by
Burkard Baschek
D iplom , U n iv ersitat zu Kiel, 1998A D isse rta tio n s u b m itte d in P a rtia l Fulfillm ent of th e R eq uirem en ts for th e D egree of
DOCTOR OF PHILOSOPHY
in the School of E arth and Ocean Sciences
fis d isse rta tio n as conform ing to th e req u ired s ta n d a rd
Dr. D.M. Farmer, Supervisor (School of E arth and Ocean Sciences)
Dr. C. Gaxre1[t, Cp-supervisor/O utside Member (D epartm ent of Physics and Astronomy)
---Dr. E. C&mack, D epartm ëntal Member (School of E arth and Ocean Sciences)
Dr. "R. hueck. D epartm ental M ember (School of E arth and Ocean Sciences)
Dr. P. Cummins, A d d i^ im l M ember (Institute of Ocean Sciences, Sidney)
_______________________________________ Dr. W. Jenkins, E t e r n a l Exam iner (Woods Hole Oceanographic Institution, MA, USA)
© B urkard Baschek, 2002 University of Victoria
All rights reserved. This thesis may not be reproduced in whole or in p art, by photocopy or other means, w ithout the permission of the author.
E xam iners:
Dr. D.M. Farmer, Supervisor (School of E arth and Ocean Sciences)
Co-supervisor/ O utside Member (D epartm ent of Physics and Astronomy) Dr. C. G atrett,
Dr. E . ^ a r nack, D epartm ental Member (School of E arth and Ocean Science)
■e
D r^R .' Lueck, D epartm ental M ember (School of E arth and Ocean Science) _________________ Dr. P. Cum m ins, A drW onal M ember (Institute of Ocean Sciences, Sidney)
Supervisors: Dr. D.M. Farmer, Dr. C. G arrett
A b stract
Strong tidal currents in the Fraser Estuary, BC, Canada, cause intense flow-topography interaction around islands and over shallow sills. At Boundary Pass, a steep sill forms a pronounced barrier for flow of dense w ater from the Paciflc Ocean into th e S trait of Georgia. T he processes at the sill control th e renewal of deep and interm ediate w ater in th e S trait. The strong flood tidal flow forces dense w ater to flow over the sill. It then meets a fresh surface layer ju st downstream of the sill crest and subducts underneath the fresh water, setting up a hydraulic sill flow with an arrested upper layer. Vertical current speeds at the downstream side of the sill reach up to 0.75 m s “ ^, and intense detrainm ent of dense water from th e lower into the upper layer causes a volume loss of 60% over a distance of 200 m. Surface waves travelling into th e convergence zone (tidal front) over the sill crest tend to steepen and break due to wave-current interaction. The breaking waves inject gas bubbles, which either rise back to the sea surface or dissolve completely, depending on their rise speed and the strength of the vertical currents. Bubbles injected close to the plunge point of the dense w ater mass are drawn down by th e extreme currents to depths of up to 160 m, enhancing air-sea gas exchange.
T he hydraulic flow, wave-current interaction, and gas bubble behaviour are described with simple models. They are used to interpret extensive ship-board measurements during two cruises in th e Fraser estuary and help in the understanding of the physical processes involved in air-sea gas exchange in tidal fronts. The oxygen flux in the tidal front a t B oundary Pass is compared w ith other oxygen sources in the Fraser E stuary and shows th a t tidal fronts
I ll
may contribute significantly to th e aeration of an estuary.
The described processes may be also applicable to other coastal areas w ith strong tidal currents like Norway, Chile, or Japan, and may be im portant in convergence zones like deep convection regimes.
Table o f C ontents
A bstract "
Acknowledgments xix
1 Introduction 1
2 Observations 14
2.1 CCGS Vector cruises 9934 and 0032 ... 15
2.2 The tidal front at Boundary P a s s ... 23
2.3 T he tidal fronts in Haro S trait ... 29
2.3.1 S tu art I s l a n d ... 29
2.3.2 Battleship I s l a n d ... 32
2.3.3 O ther tidal fro n ts ... 33
3 Model of hydraulic sill flow 37 3.1 Energy model based on th e Bernoulli-equation ... 48
4 Model for gas bubble formation by breaking waves on a current 53 4.1 Basic wave e q u a tio n s ... 55
4.2 Wave-cnrrent in te ra c tio n ... 56
4.3 Breaking waves on a c u r r e n t... 59
4.3.1 A m plitnde and energy of a breaking w a v e ... 60
Table o f Contents v
4.4 Influence of w i n d ... 67
4.5 Bnbble injection by breaking w a v e s ... 69
5 Model for th e behaviour of gas bubbles in a fluid 81 5.1 Model equations ... 81
5.2 Model application ... 85
6 Air-sea gas exchange in a convergence zone 92 6.1 Gas entrainm ent in a spatially varying flow fleld ... 92
6.1.1 Case w ith no c u r r e n t s ... 93
6.1.2 Downwelling currents ... 96
6.2 Application to the sill flow a t B oundary P a s s ... 98
7 Air-sea gas exchange in th e Fraser Estuary 107 7.1 Contribution of tidal f r o n t s ... 107
7.2 Comparison with other oxygen s o u r c e s ... 113
7.2.1 Diffusive gas f l u x ... 113
7.2.2 Biological oxygen production and c o n s u m p tio n ... 118
7.2.3 Oxygen advection ... 121
7.2.4 S u m m a r y ... 123
8 Application to other areas with strong tidal currents or flow convergence 125 8.1 S trait of Messina, I t a l y ... 126
8.2 Bay of Fundy, Eastern C a n a d a ... 128
8.3 Langmuir circulation, deep c o n v e c tio n ... 129
9 Conclusions and outlook 132
Appendix 142
B Observations 148 B .l Tidal c u r r e n ts ... 148 B.2 C T D - s ta tio n s ... 150 B.3 Comparison of 2-beam and 4-beam solutions for ADCP measurements . . . 152 B.4 Energy considerations for flow over an o b s t a c l e ... 155
v u
List o f Tables
2.1 Sensor accuracy and resolution of the Seabird CTD SBE 19-03... 19 2.2 Settings of the 100 kHz echo sounder during Vector cruises 9934 and 0032.
The values in brackets show th e settings for the first two days of cruise 9934. 21
4.1 Im portant param eters from the literature for bubble entrainm ent by breaking waves: type of study (obs.=observations), wind speed u*, significant wave height Hg, normalized depth of measurement z/H g (values in brackets are absolute values), range of radii r, spectral slope —a , entrainm ent d epth 7 (other th an exponential depth dependences are given in brackets), and void fraction... 72 4.2 Estim ates of energy loss of breaking waves from the literature. Single
break event; multiple break events... 77
5.1 Typical values for the param eters used in the model calculations... 86 5.2 Void fraction a t the sea surface calculated with the bubble model for dif
ferent vertical current speeds w . As a comparison, th e void fraction a t the measurem ent depth of 20 m depth was only 1.3 • 1G~^... 91
6.1 Total percentage of gas dissolved, Ç j, for O2, N2, CO2, and Ar as a function of entrainm ent depth 7 ... 95
6.2 P aram eter values for the model calculations of gas entrainm ent in the tidal front a t Boundary Pass. For the given 0 2 -flux values, only th e contribution to th e aeration of subsurface w ater in the S trait of Georgia is considered (see text). The lower p art of the tables shows the results from the sensitivity study; values different to the ones of the reference runs (upper p a rt of table)
are shown in bold letters. . ... 106
7.1 Average oxygen flux in tidal fronts associated with gas bubbles estim ated from model calculations and observations; values are given in m^s“ ^. The lower row shows the estim ated contribution of all fronts to the aeration of interm ediate w ater in the S trait of Georgia... 108 7.2 Surface area and calculated mean diffusive gas flux for Ju an de Fuca S trait,
Haro S trait, and S trait of Georgia calculated according to Liss Sz Merlivat (1986) and W anninkhof (1992). Taken from Thomson (1994). Estim ated. 118 7.3 Surface area, prim ary production rates, and calculated mean oxygen produc
tion by photosynthesis Qphot for Ju an de Fuca Strait, Haro S trait, and the S trait of Georgia. Taken from Thom son (1994). E stim ated... 121 7.4 Estim ated mean annual advection of oxygen into th e Fraser E stu ary and
Haro S tra it... 122 7.5 Oxygen sources [m^s“ ^] for the Fraser E stuary and the S trait of Georgia.
