBSERVATIONS OF STUDY AREA 1969-70: Observa-tions were made by the author and two B.C. Water Resources Service employees, Messrs. E. M. C o t t r e l l and L. Small, during the summers of 1969 and 1970. Temperature and dissolved oxygen readings were made with a Yellow Springs Instrument Co. Model 51 DO meter. This i n s t r u -ment combines a thermistor type temperature probe with a Clark type electrode for DO measurements. Water transparency was measured with a standard 20 cm. diameter Secchi Disk. Data gathered showed that there i s a buildup i n s t a b i l i t y , representing a lowering of the p o t e n t i a l energy of the lake, as the summer progresses. The s t a b i l i t y represents the product of the lake's mass and the distance through which i t s centre of gravity has been lowered by s o l a r energy input. As such, i t i s an i n d i c a t o r of the i n -t e n s i t y of thermal s t r a t i f i c a t i o n i n the lake. S t a b i l i t y c a l c u l a t i o n s were made by means of a computer program based on a c a l c u l a t i o n tech-23 nique developed by Symons [20]. Results.of the c a l c u l a t i o n s made using 1970 data show that a maximum s t a b i l i t y of 400 kw. hr. was reached on July 8. This represents the t h e o r e t i c a l minimum energy input required to d e s t r a t i f y the lake at that time. Accompanying the buildup of thermal s t r a t i f i c a t i o n was a decline i n the DO supply i n the hypolimnion, which began early i n June and ended at f a l l overturn i n mid-October. Observations made i n 1970 indi c a t e that the DO supply was e f f e c t i v e l y zero i n mid-August and started to recover i n l a t e September. I t i s normal for a lake that i s eutrophying to reach this point e a r l i e r each year. I n s u f f i c i e n t data i s a v a i l a b l e to e s t a b l i s h the rate of advancement of t h i s date i n Osoyoos Lake, however. Diurnal v a r i a t i o n s i n DO were observed during most of both summers, i n d i c a t i n g algae growth. However, t h i s v a r i a t i o n rare-l y exceed 0.5 mg/1, i n d i c a t i n g a f a i r l y balanced a l g a l population. Secchi Disk readings showed l i t t l e day-to-day v a r i a t i o n , but a tendency was noted for the transparency decline as the summer progressed, i n d i c a t i n g an increase i n a l g a l density. However, at no time did the transparency drop below 10 feet. Some eutrophic lakes have been observed to have transparencies of le s s than three feet [21], while r e l a t i v e l y o l i g o t r o p h i c Okanagan Lake, located i n the same watershed, recently exhibited a Secchi Disk transparency i n excess of 40 feet [22]. 2.5. THE WATER QUALITY IMPROVEMENT PROGRAM: Af t e r the decline i n water q u a l i t y was experienced i n the summer of 1968, the 24 B.C. Water Resources Service decided to.attempt to remedy one of the most bothersome of the e f f e c t s of the algae blooms. This was the de-c l i n e i n DO which took place hear the No. 9 pumphouse intake i n the study area, a f f e c t i n g the p o t a b i l i t y of the water. To t h i s end, two point source a i r bubblers, f i t t e d with "Helixor" brand aeration devices were i n s t a l l e d near the intake, with the a i r released at a depth of 35 feet. A "Helixor" i s a hollow cynd-r i c a l tube, 12 feet long and 18 inches i n diameter, f i t t e d with a h e l i -c a l diaphragm which divides the c y l i n d e r into two chambers. When i t i s fastened with i t s long axis v e r t i c a l above an a i r d i f f u s e r , the a i r -water mixture i s forced to take a tortuous v e r t i c a l path, which theore-t i c a l l y enhances oxygen transfer from bubble to water because of the i n -creased turbulence. A i r was supplied to the d i f f u s e r s v i a p l a s t i c pipe from a Gardner Denver Model 3CDL-5 rotary compressor, shown i n Plate 1, d e l i v e r i n g approximately 30 cubic feet of free a i r per minute to each d i f f u s e r . A t y p i c a l " b o i l " pattern observed on the lake surface above one of the a i r discharge points appears i n Plate 2. Figure 7 i s a plan view of the i n s t a l l a t i o n . The compressor l o c a t i o n i s also marked on Plate 3. The p r i n c i p a l requirement for design of t h i s i n s t a l l a t i o n was to e f f e c t the aeration of a small portion of the study area i n the v i c i n i t y of the pumphouse intake. The e f f e c t s of t h i s aeration would be most e a s i l y observed i f a d r a s t i c depletion i n DO, s i m i l a r to the one i n 1968, occurred again.during the testing period. Then i t would be r e l a t i v e l y simple to trace the progress of aeration by monitoring the buildup of DO around the aerators. Plate 1 Gasoline Engine and Compressor Osoyoos Lake, B.C. Plate 2 B o i l at Water Surface Due to Aerator, Osoyoos Lake, B.C. Plate 3 View of Study Area, Osoyoos Lake, B.C. (Arrow indicates compressor house) Figure 7. Plan of Aeration Installation, Osoyoos La ke, British Columbia 28 2.6. TEST RESULTS: Conditions i n 1969 and 1970 never d i d deteriorate to the l e v e l experienced i n 1968, so i t was d i f f i c u l t to evaluate the system on the basis which i t was designed. However, two test runs were made during the summer of 1969 and another was made i n 1970. The f i r s t t est began on Ju l y 14, 1969. By that date, the lake had s t r a t i f i e d and the thermocline was located at a depth of 31 fee t . The i n i t i a l DO contours, or i s o p l e t h s , are shown i n Figure 8, which i s a North-South cross-section of the te s t area. Since the a i r was released at a depth of 35 fe e t , i t was able to entrain some cold'water with a low DO concentration and bring i t to the surface. After two days, the e f f e c t on the isopleths was dramatic, though l o c a l i z e d , as shown i n Figures 9 and 10. It i s evident that, by July 16, the aerators or the wind, or both, effected an average i n -crease i n DO of 1 mg/1 i n the nearby epilimnion. However, the i s o -pleth shapes i n Figure 10 indic a t e that on July 18, a f t e r four days of aeration, the whole middle t h i r d of the area between the Helixors and most of the hypolimnion were s u b s t a n t i a l l y unaffected by the aer-ation operation. A second test was made i n mid-August, 1969. By t h i s time, the thermocline had dropped below 35 fe e t , leaving no hypolimnion water f o r the aerators to move to the surface. As pointed out i n Chapter I, there i s no point i n mixing the epilimnion only, since the wind w i l l do t h i s at a s u b s t a n t i a l l y lower cost. However, the DO of the water below the 30 foot depth was r e l a t i v e l y low (less I s o p l e f h Interval = I . O m g / l . Figure 8. Plot of Isopleths vs. Depth Osoyoos Lake, B.C. July 14,1969. ( B e f o r e A e r a t i o n ) / / / I s o p l e t h I n t e r v a l 2 I .Omg./l. F igure 9. P l o t of I s o p l e t h s vs. Depth O s o y o o s L a k e , B.C. J u l y 16 , 1 9 6 9 . (Two d a y s of A e r a t i o n ) pleth al = I.Omg./l . P l o t O f vs. Depth Lake B.C. 1 9 6 9. A e r a t i o n ) 32 than 5 mg/1), so i t was decided to try and improve this situation and test the equipment further. No dramatic changes, in water qual-i t y were observed, however. In fact, a significant drop (roughly 0.5 mg/1) in surface DO was observed within the turbulent surface-" b o i l " region near the aerators. This indicated that the dominant effect of the aerators was their transport of low DO water to the surface. Apparently contact time was too short between the bubbles and the water to permit significant oxygen transfer to occur. A third test was made in June, 1970 to evaluate the de-str a t i f i c a t i o n capabilities of the aeration system. Before the test began, the thermocline was located atta depth of 25 feet, and the air diffusers had been raised three feet to discharge at a depth of 32 feet. This meant that there would be roughly seven feet of cold water available near the aerators for mixing with the warmer epilimnion waters. Hence, i f the Helixors were efficient mixing de-vices, a significant effect on the temperature profile nearby should have been vi s i b l e . The system was run continuously, except for regular main-tenance shutdowns and an eight-hour engine overhaul, from the after-noon of June 26, 1970 to the afternoon of July 8. Observations dur-ing this period indicated that while a localized temperature effect was evident near each aerator, no appreciable change was occurring in the temperature profile .at the point mid-way between the two. Figure 11, a plot of isotherms with depth over the same cross-sec-tion as in Figures 8 to 10, illustrates this. The isotherm pattern 34 also indicates that the north aerator was operating more e f f i c i e n t l y than the south, aerator. This was probably a r e s u l t of a small d i f -ference i n t h e i r a i r supplies, caused by higher head losses i n the p i p e l i n e to the south aerator. At the end of the test period, there was no s i g n i f i c a n t d i f f e r e n c e between the temperature p r o f i l e s near the aerators and the p r o f i l e at a point well beyond t h e i r possible zone of influence. Considering the r e s u l t s of the three t e s t s , then, one has to conclude that the aeration i n s t a l l a t i o n would be inadequate to f u l -f i l l the purpose for which i t was designed. This conclusion can be reached even though the extreme conditions of 1968 did not reappear to allow a more concrete evidence to be gathered. 