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Near bottom variations of turbidity in the St Lawrence estuary
Estuarine,
Coastal
and Shelf
Near bottom St Lawrence
Bruno Institute Qukbec, Received
variations estuarya
d’Anglejan of Oceanography, H3A 2B2, Canada 17 August
(1984) 19,655-672
Science
of turbidity
in the
and R. Grant
Ingram
McGill
3620 University Street,
University,
1983 and in revised
Keywords: attenuation; suspended someters; St Lawrence estuary
form
matter;
12January
in situ
Montreal,
1984
measurements;
transmis-
Observations of the turbidity and velocity fields in the near-bottom waters of the St Lawrence estuary were obtained with a package which includes a selfrecording attenuance meter and a currentmeter. The latter also measures salinity and temperature. Time series varying in length between 26 h and 26 days, and with repetition rates between one and 15 min are discussed for 3 typical open-channel and nearshore stations. A high-frequency sampling mode provides a means to observe the passage of a frontal disturbance over the bottom during the semi-diurnal cycle. With lower frequency records having lengths of one week to one month, contributions to the turbidity fluctuations due to the spring-neap oscillations, seasonal changes in run-off, and the sudden rise in solid discharge of local tributaries following storms, can be resolved. From turbidity polar diagrams, local onshore sources of particulate suspended matter can be identified. Among other advantages, it is possible from such records to time precisely the occurrence of turbidity peaks in relation to the ebb and flow
velocities,to assess the importanceof resuspension, and to specify exactly the time rate of changeof the turbidity. On the whole, self-recordingequipments provide a wealth of information unavailablefrom more traditional hydrocast samplingtechniques. Introduction
Since the movement of sediment particles in estuaries by tidal currents largely takes place by alternation of resuspension and settling, quantitative estimates of suspended transport can only be obtained by synchronized measurement of particle concentration and velocity near the sediment surface. Ideally, the entire spectrum of velocity variations should be considered when studying particle motion. These include the very short-lived events linked to burst-sweep phenomena (Heathershaw & Simpson, 1978), fluctuations associatedwith the tidal motion and seasonalchangescontrolled by run-off. Becauseof practical limitations in shipboard collection of estuarine data, most studies of suspended sediment transport in estuarieshave been basedon hourly measurementsover one or two semi-diurnal tidal cycles at selected stations regularly spaced along the channel axis. “Contribution to the program of GIROQ (Groupe Inreruniversitaire Oceanographiques du Qubbec).
de Recherches
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656
B. d’Anglejan
& R. G. Ingram
These studies may be done either by optical attenuation profiling, or by gravimetric filtration of the suspended particulate matter (SPM), or both. This procedure may be sufficient to estimate the net particulate flux in simple, narrow tidal channels approaching unidimensional characteristics (e.g. Winters, 1981). Generally however, the determination of TSM flux in estuaries is a difficult task (McCave, 1979). For suitable examination of this flux in estuaries of complex geometries, high-frequency sampling of both velocity and particle concentrations over longer periods are necessary. This is particularly true in large estuaries, such as the St Lawrence, which are marked by topographic irregularities and by a significant cross-channel component of motion (d’Anglejan & Ingram, 1976). Development of moored systems to monitor turbidity (light scattering or attenuation) and flow stress near the sea bed has recently provided direct insight into the conditions for incipient sediment motion on continental shelves (Sternberg et ul., 1974; Lesht, 1979; Cacchione & Drake, 1979; Downing et al., 1981). However, longer time series of these two variables, particularly in estuaries, are few. This article examines records ranging in length from 24 h to four weeks, taken in the St Lawrence estuary with a simple and rugged instrumented system composed of a transmissometer and a recording current meter. The purpose was to examine, by means of high-frequency and/or long period observations, fluctuations in near-bottom turbidity (light attenuation) in relation to velocity and salinity changes due to tidal cycles and decreasing run-off. A related objective was to assess the capacity of our low-cost self-recording equipment to provide continuous series of observations on suspended matter in the estuarine environment. Physical measurements of the actual sediment resuspension were not part of this study. Instruments
and methods
The instrument package used in this experiment (the benthic tower) consists of an Aanderaa RCM-4 current meter, which records velocity, temperature, salinity and pressure, and a modified Partech Suspended Solid Monitor which is interfaced to a were mounted inside a twelve-channel Aanderaa data logger. Both instruments galvanized steel tripod stabilized on the bottom by three 90 kg anchor weights. The sensors were set at a height of 1 m above the sediment surface, free of interference from any part of the system (Figure 1). Recovery of the instrument was accomplished by release of a buoy connected to a nylon line coiled inside a cylindrical container on the tripod. The Partech Monitor records light attenuation over two dissimilar gaps by means of photoelectric cells which form the two arms of a balanced bridge. A O-100 ppm sensor was selected for this work. The source is unfiltered white light. Thus the attenuation coefficients for both dissolved and particulate solids in the water, which are dependent on wave length (Winters, 1981), cannot be determined. The recorded signal is the ratio of light attenuation received by the two cells, which minimizes the effect of voltage fluctuations in the power supply. The data, which consist of bursts of 12 successive readings taken over 1 min at predetermined intervals of 5, 10 or 15 min, are recorded on magnetic tape. Attenuation is measured on an arbitrary scale of 0 to 1023 units inside the channel recorder. This scale can be adjusted by means of internal resistors in order to maximize the sensitivity of the probe to ambient turbidity levels. Calibration in the laboratory using a Formazin turbidity standard indicated a good linear response of the recorded signal to Formazin concentration. To try interpreting the
655
Near bottom variations of turbidity
0
25
50
75
100 I
cm
Turbidimeter
//
/-
Figure bottom
Aanderaa Current
meter
1. Self-recording equipment for the simultaneous in situ monitoring turbidity and current velocity fluctuations (the benthic tower).