For the diffusive gas flux, a mean value is given... 123
B .l CTD stations during Vector cruises 9934 and 0032. A: Haro S trait and adjacent channels; B: B attleship Island; F: S tu art Island flood tidal front; P: Boundary Pass; S: S tuart Island ebb tidal front (see also Figures 2.1 and 2.2). 150 B.2 CTD stations during Vector cruises 9934 and 0032. A: Haro S trait and
adjacent channels; B: B attleship Island; F: S tu art Island flood tidal front; P: Boundary Pass; S: S tu art Island ebb tidal front (see also Figures 2.1 and 2.2). 151
IX
List o f Figures
1.1 M ap of southern British Columbia, Canada. The Fraser River supplies fresh w ater to th e estuary which flows from the S trait of Georgia through Haro S trait and Ju an de Fuca S trait into the Paciflc Ocean... 1.2 Prediction of the peak ebb tidal flow in Haro S trait on July 1, 1996, from a
fully 3-dimensional constant density flnite-element frequency-domain model by Foreman & Thom son (1997) (Figure adapted from Farm er et al. (2002)). 1.3 Aerial view of Haro S trait on September 24, 1999, showing tidal fronts (red
arrow s)... 1.4 An eddy of approxim ately 10 m diam eter formed in the S tu art Island ebb
tidal front. Gas bubbles are visible in the core of the eddy... 1.5 Gas bubble entrainm ent at Boundary Pass. The colored dots indicate areas of
high acoustic backscatter intensity (mostly gas bubbles); th e different colors represent depth. T he purple line shows the location of th e tidal front during strong flood tide and the green arrows the appoxim ate flow direction. . . . 1.6 C hart of Haro S trait and Boundary Pass. The red lines and arrows show the approxim ate locations of the ebb tidal fronts and the flow direction a t ebb tide. The green lines and arrows show fronts and currents at flood tide. . .
1.7 Sketch of th e processes im portant for air-sea gas exchange in th e tidal front at Boundary Pass. A t the early stage of the flood tide (see also Section 2.2), the hydraulic flow over th e sill is characterized by a strong convergence zone at the sea surface causing wave breaking. Bubbles generated by the breaking waves are drawn down by the currents and dissolve... 8 1.8 Hydrographic survey in the Fraser E stuary in July 2000 by D. Masson, lOS,
Sidney, Canada, a) C hart of the hydrographic stations; b) Tem perature section along the Fraser estuary. The Pacific Ocean is to the right, th e S trait of Georgia to the left. The sill a t Boundary Pass is located a t æ = 220km. c) Oxygen section... 11
2.1 Positions of the CTD -stations (green dots) and the towyos with C T D /resonator package (orange lines) during Vector cruise 9934. T he po sition of the tidal fronts a t S tu art Island and Battleship Island during peak ebb tide on O ctober 6, 1999 is shown by the purple lines... 16 2.2 C hart of the sill at Boundary Pass. The purple line shows the approxim ate
location of the flood tidal front. The green dots m ark th e location of the CTD -stations and th e orange lines show th e position of the C T D /resonator package-towyos during Vector cruise 0032... 17 2.3 Design of the acoustical resonator. One of the identical plates is used for
transm ission - the other one for detection... 22 2.4 The resonator package during Vector cruise 9934. T he resonator plates are
located at th e lower right hand side... 22 2.5 Along-strait transect over the sill at Boundary Pass at the beginning of the
flood tidal period on September 29, 2000 (for an enlarged picture of the flow a t the sill see Figure 3.2). M easurements were taken w ith ADCP, echo sounder, and towed CTD. The time is given relative to Fulford H arbour, Saltspring Island, a) N orth-south (along-strait) component of current speed; b) vertical current speed; c) acoustic backscatter intensity; d) density. . . . 24
L ist o f Figures xi
2.6 Flood tidal front at Boundary Pass during Vector cruise 0203 in January 2002. a) Echo sounder image I h lO m in after low tide (relative to Fulford H arbour); b) echo sounder image I h 33m in after low tide; c) density-profile downstream of the sill, 5 min before low tid e... 25 2.7 Average horizontal currents during flood tide a t Boundary Pass in September
2000 for a depth of 50 m and 100 m and for tim e intervals 0—2 h, 2—4 h, 4—6 h, and 6 —8 h. The thickness of th e arrows represents the current speed. The w ater depth is shown in blue, w ith darker colors representing greater depth. The islands in the lower left and right hand corners are S atu rn a and Patos Island... 27 2.8 Transect over the sill at Boundary Pass at the later stage of the flood
tide measured w ith ADCP, echo sounder, and towed resonator package on September 29, 2000. a) Along-strait current; b) vertical current; c) acoustic backscatter intensity. T he p ath of the resonator is shown by th e w hite line and th e location of th e bubble measurements in Figure 7.1 is marked by the black dot. ... 28 2.9 Sketch of th e later stage of the tidal front at Boundary Pass (about 4 —6 h
after low tide). For explanations see te x t... 29 2.10 Sketch of the S tu art Island front. The front is initially vertical (Transect A,
Figure 2.11), b u t tilts and stretches w ith tim e (Transect B) due to effects of the density gradient across the front (Figure adapted from Farm er et al.