2.7. REASONS FOR THE SYSTEM'S PERFORMANCE: The aeration system studied i n Osoyoos Lake was designed to influence only a small part of the study basin near the pumphouse intake. Other researchers [9] have found that i t i s impossible to mix one part of a water body without inf l u e n c i n g the remainder. This i s due p a r t l y to hydraulic gradients set up by lowering the thermocline, bringing i n dense water from elsewhere i n the lake, and p a r t l y to wind and aerator induced currents. These currents w i l l transport oxygen demanding material to the aerators, thereby overloading them. This e f f e c t predominates when an under-designed system i s i n s t a l l e d at the deepest point i n a lake. A second f a c t o r i n the lack of success of the i n s t a l l a -t i o n i s the tendency of aerators, when set up i n shallow water and 35 having l i t t l e or no cold water to transport to the surface, to form closed " c e l l s " of mixed water [9]. These c e l l s r e s u l t when the mixed water at the surface i s not s i g n i f i c a n t l y less dense than the water at the bottom, so the mixture can sink down to be r e c i r c u l a t e d by the aerator. C e l l s were set up to some degree i n a l l the t e s t s , re-s u l t i n g i n a small zone of influence around each Helixor. The system';s disappointing performance was also related to i t s f a i l u r e to measurably transfer oxygen d i r e c t l y from bubble to water. The reason for this lack of oxygen transfer can best be understood when i t i s r e a l i z e d that the oxygen transfer performance curves for the H e l i -xor were derived from tests run on sewage. There are two basic reasons why oxygen transfer takes place more e f f i c i e n t l y i n sewage than i n fresh water. Since bubble 'sizes are smaller i n sewage due p r i n c i p a l l y to i t s lower surface tension, more oxygen transfer w i l l take place per unit volume of a i r . This i s due simply to the higher area to volume r a t i o for smaller bubble diameters. Secondly, the rate of transfer i s d i r e c t l y proportional to the oxygen concentration gradient, which i s always lower i n natural waters than i n sewage [23]. This lack of d i r e c t oxygen transfer would not be of import-ance i f s i g n i f i c a n t quantities of water could be transported to the surface to renew the air-water i n t e r f a c e . However, t h i s was not pos-s i b l e i n t h i s case because a s i g n i f i c a n t supply of cold, low DO water was not a v a i l a b l e to be " a i r l i f t e d . " 36 2.8. CONCLUSIONS: To effectively aerate the area of water near the pumphouse intake, i t would be necessary to move the aerators closer to the intake, reduce their spacing to less than 200 feet, and at least double the number of discharge points. This should enable the aerators to "keep ahead" of the oxygen demand from outside the immediate area. Even though " c e l l s " would probably set up, the spa-, cing should be small enough to aerate a l l of the desired volume. A question now arises concerning the efficiency of perform-ing such aeration. As was pointed out in Chapter I, the wind i s cap-able of mixing the epilimnion throughout the summer, so this type of installation may not provide a significant additional oxygen input. A more practical approach would be to design a system to destratify the whole study basin, thereby raising the DO of a l l the water above zero and producing the beneficial side effects outlined in Chapter I. A sample design for such a system is worked out in Appendix D. CHAPTER III LABORATORY STUDIES 3.1. OBJECTIVES: Laboratory studies were undertaken during the summer of 1970 to investigate the ph y s i c a l behaviour of a s t r a t i f i e d body of water under the stimulus of.various mixing techniques. In p a r t i c u l a r , i t was desired to investigate the mechanisms of mixing and t h e i r r e l a t i v e magnitudes, and to compare the mechanical performance of d i f f e r e n t mix-ing devices. 3.2. THEORY OF AIR DIFFUSERS; An a i r d i f f u s e r which i s d i s -charging a i r bubbles below the surface of a body of water can be theo-r e t i c a l l y approximated by a g r a v i t a t i o n a l convection or "buoyancy" j e t [24]. The behaviour of such j e t s depends on the density difference be-tween the injec t e d and receiving f l u i d s . The other type of free turbu-lent j e t , which i s known as a "momentum" j e t , consists of a j e t of f l u i d being discharged into a receiving f l u i d having approximately the same density. The properties of t h i s kind of j e t depend on i t s momentum as i t leaves the nozzle. The rate of f l u i d entrainment or "induced flow" f o r a buoyant j e t has been rel a t e d [25] to the a i r flow rate by the equation: £ = 0.57 ( ^ g ) 1 / 3 y (1) This equation applies to a long bubble screen of length L. : Note that the flow induced per unit length of bubble screen, Q/L, varies with 37 38 the cube root of the a i r flow rate per unit length, and l i n e a r l y with y, the distance of the section i n question above the source. C o l l e c t -ing terms, the equation becomes: Q = 0.57 ( Q o g ) 1 / 3 y L 2 / 3 (2) Comparing equation (2) with the formula for a point source, [25] Q = 0.153 ( Q o g ) 1 / 3 y 5 / 3 (3) a t h e o r e t i c a l r e l a t i o n s h i p between L and y can be derived for equal Q and Q . Combining equations (2) and (3), we get: o .57 y , L 2 / 3 = .153 y 5 / 3 (4) which y i e l d s the r e l a t i o n s h i p : L = .138 y (5) Hence, i t i s t h e o r e t i c a l l y true that for a given a i r flow rate, i f L > .138 y (6) a l i n e source w i l l induce more upward water flow at a section y feet or les s above the a i r bubble source. For example, i n an impoundment 100 feet deep, the a i r d i f f u s e r need only be 14 feet long to induce more up-ward flow i n the water than a point source receiving the same a i r flow. It i s worthwhile noting that while the induced water flow rate for the point source also varies with the cube root of the a i r flow rate, i t varies with the 5/3 power of the v e r t i c a l distance of the section 39 above the source. The reason f o r t h i s can be grasped by remembering that, i f a plan view i s v i s u a l i z e d , flow from the point source can spread out i n two dimensions, while flow from the l i n e source i s l i m i t e d to one. When an a i r d i f f u s e r i s operated with the object of d e s t r a t i f y -ing a body of water, two mixing mechanisms w i l l be involved. The primary mechanism involves the entrainment of denser hypo-limnion water by the a i r bubbles and the mixing of th i s water with e p i l -imnion water as the bubble stream r i s e s through i t . From the formulae previously set out, t h e o r e t i c a l flow rates can be calculated to estimate how much dense water should be moved by a given a i r flow rate f o r any thermocline l e v e l . S i m i l a r l y , the same formulae can be used to calculate the induced flow rate j u s t at the water surface, which w i l l show what de-gree of mixing has occurred. Further mixing w i l l occur as the plume spreads out and f a l l s back down to some intermediate depth. The secondary mixing mechanism involves turbulent d i f f u s i o n across the i n t e r f a c e of the denser water into the upper la y e r . Rouse and Dodu [26], i n t h e i r experiments with a f r e s h - s a l t water layered sys-tem, showed that turbulence generated i n the upper layer did not pene-tra t e the lower la y e r . Rather, i t "produced i r r e g u l a r i n t e r f a c i a l cusps from which streamers were r a p i d l y l i f t e d and dif f u s e d through the upper stratum." [26]. Their findings showed that as mixing progressed, the in t e r f a c e sank lower, with the upper layer's density approaching some neutral value and the lower layer's density remaining unchanged. This c l o s e l y p a r a l l e l s f i e l d observations of d e s t r a t i f i c a t i o n devices oper-40 ating i n lakes. In such cases, the turbulence near the i n t e r f a c e i s generated by the movement of the return current induced by the mixing device. To obtain a t h e o r e t i c a l minimum energy input required to com-press a given volume of a i r to be released at a c e r t a i n depth, assume we have a 100% e f f i c i e n t compressor which compresses a i r a d i a b a t i c a l l y and d e l i v e r s i t to the d i f f u s e r with no f r i c t i o n losses. The use of the t h e o r e t i c a l work required to accomplish t h i s w i l l i n l a t e r c a l c u l a -tions eliminate the e f f e c t s of i n e f f i c i e n t l y operating compressors and piping systems and allow comparison of mixing techniques on an equal basi s . For adiabatic compression, PV Y = constant (7) where y f o r a i r = 1.4. Hence, the r e l a t i o n s h i p between the volumes be-fore and a f t e r compression can be calculated from the r e l a t i o n s h i p P2 1 / y V l = (P ) V2 C8) 1 and a r e simply atmospheric and atmospheric and hydrostatic pres-sures, so equation (8) can be modified to: H A 1/Y V l - ( H + D > V2 ^ A where H^ represents atmospheric pressure i n feet of water and D i s the hydrostatic head i n f e e t . H i s usually assumed to be constant at 33.9 f e e t . 41 Work done on the gas during adiabatic compression can be ex-pressed as: P V - P V w = 1 1 2 2 (10) Y-1 3 If we substitute the free a i r flow rate, Q^, i n f t /min. for and con-vert the work units to horsepower, equation (10) becomes: H 1/Y W = .00472 Q A [ (HA+D) - (11) A If i t i s desired to express work as a function of the a i r flow rate at the nozzle, equation (11) becomes: - 1/Y -W = - .00472 Q [(H.+D) - H ( HA + D) ' ] (12) O A A „ H A 3.3. THEORY OF MECHANICAL PUMPING: There are two mechanisms involved i n the mechanical pumping technique of a r t i f i c i a l d e s t r a t i f i c a -t i o n . They are s i m i l a r to the mechanisms previously described f o r a i r d i f f u s e r s . Primary mixing, or phy s i c a l movement of the dense water from bottom to surface i s accomplished by t h i s water passing through the pump. It i s discharged h o r i z o n t a l l y at the water surface, and i t s c h a r a c t e r i s t i c s can be t h e o r e t i c a l l y approximated by a free turbulent "momentum j e t . " A momentum j e t , l i k e a buoyancy j e t , induces flow i n the receiving f l u i d so that the j e t spreads out and the v e l o c i t y de-creases downstream from the source. This ensures mixing of the colder water from the outlet with the receiving water. The e f f e c t of varying density differences can be combined with 42 that of the discharge velocity through the use of the densimetric Froude Number: (13) Sharp [28] has found that for F^ greater than 4, the jet can be cla s s i -fied as a "momentum" jet, and that in general, the larger F^ i s , the greater the dispersion of the heavier f l u i d at a given distance from the discharge point. From equation (13), i t can be seen that an increase in F^ can be accomplished by increasing the discharge, decreasing the diameter of the discharge jet (thereby increasing i t s velocity), or by reducing the density difference between the two fluids. An application of the use of the approach is demonstrated in Appendix D. Secondary mixing is induced by the momentum imparted to the epi-limnion by the