of the near-
field turbidity data in terms of weight concentration of the particles, a further calibration was carried out on station as follows: water was pumped on deck through a nylon hose from the near-bottom layer using a submersible pump and centrifuged on a continuousflow De Lava1 Clarifier (3450 rpm). The rate of flow in the centrifuge was adjusted so as to extract 95”,, or more of particles with d < 45 pm. Aliquots of the extracted particulate
658
B. d’rlnglejan
c3 R. G. Ingram
95-
5 2
8-
2-
o Calibration at station 80-I x Measurements at various locations in the estuary
I-
0
I IO
I 20
I 30
I 40
I 50
I 60
I 70
I 80
I 90
I 100
SPM (mg L-‘)
Figure 2. Relation between the light suspended particulate matter (SPM).
attenuation
and the weight
‘<, concentration
of the
material were added to in steps, and thoroughly mixed into, a 20 1 volume of clarified water, so as to include the range of particle concentrations observed in the near-bottom layer. Light attenuation was measured after each addition in a container well protected from ambient light, and subsamples were taken and filtered to determine particle concentration. The linear relationship found between relative attenuation and particle concentration (Figure 2) was excellent for a given location in the estuary where the range of particle properties (size, shape, mineral composition) remains narrow. However, the correlation was not as good between in situ particle concentration and light attenuation for isolated sampling points taken randomly over the entire estuary (Figure 2). This suggests that a new field calibration should be made for each station: clearly, attenuation measurements are not a good measure of particle concentration in very turbid environments, where particle size and composition fluctuate widely in time and space, or near river entrances where the concentration of humic compounds may be high. One problem encountered with in situ continuous optical measurements is that of marine growth and organic film deposition on the windows of the probe. The manufacturer suggests that cleaning these windows every two weeks is sufficient for sewage effluent conditions. In clearer natural waters a better performance may be expected. After a two-month immersion period, light transmission appeared to be good in spite of some film deposition on the windows. The record did not indicate any significant change in the tidal range of attenuation values during this period. In the three moorings discussed below, the depth of immersion was large enough for the effects of ambient light to be negligible. The probe was covered with a wire screen to prevent any floating debris from entering the light path. Furthermore, tests undertaken in the laboratory indicated that the instrument readings were not influenced by temperature or salinity changes.
Near bottom variations of turbidity
North
459
Chonnei
----
Mean
lower
IOW water
7oooo’ 1
Figure 3. General bathymetry three benthic moorings.
of the St Lawrence
upper
estuary,
and location
of the
Field observations Observations were made in the South Channel of the St Lawrence middle estuary between Quebec and the Saguenay (Figure 3). This area lies on the main path of the seaward transport of suspended sediments derived from the St Lawrence drainage basin (d’Anglejan, 1981). The waters flowing north-eastward over the South Channel are less saline and more turbid than those in the North Channel, as they carry a load of particles
660
B. d’dnglejan
~5 R. G. Ingram
originating in the upstream turbidity maximum. Previous work (Muir, 1979a, 6; d’Anglejan & Ingram, 1976) has shown that a distinct two-layer circulation exists in the South Channel, and that the net residual circulation is downstream near the surface and upstream near the bottom. The existence of a significant cross-channel component of the particulate flux was also demonstrated by d’Anglejan & Ingram (1976). The South Channel has an approximate width of 3 to 4 km, with depths lessthan 30 m, and a gentle topography ending into a steep slope over the Laurentian Channel. Most of the South Channel floor is lined with a thin layer of mixed residual gravels and coarse sands on glacial origin, resting over late glacial compact marine clays. No muddy surfaces exist except on isolated silty patches close to a few areas of intertidal mudflats on the south shore. The well-sorted sandy deposits are often reworked into various bedforms by tidal currents, which range between 50 cm s ’ and 100 cm s- ‘. Instrument mooring took place at three stations, shown on Figure 3: The first station, 79-1, was located on the slope of the South Channel (depth: 9 m), facing Baie de Ste Anne, a major embayment along the south shore with large intertidal mudflats (d’Anglejan et al., 1981). Current velocities and light attenuance measurements were obtained over the bottom during 48 h and 13 h, respectively, between 21 and 23 August 1979. The second station, 80-1, was at the centre of the South Channel, a few miles upstream of its opening into the Laurentian Channel. The station depth was 31 m (MLLW) over well-sorted coarse sands reworked