(2002)... 30
2.11 Currents at the S tu art Island front. Colors indicate the along-front current speed and the arrows the currents along th e transect (Figure adapted from Farmer et al, 2002). For the location of transects A and B see Figure 2.10. . 30
2.12 M easurements in Haro Strait. The purple lines show the location of th e tidal fronts at S tu art and Battleship Island during strong ebb tide. The colored dots indicate high acoustic backscatter intensity (mostly gas bubbles). The different colors represent the tim e of measurement relative to th e max. ebb current. T he green arrows show the mean ebb tidal current... 31 2.13 Aerial view of the tidal front at B attleship Island, Haro S trait. T he green
arrows indicate th e flow direction. The red line and purple arrow m ark the transect and frontal location shown in Figure 2 . 1 4 ... 33 2.14 Cross section through the tidal front a t B attleship Island (Figures 2.13, 2.15)
measured w ith ADCP, echo sounder, and towed CTD. T he location of the tidal front is marked by the purple arrow, a) Along-strait current, b) vertical current, c) acoustic backscatter intensity, d) density... 34 2.15 Sketch of the tidal front a t B attleship Island. Note th a t it is oriented the
opposite way as Figure 2.13 w ith N orth to the left. The purple lines indicate the location of the tidal fronts, the green arrows the direction of flow, and the orange dots locations of gas bubble entrainm ent by eddies and subducting flow. The red line marks th e section across the tidal fronts shown in Figure 2.14. 35
3.1 Sketch of barotropically forced sill flow (adapted from Farm er & Armi (1986)). 38 3.2 Flow over the sill at Boundary Pass during the early stage of the flood tide
(~ 1.5 h after low tide) on September 29, 2000. The colors show the acoustic backscatter intensity measured w ith an echo sounder and the black arrows indicate the current speed perpendicular to th e sill crest. Note th a t distances in th e horizontal and vertical are the sam e... 40 3.3 Density (color) of the transect shown in Figure 3.2. M easurements are shown
along the p ath of th e instrum ent. The green line indicates th e botto m depth, the white line the interface location... 42 3.4 Sketch of th e flow over the sill at Boundary Pass. For explanations see text. 43
L ist o f Figures xiii
3.5 Observed layer-averaged horizontal (a) and vertical (b) flow com ponent u and w, c) upper and lower layer Fronde numbers F f and F | ; d) detrainm ent velocity V2 1', e) prescribed densities p \ and p2... 47 3.6 Sketch for the calculation of the vertical flow component w. T he current
vector is u, the horizontal flow component u, the detrainm ent velocity V2 1, and the slope of the interface a ... 50 3.7 Observed (black line) and modeled interface depth for the model w ithout de
trainm ent (white line; Eq. 3.3). a) Horizontal currents; b) vertical currents; c) prescribed density for upper (red) and lower layer (blue)... 51 3.8 Observed (black line) and modeled interface depth (white line) for the model
w ith detrainm ent and variable g '{x) (white line; Eq. 3.8). a) Horizontal currents; b) vertical currents; c) prescribed density for upper (red) and lower layer (blue)... 52
4.1 Wave breaking in the convergence zone of th e S tuart Island ebb tidal front. 53 4.2 Sketch of wave-current interaction in a convergence zone (77: sea surface
elevation). The transition is from fast current (in the same direction) to still w ater (n = 0)... 54 4.3 Wave energy as a function of current speed. Current speed and am plitude
are scaled w ith the values for a medium a t rest (u = 0). The wave energy in the absence of wave breaking (black curve) is plotted together w ith the wave energy for a breaking wave of initial steepness kouo (gray curves, Equation 4.16)... 58 4.4 Relative change of am plitude of a breaking wave as function of th e initial
wave steepness. T he numbers indicate th e corresponding current speed u / c q . 61 4.5 The gray curves show the normalized am plitude of a wave of initial steepness
koao as a function of current speed. After the wave reaches the critical value of = 1/2 it is given by the black curve, or in the absence of breaking by the dashed gray curves... 61
4.6 Sketch of a) spilling breaker and b) plunging breaker... 62 4.7 Development of a wave packet of deep w ater waves in an ocean a t rest for
times t = 0, t = r , and t = 2 r. Wave breaking occurs when th e waves reach the critical steepness of A;a = 1 /2 ... 64 4.8 Behaviour of a quasi-monochromatic wave on an opposing current for tim e
t — 0 and t — 2T. The normalized energy is given for the case w ith breaking (red curves T j ) and w ithout breaking (blue curves F j)... 65 4.9 Sketch of the break behaviour of a group of waves on a current. For expla
nations see te x t... 67 4.10 a) Limiting fetch .Fiim (equation 4.22) for a wind duration of 0.5 h, 1 h, 2 h,
and 3 h. The fetch in the S trait of Georgia (20 km) is shown by th e gray box. b) Significant wave height H s for a fetch F = 2 0 km ... 69 4.11 Sketch of the initial bubble size distribution (G arrett et al. (2000); Deane &
Stokes (2002))... 75 4.12 Example of a wave travelling from left to right into a linearly increasing
current, a) C urrent speed; b) wave am plitude with breaking (solid line) and w ithout breaking (dashed line); c) wave energy lost by breaking; d) void fraction of initially injected bubbles... 79
5.1 Rise speed Wb of dirty (solid line) and clean (dashed line) gas bubbles. . . . 84 5.2 The color image shows acoustic backscatter intensity measured with a
100 kHz echo sounder on September 24, 2000, at Boundary Pass. The cor responding resonance radius is indicated by the red curve and the blue lines show th e “p a th ” of th e injected bubbles for a vertical current speed o f —0.5 m s“ ^... 87 5.3 a) Resonance radius of a 100 kHz echo sounder (solid line) and “bubble p ath s”
(dashed lines) for gas bubbles of different initial radius and corresponding m inimal vertical current speed |w| = |tCbo|- b) Resulting relationship between minimal current speed and depth of the observed bubbles... 89
L ist o f Figures xv
5.4 Bubble size distribution measured in th e tidal front at B oundary Pass on September 29, 2000, at 20m depth (black). The bubble model was used to back-trace the size distribution to th e surface assuming different verti cal current speeds of - 0 .1 m s"^ (blue), —0.3m s~^ (green), and —0 .5 m s “ ^
(orange)... 90
6.1 Percentage of gas d {r ,z ) dissolving from a bubble while it rises back to the surface. It is shown for the gases O 2, Ng, CO2, and Ar as function of initial bubble radius and depth (d is shown for the upper 15 m, although most bubbles are entrained in the upper 2 m). The gray shaded area m arks the bubbles which dissolve c o m p le te ly ... 94 6.2 Ratio G j of th e am ount of dissolved gas to th e total injected gas per 1 fxm-
increment for O 2, N2, CO 2, and Ar. The curves are plotted for different entrainm ent depths: 7 = 0.1 m (red), 7 = l m (green), and 7 = 2 m (blue). . 95 6.3 Percentage d{r, z) of gas dissolving from a bubble while it rises back to the
surface. It is shown for the gases O 2, N2, CO2, and Ar as a function of initial bubble radius and depth and for a current speed of u; = —0.2 m s “ ^. T he gray shaded area marks the bubbles which dissolve completely. ... 97 6.4 R atio of the total am ount of dissolved gas G j to the total injected gas
per 1 //m-increment for O 2, N2, CO2, and Ar for a current speed of w = —G .2m s“ ^. The curves are plotted for different entrainm ent depths: 7 = 0.1 m (red), 7 = l m (green), and 7 = 2 m (blue)... 98 6.5 Percentage of the total am ount of dissolved gas Ç for O 2, N2, CO 2, and Ar
as function of current speed w. It is plotted for different entrainm ent depths: 7 = 0.1 m (red), 7 = l m (green), and 7 = 2 m (blue)... 99
6.6 a) Horizontal (blue) and vertical (red) flow component in the tidal front a t B oundary Pass. Surface currents are shown by dashed lines, vertically averaged lower layer currents by solid lines. b)-c) A m plitude and dissipated energy for waves I (red), II (blue), and III (green) approaching the front from the E ast (S trait of Georgia); d) am ount of gas injected by the three waves; e) percentage of oxygen dissolved for detrainm ent depths 0.05 m (green), 0.5 m (magenta), and I m (cyan). ... 101 6.7 a)-b) A m plitude and dissipated energy for waves I (red), II (blue), and III