discharge jet of the pump. A return current which skims along above the thermocline induces turbulence and consequent mixing in a manner similar to that outlined for the air diffuser method. Actual work done in performing this mixing can be calculated using standard'pump formulae. The static head over which the pump is operating is given by the formula: H s ' = > HS ( PB " P T> + E < " ) where H is the vertical distance in feet from the water surface to the pump intake, p D and p. are the densities of the bottom and top waters a t respectively, and E is the elevation in feet above the water surface 43 of the discharge pipe's center line. Since E i s usually zero, the only static head that the pump must overcome is due to the difference in den-sity between the top and bottom waters. The static head i s a small frac-tion of water depth, and is usually small compared to the velocity head, which is calculated from the formula: 2g As mixing progresses, the static head drops even lower, and so i t i s often ignored in calculations. Useful work input for mixing purposes, which does not include head losses due to fr i c t i o n and other sources, can now be calculated from the standard formula: W = Q y H (16) w where H = H g + (17) If H is ignored, equation (16) reduces to: L3 W -•= 0.97 ^  (18) 3.4. APPARATUS AND INSTRUMENTATION: The test apparatus, pictured in Plate 4, consisted of a 12' x 12' x 6' deep tank constructed with a steel skeleton and plywood panels on three sides and the bottom. The fourth side consisted of three plexiglass sheets to provide an unobstruc-ted view of the tank's contents. 44 Plate 4 Laboratory Tank, U.B.C. 45 The s t r a t i f i e d system was simulated using a s a l i n e s o l u t i o n as the dense bottom layer, and fresh water from the Vancouver water mains as the less dense upper.layer. The bottom s a l i n e layer was thoroughly mixed, then the fresh water was added very slowly-. It was thus possible to achieve a s t r a t i f i e d system such that the density t r a n s i t i o n occurred over a v e r t i c a l distance of less than three inches. Fresh-salt water density s t r a t i f i c a t i o n has been used previous-l y to simulate thermal s t r a t i f i c a t i o n [26]. The chief disadvantages of d i r e c t simulation of thermal s t r a t i f i c a t i o n are the cost of the equip-ment necessary and heat losses through the tank walls. Neither of these i s a f a c t o r i n the s a l t water - fresh water method, although i t does have one drawback. It i s impossible to duplicate the actual d e n s i t i e s which occur due to thermal s t r a t i f i c a t i o n , since the s a l t solution's density i s always greater than 1.0. However, the density differences which cause thermal s t r a t i f i c a t i o n are e a s i l y duplicated. Before each test was run, a density p r o f i l e of the system was constructed i n d i r e c t l y using a Barnstead PM-70CB Conductivity Bridge. This instrument was c a l i b r a t e d to y i e l d a unique r e l a t i o n s h i p between the solution's conductivity and i t s s a l t concentration. This r e l a t i o n -ship was combined with standard s a l t concentration - density r e l a t i o n -ships to y i e l d the family of curves i n Figure 12, Appendix C. T y p i c a l density p r o f i l e s can also be found i n Appendix C. To provide a continuous record of the progress of mixing, Rho-damine "B" fluorescent dye was mixed with the bottom layer of water. Its concentration i n the upper l a y e r , which was a function of the de-46 gree of mixing achieved, was monitored continuously by a Turner Model 111 Fluorometer. A small pump drew a sample at a constant rate from a representative point i n the upper layer and passed i t through a "flow-through door" attached to the fluorometer. Knowing the delay time from the tank to the fluorometer and the chart speed of the recording device, i t was possible to ascertain the time at which complete mixing was achieved with an accuracy of - 30 seconds. For purposes of compari-son at l e s s e r degrees of mixing, points were taken from the curves at 75% and 95% of maximum dye concentration. A t y p i c a l fluorescence vs. time curve i s reproduced i n Figure 13, Appendix C. An added advantage of "tagging" the denser water with fluorescent dye was the added contrast i t caused between the l a y e r s . This f a c i l i t a t e d observation of the i n t e r f a c e and the d i f f u s i o n of the denser water into the upper l a y e r . A t y p i c a l "before" p i c t u r e , Plate 5, demonstrates how well the contrast shows up i n photographs and also how sharp a discon-t i n u i t y i t was possible to achieve. Three mixing devices were compared i n the t e s t i n g program. Diffused a i r mixing was t r i e d two ways: f i r s t l y , with a 3" d i a -meter, 18" long model "Helixor" mounted above two 1/8 i n . "Poly F l o " nozzles as shown i n Plate 6; and secondly, with the same nozzles but no Helixor. In both cases, the laboratory compressed a i r supply was used, t h r o t t l e d down and measured by a Lab Crest Model 100H Standard Century Flowmeter. A t y p i c a l bubble pattern from an aeration test i s i l l u s t r a t e d by Plate 7. Mixing by means of mechanical pumping was modelled using a Para-mount Model 11/2 V6B1 pump, t h r o t t l e d down to provide an energy input Plate 5 S t r a t i f i e d System Prior to Mixing Plate 6 Model Helixor i n Operation Plate 7 Bubble Column Breaking Surface Plate 8 Discharge Jet From Mechanical Pump 49 close to that calculated f o r the a i r d i f f u s e r t r i a l s . Its suction l i n e was on the tank bottom, as shown i n Plate 5, and i t discharged the dense water into the surface l a y e r , as shown i n Plate 8. The three d i f f e r e n t mixing devices were tested under two com-p l e t e l y d i f f e r e n t conditions of s t r a t i f i c a t i o n . Tests 1 to 4, 8 and 9 were run using one foot of dense s a l t water, o v e r l a i d by 4.5 feet of fresh water. Three tests.:were subsequently made using two feet of dense s a l t water, ov e r l a i d by 3.5 feet of fresh water. The system's i n i t i a l s t a b i l i t y , or i t s drop i n p o t e n t i a l energy induced by s t r a t i f i c a t i o n , was calculated f o r each test using the den-s i t y p r o f i l e previously mentioned. It was calculated by summing the moments of 0.5 foot t h i c k sections of the water about the centre of gra-v i t y of the completely mixed system. As pointed out previously, s t a b i l -i t y represents the t h e o r e t i c a l minimum energy required to d e s t r a t i f y a given system, and as such has been used ext e n s i v e l y . i n the l i t e r a t u r e to compare;the performance of various mixing devices. Estimates of average primary and secondary mixing rates were made for each method by d i v i d i n g the volume of dense water moved by one of the mechanisms by the time over which that mechanism could be assumed to be operating. For the a i r d i f f u s e r devices, i t was assumed that secondary mixing alone was i n e f f e c t a f t e r the i n t e r f a c e had dropped below the l e v e l of the nozzles. The v a r i a b l e denoted t ^ i n Table 2 i s the time at which t h i s occurred. The average secondary mixing rate was then c a l -culated by d i v i d i n g the volume of water below the nozzles by the d i f f e r -ence between the complete mixing time and t 1 . The primary mixing rate TABLE 2 •SUMMARY OF LABORATORY TEST RESULTS August-September, 1970 TEST NO. DEPTH OF DENSE LAYER f t . STABILITY f t . - l b . MIXING DEVICE ENERGY INPUT RATE ft.-lb./rain. 100% min. TIMES 75% rain. u l min. ENERGY IN 100% 75% f t . - l b . f t . - l b . AVERAGE MIX RATE Primary Secondary ft /min. f t /min. DESTRAT. 100% % EFFICIENCY 75% % 1.0 1.0 1.0 1.0 1.0 2.0 112. 100. 118. 91. 122. 148. Helixor + Bubbler Helixor + Bubbler Bubbler Pump Pump Helixor + Bubbler 221. 221. 221. 353. 353. 221. 30. 32. 35. 26. 23. 50. 14. 13. 15. 15. 22. 13. 12.5 — 11. 22. 28. 6,630. 7,070. 7,740. 9,160. 8,120. 11,050 3,095. 3,310. 4,855. 4,410. 3,885. 4,855. 8.4 7.3 8.4 5.7 5.7 8.8 1.6 1.6 1.3 9.0 7.4 1.3 1.69 1.42 1.53 1.00 1.50 1.34 3.62 3.02 2.43 2.06 3.13 3.04 2.0 2.0 1.0 165. Bubbler 221. 37. 16.5 15. 8,180. 3,650. 16.5 1.3 152. Pump 353. 34. 14.5 — 12,000. 5,110. 5.7 9.5 96. Bubbler 116. 75. 38. 34. 8,700. 4,405. 3.2 0.7 2.02 1.27 1.10 4.52 2.98 2.18 51 for the mechanical pump was. directly measurable, so the secondary mix-ing rate could be found by solving an equation of the form: Pr*:t7 * ' i o o + SecR«ry * ci 1 0 0 - v - v o i . of ci9) dense water where t^ represents the time necessary for the return current from the pump discharge to begin i t s mixing operation. This time was estimated from visual observations during the tests. 3.5. DISCUSSION OF RESULTS: Examination of Mixing Mechanisms: On examination of the mixing rates recorded in Table 2, i t becomes apparent that secondary mix-ing does play a significant part in the destratification process. The figures show that for aeration devices, the secondary mixing rate changes from 15 to 22% of the primary rate. This rate remains roughly constant, but i t s significance increases for aeration devices as the thermocline drops lower and the primary mixing rate drops. For mechanical pumping, primary and secondary mixing rates remain approximately constant through-out the mixing process. Laboratory results show that secondary mixing plays a proportionately larger part in the pumping system, possibly due to scaling problems which w i l l be discussed later. Secondary mixing, involving turbulent diffusion of the dense water into the upper layer, is illustrated well by Plate 9. This photo-graph is a closeup of the interface between the two layers, i l l u s t r a t i n g the irregular "cusps" which are diffusing upwards. These cusps are l i f t e d by the turbulence induced at the interface by the movement of 52 Plate 9 Fresh-Salt Water Interface, I l l u s t r a t i n g Secondary Mixing 53 the secondary current over i t . This current moves r a d i a l l y towards an aerator when such a device i s used, and i t moves i n the opposite d i r e c -t i o n to the discharge j e t of a mechanical pump. Occasionally a wave i s v i s i b l e t r a v e l l i n g along the i n t e r f a c e i n the same d i r e c t i o n as the secondary current. Plates 10 and 11 i l l u s t r a t e such waves for aerator and pump re s p e c t i v e l y . Bases f o r Comparison of Mixing Devices: There are numer-ous ways to compare the performance of mixing devices. One method i n -volves an attempt to eliminate the e f f e c t of the depth and density of the lower layer so that a l l test r e s u l t s can be compared on the same basis. This i s c a l l e d the " D e s t r a t i f i c a t i o n E f f i c i e n c y " method. The percentage D e s t r a t i f i c a t i o n E f f i c i e n c y i s calculated by d i v i d i n g the system's i n i t i a l s t a b i l i t y by the work input required f or mixing, then multiplying by 100. A second method separates the tests into "blocks," one f o r each depth of dense l a y e r . Energy inputs are then compared with-i n each block. 