into irregular sand waves. Ambient suspended matter concentrations were low ( < 15 mg l- ‘). Local resuspension was minimal, and the turbidity variations were essentially controlled by tidal advection. A continuous record of turbidity, salinity, temperature and current direction was obtained between 1 and 26 June 1980. The third station, 81-1, was at the shelf edge along the south shore, about 4 km due east of station 80-l (water depth: 15 m MLLW), and 0.5 km downstream of the Gros Cacouna deep water harbour entrance on a heterogenous bed of silty sandsand gravels. The observations (velocity, S, T, turbidity) were made in connection with studies of silting rates in the harbour basin (d’Anglejan & Ingram, 1982). A six-day continuous record was obtained between 10 and 18 June 1981. Results Station
79-1
Light attenuation and salinity records at this site are shown on Figure 4. Attenuation measurements were carried out in the continuous mode (12 observations mini). This makes it possible to precisely time the initial appearanceof a turbidity front, marked by a rapid rise in attenuance at 13 : 45 EST, lessthan 2 h after the start of the flood at a depth of 8 m. This increase occurred simultaneously with a sharp salinity rise of over 5%. It took a full hour for the front to migrate across the station, and for the attenuation to return to background values. The velocity vector diagram (Figure 5) showed that there was no significant change in current direction during the event, the velocity pointing upstream along the major axis of the estuary. The polar turbidity plot (Figure 6) provides an additional illustration of the fact that the highest turbidity values within the front were associated with upstream transport. It also indicates a counterclockwise rotation of the turbidity flux during low tide slack. Previous observations have shown that at this phase of the tidal cycle particle laden waters from Baie de Ste Anne are
Near
bottom
variations
Ebb 017” -~
of turbidity
Slack ~~~ +
Flood --.-
661
220” -~
-
^.._.. ~~. -~
-
1,
I -t
45
29
E
40
E ;;
35
27
2 f a
30
25
25
1
20
-/
End,
3 ” ‘I
21
c ::
1 19
IO
17
5
15
I
250’
23
15
Time
Predicted low tide
V Direction,
I
Figure 1979.
4. Time
variations
Figure
5. Progressive
of light
(E ST 1 Predicted high tide
attenuation
and salinity
N
23 Aug 1979
‘g:OOE.S.T.
vector
diagram
for station
79- 1.
at station
79-l
on 23 August
662
B. d’Anglejan
& R. G. Ingram
N
5801
Figure 6. Polar turbidity diagram instrument relative scale units.
for station
79-l.
Attenuation
values
given
in the
injected into the South Channel, enhancing the offshore turbidity (d’Anglejan et al., 1981). Earlier surveys have also indicated that within the study area the large estuarine turbidity maximum zone has a downstream limit which exhibits frontal characteristics at the surface (d’Anglejan, 1981). The exact location of the turbidity front changes as a function of tidal motion on a small scale and with freshwater discharge on a larger scale. Turbidity appears to be dissipated by tidal mixing with more transparent and saltier waters in the area of the front. At station 79-1, the earlier flood appearance at depth coinciding with a sharp rise in both salinity and turbidity suggests the existence of a retrograde front which provokes upward movement of the sediment laden (and lighter) waters adjoining the bottom along the frontal interface (Bowman & Iverson, 1978). Lobe-like features as found by Garvine & Monk (1974) or entrainment processes immediately behind the leading edge of the front may account for the double peak observed. A similar sharp peak in turbidity along the bottom at early flood was noted during a previous survey at a nearby 25 m-deep station in the axis of the South Channel (d’Anglejan & Ingram, 1976, Figure 4).
663
Near bottom variations of turbidity
Sprrng
Neap
-7
23 22
“‘I
I
2
3
4
5
6
7
8
9
IO
II
Time
Figure salinity
Station
7. Twenty-six at station 80-l.
day
time
12
13
(E ST),
series
14 June
15 16
17
18 19 20
21
I
1
/
1
22
23
24
2526
27
1980
observations
for
turbidity,
temperature
and
80-l
In June 1980, the benthic water tower was moored in the middle of the South Channel, some 50 km downstream of station 79-l. This station was selected because its coarse sandy bottom rules out local resuspension as a source of particles to the water column. Similar conditions are found over long stretches of the St Lawrence estuary both the North and South Channels, which are lined by reworked morainal deposits and are devoid of recent mud accumulation. Indeed, Coulter counter particle volume spectra from various regions of the middle estuary indicate the dominance of particles below 10 pm mean diameter (Kranck, 1979). In regard to the field experiment, the time release controlling the surface buoy malfunctioned causing the system to remain on site for an extended period. Becauseof limits in the recording capacity, only 26 days (June 1 to 26) of data were obtained. Because of an additional malfunction in the rotor bearings of the current meter, no current speed was obtained. The complete record for the 26 days of observations is shown on Figure 7.