(green) approaching the front from the West; c) am ount of gas injected by the three waves... 102
7.1 a) The void fraction during the transect shown in Figure 2.8. b) Volume scaled bubble size distribution 4 /3 n r ^ N { r ) (black curve) of the bubble plume in th e front in approxim ately 20 m depth (marked in red in the upper panel and by the black dot in Figure 2.8). The orange, green, and blue curves show the bubble size distribution at th e surface, calculated w ith th e bubble model (Section 5) for vertical current speeds of 0.1 m s " \ 0.3m s~^, and 0.5m s~^ . 110 7.2 Diffusive gas flux per square m eter for O 2 (blue) and N 2 (red) as function of
wind speed according to Liss & M erlivat (1986) (solid line) and W anninkhof (1992) (dashed line) for an assumed saturation of 95%... 115 7.3 0 2 -saturation in 1968 for Juan de Fuca S trait (*, solid line) and th e S trait
of Georgia ({>, dashed line) estim ated from Crean & Ages (1971) and shown together w ith a fitted polynomial of third order... ... 116 7.4 a) W ind speed in the Fraser E stuary measured a t Sandhead in 1998. b) Dif
fusive oxygen flux calculated for Ju an de Fuca S trait (red). S trait of Georgia (green), and Haro S trait (blue; positive values are oxygen gain), c) T h e oxy gen flux for the to tal estuary (blue) is compared to the oxygen flux according to Liss & Merlivat (1986). The corresponding saturation levels are shown in Figure 7.3... 117
L ist o f Figures xvii
7.5 Chlorophyll-a concentration in th e Fraser E stuary in July 2001 (Figure pro vided by R.Pawlowicz, UBC, Canada, as p art of STRA TO GEM )... 119 7.6 P rim ary production rates in P uget Sound, WA, USA, in 1965, estim ated
from Koblents-Mishke (1965)... 120 7.7 Sketch of the oxygen sources and sinks in the Fraser E s t u a r y . ... 124
8.1 M ap of the S trait of Messina, Italy (adapted from B randt et al. (1999)). . . 126 8.2 Northw ard tidal flow through the S trait of Messina on O ctober 25, 1995 (data
from P. B randt, IfM Kiel, Germany), a) Along-strait current; b) across-strait current; c) vertical current; d) acoustic backscatter intensity. ... 127 8.3 Aerial view of the flood tidal front at th e N orthern tip of G rand M anan
Island, Bay of Fundy, NB, Canada. The main flow direction is indicated by the red arrows (photo: Dave Johnston, Duke Marine Lab, NC, USA). . . . 129 8.4 ADCP-section through the tidal front at G rand M anan Island (Figure from
Dave Johnston, Duke M arine Lab, NC, USA). The section is taken from Southeast (left) to Northwest (right), a) Velocity m agnitude [m m s“ ^]; b) velocity direction [°]; c) backscatter intensity [dB]... 130
B .l Tidal currents at Turn Point (S tuart Island) and Boundary Pass during CCGS Vector cruise 9934 (4-14 O ctober 1999). C urrents speeds above 0.75 kt and b e lo w —1.5 kt are marked in green and red, respectively, currents . . . 148 B.2 Tidal currents at Turn Point (S tuart Island) and B oundary Pass during
CCGS Vector cruise 0032 (18-30 September 2000). C urrents speeds above 0.75 kt and b e lo w —1.5 kt are marked in green and red, respectively. . . . . 149 B.3 The current vector U (green) w ith its components u and w (red) is calculated
from th e currents in beam coordinates (n # i, blue). The angle between current vector and vertical z-axis is /3 and the beam angle of the ADCP is a . 153
B.4 A D C P-transect a t Boundary Pass, a) The horizontal current speed u cal culated from the 4-beam solution, b) the vertical current speed w from the 4-beam solution, c) w from the 2-beam solution, d) the difference between w from th e 2- and 4-beam solutions... 154
XIX
A cknow ledgem ents
Many thanks to my supervisors Dr. David Farm er and Dr. Chris G arrett for the oppor tunity to participate in an exciting project at the beautiful coast of British Columbia. I am very grateful for the many stim ulating discussions and critical comments and highly appreciate their great support during my tim e a t the University of Victoria.
I also would like to thank my committee members Dr. Patrick Cummins, Dr. Rolf Lueck, and Dr. Eddy Carmack for their helpful suggestions and discussions to improve this thesis, and I am grateful to Dr. William Jenkins for coming to Victoria as my external examiner. I gratefully acknowledge the support and cooperation of the captains and crew of CCGS Vector, which was essential in collecting d a ta in a navigationally very challenging environ ment.
I am indebted to the Ocean Acoustics G roup for their wonderful support during the project and the pleasant work atm osphere a t the Institu te of Ocean Sciences and during th e ex perim ents on sea. Thanks also to my fellow students Frank Gerdes and Roblyn Kendall for their support in many scientific and non-scientihc respects.
Many thanks to my father Bodo and brother Bjorn for the careful proofreading of my thesis. B ut most of all, I would like to thank my parents Elke and Bodo for their enduring support throughout my studies.
Coastal environments are often characterized by productive b ut fragile ecosystems and are often im portant for hum an food resources and for recreational activities. The excess supply of nutrients and waste w ater by the surrounding communities can harm these ecosystems and diminish commercially valuable fish populations. It is therefore im portant to b et ter understand the mechanisms responsible for the mixing of (polluted) w ater in coastal environments and for supplying oxygen to the water below the surface layer where the decomposition of organic m aterial can cause oxygen depletion.
T he estuary of th e Fraser River in British Columbia, C anada (Figure 1.1), is a highly productive ecosystem renowned for its salmon populations. These support the fishery fleets of C anada and the USA and are the main food source of resident orcas (killer whales). Oxygen concentrations, however, can drop in the deeper regions of the S trait of Georgia (Figure 1.1) to levels at which fish are affected negatively and may be forced to move away. In this thesis particular attention is given to the mechanisms which supply oxygen to the Strait. It is suggested th a t they are as follows: pronounced interaction of strong (tidal) currents w ith topography in p arts of the estuary (Haro Strait) mixes dense w ater from the Pacific Ocean w ith fresh, oxygenated surface w ater (a significant portion of this oxygen is contributed by th e generation and subduction of gas bubbles in tidal fronts). At the entrance to th e S trait of Georgia (Boundary Pass), this water flows over a sill down to interm ediate depths, from where the oxygen is likely to be transported by diffusion to the deeper p arts of th e Strait. Oxygen input by rare deep water renewal events of somewhat oxygenated w ater in spring (Masson, 2002) will not be considered.
1. Introduction 2
This thesis will describe th e mechanisms responsible for the supply of interm ediate water in the S trait of Georgia and it will be shown th a t air-sea gas exchange in tidal fro n ts can contribute significantly to th e aeration of an estuary.
Estuarine Circulation
The two big straits in the Fraser Estuary, Ju an de Fuca S trait (160—280 m w ater depth) and th e S trait of Georgia (200 —440m w ater depth), are connected by various smaller straits and channels (Figure 1.2). The most im portant one for the estuarine dynamics is Haro S trait (250 —300 m deep, 3 —4 km wide) which is separated from Ju an de Fuca S trait by Victoria Sill (60—70 m w ater depth) and from the S trait of Georgia by a sill at Boundary
51°N 50°N 49°N 48°N Pacific Ocean Juan . Fuca S Seattle 128% 127°W 126% 125% 124% 123% 122%
F ig u r e 1.1: Map of southern British Columbia, Canada. The Fraser River supplies fresh water to
the estuary which flows from the Strait of Georgia through Haro Strait and Juan de Fuca Strait into the Pacific Ocean.
Pass (60m w ater depth). The to tal area of the estuary is 10950km^, w ith the areas of the S trait of Georgia 68G0km^, Ju an de Fuca S trait 3 700km^ (Thomson, 1994), and Haro S trait 450 km^.