'No attempt i s made to compare i n d i v i d u a l r e s u l t s d i r e c t -l y from within one block to another, but trends i n mixing performance can be evaluated by c a r e f u l comparison. A t h i r d method involves the c a l -c u l a t i o n of mixing rates as mentioned e a r l i e r , usually to supplement i n -formation obtained from one of the two aforementioned techniques. The use of D e s t r a t i f i c a t i o n E f f i c i e n c y has one serious drawback. When comparing two devices operating i n dense layers of the same t h i c k -ness but of s l i g h t l y d i f f e r e n t d e n s i t i e s , a misleading E f f i c i e n c y value may r e s u l t . This i s because s t a b i l i t y , the numerator i n the c a l c u l a t i o n , varies not only with the depth of the lower l a y e r , but also with i t s den-Plate 10 Wave Travelling Left to Right Along Interface Towards Aerator Plate 11 Wave Travelling Right to Left Along Interface. Pump Discharges Left to Right at Surface 55 s i t y . However, a small change i n density i s u n l i k e l y to greatly a f f e c t the performance of the mixing device. The r e s u l t s of t h i s deficiency can be seen by comparing the r e s u l t s of Tests l-.-and 2, as recorded i n Table 2. Here a l l conditions were i d e n t i c a l except f o r a s l i g h l y l e s s dense lower layer i n Test 2, y i e l d i n g a calculated s t a b i l i t y that was 11 per cent lower than that calculated for Test 1. Comparing the energy inputs required f o r d e s t r a t i f i c a t i o n , i t can be seen that the system with the higher s t a b i l i t y a c t u a l l y required s i x per cent less energy to de-s t r a t i f y i t . Comparing D e s t r a t i f i c a t i o n E f f i c i e n c i e s , however, one would get the impression that the device used i n Test 1 was 17 per cent more e f f i c i e n t than that i n Test 2. In f a c t , they were the same device, so any difference i n performance must have been due to experimental e r r o r , which i s better r e f l e c t e d by the s i x per cent diffe r e n c e shown by energy comparisons. A s i m i l a r conclusion can be drawn from a comparison of the r e s u l t s of Tests 4 and 8. Here there was a 29 per cent diffe r e n c e i n s t a b i l i t y , a 12 per cent difference i n power input, and a 40 per cent differ e n c e i n D e s t r a t i f i c a t i o n E f f i c i e n c y . D e s t r a t i f i c a t i o n E f f i c i e n c y , when calculated using useful energy input as i t s denominator, and used c a r e f u l l y by someone who knows i t s l i m -i t a t i o n s , can be useful i n comparing equipment performance from one body of water to another. In t h i s case, where one tank i s being used, i t i s better to r e l y on comparison of actual energy inputs required f o r de-s t r a t i f i c a t i o n of a system having a given dense layer thickness. Comparison of Aeration Devices: Tests 1 to 3 and Test 8 were made using s t r a t i f i e d systems containing a one foot t h i c k layer of s a l t water o v e r l a i d by a 4.5 foot layer of fresh water. Based on a 56 comparison of input energy used to attain 75% and 100% mixing, the Heli-xor-bubbler combination required between 12 and 41 per cent less energy than did the bubbler alone. Tests 6 and 7, performed with two feet of dense water, showed that the bubbler-alone required approximately 30 per cent less energy to mix this system. A comparison of the mixing rates and operational characteristics of the two air diffuser methods explains the apparent discrepancy in their performance. Mixing rates recorded in Table 2 show that the Heli-xor seems to induce a slightly higher secondary mixing rate when i t is attached above the bubbler. This is probably due to a "sucking" action caused by forcing the air-water mixture into the constriction of the tube. The mixture w i l l be less dense than i f i t were unconfined, so a current w i l l set up to " p u l l " more water into the bottom of the tube. When the layer of dense water is comparatively thick, however, the bub-bler alone w i l l mix the system more eff i c i e n t l y . This is because of i t s greater capacity for primary mixing, as illustrated by the calculated mixing rates recorded in Table.2. For the two foot layer of dense water, 3 the bubbler displayed an average primary mixing rate of 16.5 ft /min., and 3 a theoretical maximum rate of 19 f t /min, asvcalculated from equation (3). The Helixor-bubbler combination, however, is restricted in i t s capacity for primary mixing by i t s enclosure of the air bubbles in a tube over a significant portion of the dense layer's thickness. In Test 7, i t s 3 average primary mixing rate was calculated to be 8.8 ft /min., which is almost equal to i t s primary rate in Tests 1 and 2, but approximately one half the rate displayed by the bubbler alone. 57 Comparison of Pump With Aerators: Tests 4, 5 and 8 were conducted using the mechanical pumping to destratify systems which were layered similarly to those in which the aerators were tested. Tests 4 and 8 were performed using the one foot thick layer of dense water and Test 5 was performed using the two foot layer. Comparison of real power inputs recorded in Table 2 shows that the pump was operating in the same efficiency range as the aerators were. Edge and Scale Effects; Comparison of mixing rates for the various methods shows that the pumping system had the lowest primary mixing rate and the highest secondary rate. The high secondary rate resulted from the high velocity of the discharge jet relative to the size of the tank and the discharge capacity of the pump. This could have distorted the test results in two ways. The fast jet, imparting a large amount of momentum to the surface waters, could have had i t s effect largely dissipated through current reflection from the tank wall opposite the jet, thereby wasting energy which might have been.used in transporting more dense water to the surface. Conversely, the wall's edge effect could have been to contain and return much of the energy to the zone of secondary mixing by forcing the current to drop down the tank wall. This effect could not be duplicated in a lake and would yield misleadingly good performance figures. The importance of these edge effects can only be determined through further research with d i f -ferent velocity and discharge magnitudes, combined with f i e l d tests of pumping systems designed expecially for destratification, as outlined in Appendix D. It is possible that such tests w i l l show that a low 58 v e l o c i t y discharge j e t would s t i l l permit adequate -mixing at the sur-face and pump much more water f o r the same energy, making the mechanical pumping technique look more a t t r a c t i v e than aeration methods. In order that the magnitude and trend of the s c a l i n g e f f e c t might be evaluated, a low flow test of the Helixor-aerator combination was set up to duplicate the conditions which prevailed i n the f i r s t "block" of t e s t s , with the a i r flow cut down to produce approximately one h a l f the energy input rate. Comparison of the performance parameters for t h i s run, Test 9, with those reported f o r Test 3, i n d i c a t e that the device performed s i m i l a r l y under lower flow conditions. This indicates that edge e f f e c t s which should have been l e s s s i g n i f i c a n t i n Test 9, produced only a small change i n the energy input required for d e s t r a t i f i c a t i o n . It can also be shown that the s c a l i n g e f f e c t s are not large by comparing the performance parameters of the laboratory tested devices with those of a s e l e c t i o n of f i e l d tests reported i n the l i t e r a t u r e ,[12, 29J compiled i n Table 3. Note that the performance figures have been adjusted to show r e a l energy input using the formulae developed e a r l i e r i n the chap-t e r . Such a comparison shows:,: that f i e l d t r i a l s have demonstrated equip-ment e f f i c i e n c i e s of the same order of magnitude as the laboratory tests reported herein. I t also shows that mechanical pumping i s closer to the a i r d i f f u s e r method i n performance than would be concluded by comparing D e s t r a t i f i c a t i o n E f f i c i e n c i e s calculated from rated power figures f o r the equipment. Conclusions: The mechanisms of mixing can be c l a s s i f i e d as primary mixing, involving p h y s i c a l transport of dense water from TABLE 3 PERFORMANCE DATA FOR FIELD INSTALLATIONS AS REPORTED IN THE LITERATURE POWER EQUIP. ENERGY INPUT STABILITY DESTRATIFICATION LAKE NAME VOLUME acre f t . AREA acres DEPTH f t . .DESTRATIFICATION METHOD KW Real Rated EFF'Y. % KW-Real -hr. Rated CHANGE KW-hr. EFFICIENCY % Real Ratei Wahnback [29] 33,740 530 141 Air 14.9 57 .6 31 32,800 104,800 400 1.22 0.39 Cox Hollow [21] 1,190 96 25 Air 1.5 5 .6 27 570 2,140 7.9 1.39 0.37 Boltz [14] 2,900 96 62 Pump 0.6 16 .0 3.8 550 14,400. 18 3.3 0.13 Boltz [12] 2.900 96 62 Air 4.8 22 .4 21 435 1,760 27 6.2 1.53 Boltz [12] 2,900 96 62 Air 4.8 22 .4 21 414 1,680 23 5.6 1.37 Boltz [12] 2,900 96 62 Air 4.8 22 .4 21 546 2,200 33 6.0 1.50 Falmouth [12] 4,600 225 42 Air 3.5 22 .4 16 . 