664
B. d’Anglejan
& R. ti. Ingram
27 June
Figure 8. Low frequency 80-l; June 1980.
June
Tlme(EST)
variability
of turbidity,
temperature
and salinity
at station
A seasonal trend, as well as tidal fluctuations, in light attenuation, salinity and temperature are noticeable. The downward trend in turbidity during the month of June is matched by an increase in salinity. The depth of the observations (> 30 m), and the generally low levels of primary production in the surface waters of the upper estuary dismiss the possibility that the trend may have happened as a response to a major plankton bloom slowly decaying in June. So does the marked relationship between attenuation and salinity. This relationship becomes even more evident on comparing the corresponding low frequency variability (Figure 8). The original data were filtered to remove variability with periods between 0 and 25 h to produce these ‘ low pass ’ plots. For a 95”,, confidence level, the correlation coefficient between attenuation and salinity is greater than 0.95. Similar calculations on the original data smoothed to hourly values gave an optimum correlation of 0.64 for a 1 h phase difference. A tidal analysis of the hourly data indicated that the relative amplitudes of the tidal period attenuation fluctuations were in the following order: M2 > 0 I> M4 > K 1. Figure 9(a), shows in more detail the data obtained during the spring tides on 13 and 14 June.
Near
bottom
-3
I 0:oo
variations
I 500
I IO:00
665
of turbidity
I 15:oo
I 20:oo
I I:00
I 6100
I3 June
I II:00
I 16:OO
I 21:00
-~.
14 June Tlme(EST)
Figure spring dCjdt,
9. (a) Details of the turbidity (attenuation) tide (13 and 14 June 1980) at station 80-l. the local time rate of change of the turbidity.
fluctuations, C, during 2 days (b) Corresponding variations
of of
The effect of the diurnal inequality on the attenuation levels is clearly indicated. On comparing neap and spring tide conditions one finds an increase in the amplitude of both turbidity and salinity fluctuations during the latter. This may result from the increased energy available for resuspension in the source areas and/or the much larger tidal excursion length which may allow for the advection of turbid waters over the site. There is also weak evidence of a flattening of the seasonal downward trend in turbidity during spring tides which may result from higher levels of mixing. The dominance of particles below 10 pm in average size and the absence of local resuspension as discussed before was confirmed by Coulter counter particle size measurements at station 80-l. These measurements indicated that the particle concentrations were less than 10 mg l- t and that the range of particle size remained uniform over a tidal cycle, ruling out any potential effect of flocculation on attenuation levels. Such observations suggest that most particles in suspension have long settling times and are conservative elements in the water mass. Since the distribution of these particles is mainly governed by advection and diffusion processes within the water column, it could (in theory) be adequately described by means of the general conservation equation. Particle distribution models based on this equation have been previously applied to estuarine studies (Schubel & Okubo, 1972).
666
B. d’Anglejan
& R. G. Ingram
From high density continuous data, such as we have collected, the instantaneous value of the local time rate of change of the turbidity C, dC/dt, can be obtained providing one essential term of the conservative equation. Figure 9(b) gives the fluctuations of dC/dt at station 80-l for two days during the spring tides of 13 and 14 June. An examination of Figures 7 and 9(a) confirms the importance of advection in determining particle concentration, since maximum values in C (dC/dt = 0) were found at the end of the ebb, near low tide slack, when the velocity approaches zero. Larger values of dC/dt were recorded during flood and ebb, when particle transport is most efficient [u(dC/dt) increasing] (Schubel et al., 1977). The concentration peak near low tide slack water is a general feature of the particle distribution observed everywhere in the estuary (d’Anglejan & Ingram, 1976; Silverberg & Sundby, 1980). The rapid fluctuations of dC/dt seen in Figure 9(b) also illustrate the importance of frontal migrations over the station, emphasizing the impossibility of estimating the net advective flux of suspended matter in large estuaries from discrete hourly observations. The resolution of dC/dt would improve considerably with higher frequency sampling, as was obtained at station 79- 1. The background trends in both attenuation and salinity during the month long mooring at station 80- 1 are considered to reflect the reduction in freshwater discharge during June, provoking a progressive upstream penetration of saltier, clearer waters from the lower estuary. Evidence for this was found when comparing the Lowpass curves shown on Figure 8 with the daily discharge measurements made in the St Lawrence river 250 km upstream of station 80-l at La Salle (Quebec) during the spring freshet (Water Survey of Canada, Department of the Environment, unpublished data). A plot of the hydrograph for April, May and June is shown in Figure 10. It indicates that the last and most pronounced decrease in the St Lawrence discharge at La Salle occurred during a few days at the end of April. To estimate the time required for this disturbance to propagate downstream and reach the Gros Cacouna cross-section, we used the mean river flow velocities computed by Forrester (1972) for successive segments of the lower river and estuary. The travel time thus calculated is about 45 days. We further note in Figure 8 the major drop towards lower attenuation and higher salinities around 16 June, which is from 45 days to 2 months after the sharp decrease in discharge at La Salle. This suggests a relationship between the two variables. The seasonal decrease in attenuation during June is large (-SO”,,) compared with the most extreme tidal fluctuations observed during the month. In Figure 11, the attenuation data are plotted on a polar diagram, giving turbidity levels as a function of flow direction. Such plots are useful to identify locally important sources of fine particles. At station 80-1, the highest attenuation values are for southwesterly flow or from the general direction of the nearby Ile Verte mud flats (Figure 3). These findings suggest this area as a possible point source of suspended particulate matter in the lower South Channel. Further work is needed to substantiate this speculation. The polar plot, using 22.5. sector units, indicates that the turbidity flux is mostly from the north with 33”,, of the recorded values between 157.5” and 202.5 . Station 81-l The third mooring of the benthic tower to be discussed took place between 11 June and 15 July 1981 (Figure 3). Attenuation was measured at 15 min intervals in bursts of 12 samples over 1 min. Velocity was recorded every 10 min. Because of a failure in the rechargeable batteries energizing the transmissometer, the 6.5 day record shown 11 to 17
Near
bottom
variations
667
of turbidity
80-I
I M
I J
I J
I A
Figure 10. Hydrograph for the St Lawrence 1980 observed at LaSalle, 250 km upstream
river for the spring of station 80-l.