T he estuary is greatly influenced by th e fresh water delivered by th e Fraser River, which drains an area of 217 300 km^ and has a mean annual volume tran sp o rt of 4400m ^s“ ^ (Thomson, 1994). This fresh water flows at th e surface through Haro S trait and Ju an de Fuca S trait into th e Pacific Ocean, while, beneath it, saltier and denser water enters the estuary from the Pacific Ocean setting up a classical estuarine circulation. Superimposed on this mean circulation (the mean current speed in Haro S trait is about 0.2 m s “ ^ and in Ju an de Fuca S trait 0.05 m s"^ (Pawlowicz & Farmer, 1998), is a strong tidal regime with pronounced spring-neap tidal variability (Pawlowicz, 2002) (see also A ppendix B .l). Predictions from a fully 3-dimensional constant density finite-element frequency-domain model (Foreman & Thomson, 1997) give some idea of the peak ebb tidal flow in Haro S trait (Figure 1.2), where current speeds can reach up to 1.3m s “ ^ like a t Turn Point, S tu art Island (Fisheries and Oceans Canada, 2000). M easurements during two cruises (Section 2) showed even higher values of up to 3 m s“ ^ a t the sill a t Boundary Pass.
Mixing and tidal fronts
As the currents in this region interact strongly with the topography, Haro S trait plays an im portant role in th e mixing of dense w ater from the Pacific Ocean and fresh surface water from the Fraser River (Pawlowicz & Farmer, 1998; Farm er et a l, 1995). T idal fronts (sharp transition zones of w ater masses w ith different current speed and w ater mass properties) develop due to flow separation past headlands (Farmer et al., 2002) or flow over shallow sills.
These tidal fronts are clearly visible a t the sea surface (Figure 1.3). Energetic eddies (Figure 1.4) w ith diameters of typically 10-150 m are formed in the front by strong horizontal shear. Front and eddies may tilt further downstream due to density differences across the
I. Introduction
Strait of G eorgia
Stuart Island Haro Strait JuanVictoria
' Juan d e Fuca Strait ‘ , /
5- I23ŒW
F igu re 1.2: Prediction of the peak ebb tidal flow in Haro Strait on July 1, 1996, from a fully 3-dimensional constant density finite-element frequency-domain model by Foreman & Thomson (1997) (Figure adapted from Farmer et al. (2002)).
front, enhancing th e eddy circulation speed and transform ing horizontal into vertical mixing (Farmer et a i, 2002).
Observations show th a t in these fronts huge clouds of gas bubbles are draw n into the ocean (Figure 1.5). They can reach depths of up to 160m (Figure 5.2), which is only slightly deeper th a n observed by Farm er et al. (2002) in Haro S trait, b u t several times th e depth
Stuart Island
Haro Strait
Battleship Island
San Juan Island
F igure 1.3: Aerial view of Haro Strait on September 24, 1999, showing tidal fronts (red arrows).
F igu re 1.4: An eddy of approximately 10 m diameter formed in the Stuart Island ebb tidal front. Gas bubbles are visible in the core of the eddy.
of bubble plumes in the open ocean, where maximal entrainm ent depths of 15 —20 m have been measured (Vagle, 1989; Crawford 6 Farmer, 1987; Wallace & Wirick, 1992). Based on our ship and aerial surveys in Haro S trait (Section 2), it has been assessed th a t about 9 pronounced fronts develop a t ebb tide and 6 a t flood tide (Figure 1.6).
1. Introduction 6 E TOO 48’47.40’N 48 48.00 N 48 48.60'N 123'02.40’W 123*01.20’W 123'00.00'W 122*58.80’W 48*49.20’N
F igure 1.5: Gas bubble entrainment at Boundary Pass. The colored dots indicate areas of high acoustic backscatter intensity (mostly gas bubbles); the different colors represent depth. The purple line shows the location o f the tidal front during strong flood tide and the green arrows the appoximate flow direction.
This thesis has been motivated by the observations of bubble plumes in tidal fronts suggest ing th a t the processes in the fronts may play an im portant role in air-sea gas exchange in an estuary. Oxygen is an especially im portant param eter for m arine life and will be studied in more detail. Some of th e described processes are also applicable to larger scales in the open ocean where gases like COg, a m ajor contributor to global warming, may be more im portant.
Saturna Island Boundary P a s s /. i 9 * % S tu art slan d B attleship Island s and
F ig ure 1.6: Chart of Haro Strait and Boundary Pass. The red lines and arrows show the approxi mate locations of the ebb tidal fronts and the flow direction at ebb tide. The green lines and arrows show fronts and currents at flood tide.
Sill flow a t Boundary Pass
T he processes at Boundary Pass deserve special attention; the sill controls the renewal and aeration of deep and interm ediate w ater in the S trait of Georgia, as it forms a pronounced barrier for the flow into the Strait, causing intense mixing and hydraulic flow over the sill. B ut it is also the location of one of the most energetic tidal fronts in th e estuary and is therefore im portant for air-sea gas exchange. The front is generated by the hydraulic sill flow characterized by a strong convergence zone at the sea surface (Figure 1.7). Waves
I. Introduction 8
travelling into the front tend to steepen and break, creating gas bubbles which are then carried down by the currents.
breaking waves , tidal front
Boundary
Pass
o° 0OOO„ A m ixingStrait of
Georgia
F igure 1.7: Sketch of the processes important for air-sea gas exchange in the tidal front at Boundary Pass. A t the early stage of the flood tide (see also Section 2.2), the hydraulic flow over the sill is characterized by a strong convergence zone at the sea surface causing wave breaking. Bubbles generated by the breaking waves are drawn down by the currents and dissolve.
A t flood tide, th e flow over the sill at Boundary Pass can be considered as a two-layer flow w ith an active lower layer w ith strong tidal currents of up to 3 m s “ ^ - a situation which is in many respects similar to th e one a t Knight Inlet, BC, Canada. Arm! & Farm er (2002) describe in detail th e hydraulic response of the flow during the transition from interm ediate to strong and flnally (as the tide slackens) m oderate forcing (see also Farm er & Armi
The observed flow a t Boundary Pass at the early stage of the flood tide fits into the category of strong tidal forcing w ith an arrested upper layer. The plunge point (the tidal front), where the dense lower layer is forced to subduct underneath th e fresh surface layer is located downstream of th e sill crest; the supercritical flow follows the bottom topography to the bottom of th e sill where a hydraulic jum p marks the transition to subcritical conditions (Figure 2.5).
Dense w ater is detrained from the lower layer into the passive, wedge-shaped surface layer. Tank experiments by Pawlak & Armi (1986) show th a t the volume of detrained fluid depends on the slope of the sill and the distance over which it is detrained.
At even stronger forcing (peak flood tid e), th e flow a t Boundary Pass forms an overshooting jet, which separates at the sill crest. The plunge point is shifted far downstream by the currents and is not marked by a clear front line any more as turbulence and eddy activity seem to be more im portant.
The hydraulic flow described is connected to air-sea gas exchange processes by setting up a flow convergence zone a t the surface (plunge p o in t/tid al front) which favours wave breaking. T he breaking waves inject gas bubbles into the ocean, which are then draw n down by the currents and dissolve.