529 2,740 24 4.5 0.87 Falmouth [12] 4,600 225 42 Air 3.5 22 .4 16 722 3,740 35 4.8 0.94 Falmouth [12] 4,600 225 42 Air 3.5 22 .4 16 685 3,550 28 4.1 0.79 Falmouth [12] 4,600 225 42 Air 3.5 22 .4 16 491 2,540 8 3.1 0.39 60 bottom to top of the water body; and secondary mixing, which involves d i f f u s i o n of dense water across the i n t e r f a c e i n t o the upper l a y e r . Primary mixing i s generally the most important, but secondary mixing i s s i g n i f i c a n t , p a r t i c u l a r l y when aerators are used and the thermo-c l i n e drops down close to the a i r discharge point. D e s t r a t i f i c a t i o n E f f i c i e n c y can be a misleading basis f o r compar-ison of aeration devices, p a r t i c u l a r l y when they are being compared for the same depth of in t e r f a c e but s l i g h t l y d i f f e r e n t d e n s i t i e s . When c a l -culated using r e a l energy input, however, i t i s useful f o r comparison of devices from one water body to another. The bubbler alone performed better than the Helixor-bubbler did under conditions which are most l i k e l y to be duplicated i n the f i e l d , i . e . , a s i g n i f i c a n t l y t h ick layer of water above the a i r discharge point. The mechanical pump performed at the same l e v e l of e f f i c i e n c y as the aerators, but no conclusive evidence was obtained to show which basic method was superior. The pumping method does deserve further i n -ve s t i g a t i o n ,.preferably including f i e l d tests of a system designed s p e c i -f i c a l l y f o r d e s t r a t i f i c a t i o n . The s c a l i n g e f f e c t was found to be small, with a tendency ex-h i b i t e d f o r a system to perform s l i g h t l y more e f f i c i e n t l y under lower energy inputs. This was also evident i n comparing laboratory r e s u l t s with f i e l d r e s u l t s reported i n the l i t e r a t u r e . SUMMARY OF CONCLUSIONS The aging process in fresh water lakes is a natural one, which, under natural conditions, should take thousands of years to complete in any one body of water. Since our fresh water lakes vary in chronologi-cal age, a balanced condition should result in which they are equally diverse in biological age. Cultural nutrient inputs resulting from concentrated human activity have upset this balance, accelerating the biological aging process in many lakes. Often i t is these very lakes which, by their proximity to population concentrations, offer the most potential for uses which demand good quality water, such as recreation or water supply. The f i r s t sign of oncoming "eutrophic" conditions is usually a summer "bloom" of algae, often of the nuisance blue-green type. The physical signs of such a growth.are a change in water colour, increase in i t s turbidity, and wide diurnal variations in pH, dissolved oxygen concentration and carbon dioxide concentration. While algae are useful organisms when their growth is moderate, a summer:, bloom can k i l l f i s h due to the wide DO variation i t causes, i t can cause "swimmer's it c h " and other problems which render the lake undesirable for recreation, and i t can create further problems when i t dies. The dead cells settle to the lake bottom to decompose, exerting an oxygen demand which depletes the DO supply in the bottom waters. This supply cannot be replenished because the bottom waters, known as the hypolimnion,,are isolated from the atmosphere by the upper, warmer layer 61 62 known as the epilimnion. If the oxygen demand is sufficient to create anaerobic conditions in the hypolimnion, a reducing environment is cre-ated in which certain algal nutrients are reduced to their soluble states, and toxic hydrogen sulfide is released. These conditions w i l l persist until f a l l overturn occurs at which time the lake is completely mixed. The best long term solution to the problem of man's acceleration of the natural eutrophication trend i s to substantially reduce cultural nutrient inputs through treatment or land disposal of wastes and better control of agricultural f e r t i l i z a t i o n , irrigation and drainage. This solution usually requires years to take effect. In the meantime, there are'two temporary methods available for the improvement of summer water quality. "Slug" or "dilution" flushing of lakes can reduce nutrient concentrations, but usually the required volumes or peak flows are not available. The alternative method ava i l -able is the a r t i f i c i a l elimination of summer thermal st r a t i f i c a t i o n . This is accomplished through the use of diffused air injected at the lake bottom or a mechanical pump which draws the bottom waters to the surface to be mixed with the epilimnion water. Both methods have the same effect on water quality. Such mixing w i l l permit the atmospheric reaeration of the bottom waters, preventing the previously mentioned reducing environment from occurring. Also, the mixing w i l l enable fi s h to respire in the whole lake,give rise to lower evaporation losses and enhance biodegradation of synthetic organics such as 2-4-D and linear alkylate sulfonate. The only possible detrimental effects of a r t i f i -63 f i c i a l mixing would seem to be long term ones induced by the change in the lake's summer environment. Such effects have not yet been detected. Osoyoos Lake i s subject to large cultural nutrient inputs, from agricultural runoff, tributary septic tanks and upstream communities and industries. It has been classified by one researcher as mesotrophic, tending towards a eutrophic state. It suffered a major algae bloom in 1968, which adversely affected the tourist-oriented local economy. It also caused a potability problem in the lake water, some of which is used for domestic purposes. Similar blooms did not occur in 1969 or 1970. An aeration program was initiated in 1969 by the B.C. Water Resources Service in the vi c i n i t y of the pump intake affected in 1968. Although the severe DO depletion of 1968 did not recur, three test runs made in 1969 and 1970 indicated that the system could not adequate-ly aerate the water entering the intake. Five interrelated factors ex-plain why this is true. 1. The aerators were located at too shallow a depth to provide a significantly large volume of cold, dense hypolimnion water for transport to the surface. 2. The Helixor deprived the air column of what l i t t l e hypolimnion water there was available for vertical transport. 3. Direct bubble to water oxygen transfer did not occur to a significant degree. Most oxygen transfer occurs after the water reaches the lake surface. 64 4. When there was no significant density d i f -ference between the surface and bottom waters, mixing " c e l l s " would set up and the aerators' effects became very localized. 5. If the density difference were significant, the effect of the aerators would be diffused to a great de-gree, since i t is impossible to destratify only a small por--tion of an open body of water. Efforts should be made to control the cultural nutrient i n -puts to Osoyoos Lake, and consideration given to a mixing system designed to destratify the middle basin of the lake. The former should provide a long term solution to the water quality problems being experienced, and the latter should provide a better short term solution to the problem of oxygen-poor bottom waters. Laboratory tests were conducted to discover why the Helixor was not performing to expectations, and to investigate certain other mixing techniques. The theoretical discussion showed that free turbulent jets of the gravitational convection type gave an approximation of the flow en-trainment properties of an air column. A comparison of a bubble line source with a point source showed that the line source theoretically i n -duces more water flow i f i t s length exceeds 0.14 of the distance from the lake bottom to the thermocline. An investigation into the two mixing mechanisms involved in de-strat i f i c a t i o n showed that secondary mixing, which involves diffusion of the dense water across the thermocline into the less dense epilimnion, 65 played a s i g n i f i c a n t part i n the aeration and mechanical pumping mixing processes tested. Primary mixing, which i s the phy s i c a l trans-port to and subsequent dispersion of the dense water at or near the water surface, played a proportionately larger part i n the aeration pro-cess. This was possibly due to s c a l i n g problems with the mechanical pumping system tested. The H e l i x o r - a i r bubbler combination appeared to induce a s l i g h t -l y higher secondary mixing rate than did the bubbler alone; However, the Helixor s i g n i f i c a n t l y reduced the primary mixing capacity of the a i r column. The pumping system operated i n the same performance range as the aerators. Possible edge e f f e c t s prevented a d e f i n i t e comparison between the two basic techniques; however, i t was concluded that the mechanical pumping technique deserves further f i e l d and laboratory study. " D e s t r a t i f i c a t i o n E f f i c i e n c y " as currently used i n the l i t e r a t u r e was found to be a misleading basis f or the comparison of the performance of mixing devices. S t a b i l i t y , the numerator i n t h i s parameter, depends not only on the volume of dense water to be mixed, but also on i t s den-s i t y , which can be a minor f a c t o r i n the performance of a d e s t r a t i f i c a -t i o n system. The power input required f o r the change i n s t a b i l i t y i s calculated by m u l t i p l y i n g the rated power of the prime mover by the number of hours i t operated. This number serves as the denominator i n the c a l c u l a t i o n , but i t s s i z e r e l a t i v e to the r e a l work done depends on the design of the mechanical system, as well as on the merits of a par-t i c u l a r configuration of a i r or water j e t s . 