and summer
months
of
N 360.
Figure 11. Polar turbidity diagram instrument relative scale units.
for station
80-1, attenuation
values
given
in the
Neap
Spring
I i i b”J’
/I\: I )JL 14
13 Time
(E.S.T.),
Figure 12. Five-day time series of turbidity, direction at station 81-1; 12-17 June 1981.
15 June
16
I7
1981
temperature,
salinity,
current
speed and
June was the only portion of the entire record which includes both turbidity and current meter data. Fortunately, the record covers a full interval between neap and spring tide. In Figure 12, semi-diurnal fluctuations in turbidity are present, but not as apparent as at the earlier stations because of high-frequency noise related to wave turbulence and day-to-day changes in wind conditions. However, no simple relationship between daily wind velocities (recorded at Rivitre-du-Loup) and bottom attenuation levels could be found. On comparing the hourly averaged salinity and turbidity time series, the former led the latter in most cases. Highest values of attenuation often occurred near low tide slack, when the flow was generally downstream. In contrast to station 80-1, data at this site indicate that the longshore transport downstream controls the movement of suspended matter along the shallow southern shelf. Further evidence for this can be found in Landsat imagery in which a stream of particle-rich waters is generally found adjacent to the South shore off Riviere du Loup. Furthermore, a previous nephelometric survey of the bottom waters between Riviere du Loup and Gros Cacouna indicated the downstream propagation of a turbid plume on the shelf (d’Anglejan & Ingram, 1982). An important local source of turbidity in the region may be the Riviere du Loup with peak instant discharges above 100 m3 s-i (Figure 3). This small river carries a significant suspended load into the estuary, particularly following brief intense summer storms. Figure 13 shows the effects of a sudden rise in the river discharge around 25 June on bottom salinity values at station 81-1. Although attenuation data were not obtained,
Near
bottom
variation
669
of turbidity
Spring
60 k
14 Time
(E.s.T.),
16 June
1981
the sharp decrease in salinity probably led to a rise in turbidity because of the inverse correlation generally found between these variables. The polar frequency diagram (not shown) showed a higher dispersion of the attenuation data than at the previous station, but with a strong mode to the northeast (23”,,). Progressive vector diagrams constructed from the current meter data at 81-1 show a definite eastward trend, with a net velocity around 7 cm s I. These findings indicate a net turbidity flux towards the entrance of the Gros Cacouna deep water basin, superimposed over the general downstream drift alongshore. This flux is thought to play an important role towards the silting of the harbour basin (d’Anglejan & Ingram, 1982). A simple computation of the bottom shear stress (r) at station 81-1 was made by applying the quadratic relation, ‘t = A x u”, where A is the friction coefficient (= 3 x 10e3) and U2 is the mean of squared bottom velocity at 1 m above the interface (Sternberg, 1972). Figure 14 shows the contrast in the r values between neap and spring conditions. However, no simple relationship between r and the near bottom attenuation levels recorded between 11 to 17 June was found, suggesting no local resuspension effects on the water column turbidity.
670
B. d’ Auglejan
& R. G. Ingram
June
July TimeCEST)
Figure 13. Relation between the daily discharge of Riviere-du-Loup monitored at station 81-l; 7 June to 14 July 1981.
7.0
and the variables
1
Spring I
Neop I
Neap I
Spring I
Neop I
6.0
01
I 8
12
I6
20
24
28
2
6
IO
14
I8
July
June Ttme
(E ST)
Figure 14. Near bottom shear stress at station 81-1 calculated from the current velocity measurements, June and July 1981. Values are smoothed over a tidal cycle from 6 phase lag per cycle.