Gas bubbles
Gas bubbles are generated by breaking waves, precipitation, and in supersaturated condi tions (Blanchard & Woodcock, 1957). Bubbles have been studied in the open ocean, surf zone, or in tank experiments for various reasons: bubbles can significantly change th e opti cal properties of w ater (Terrill et a l, 1998) as well as the transm ission of sound (Deane & Stokes, 1999) - sound speeds as low as 500 m s~^ have been observed under breaking waves in the surf zone (Deane, 1999). Farmer &: Vagle (1988) and Deane (1997) used the sound radiated by bubbles to identify wave breaking events, and Thorpe (1982), Vagle (1989), as well as Crawford & Farmer (1987) connected the penetration depth of bubbles generated
1. Introduction 10
by white caps w ith wind speed. It is on the order of 1 — 2 wave heights (Hwang et a i, 1990; Loewen et al., 1996) and can be increased by the downwelling currents in Langmuir circulations (Thorpe, 1982). Keeling (1993) suggests th a t bubbles make an im portant con tribution to air-sea gas exchange at wind speeds above 1 0 m s “ ^, as they can increase gas transfer rates by a factor of up to three (Merlivat &: Memery, 1983) and hence also the oxygen content in th e surface layer (Wallace &: Wirick, 1992).
In tidal fronts, however, gas bubbles are also generated and entrained into th e ocean on calm days w ith very low wind speeds (a process which is greatly enhanced a t higher wind speeds though). Tidal fronts are usually characterized by a flow convergence zone a t the sea surface. (Small) waves travelling into the convergence zone tend to steepen and break due to wave-current interaction (Bretherton & G arrett, 1969; Phillips, 1977). Gas bubbles which are generated by th e breaking waves are carried down by downwelling currents. Vertical velocities are usually in th e order of 0.15—0.2 m s “ ^ (Farmer et a l, 1995), although observations in Haro S trait (Farmer et a l, 2002) and at the sill at B oundary Pass show values as high as 0.5 ms~^ and 0.75 m s “ ^ respectively. These strong downwelling currents are responsible for the great bubble entrainm ent depths observed in th e tidal fronts in Haro Strait.
Oxygen supply to th e deep and interm ediate w ater in th e S trait of Georgia
While oxygen is abundant in the surface layer, it may be a restrictive param eter for m arine life in greater depths, especially when oxygen is consumed by the decomposition of organic material. At oxygen levels of less th an 2m ll~^ fish cannot survive, b u t at values o f 2 — 4 m ll“ ^ they are also negatively affected and might move away (The Swedish Environm ental Protection Agency, 2003).
Monthly measurements in 1969 by Crean & Ages (1971) show th a t oxygen levels in the S trait of Georgia below 200 m are always less th an 4 m ll“ ^, w ith minimal values of < 3 m ll“ ^ in January to April for depths below 250—300 m. However, newer measurements by D. Masson,
lOS, Sidney, C anada (pers. communication; Figure 1.8) show even lower values for summer 2000 close to th e critical value of 2 m ll“ ^ at depths below 200—250 m.
D istancefknil
i r —
200
D istan ce Ikm ]
F ig ure 1.8: Hydrographic survey in the Fraser Estuary in July 2000 by D. Masson, lOS, Sidney, Canada, a) Chart of the hydrographic stations; b) Temperature section along the Fraser estuary. The Pacific Ocean is to the right, the Strait of Georgia to the left. The sill at Boundary Pass is located at x = 220 km. c) Oxygen section.
In the S trait of Georgia, as in most other estuaries, local formation of (oxygenated) deep w ater by convection is inhibited by the stratification of th e w ater column. Oxygen in the subsurface w ater masses is therefore renewed only by advection or by slow diffusion from th e surface layer. The Fraser E stuary is characterized by two shallow sills, V ictoria Sill and the sill a t Boundary Pass, over which the dense water from th e Pacific Ocean has to flow
1. Introduction 12
before entering th e S trait of Georgia. B oth sills are regions of vigorous mixing of this dense w ater mass w ith the fresh w ater of th e surface layer and are therefore im portant for deep w ater renewal in th e S trait as suggested by LeBlond et al. (1991) and Masson (2002). Deep water renewal events are most likely to happen in spring (April) and fall (Septem b er/O cto ber). Masson (2002) distinguishes intrusions of cold, saline, and oxygen rich wa ter in spring and warm, saline, low oxygen w ater in fall. During these periods, the up- welling region off the coast of British Columbia brings dense water into Juan de Fuca S trait (Waldichuk, 1957) and the am ount of fresh w ater brought in by th e Fraser River is not too large. If also the tidal mixing between these w ater masses is minimal (at neap tide), the dilution of the dense w ater is limited, keeping it dense enough to reach the bottom of the S trait of Georgia. LeBlond et al. (1991) suggest th a t renewal events require a current speed of no more th a n 0.5 m s “ ^ at Boundary Pass.
At spring tide, tidal mixing is stronger in Haro S trait and Boundary Pass. The water flowing into the S trait of Georgia is now too diluted with fresh w ater to reach the bottom and subducts only to interm ediate depths of 150—250 m. However, this w ater mass carries high concentrations of oxygen (Figure 1.8) as the mixed surface w ater has had recent contact w ith the atmosphere. From interm ediate depths, the oxygen might be supplied to th e deeper p arts of th e S trait by turbulent diffusion.
Overview
The objectives of this thesis are to understand the physical processes which are im por ta n t for the generation and entrainm ent of gas bubbles in tidal fronts and to estim ate the contribution of tidal fronts to the air-sea gas exchange in the Fraser Estuary.
The thesis is structured as follows: In Section 2, the research cruises and measurements are described. The (hydraulically controlled) flow over the sill at Boundary Pass will be discussed in Section 3 by using a simple model, based on the Bernoulli equations. Wave- current interaction in a convergence zone as well as the formation of gas bubbles by breaking
waves are the subjects of Section 4. The behaviour of these gas bubbles is then described in Section 5. In Section 6, the models of hydraulically controlled flow, wave-current interaction, and bubble behaviour are combined in order to explain th e processes a t th e flood tidal front at Boundary Pass. The contribution of this and other tidal fronts to the aeration of the Fraser E stuary will then be compared to other oxygen sources in Section 7. It will be finally shown in Section 8 th a t th e physical processes described in this thesis may be also applicable to other areas in th e world w ith strong flow-topography interaction or to open ocean processes like deep convection.
14
2 Observations
Tidal fronts can be defined as sharp transition zones between two w ater masses of different density and (tidal) current speed. In the Fraser Estuary, several of these fronts develop due to a pronounced interaction of strong tidal currents in the straits and adjacent channels w ith the topography. Tidal fronts can be grouped into three main categories, w ith typical examples at S tu art Island, B attleship Island, and Boundary Pass:
• flow over a shallow sill, i.e. flow-topography interaction at a horizontal boundary (e.g. sill at Boundary Pass, Section 2.2)
• flow separation processes past a headland, i.e. flow-topography interaction a t a ver tical boundary (e.g. S tu art Island front. Section 2.3.1)
• inflow of dense w ater from an adjacent channel and plunging flow into interm ediate depths (e.g. Battleship Island front. Section 2.3.2)
All these fronts are characterized by converging flow a t the surface, which enhances wave breaking and th e formation of gas bubbles which are drawn down by the downwelling currents sometimes reaching significant depths (~ 160 m a t Boundary Pass, Section 2.2). The tidal fronts at S tu art Island, Battleship Island, and Boundary Pass were investigated during CCGS Vector cruises 9934 and 0032 in O ctober 1999 and September 2000. The measurements and instrum entation will be described below in Section 2.1. This thesis, however, will concentrate on th e processes at Boundary Pass, while the tidal fronts at S tu art and B attleship Island are only described briefly. In particular, th e research objectives to investigate are:
• hydraulical flow over a steep sill and detrainm ent of w ater from the active lower layer into the the passive upper layer
• tem poral evolution of th e tidal front
• wave-current interaction in a convergence zone and bubble formation by wave breaking
• subduction of gas bubbles by downwelling currents/energetic eddies and their contri bution to the aeration of water
• contribution of tidal fronts to the aeration of th e Fraser estuary
2.1 CCGS V ector cruises 9 9 3 4 and 0032
T he measurements presented in this thesis were mainly collected during two research cruises in Haro S trait and Boundary Pass.