66 Hence, i t is suggested that, when possible, the use of Destra-t i f ication Efficiency should be avoided. If a comparison is being made of two devices in the same body of water, an adjustment can be made for thermocline level and theoretical energy inputs can be compared. These can be calculated by assuming adiabatic compression for air pumping or assuming that the velocity head represents the only real energy input for mechanical pumping. Comparison of devices from one lake to another w i l l probably necessitate the use of Destratification Efficiency, modi-fied to use theoretical energy input as i t s denominator. Whatever the basis for comparison, every effort should be made to separate the effects of energy losses within the equipment from the mixing effectiveness of the technique under consideration. Equipment losses w i l l be an important design consideration, but logically i t i s best to separate them from the losses associated with the mixing i t s e l f , especially when comparing various possible or existing installations. B I B L I O G R A P H Y Bain, R. C , J r . "Predicting DO Variations Caused by Algae," Journal of the Sanitary Engineering Division, ASCE, XCIV, No. SA5 (October, 1968), 867-881. Symons, J . M., S. R. Weibel, and G. G, Robeck. "Influence of Im-poundments on Water Quality — A Review of L i t e r a t u r e and Statement of Research Needs," PHS Publ. No. 999-WP-18 (Oct-ober, 1964). Revised January, 1966. Sawyer, C. N. " F e r t i l i z a t i o n of Lakes by A g r i c u l t u r a l and Urban Drainage," Journal of the New England Water Works Associa-tion, LVI (June, 1947), 109-127. Thackston, E. L. "Discussion of 'Management and Measurement of DO i n Impoundments'," Journal of the Sanitary Engineering Division, ASCE, XCIV, No. SA4 (August, 1968), 748-752. Brooks, N.H., and R. C. Y. Koh. "Selective Withdrawal From Den-s i t y S t r a t i f i e d Reservoirs," Journal of the Hydraulics Division, ASCE, XCV, No. HY4 (July, 1969), 1369-1400. Oglesby, R. T., and W. T. Edmondson. "Control of Eutrophication," Journal of the Water P o l l u t i o n Control Federation, XXXVIII, No. 9, 1452-1460. Nunnallee, D. A. "Engineering Aspects of Nuisance Algae Control i n Moses Lake." Unpublished Master's d i s s e r t a t i o n , Uni-v e r s i t y of Washington, 1968. Symons, J . M. "Reservoir Water Quality Control by A r t i f i c i a l D e s t r a t i f i c a t i o n — A Summary Report." Presented at the Seminar on Reservoir D e s t r a t i f i c a t i o n , sponsored by the C a l i f o r n i a Department of Water Resources, Los Angeles, January 26, 1968. , W. H. Irwin, and G. G. Robeck. "Control of Reservoir Water Quality by Engineering Methods." In Proceedings of the Specialty Conference on Current Research Into the E f f e c t s of Reservoirs on Water Quality. Tech. Rep. No. 17. Dept. of Environmental and Water Resources Engineering, Vanderbilt U n i v e r s i t y , 1968, 335-390. 67 68 [10] Koberg, G. E., and M. E. Ford, J r . "Elimination of Thermal S t r a t i -f i c a t i o n i n Reservoirs and the Resulting Benefits." Geologi-c a l Survey Water Supply Paper 1809-M, U.S. Government P r i n t -ing O f f i c e , 1965. [11] Irwin, W. H., J . M. Symons, and G. G. Robeck. "Water Quality i n Impoundments and Modifications From D e s t r a t i f i c a t i o n . " In Proceedings of the Reservoir Fishery Resources Symposium, 1967, d i s t r i b u t e d by American F i s h e r i e s Society, Washington, D.C., 130-152. [12] Symons, J. M., W. H. Irwin, E. L. Robinson, and G. G. Robeck. "Impoundment D e s t r a t i f i c a t i o n f o r Raw Water Quality Con-t r o l Using E i t h e r Mechanical or Diffused-Air Pumping," Journal of the American Water Works Association, LIX (Oct-ober, 1967), 1268-1291. [13] Irwin, W. H., J . M. Symons, and G. G. Robeck. "Impoundment De-s t r a t i f i c a t i o n by Mechanical Pumping," Journal of the San-itary Engineering Division, ASCE, XCII, No. SA6 (December, 1966), 21-40. [14] De Marco, J . , J. M. Symons, and G. G. Robeck. "Behaviour of Syn-t h e t i c Organics i n S t r a t i f i e d Impoundments," Journal of the American Water Works Association, LIX (August, 1967), 965-976. [15] Symons, J. M., De Marco, J . , W. H. Irwin, and G. G. Robeck. "En-hancing Biodegradation of Synthetic Organics i n S t r a t i f i e d Impoundments by A r t i f i c i a l M o d i f i c a t i o n of the Thermal Pro-f i l e . " In Proceedings of Symposium of Garda-Hydrology of Lakes and Reservoirs. Pub. No. 70, L'Association Inter-nationale D'Hydrologie S c i e n t i f i q u e , Belgium, 1966, 383-391. [16] , W. H. Irwin, and G. G. Robeck. "Impoundment Water Quality Changes Caused by Mixing," Journal of the Sanitary Engineering Division, ASCE, XCIII, No. SA2 ( A p r i l , 1967), 1-20. . [17] Robinson, E. L., W. H. Irwin, and J. M. Symons. "Influence of A r t i f i c i a l D e s t r a t i f i c a t i o n on Plankton Populations i n Im-poundments." Transactions of the Kentucky Academy of S c i -ence, XXX, Nos. 1 & 2, 1969, 1-18. 69 [18] Booth, D. M. "Water Quality Studies i n Osoyoos Lake, B.C." Un-published Master's d i s s e r t a t i o n , U n i v e r s i t y of B r i t i s h Columbia, 1969. [19] Coulthard, T. L., and J . R. Stein. "A Report on the Okanagan Water Investigation 1968-69." Report prepared for the Water Investigations Branch., B.C. Water Resources Service, Univer-s i t y of B r i t i s h Columbia, 1969. [20] Symons, J . M., and G. G. Robeck. " C a l c u l a t i o n Technique for De-s t r a t i f i c a t i o n E f f i c i e n c y . " Unpublished mimeographed re-port, November, 1966. [21] Brezonik, P. L., J . J. D e l f i n o , and G. F. Lee. "Chemistry of N and Mn i n Cox Hollow Lake, W i s e , Following D e s t r a t i f i c a -t i o n , " Journal of the Sanitary Engineering Division, ASCE, XCV, No. SA5 (October, 1969), 9.29-940. [22] Alcock, F. R. "The Threat of Eutrophication of Okanagan Val l e y Lakes." Unpublished mimeographed report, October, 1968. [23] ?Barnhart, E. L. "Transfer of Oxygen i n Aqueous Solutions," Journal of the Sanitary Engineering Division, ASCE, XCV, No. SA3 (June, 1969), 645-661. [24] Iamandi, C., and H. Rouse. "Jet Induced C i r c u l a t i o n and D i f f u s i o n , " Journal of the Hydraulics Division, ASCE, XCV, No. HY2 (March, 1969), 589-601. [25] Rouse, H., C. S. Yih, and H. W. Humphreys. " G r a v i t a t i o n a l Convec-t i o n From A Boundary Source," Tellus, IV, No. 3 (August, 1952), 201-210. [26] , and J. Dodu. "Turbulent D i f f u s i o n Across A Density Di s c o n t i n u i t y , " La Houille Blanche (August-September, 1955), 522-532. [27]. Ages, A. B. "The Use of A i r Bubblers to Prevent Shoaling at Wharves i n Navigable Rivers." Unpublished Master's d i s s e r -t a t i o n , U n i v e r s i t y of B r i t i s h Columbia, 1967. 70 [28] Sharp, J. J. "Physical Interpretation of Jet D i l u t i o n Parameters," Journal of the Sanitary Engineering Division, ASCE, XCIV, No. SA1 (February, 1968), 55-63. [29] Bernhardt, H. "Aeration of Wahnbach Reservoir Without Changing the Temperature P r o f i l e , " Journal of the American Water Works Association, LIX (August, 1967), 943-964. [30] Bailey, T. E. "Measurement and Detection of Eutrophication," Journal of the Sanitary Engineering Division, ASCE, XCIII, No. SA6 (December, 1967), 121-132. [31] C h u r c h i l l , M. A., and W. R. Nicholas. " E f f e c t s of Impoundments on Water Quality," Journal of the Sanitary Engineering Division, ASCE, XCIII, No. SA6 (December, 1967), 73-90. [32] De Marco, J . , J. K u r b i e l , J. M. Symons, and G. G. Robeck. "In-fluence of Environmental Factors on the Nitrogen Cycle i n Water," Journal of the American Water Works Association, LIX (May, 1967), 580-592. [33] Halsey, T. G. "Autumnal and Over-Winter Limnology of Three Small Eutrophic Lakes With P a r t i c u l a r Reference to Exper-imental C i r c u l a t i o n and Trout M o r t a l i t y . " Unpublished Master's d i s s e r t a t i o n , U n i v e r s i t y of B r i t i s h Columbia, 1967. [34] Lachmann, G. V. (ed.). "Boundary Layer and Flow Control — Its P r i n c i p l e s and A p p l i c a t i o n , " Pergammon, I, 1961, 232-240. [35] Odum, E. P. "Factors Which Regulate Primary Productivity and Heterotrophic U t i l i z a t i o n i n the Ecosystem." Transactions of the 1960 Seminar, "Algae and Metropolitan Wastes," USPHS Report W-61-3, 65-71. [36] S c h l i c h t i n g , H. (Translated by J. Ke s t i n ) . Boundary Layer-Theory. 1968, 681-707. [37] Symons, J. M., W. H. Irwin, R. M. Clark, and G. G. Robeck. "Management and Measurement of DO i n Impoundments," Journal of the Sanitary Engineering Division, ASCE, XCIII, No. SA6 (December, 1967), 181-209. 71 [38] Symons, J. M., W. H. Irwin, J. De Marco, and G. G. Robeck. "Effects of Impoundments on Water Quality: A Research Summary." Transactions of the 17th Annual Sanitary En-gineering Conference, Bulletin of Engineering and Archi-tecture, University of Kansas, No. 57, 1967, 28-36. [39] , and G. G. Robeck. "Impoundment Research: Key to Streamflow Regulation Problems," Water and Wastes En-gineering, III, No. 1 (January, 1966), 42-44; III, No. 2, (February, 1966), 66-68. [40] y S. R. Weibel and G. G. Robeck. "Impoundment In-fluences on Water Quality." Journal of the American Water Works Association, LVII (January, 1965), 51-75. [41] Webster, L. F. "Happening at Cox Hollow," Water and P o l l u t i o n Control, CV, No. 1 (January, 1967), 17-19. A P P E N D I C E S APPENDIX A Notation: The following symbols have been used. A = area of pump discharge nozzle, in ft . D = depth below water surface at which air is discharged, in f t . E = elevation of discharge pipe L above water surface; in f t . F. = densimetric Froude number, dimensionless. A ' = atmospheric pressure, in f t . of water. Hg = depth below surface at which pump suction line draws water, in f t . Hg' = static head over which pump operates in moving dense water to surface, in f t . Hy = velocity head of pump discharge, in f t . H => total head over which a pump operates, in f t . K = constant in isothermal expansion equation, in f t . - l b . L = length of line source air diffuser, in f t . = air flow at atmospheric pressure, for a line ^ source, in ft^/min, or for point source, in f t /min. 3 QQ = air flow at nozzle, In ft /sec. 73 APPENDIX A (Continued) f l u i d flow rate, in f t /sec. 3 volume of air at water surface, at f t . 3 volume of air at nozzle, in f t . velocity of water jet, in ft./sec. work input, in ft-lb./min or H.P. diameter of water jet , in f t . 2 acceleration due to gravity, 32.2 ft./sec distance above nozzle to section, in f t . 3 unit weight of fresh water, 62.4 l b . / f t . , density of water at lake bottom, in g./ml. density of water at lake surface, in g./ml i n i t i a l density of water jet, in g./ml. density of receiving water, in g./ml. APPENDIX B FIELD DATA FROM OSOYOOS LAKE SAMPLE POINT L O C A T I O N 1 250' North of North. Helixor 9 125' North of North Helixor 2 125' South of North Helixor Intake 250' South of North Helixor, at "Tee" 3 125' North of South Helixor 4 250' South of South Helixor JULY 14, 1969 STATION DEPTH TEMPERATURE DISSOLVED OXYGEN ( f t . ) (°C) (mg./l) 1 0 20.2 7.4 1 10 20.1 7.2 1 20 19.7 7.3 1 30 19.9 7.2 1 32.5 19.4 5.5 1 35 15.4 2.7 1 37.5 14.6 1.8 North Helixor 0 19.7 7.9 North Helixor 10 20.2 7.4 North Helixor 20 20.0 7.2 North Helixor 30 19.7 7.4 North Helixor 32.5 18.3 3.9 North Helixor 35 15.5 2.0 2 0 20.2 7.5 2 10 20.2 7.5 2 20 20.0 7.5 2 30 19.6 7.1 2 32.5 17.2 3.0 2 35 15.4 3.2 75 76 APPENDIX B (Continued) July 14, 1969 STATION DEPTH TEMPERATURE DISSOLVED ( f t . ) (°C) (mg./] Intake 0 20.3 7.2 Intake 10 20.2 7.0 Intake 20 20.1 7.1 Intake 30 19.9 6.9 Intake 32.5 19.3 5.8 Intake 35 15.5 3.3 3 0 19.9 7.6 3 10 20.1 7.6 3 20 20.0 7.6 3 30 20.0 7.2 3 32.5 19.7 5.2 3 35 16.3 3.1 South Helixor 0 20.3 7.7 South Helixor 10 20.3 7.1 South Helixor 20 20.2 7.5 South Helixor 30 20.1 7.2 South Helixor 32.5 17.1 3.8 4 0 20.4 7.5 4 10 20.4 7.3 4 20 20.3 7.3 4 25 20.2 7.4 JULY 16. 