Near
bottom
Summary
variation
671
of turbidity
and conclusions
Deployment of, a benthic tower, recording light attenuation (turbidity), temperature, salinity and velocity at three different sites in the St Lawrence estuary has demonstrated the importance of various phenomena on the turbidity regime. In addition to semi-diurnal fluctuations of turbidity, the influence of fronts, river runoff (both the St Lawrence and smaller tributaries), and local particle sources has been documented. The area sampled, downstream of the turbidity maximum zone, shows a dominance of advection over local resuspension in determining the medium time scale variations in turbidity. Another important finding in this study was the observation that high frequency changes in turbidity occur over this area of the St Lawrence. Therefore, previous studies employing hourly or half-hourly sampling of the turbidity field were subject to a serious aliasing problem. The extended time series taken at station 80-l indicates the importance of river runoff to the long-term variations in turbidity. A major shift to lower turbidity values occurred during the month of June in the downstream part of the survey area. Closer to shore, the importance of local sources of turbidity, such as Baie de Ste Anne, Riviere du Loup and the Ile Verte mud flats, has been shown. In regard to tributary effects, consonant (but opposing) changes of both attenuation and salinity were observed. The influence of Baie de Ste Anne and the Ile Verte mudflats on offshore bottom turbidity was found to result from the local circulation patterns emphasizing the importance of cross-channel effects in particle flux through large estuaries such as the St Lawrence (d’Anglejan & Ingram, 1976). In summary, a self-recording benthic tower has been shown to be of great value in any study of turbidity distribution in a large estuarine system. Burst sampling of the turbidity field shows importance high frequency variability. The complexity of the various features influencing the local bottom turbidity values in the St Lawrence estuary amply demonstrates the need for high frequency sampling of attenuation, velocity and salinity over long periods at a number of stations. Traditional profiling techniques using low frequency sampling were found to be deficient in both describing the variability and allowing for a rigorous analysis of the causes responsible for the observed changes.
Acknowledgements We are particularly grateful to Jean-Pierre Savard who helped us throughout the various phases of this study, and to Paul Peltola for technical assistance. This work was supported financially by a Strategic Grant of the NSERC of Canada to both authors, and by grants from the Ministry of Education of the Province of Quebec (FCAC Programme) to GIROQ (Groupe Interuniversitaire de Recherches Oceanographiques du Qukbec). References d’Anglejan, B. 1981 On the advection of turbidity in the Saint Lawrence estuary. Estuaries d’Anglejan, B. & Ingram, R. G. 1976 Time-depth variations in tidal flux of suspended Lawrence estuary. Estuarine and Coastal Marine Science 4,401-416. d’Anglejan, B. & Ingram, R. G. 1982 Investigation of natural sediment and dredge spoil vicinity of Gros Cacouna harbour (St Lawrence Estuary). Department of Supply & Unpublished manuscript. 50 pp. d’Anglejan, B., Ingram, R. G. & Savard, J. I’. 1981 Suspended sediment exchanges Lawrence estuary and a coastal embayment. Marine Geology 40,85-100.
4(l), Z-15. matter in the St movement in the Service. Ottawa. between
the St
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B. d’ Auglejarr
& R. G. Ingrum
Bowman, J. & Iverson, R. I. 1978 Estuarine and plume fronts. In Oceanic Fronts and Coastal Processes (M. J. Bowman & W. E. Essaias, eds). Proceedings of a workshop, Marine Science Research Center, 25-27 May 1977. Springer Verlag, Heidelberg. Chapter 10. Cacchione, D. A. & Drake, D. E. 1979 A new instrument to investigate sediment dynamics on continental shelf. Journal of Geophysical Research 84, 1187-1205. Downing, J. P., Sternberg, R. W. & Lister, C. R. B. 1981 New instrumentation for the investigation of suspended sediment suspension processes in the shallow marine environment. Marine Geology 42, 19-34. Forrester, W. S. 1972 Tidal transports and streams in the St Lawrence river and estuary. International Hydrographic Review 49,95-108. Garvine, R. W. St Monk, J. D. 1974 Frontal structure of a river plume. Journal of Geophysical Research 79, 2251-2259. Heathershaw, D. C. & Simpson, J. H. 1974 Fine structure of light attenuation and its relation to temperature in the Irish Sea. Esruarine and Coastal Marine Science 2,91-103. Kranck, K. 1979 Dynamics and distribution of suspended particulate matter in the St Lawrence estuary. Le Naturaliste Canadien 106,1063-1073. Lesht, B. M. 1979 Relationship between sediment resuspension and the statistical frequency distribution of bottom sediment shear stress. Marine Geology 32, M19-M27. McCave, I. N. 1979 Suspended sediment. In Estuarine Hydrography and Sedimentation (K. R. Dyer, ed.). Cambridge University Press, Cambridge. Muir, L. R. 1979a St Lawrence River oceanographic survey, 1977 data report. Vol. 1: tidal, meteorological and current meter data. Government of Canada; Ocean and Aquatic Sciences. Burlington, Ont. Unpublished manuscript. 