CCGS Vector cruise 9934 was carried out in Haro S trait from O ctober 4 to 14, 1999 and focused on the ebb tidal fronts a t S tuart and B attleship Island. D uring this time, the predicted tidal currents a t Turn P oint/B oundary Pass reached speeds of up to 0.45 m s^ ^ at flood tide and 1.0 ms~^ at ebb tide (Figure B .l). Locally, these currents are much stronger, developing pronounced tidal fronts.
T he second research cruise took place from September 18 to 30, 2000 (CCGS Vector cruise 0032). M easurements concentrated on the flood tidal front at Boundary, although additional measurements were carried out at the same stations and transects at S tu art and Battleship Island as during the first cruise (for a list of CTD station see A ppendix B.2). D uring the second cruise, the tidal currents were slightly stronger reaching speeds of up to 0.5 m s ” ^ at flood tide and 1.1 m s “ ^ at ebb tide (Turn P oint/B oundary Pass, Figure B.2). Flood tidal currents were of similar am plitude during th e whole cruise, which allowed th e neglect of spring-neap tidal variations and the comparison of measurements from one day to the next. Ship-board measurements w ith a conductivity-tem perature-depth recorder (CTD ), a
2. Observations 16
150 kHz vessel-mounted acoustic Doppler current profiler (ADCP), a 100 kHz and a 12 kHz echo sounder, as well as a resonator package for measuring size distributions of gas bub bles provided inform ation about the physical processes in the tidal fronts as well as their im portance for the aeration of water.
4844'N 48'43’N 48°42’N 4841'N 48 40'N -48°38’N 48’37’N 48°36’N 48°35’N 123"18’W 123°15’W 123'12'W 1231 O'W 12308'W 123*05’W
F igure 2.1: Positions of the CTD-stations (green dots) and the towyos with CTD/resonator pack age (orange lines) during Vector cruise 9934- The position of the tidal fronts at Stuart Island and Battleship Island during peak ebb tide on October 6, 1999 is shown by the purple lines.
■ f.
■ M
T he instrum ents were either used on single stations at a given location or on transects through the frontal area. Both single stations and transects were repeated several times over a tidal period to m ap the tem poral evolution of the tidal fronts. A dditional measurements
a t slack tide were used to m ap the hydrographic stru ctu re in Haro S trait and adjacent channels. Air photos taken from a small plane provided inform ation on the spatial extent of th e fronts. Figures 2.1 and 2.2 show the locations of the measurements during b o th research cruises. The instrum entation was the same on bo th cruises and will be described in th e following in more detail.
Strait of Georgia
^ a e e ^ A u i iv w A / v o ir a
p&btlJQN B /yfflC ^J TENTlON 9
u a u m w w A / v o w ATTENTION f
\ /W \ -I
Saturna Island
fBounda
F igure 2.2: Chart of the sill at Boundary Pass. The purple line shows the approximate location of the flood tidal front. The green dots mark the location of the CTD-stations and the orange lines show the position of the CTD/resonator package-towyos during Vector cruise 0032.
A dditional brief measurements w ith echo sounder and CTD were carried out on Jan uary 14 and 15, 2002 as p art of CCGS Vector cruise 0203. T he d ata were collected during flood tide at B oundary Pass.
2. Observations 18
Global Positioning System
A differential GPS-receiver (Global Positioning System) was used to determ ine th e time and ship’s position during the measurements. The d a ta were recorded in 10 s-intervals and the accuracy was generally b e tte r th an 10 m.
In addition, th e locations of the tidal fronts at B attleship and S tu art Island were m apped with a small boat (Zodiac) by driving along the front lines. Also here, th e position was determ ined w ith a differential GPS w ith recording intervals of 1 s. Due to the high speed of th e Zodiac these measurements provided a quasi-synoptic picture of the location of the tidal front.
Acoustic Doppler current profiler
For measuring the current speed, a 4 beam 150 kHz vessel-mounted broadband acoustic Doppler current profiler (ADCP) from RDI w ith a bandw idth of 39 kHz and a beam angle of 20° was used.
The instrum ent was m ounted on a stru t on the starboard side of the b oat allowing for a cruising speed of up to 6 kt. M easurements were therefore only taken in th e frontal zones. T he stru t was taken out of the w ater for longer transits in order to increase the ship’s speed. T he transducer depth was about 1 m. The ADCP was oriented w ith transducer # 3 to the front, so th a t beam s # 3 and # 4 were located in a plane parallel to the ship and beam s # 1 and # 2 in a plane orthogonal to it.
T he ADCP uses two kinds of pulses: a short 6 ms pulse repeated three times for m easur ing the current speed and backscatter intensity and a single long 79 ms pulse for bottom tracking. B ottom tracking was used to convert the currents from ship-coordinates into earth-coordinates allowing for a direct correction of the alignment angle of the A DCP and of Schuler oscillations of the gyro compass due to N orth-South accelerations of the ship. The ship’s position was provided by th e differential CPS-receiver of the Vector and the
ship’s heading was determ ined w ith a flux gate compass which was m ounted on the upper deck. The compass was calibrated at the beginning of the cruise by running th e vessel twice in a tight circle. W ith a bin size of 4 m, the ADCP measures the upper 200—300 m of the w ater column, which is close to th e maximum w ater depth in the area of investigation. The d a ta were collected at a rate of one ping per second. D ata acquisition and processing was carried out w ith the software package Tm nsect from R D I. Bad bins and profiles were edited and the d ata were averaged to 10 s-ensembles.
T he calculation of the current speed is based on the assumption of horizontal homogeneity. However, due to th e high horizontal gradients in the flow fleld of th e tidal fronts, it can be questioned if this assum ption is valid. In a depth of 100 m, the beam s of the ADCP spread horizontally over a distance of 73 m (at an beam angle of 20°). Over this distance, significant horizontal changes of the flow field can be observed (Figure 3.2). However, in A ppendix B.3 it is shown th a t th e usual 4-beam solution for deriving the current speed can provide useful results in tidal fronts.
C T D
A Seabird CTD system (model SBE 19-03) was used to measure the hydrographic structure of th e water. The CTD carried a Paine strain-gauge pressure sensor, a pressure-protected therm istor, and a Pyrex conductivity cell (for sensor accuracy and resolution see Table 2.1).
Sensor Accuracy Resolution
Pressure 1.25 dbar 0.75 dbar
Tem perature
o.orc
0.001°CConductivity
1
mS m~^ O .lm S m ” ^Table 2.1: Sensor accuracy and resolution of the Seabird CTD SBE 19-03.
2. Observations 20
different depths (water masses) to recalibrate it after th e cruises. T he d a ta were directly transferred to a com puter via cable and then processed with th e software package Seasoft 4 . 2 from Seabird Electronics.
T he CTD was attached to a wire on the A-frame of the ship and was used in two different modes. One mode consisted of single stations a t a given location w ith casts reaching to the bottom (for a list of CTD stations see A ppendix B.2). These m easurements were carried out before and after the strongest ebb and flood tide in order to measure the changes of the hydrographic stru ctu re in the frontal area and surrounding waters.