1969 STATION 1 1 1 1 1 1 1 DEPTH ( f t . ) 0 10 20 30 32.5 35 37.5 TEMPERATURE (°C) 21.1 20.2 19.9 19.3 18.5 17.4 16.4 DISSOLVED OXYGEN (mg./l) 8.5 8.3 8.4 7.7 7.6 4.3 2.7 77 July 16, 1969 APPENDIX B CContinued) STATION DEPTH (ft.) TEMPERATURE (°C) DISSOLVED OXYGEN (mg./l) 9 9 9 9 9 9 0 10 20 30 32. 35 21.1 20.3 20.1 19.3 18.6 17.7 8.5 8.3 7.9 7.1 5.3 3.4 North Helixor 0 North Helixor 10 North Helixor 20 North Helixor 30 North Helixor 32. North Helixor 34 20.2 20.2 20.1 19.3 18.5 17.8 8.6 8.5 8.4 7.5 5.8 4.1 2 2 2 2 2 0 10 20 30 32.5 21.1 20.6 20.1 19.3 18.6 8.5 7.8 7.1 6.2 5.2 Intake Intake Intake Intake Intake Intake 0 10 20 30 32. 36 21. 20, 20. 19, 18. 17. 8.'6 7.9 7.5 6.3 5.0 3.0 3 3 3 3 3 3 0 10 20 30 32. 35 20.2 20.7 20.1 19.1 18.4 17.4 8.5 8.3 7.9 6.7 5.3 3.2 South Helixor 0 South Helixor 10 South Helixor 20 South Helixor 30 South Helixor 32. South Helixor 35 20.2 20.3 19.8 19.3 18.8 18.1 8.4 8.5 8.1 7.2 5.9 4.1 78 APPENDIX B (Continued) July 16, 1969 STATION DEPTH TEMPERATURE DISSOLVED OXYGEN (ft.) C"C) Ong./l) 4 0 22.1 8.2 4 10 20.6 8.1 4 20 20.0 7.5 4 25 19.4 7.1 JULY 18, 1969 STATION DEPTH TEMPERATURE DISSOLVED OXYGEN 1 0 21.3 8.4 1 10 21.1 8.5 1 20 20.6 8.5 1 30 19.4 7.3 1 32.5 18.2 4.1 1 35 16.0 1.9 1 37.5 15.3 0.7 9 0 21.3 8.5 9 10 21.0 8.5 9 20 20.7 8.5 9 30 19.2 6.8 9 32.5 18.1 4.3 9 35 15.7 2.0 North Helixor 0 20.7 8.8 North Helixor 10 20.8 8.8 North Helixor 20 20.6 8.8 North Helixor 30 20.1 7.0 North Helixor 32.5 18.4 6.5 North Helixor 35 17.9 1.9 2 0 21.8 8.8 2 10 21.3 8.5 2 20 21.6 8.3 2 30 19.3 . 7 . 5 2 32.5 18.3 4.9 79 APPENDIX B (Continued) July 18. 1969 STATION DEPTH (ft.) TEMPERATURE (°C) DISSOLVED OXYGEN (mg./l) Intake Intake Intake Intake Intake Intake 0 10 20 30 32. 35 21.3 21.1 20.4 19.4 17.9 16.3 8.7 8.3 8.3 7.1 4.0 1.4 3 3 3 3 3 3 0 10 20 30 32. 35 22.1 21.3 20.2 18.8 16.9 15.5 8.6 8.5 8.4 7.0 1.9 2.1 South South South South South South Helixor Helixor Helixor Helixor Helixor Helixor 0 10 20 30 32. 35 20.7 21.1 20.2 18.3 16.4 15.7 8.8 8.5 8.2 6.3 2.1 2.0 4 4 4 4 0 10 20 25 23.1 21.3 20.3 19.5 8.5 8.5 8.5 7.8 JULY 5, 1970 STATION DEPTH (ft.) TEMPERATURE (°C) DISSOLVED OXYGEN (mg./l) North Helixor 0 North Helixor 10 North Helixor 20 North Helixor 25 21.6 21.3 20.2 18.7 9.0, 9.3 9.4 9.0 80 July 5, 1970 APPENDIX B CContlnued) STATION DEPTH (ft.) TEMPERATURE C°c) DISSOLVED OXYGEN (mg./l) North Helixor 27.5 North Helixor 30 North Helixor 32.5 North Helixor 35 17.8 15.4 13.9 13.6 8.1 7.2 6.5 6.1 Intake Intake Intake Intake Intake Intake Intake Intake Intake Intake 0 5 10 15 20 25 27. 30 32. 35 24.9 22.4 22.1 21.8 20.8 18. 16. 15, 15, ,3 ,5 ,5 ,2 14.4 9.2 9.3 9.4 9.5 9.7 8.3 8.0 7.9 7.3 6.8 South South South South South South South South Helixor Helixor Helixor Helixor Helixor Helixor Helixor Helixor 0 10 20 25 27. 30 32. 35 21.2 21.5 20.1 19.3 18.2 16.4 14.6 12.4 9.0 9.6 8.8 8.7 8.4 7.7 7.1 5.2 APPENDIX C LABORATORY DATA TEST NO. 1 AUGUST 14, 1970 Bubbler and Helixor 85.0 lb. Stock Salt in 1 f t . of water = .388 g. Rhodamine "B" in 1 f t . of water Add 4.5 f t . fresh water - total depth >Theor. cone. = 10,000 mg./l. >Theor. cone. = 100 p.p.b. 5.5 f t . BEFORE MIXING AFTER MIXING DEPTH TEMP. CONDUCTIVITY DENSITY TEMP, (ft.) (°C) (xlO millimhos) (g./ml.) (°C) CONDUCTIVITY DENSITY (xlO millimhos) (g./ml.) 0.0 13.0 0.540 0.9997 12.7 1.860 1.0007 0.5 12.2 0.500 0.9997 1.0 12.2 0.495 0.9997 12.7 1.850 1.0007 1*5 12.2 0.490 . 0.9997 2.0 12.2 0.485 0.9997 12.7 1.850 1.0007 2.5 12.4 0.480 0.9997 3.0 12.5 0.490 0.9997 12.7 1.850 1.0007 3.5 12.8 0.490 0.9997 4.0 13.0 0.495 0.9997 12.7 1.840 1.0007 4.5 13.8 0.520 0.9997 4.75 14.4 6.540 1.0048 5.0 14.9 8.100 1.0060 12.7 1.840 1.0007 5.2 15.1 8.230 1.0061 5.4 15.1 8.300 1.0062 12.7 1.840 1.0007 81 81a APPENDIX C CContlnued) Test No. 1 August 14, 1970 Energy input: From equation (11) W - .00472 Q A[(H A + D) -H A] .00472 x 1.01 [(33.9 + 5.3) (_„ 3 3 ' 9 . _) / - 33.9] 3o;S-+ 5.3 = 221. ft.lb/min. Time for Destratification From graph N.B. 100%. . . 3 1 - 1 75% . . 1 5 - 1 - 1 min. delay for sample = 30 min. = 14 min. 83 84 Density -Cg./m/.) - 7~AeoraA'ca/ 0.393 /.OOO /.OOt /.0O2. /.003 /.OO* /.005~ /.O06, TQNk BOTTOM Figure 14 Laboratory Test Density P r o f i l e Test No. 4 September 3, 1970 N.B. Densities derived from Salt c o n c -density r e l a t i o n s h i p * 0 ^'Before Mixing 6.939 LOOO Density - C<j.//»!•) -Ttteaf-e.-fie.eJ /.OOt f.OOZ. /.<X>3 I,CO?- /.COS" Offer Mini* TflNK BOTTOM Figure 15 Laboratory Test Density Profile Test No. 6 September 15, 1970 N.B. Densities derived from Salt Conc-density relationship 86 APPENDIX C CContinued) Test No. 1 August 14, 1970 DIST. LAYER (ft.) AT 4°C WEIGHT (lb.) CENTER OF LAYER TO ISOTHERMAL C.G. (ft.) P.E. CHANGE (2 x 3) f t . - l b . S.G. FACTOR (S.G'.-l.OOO) ENERGY LOST (4 x 5) f t . - l b 0.0 - 0.5 4273.3 - 2.50 -10,683.2 - .0003 + 3.2 0.5 - 1.0 4273.3 - 2.00 - 8,546.6 - .0003 + 2.6 1.0 - 1.5 4273.3 - 1.50 - 6,409.9 - .0003 + 1.9 1.5 - 2.0 4273.3 - 1.00 - 4,273.3 - .0003 + 1.3 2.0 - 2.5 4273.3 - 0.50 - 2,136.6 - .0003 + 0.6 2.5 - 3.0 4273.3 0.00 0.0 - .0003 0.0 3.0 - 3.5 4273.3 0.50 2,136.6 - .0003 - 0.6 3.5 - 4.0 4273.3 1.00 4,273.3 - .0003 - 1.3 4.0 - 4.5 4273.3 1.50 6,409.9 - .0003 - 1.9 4.5 - 5.0 4273.3 2.00 8,546.6 + .0048 +41.0 5.0 - 5.5 4273.3 2.50 10,683.2 + .0061 +65.2 Total Stability = 112.0 f t . l b . Air Flow Calculations Flow meter tube no. 448-324 Rdg. const, at 25.0 Manometer - avg. rdg. 15.0 in. Hg. = 7.36 psig. From calib. curve no. C658,16 flow-with 0 back press. = 23,400 ml./min @ STP. = 0.826 ft 3/min @ STP. Correction curve no. GI 1313 shows corr'n. factor of 1.22 making corr. free air flow = 1.01 ft /min @ STP. 87 APPENDIX C CContinued) TEST NO . 2 AUGUST 18, 1970 Bubbler and Helixor 114.6 l b . Stock Salt i n 1 .388 g. Rhodamine "B" i n 1 Add 4.5 f t . fresh water -f t . of water f t . of water t o t a l depth = 5.5 f t . BEFORE MIXING AFTER MIXING DEPTH ( f t . ) TEMP (°c) CONDUCTIVITY (xlO millimhos) DENSITY (g./ml.) TEMP. (°C) CONDUCTIVITY (xlO millimhos) DENSIT (g/lml.: 0.0 11.5 1.03 1.00040 10.8 2.18 1.0017 0.5 11.0 1.09 1.00045 1.0 11.0 1.11 1.00050 10.8 2.18 1.0017 1.5 11.0 1.14 1.00055 2.0 11.0 1.17 1.00060 10.8 2.18 1.0017 2.5 11.0 1.24 1.00065 3.0 11.0 1.28 1.00070 10.8 2.18 1.0017 3.5 11.0 1.35 1.00080 4.0 11.2 1.50 1.00090 10.8 2.18 1.0017 4.5 11.4 3.05 1.00230 5.0 11.7 7.26 1.00620 10.8 2.18 1.0017 5.25 11.7 7.28 1.00630 5.5 11.7 7.28 1.00630 10.8 2.18 1.0017 I n i t i a l S t a b i l i t y = 100.ft.lb. F i n a l S t a b i l i t y = 0. f t . l b . A i r Flow Rate = 1.01 f t 3 / m i n . Energy Input Rate = 221 ft.lb./min. Time f o r D e s t r a t i f i c a t i o n N.B. - 1 min. delay f o r sample. 32 min. 15 min. 88 APPENDIX C (Continued) TEST NO. 3 AUGUST 25. 1970 Bubbler 87.29 lb. Stock Salt in 1 f t . of water .388 g. Rhodamine "B" in 1 f t . of water Add 4.5 f t . fresh, water - total depth =5.5 f t . BEFORE MIXING AFTER MIXING DEPTH TEMP. CONDUCTIVITY DENSITY TEMP. CONDUCTIVITY DENSITY (ft.) (°C) (xlO millimhos) (g./ml.) (°C) (xlO millimhos) (g./ml.) 0.0 15.0 0.20 0.9990 13.1 1.61 1.00065 0.25 15.0 0.24 0.9992 0.5 13.9 0.24 0.9994 0.75 13.7 0.24 0.9994 1.0 13.1 0.24 0.9995 13.1 1.61 1.00065 1;5 13.1 0.24 0.9995 .2.0 13.1 0.24 0.9995 13.1 1.61 1.00065 2.5 13.1 0.24 0.9995 3.0 13.1 0.24 0.9995 13.1 1.61 1.00065 3.5 13.2 0.26 0.9995 4.0 13.2 0.26 0.9995 13.1 1.61 1.00065 4.25 13.2 0.41 0.9996 4.5 13.2 1.73 1.0008 4.75 14.0 7.07 1.0054 5.0 14.2 7.07 1.0054 13.1 1.61 1.00065 5.25 14.1 7.07 1.0054 5.5 14.1 7.07 1.0054 13.1 1.61 1.00065 I n i t i a l Stability = 118. f t . l b . Final Stability = 0. f t . l b . Air flow rate = 1.01 ft-Vmin. Energy Input rate = ."2'21 ft.lb./min. Time for Destratification N.B. 1 min. delay for sample. 35 min. 22 min. 89 APPENDIX C (Continued) TEST NO. 4 SEPTEMBER 3, 1970 Mechanical Pump 85.29 lb. Stock Salt in 1 f t . of water. .388 g. Rhodamine "B" in 1 f t . of water. Add 4.5 f t . fresh water - total depth =5.5 f t . BEFORE MIXING AFTER MIXING DEPTH TEMP. CONDUCTIVITY (ft.) (°C) (xlO millimhos) DENSITY TEMP. CONDUCTIVITY DENSITY (g./ml.) (°C) (xlO millimhos) (g./ml.) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 4.75 5.0 5.25 5.5 14.2 14.0 14.0 14.0 14.0 14.0 14.0 14.1 14.2 14.6 14.8 15.0 15.1 15.2 0.39 0.40 0.40 0.42 0.43 0.48 0.52 0.60 1.05 1.12 2.71 7.28 7.35 7.35 0.9995 0.9995 0.9995 0.9995 0.9995 0.9996 0.9997 0.9997 0.9999 1.0000 1.0012 1.0051 1.0052 1.0052 14.1 14.1 14.1 14.1 14.1 14.1 14.1 1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.0005 1.0005 1.0005 1.0005 1.0005 1.0005 1.0005 I n i t i a l Stability = 91 f t . l b . Final Stability = O.ft. lb. Water Flow Rate =-5.67 ft 3/min. T _ ^ T 5.67 x 144 ,., . . Jet Velocity = —60 x 2 4 = ft/sec. Pump Head = Static Head + Velocity Head = 1.0 f t . Energy Input Rate = Q + H =5.67 x 62.4 x 1.0 = 353 ft.lb./min. Time for Destratification N.B. - 1 min. delay for sample, 2 min. delay for pump 100 %. . . 29 - 3 = 26 min. 75 %. . . 14 - 3 = 11 min. 90 APPENDIX C (Continued) TEST NO. 5 SEPTEMBER 10? 1970 Mechanical Pump 170.0 .388 g Add 3. lb. Stock Salt in 2 Rhodamine "B" in ,5 f t . fresh water -f t . of water. 2 f t . of water, total depth = 5 .5 f t . BEFORE MIXING AFTER MIXING DEPTH (ft.) TEMP (°C) CONDUCTIVITY (xlO millimhos) DENSITY Cg/ml.) TEMP. (°c) CONDUCTIVITY (xlO millimhos). DENSIT (g. /mi.; 0.0 12.8 0.41 0.9995 13.3 2.73 1.0016 0.5 12.8 0.41 0.9995 1.0 12.8 0.41 0.9995 13.3 2.73 1.0016 1.5 12.8 0.42 0.9995 2.0 12.8 0.50 0.9996 13.3 2.73 1.0016 2.5 12.9 0.54 0.9997 3.0 13.0 0.62 0.9998 13.3 2.73 1.0016 3.25 13.0 0.84 0.9999 3.5 13.0 1.33 1.0003 3.75 13.2 4.54 1.0032 4.0 14.0 6.30 1.0048 13.3 2.73 1.0016 4.5 14.0 6.30 1.0048 5.0 14.0 6.30 1.0048 13.3 2.73 1.0016 5.5 14.0 6.30 1.0048 13.3 2.73 1.0016 I n i t i a l Stability = 152. f t . l b . Final Stability = 0. f t . l b . Energy Input Rate = 353 ft.lb./min. Time for Destratification N.B. - 1 min. delay for sample 1 min. delay for pump. 100%. . . 36 - 2 = 34 min. 75%. . . 16.5 - 2 = 14.5 min. 91 APPENDIX C (Continued) TEST NO. 6 SEPTEMBER 15, 1970 Bubbler 170.0 .388 g Add 3. lb. Stock Salt in 2 ;. Rhodamine "B" in 5 f t . fresh water -f t . of water. 