199 pp. Muir, L. R. 19796 St Lawrence River oceanographic survey, 1977 data report. Vol. 2: profile data. Government of Canada; Ocean and Aquatic Sciences. Burlington, Ont. Unpublished manuscript. 278 pp. Schubel, J. R. & Okubo, A. 1972 Comments on the dispersal of Suspended Sediments across the Continental Shelves. In Shelf Sediment Transport: Process and Patrern (J. P. Swift, D. B. Duane & 0. H. Pilkey, eds). Dowden, Hutchinson & Ross, Stroudsburg, PA. 333-346. Schubel, J. R., Wilson, R. E. & Okubo, A. 1977 Vertical transport of suspended sediment in upper Chesapeake Bay. In 7th Baruch Research Symposium ‘Transport Processes in Estuarine Environments’ (B. Kjerfve, ed.). University of South Carolina Press, Columbia, S.C. pp. 161-175. Silverberg, N. & Sundby, B. 1979 Observations in the turbidity maximum of the St Lawrence estuary. CanadianJournal of Earth Sciences 16,939-950. Sternberg, R. W. 1972 Predicting initial motion and bedload transport of sediment particles in the shallow marine environment. In Shelf Sediment Transport: Process and Pattern (J. P. Swift, D. B. Duane & 0. H. Pilkey, eds). Dowden, Hutchinson & Ross, Stroudsburg, PA. pp. 61-82. Sternbere. R. W.. Baker. E. T., McManus, D., Smith, S. & Morrison, D. R. 1974 An integrating nephelometerfor measuring particle concentrations in the deep sea. Deep Sea Research 21,887-892. Winters, G. V. 1981 Environmental geology of the Miramichi estuary: suspended sediment transport. Geological Survey of Canada, Paper 8 l-l 6,12 pages. Winters, G. V. & Buckley, D. E. 1978 in situ determination of suspended particulate matter and dissolved organic matter concentrations in an estuarine environment by means of an optical beam attenuance meter. Estuarine and Coastal Marine Science 10,455-466.
Coastal
and Shelf
Near bottom St Lawrence
Bruno Institute Qukbec, Received
variations estuarya
d’Anglejan of Oceanography, H3A 2B2, Canada 17 August
(1984) 19,655-672
Science
of turbidity
in the
and R. Grant
Ingram
McGill
3620 University Street,
University,
1983 and in revised
Keywords: attenuation; suspended someters; St Lawrence estuary
form
matter;
12January
in situ
Montreal,
1984
measurements;
transmis-
Observations of the turbidity and velocity fields in the near-bottom waters of the St Lawrence estuary were obtained with a package which includes a selfrecording attenuance meter and a currentmeter. The latter also measures salinity and temperature. Time series varying in length between 26 h and 26 days, and with repetition rates between one and 15 min are discussed for 3 typical open-channel and nearshore stations. A high-frequency sampling mode provides a means to observe the passage of a frontal disturbance over the bottom during the semi-diurnal cycle. With lower frequency records having lengths of one week to one month, contributions to the turbidity fluctuations due to the spring-neap oscillations, seasonal changes in run-off, and the sudden rise in solid discharge of local tributaries following storms, can be resolved. From turbidity polar diagrams, local onshore sources of particulate suspended matter can be identified. Among other advantages, it is possible from such records to time precisely the occurrence of turbidity peaks in relation to the ebb and flow
velocities,to assess the importanceof resuspension, and to specify exactly the time rate of changeof the turbidity. On the whole, self-recordingequipments provide a wealth of information unavailablefrom more traditional hydrocast samplingtechniques. Introduction
Since the movement of sediment particles in estuaries by tidal currents largely takes place by alternation of resuspension and settling, quantitative estimates of suspended transport can only be obtained by synchronized measurement of particle concentration and velocity near the sediment surface. Ideally, the entire spectrum of velocity variations should be considered when studying particle motion. These include the very short-lived events linked to burst-sweep phenomena (Heathershaw & Simpson, 1978), fluctuations associatedwith the tidal motion and seasonalchangescontrolled by run-off. Becauseof practical limitations in shipboard collection of estuarine data, most studies of suspended sediment transport in estuarieshave been basedon hourly measurementsover one or two semi-diurnal tidal cycles at selected stations regularly spaced along the channel axis. “Contribution to the program of GIROQ (Groupe Inreruniversitaire Oceanographiques du Qubbec).