In the other mode, the CTD was used for continously repeated rapid up- and down casts while the ship was moving along a transect ( “CTD-towyo”). The towyos were used in the frontal region w ith the ship slowly moving in the direction of the tidal currents so th a t th e ship’s speed over ground was significant while the wire angle of th e CTD, which is determ ined by th e speed through the water, could be minimized. Usually, the instrum ent was lowered to a depth of 15 m above the bottom , although the tidal conditions m ade it sometimes impossible to avoid a big wire angle so th a t the instrum ent could not be lowered far enough.
Echo sounder
T he acoustic backscatter intensity of the water column was measured w ith two echo sounders w ith a frequency of 12 kHz and 100 kHz. The transducers were m ounted in the hull of the Vector at a depth of about 2 m. A board unit from B iosonics was used to set pulse w idth, sample frequency, ping interval, etc. (Table 2.2). The d a ta were acquired w ith the program S ounder of the IQS Ocean Acoustics group and processed with Matlab.
T he echo sounder images show areas of different backscatter intensity. High backscatter intensity indicates the sea floor as well as gas bubbles, turbulence, zooplankton aggregations, or fish.
Param eter Cruise 9934 Cruise 0032 Transm itter pulse w idth
Receiver gain
Receiver band w idth Sample frequency Pulse w idth Ping interval Trigger interval 0.5 (0.3) s 18 (12) dB 5 (10) kHz 12 kHz 0.3 s 0.5 s 0.5 s 0.2 s 24 dB 5 kHz 12 kHz 0.2 s 0.7s 0.5 s
Table 2.2: Settings of the 100kHz echo sounder during Vector cruises 9934 and 0032. The values in brackets show the settings for the first two days of cruise
9934-one (Medwin, 19776) and independent measurements w ith an acoustical resonator showed th a t gas bubbles are present in th e downwelling regions of tidal fronts, it can be assumed th a t the high backscatter intensity is mainly due to gas bubbles and not zooplankton or turbulence.
Acoustical resonator
An acoustical resonator was used for measuring bubble size distributions in the tidal fronts. T he instrum ent consists of two identical steel plates w ith a thickness of 1.27 cm and a diam eter of 26.5 cm, which are m ounted a t a distance of 15.0 cm from each other (Figure 2.3). T he steel plates are covered w ith a layer of piezoelectric polyvinylidenedifluoride (PVDF) and a pc-compound (Farmer et a l, 1998). One of the plates produces broadband noise setting up 37 resonant modes in th e resonant cavity between the plates. The other plate is used as a hydrophone, which detects these modes as well as the dam ping (attenuation) by gas bubbles. The frequency range of 6 —196 kHz allows it to detect gas bubbles w ith radii of 15—550 pm .
T he resonator was m ounted on a frame which also held the b attery case, th e recording unit, and a CTD (Figure 2.4). During Vector cruise 9934, this resonator package was equipped
2. Observations 2 2 PVDF (Piezoelectric material) I S 15.0 cm Pc-compound
F igu re 2.3: Design of the acoustieal res onator. One of the identical plates is used for transmission - the other one for detection.
F igure 2.4: The resonator package during Vector cruise 9934- The res onator plates are located at the lower right hand side.
with an additional Sensortec acoustic current m eter (model UCM 40 Mk II) and during cruise 0032 w ith a Seabird dissolved oxygen sensor (model 23-01-Y). However, th e d a ta from the current m eter was not used due to a broken tilt sensor.
T he resonator package was attached to a wire at the A-frame of th e Vector. M easurements were taken while it was continuously profiling up and down in the upper 60 m of the water column - either along a transect through th e frontal region or while th e b oat was sitting (more or less stationary) in th e tidal front.
T he distance between th e resonator and the hull-mounted echo sounder (sometimes > 50 m) has to be taken into account when the measurements are compared w ith each other. The measured bubble size distributions in the upper 10 m were not used as they might be
“contam inated” w ith bubbles of th e ship’s wake.
T he d ata were acquired and processed w ith a software package from th e lOS Ocean Acous tics group.
Aerial photography
A week before Vector cruise 9934 (on September 24) as well as during Vector cruise 0032, air photos were taken w ith a 35 mm photo cam era from a small plane to provide additional information on the surface signals (front lines, eddies, waves) of th e tidal fronts. This inform ation was used to estim ate the spatial extent of the tidal fronts in th e region.
2 .2 T h e tidal front at Boundary Pass
Between S atu rn a and Patos Island (Figure 2.2), a steep sill forms a pronounced barrier for the w ater exchange between Boundary Pass and th e S trait of Georgia. It rises from 200 m to 60 m depth and blocks about 80% of th e w idth of passage, which is also about 200 m deep. The mean slope of the sill is about 30°.
M easurements at Boundary Pass were carried out in September 2000 during Vector cruise 0032 and also briefly in January 2002 during Vector cruise 0203. In th e following, the observations from cruise 0032 are described and are then compared to the ones from cruise 0203 to get an idea of the seasonal variations of the observed processes.
The times which are given in this section in order to describe the tem poral evolution of the frontal system are relative to th e preceding low tide a t Fulford H arbour (Saltspring Island). Slack tide at Boundary Pass is about 1.5 h later.
The strong flood tidal flow in Boundary Pass can reach current speeds of up to 3 m s~ ^. It forces dense w ater from Boundary Pass over the sill (and partly through the gap at the eastern side of the passage) into the S trait of Georgia. This water mass meets a fresher surface layer in the S trait of Georgia ju st downstream of the sill crest (Figures 1.7, 2.5) and subducts underneath it due to the density difference between the two water masses (about 2(7g-units in September 2000).
2. Observations 24 "W 100 --100 02:52 03:00 03:07 02:38 02:45 02:38 02:45 02:52 03:00 03:07 80 2 , 02:38 02:45 02:52 03:00 03:07 02:38 02:45 02:52 03:00 T im e re la tiv e io w tid e [h] 03:07
F igure 2.5: Along-strait transect over the sill at Boundary Pass at the beginning of the flood tidal period on September 29, 2000 (for an enlarged picture of the flow at the sill see Figure 3.2). Measurements were taken with ADCP, echo sounder, and towed CTD. The time is given relative to Fulford Harbour, Saltspring Island, a) North-south (along-strait) component of current speed; b) vertical current speed; c) acoustic backscatter intensity; d) density.
the sill while a t a later stage flow separation a t the sill crest can be observed (next Section). At the surface, where th e two w ater masses meet, a distinct tidal front line forms and eddies are generated by the horizontal shear. Waves, which travel into this convergence zone tend to steepen and break (Section 4) creating gas bubbles in th e surface layer (Figure 1.7).
T he bubbles are draw n down by the strong vertical currents of up to 0.75 m s t o depths > 160 m (Figure 5.2). These current speeds are higher th a n the rise speed of gas bubbles, which is even for th e bigger bubbles only in the order of 15 —3 0 c m s “ ^ (Figure 5.1). This means th a t all gas bubbles, which are trapp ed in th e downwelling currents are draw n into the ocean where they completely dissolve.
a) 15 January, 00:42 g 100 ■g. =>■ 120 c) 14 January, 23:27 0 20 40 60 80 200 b) 15 January, 01:05 100 "S 22 23 24 Density [c^-unitsj 0
F igu re 2.6: Flood tidal front at Boundary Pass during Vector cruise 0203 in January 2002. a) Echo sounder image 1 h lO m in after low tide (relative to Fulford Harbour); b) echo sounder image I h S S m in after low tide; c) density-profile downstream o f the sill, 5m in before low tide.
M easurements taken in January 2002 during Vector cruise 0203, show a frontal system at Boundary Pass which is very similar to the one in September 2000. Echo sounder images (Figure 2.6a-b) indicate hydraulically controlled flow over the sill at about 1 h after low tide