2 f t . of water, total depth = 5 .5 f t . BEFORE MIXING AFTER MIXING DEPTH (ft.) TEMP. (°C) CONDUCTIVITY (xlO millimhos) DENSITY (g./ml.) TEMP. (°c) CONDUCTIVITY (xlO millimhos) DENS IT1 (g./ml.! 0.0 13.2 0.28 0.99935 13.2 2.63 1.0015 0.5 13.2 0.29 0.99935 1.0 13.2 0.31 0.99935 13.2 2.63 1.0015 1.5 13.2 0.36 0.99945 2.0 13.2 0.40 0.99950 13.2 2.63 1.0015 2.5 13.2 0.50 0.99965 3.0 13.3 0.58 0.99970 13.2 2.63 1.0015 3.25 13.4 0.85 0.9999 3.5 13.5 3.05 1.0018 3.75 13.7 6.29 1.0046 4.0 13.8 6.64 1.0049 13.2 2.63 1.0015 4.5 13.8 6.64 1.0049 5.0 13.8 6.64 1.0049 13.2 2.63 1.0015 5.5 13.7 6.64 1.0049 13.2 2.63 1.0015 I n i t i a l Stability = 165. f t . lb. Final Stability = 0. f t . l b . Air flow rate = 1.01 ft^/min. Energy input rate = 221- f t . lb./min. Time for Destratification N.B. - 1 min. delay for sample. 100%. . . 38 - 1 75%. . . 17.5 - 1 = 37 min. 16.5 min. 92 APPENDIX C (Continued) TEST NO. 7: SEPTEMBER 16, 1970 Bubbler and Helixor 173.4 l b . Stock Salt i n 2 f t . of water. .388 g. Rhodamine "B" i n 2 f t . of water. Add 3.5 f t . fresh water - t o t a l depth = 5.5 f t . BEFORE MIXING AFTER MIXING DEPTH TEMP. CONDUCTIVITY DENSITY TEMP. CONDUCTIVITY DENSITY (ft . ) (°C) (xlO millimhos) (g./ml.) (°C) (xlO millimhos) (g./ml.) 0.0 13.0 0.28 0.99940 13.0 2.43 1.00135 0.5 13.0 0.29 0.99940 1.0 13.0 0.30 0.99940 13.0 2.43 1.00135 1.5 13.0 0.30 0.99940 2.0 13.0 0.32 0.99945 13.0 2.43 1.00135 2.5 13.0 0.35 0.99950 3.0 13.0 0.41 0.99960 13.0 2.43 1.00135 3.25 13.0 0.50 0.99965 3.5 13.0 1.71 1.00065 3.75 13.0 5.85 1.00435 4.0 13.0 5.85 1.00435 13.0 2.43 1.00135 4.5 13.0 5.85 1.00435 5.0 13.0 5.85 1.00435 13.0 2.43 1.00135 5.5 13.3 5.85 1.00435 13.0 2.43 1.00135 I n i t i a l S t a b i l i t y =148. f t . l b . F i n a l S t a b i l i t y = 0. f t . l b . A i r flow rate = 1.01 f t /min. Energy input rate = 306 ft.lb./min. Time f or D e s t r a t i f i c a t i o n N.B. - 1 min. delay for sample 100%. . . 51 - 1 = 50 min. 75%. . . 23 - 1 = 22 min. 93 APPENDIX C (Continued) TEST NO, . 8: SEPTEMBER 22 , 1970 Mechanical Pump 89.9 l b . .388 g. Add 4.5 , Stock Salt i n 1 Rhodamine "B" i n f t . fresh water -f t . of 1 f t . - t o t a l water. of water, depth = 5.5 f t . BEFORE MIXING AFTER MIXING DEPTH (f t . ) TEMP. (°C) CONDUCTIVITY (xlO millimhos) DENSITY (g./ml.) TEMP. CONDUCTIVITY (°C) (xlO millimhos) DENS IT" (g./ml.! 0.0 13.0 0.19 0.99930 13.6 1.44 1.0004 0.5 13.0 0.19 0.99930 1.0 13.0 0.20 0.99930 13.6 1.44 1.0004 1.5 13.0 0.22 0.99935 2.0 13.0 0.24 0.99940 13.6 1.44 1.0004 2.5 13.0 0.27 0.99940 3.0 13.1 0.30 0.99940 13.6 1.44 1.0004 3.5 13.1 0.35 0.99950 4.0 13.2 0.43 0.99960 13.6 1.44 1.0004 4.25 13.3 0.54 0.99970 4.5 13.7 2.52 1.00140 4.75 14.0 7.10 1.00540 5.0 14.3 7.20 1.00550 13.6 1.44 1.0004 5.5 14.3 7.20 1.00550 13.6 1.44 1.0004 I n i t i a l S t a b i l i t y = 122. f t . l b . F i n a l S t a b i l i t y = 0. f t . l b . Energy input rate = 353 ft.lb./min. Time f or D e s t r a t i f i c a t i o n N.B. - 1 min. delay for sample 1 min. delay f o r pump, 23 min. 11 min. £94 APPENDIX C (Continued) TEST NO . 9 SEPTEMBER 28, 1970 Bubbler 85.0 lb .388 g. Add 4.5 . Stock Salt i n 1 Rhodamine "B" i n f t . fresh water -f t . of water. 1 f t . of water. - t o t a l depth = 5.5 f t . BEFORE MIXING AFTER MIXING DEPTH ( f t . ) TEMP.• (°C) CONDUCTIVITY (xlO millimhos) DENSITY (g./ml.) TEMP. (°C) CONDUCTIVITY (xlO millimhos) DENSIT (g./mi.; 0.0 12.3 0.17 0.99940 13.1 1.30 1.0004 0.5 12.3 0.17 0.99940 1.0 12.3 0.17 0.99940 13.1 1.30 1.0004 1.5 12.3 0.17 0.99940 2.0 12.3 0.18 0.99940 13.1 1,30 1.0004 2.5 12.3 0.19 0.99950 3.0 12.3 0.21 0.99950 13.1 1.30 1.0004 3.5 12.3 0.24 0.99955 4.0 12.3 0.28 0.99955 13.1 1.30 1.0004 4.25 12.3 0.37 0.99960 4.5 12.3 2.90 1.00180 4.75 12.5 5.40 1.00420 5.0 12.8 5.46 1.00430 13.1 1.30 1.0004 5.5 12.8 5.46 1.00430 13.1 1.30 1.0004 I n i t i a l S t a b i l i t y = 95. f t . l b . F i n a l S t a b i l i t y =0. f t . l b . A i r flow rate = 0.53 ft.^/min. Energy input rate = 160 ft.lb./min. Time f o r D e s t r a t i f i c a t i o n N.B. - 1 min. delay f o r sample 1 min. delay for a i r supply. malfunction 75 min. 38 min. APPENDIX D DESIGN OF A DESTRATIFICATION SYSTEM Two design techniques for destratification systemsrhave been proposed by Symons [9]. They are presented here with some modifica-tions by the author consistent with some of the findings reported in this paper. Since the design methods described depend in part on em-p i r i c a l data, they should be used with caution. They w i l l provide a rough estimate of the size of the power unit required for a given situation; this estimate should be modified to satisfy constraints peculiar to the lake in question. The design procedures presented are based in part on rated power input in one case and Destratification Efficiency on the other, both of which were discredited as c r i t e r i a for device comparison earlier in this paper. Both are presented because of the lack of a better alternative at present. 95 96 Oxygenation Capacity Method [9] Using DO curves from recent summer observations, p l o t a graph of pounds of DO Present i n the hypolimnion versus time. Next, decide on a "target" DO concentration d i s t r i b u t i o n over the summer, convert i t to pounds of DO, and plo t i t on the same graph. The area between these curves, up to the time at which the DO content n a t u r a l l y begins to i n -crease, represents the amount of oxygen to be supplied by the mixing device. From the l i t e r a t u r e [9], assume an average "Oxygenation Capa-c i t y , " or "OC," of 1.5 pounds DO per Kilowatt-hour of power input. This f i g u r e applies to a i r d i f f u s i o n devices only and refer s to power input calculated by measuring the area under the power input curve, which has a slope equal to the rated power of the device. I t i s poss-i b l e to generalize the compressor e f f i c i e n c y f o r such devices because they consistently appear i n the l i t e r a t u r e i n the 30 to 40 per cent range. The oxygen transfer properties of mechanical pumps have not been studied as thoroughly, so th i s OC cannot be assumed for them. The area between the DO curves should be divided by the assumed OC to y i e l d a value which w i l l represent the area below the 2 power input curve, and w i l l have units of Kilowatt (hours) . This curve can be assumed to be tri a n g u l a r i n shape, so i t s slope can be 2 calculated by d i v i d i n g i t s area by 1/2 t . Figure 16 "Oxygenation Capacity" Design Curves for Central Basin, Osoyoos Lake, B.C. vD 98 Osoyoos Lake Design Figure 16 is a DO curve for the hypolimnion:of the middle basin of Osoyoos Lake. Q D.O. Area = 1.49 x 10 lb. D.O. - hours „ _ . 1.49 x 10 8 lb. D.O. — hr. . Power Curve Area = c • / V_ T .-1 1.5 lb. D.O. - (KW-hr) = 0.993 x 10 8 KW - C h r ) 2 Assume operating period i s from 15 June to 30 September. t = 107 days - 2568 hr. 1/2 t 2 = 3.30 x 10 6 (hr) 2 Power = 99.3 x 1Q6 KW - (hr) 2 . 3.3 x 10 6 (hr) = 30.0 KW This can be used as a design capacity for a compressor unit. 99 D e s t r a t i f i c a t i o n E f f i c i e n c y Method [9] On a recent summer plo t of s t a b i l i t y with time f o r the design lake, superimpose a desired s t a b i l i t y curve. Usually such a curve takes the form of Eigure 17 which i s a s t a b i l i t y p l o t f o r the middle basin of Osoyoos Lake. Here __ ± s assumed that the mixing device begins operating on June 1, and operates s t e a d i l y u n t i l mid-August, at which time the s t a b i l i t y trend n a t u r a l l y turns downward. This i s probably a more l i k e l y operation than the one quoted for the OC method. The shaded area between the curves represents the t h e o r e t i c a l energy i n -put required from the mixing device. D i v i s i o n of t h i s f i g u r e by the device's actual power input D e s t r a t i f i c a t i o n E f f i c i e n c y w i l l y i e l d the area under the device's power input curve. D i v i s i o n of t h i s value by 2 1/2 t w i l l y i e l d the curve's slope, which w i l l be the energy input at the nozzle for an aeration device or at the entry point of the discharge j e t f o r a mechanical pump. Osoyoos Lake Design 5 2 Shaded area i n F i g . 17 = 6.90 x 10 KW - (hr) Using an average D e s t r a t i f i c a t i o n E f f i c i e n c y of 1.5%, area , 6.90 x 10 7 . , , f t 7 .__ .2 under power curve = ^ ,. = 4.6 x 10 KW - (hr) Assume operating period i s from 1 June to 15 August. t = 7 6 days = 1824 hr. 1/2 t 2 = 1.665 x 10 6 ( h r ) 2 • T> 4.6 x 10 7 r _ . . Power = -—-TTZ T776 = 27.4 KW. 1.655 x 10 101 For an air diffuser system, assume equipment efficiency = 40% .". capacity required = -^ x 27.4 = 68.5 KW To design compressor from energy input at nozzle, refer to equation (8), solving for Q for D = 90 f t . ri. 27.4 KW = 36.6 HP = .148 Q l o g * Z~ Z a 10 33.9 Q A = 440 ft 3/min. 3 .". the compressor should be designed to deliver 221 f t /min. of free air to a discharge point located at a depth of 90 feet. For a mechanical pumping system, Energy input rate = 27.4 KW = 20,260 f t . l b = Q y H QH =2%|20_ ! e C 3 2 5 ^ 4 62.4 sec. Criteria for discharge line sizing: (1) Maximize densimetric Froude Number to maximize mixing of cold discharge jet in warmer surface waters. d 2 . i 4 y g (—-2-) d 2 when g = 32.2 ft/sec p = 0.99975 (T = 10°C) P q = 0.99825 (T = 20°C) 102 then. FL - -5^2 d To maintain a "momentum" jet, F^ must be greater than 4, in which case: d <_ 1.15 Q 2 / 5 2 If we ignore static head, and assume H = V /2g, for Osoyoos Lake we have: 2 f t 5 QV = 20980. ^ =--sec. V - 4 % . 1 . 2 7 % . * d 2 d 2 which f i n a l l y gives us d <_ 31.4 f t . for a Q of 1800 cfs. That i s , any reasonable size of discharge line w i l l give sufficient mix-ing. (2) Maximize primary and secondary mixing. Primary rate = Q Secondary rate = f (Momentum imparted by jet) = f (MV) - f 4 d This criterion indicates that we must find an optimal balance between the primary and secondary mixing rates, since by increasing one we de— 103 crease the other for a constant energy input. It should.be possible to provide a rough estimate of this balance through further research with both model and prototype. (3) Minimize pump and pump fr i c t i o n losses. 2 Generally, a V Hence, an optimally designed pump w i l l have as large a suction line as is economically feasible, a discharge line sized to balance mixing induction against f r i c t i o n losses, and w i l l probably be of the low head-high flow type. 

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