de Recherches
655 0172-7714’84:1?0655+
18 $03,OOiO
01984 Academic Press Inc. (London) Limired
656
B. d’Anglejan
& R. G. Ingram
These studies may be done either by optical attenuation profiling, or by gravimetric filtration of the suspended particulate matter (SPM), or both. This procedure may be sufficient to estimate the net particulate flux in simple, narrow tidal channels approaching unidimensional characteristics (e.g. Winters, 1981). Generally however, the determination of TSM flux in estuaries is a difficult task (McCave, 1979). For suitable examination of this flux in estuaries of complex geometries, high-frequency sampling of both velocity and particle concentrations over longer periods are necessary. This is particularly true in large estuaries, such as the St Lawrence, which are marked by topographic irregularities and by a significant cross-channel component of motion (d’Anglejan & Ingram, 1976). Development of moored systems to monitor turbidity (light scattering or attenuation) and flow stress near the sea bed has recently provided direct insight into the conditions for incipient sediment motion on continental shelves (Sternberg et ul., 1974; Lesht, 1979; Cacchione & Drake, 1979; Downing et al., 1981). However, longer time series of these two variables, particularly in estuaries, are few. This article examines records ranging in length from 24 h to four weeks, taken in the St Lawrence estuary with a simple and rugged instrumented system composed of a transmissometer and a recording current meter. The purpose was to examine, by means of high-frequency and/or long period observations, fluctuations in near-bottom turbidity (light attenuation) in relation to velocity and salinity changes due to tidal cycles and decreasing run-off. A related objective was to assess the capacity of our low-cost self-recording equipment to provide continuous series of observations on suspended matter in the estuarine environment. Physical measurements of the actual sediment resuspension were not part of this study. Instruments
and methods
The instrument package used in this experiment (the benthic tower) consists of an Aanderaa RCM-4 current meter, which records velocity, temperature, salinity and pressure, and a modified Partech Suspended Solid Monitor which is interfaced to a were mounted inside a twelve-channel Aanderaa data logger. Both instruments galvanized steel tripod stabilized on the bottom by three 90 kg anchor weights. The sensors were set at a height of 1 m above the sediment surface, free of interference from any part of the system (Figure 1). Recovery of the instrument was accomplished by release of a buoy connected to a nylon line coiled inside a cylindrical container on the tripod. The Partech Monitor records light attenuation over two dissimilar gaps by means of photoelectric cells which form the two arms of a balanced bridge. A O-100 ppm sensor was selected for this work. The source is unfiltered white light. Thus the attenuation coefficients for both dissolved and particulate solids in the water, which are dependent on wave length (Winters, 1981), cannot be determined. The recorded signal is the ratio of light attenuation received by the two cells, which minimizes the effect of voltage fluctuations in the power supply. The data, which consist of bursts of 12 successive readings taken over 1 min at predetermined intervals of 5, 10 or 15 min, are recorded on magnetic tape. Attenuation is measured on an arbitrary scale of 0 to 1023 units inside the channel recorder. This scale can be adjusted by means of internal resistors in order to maximize the sensitivity of the probe to ambient turbidity levels. Calibration in the laboratory using a Formazin turbidity standard indicated a good linear response of the recorded signal to Formazin concentration. To try interpreting the
655
Near bottom variations of turbidity
0
25
50
75
100 I
cm
Turbidimeter
//
/-
Figure bottom
Aanderaa Current
meter
1. Self-recording equipment for the simultaneous in situ monitoring turbidity and current velocity fluctuations (the benthic tower).
of the near-
field turbidity data in terms of weight concentration of the particles, a further calibration was carried out on station as follows: water was pumped on deck through a nylon hose from the near-bottom layer using a submersible pump and centrifuged on a continuousflow De Lava1 Clarifier (3450 rpm). The rate of flow in the centrifuge was adjusted so as to extract 95”,, or more of particles with d < 45 pm. Aliquots of the extracted particulate
658
B. d’rlnglejan
c3 R. G. Ingram
95-
5 2
8-
2-
o Calibration at station 80-I x Measurements at various locations in the estuary
I-
0
I IO
I 20
I 30
I 40
I 50
I 60
I 70
I 80
I 90
I 100
SPM (mg L-‘)
Figure 2. Relation between the light suspended particulate matter (SPM).
attenuation
and the weight
‘<, concentration
of the
material were added to in steps, and thoroughly mixed into, a 20 1 volume of clarified water, so as to include the range of particle concentrations observed in the near-bottom layer. Light attenuation was measured after each addition in a container well protected from ambient light, and subsamples were taken and filtered to determine particle concentration. The linear relationship found between relative attenuation and particle concentration (Figure 2) was excellent for a given location in the estuary where the range of particle properties (size, shape, mineral composition) remains narrow. However, the correlation was not as good between in situ particle concentration and light attenuation for isolated sampling points taken randomly over the entire estuary (Figure 2). This suggests that a new field calibration should be made for each station: clearly, attenuation measurements are not a good measure of particle concentration in very turbid environments, where particle size and composition fluctuate widely in time and space, or near river entrances where the concentration of humic compounds may be high. One problem encountered with in situ continuous optical measurements is that of marine growth and organic film deposition on the windows of the probe. The manufacturer suggests that cleaning these windows every two weeks is sufficient for sewage effluent conditions. In clearer natural waters a better performance may be expected. After a two-month immersion period, light transmission appeared to be good in spite of some film deposition on the windows. The record did not indicate any significant change in the tidal range of attenuation values during this period. In the three moorings discussed below, the depth of immersion was large enough for the effects of ambient light to be negligible. The probe was covered with a wire screen to prevent any floating debris from entering the light path. Furthermore, tests undertaken in the laboratory indicated that the instrument readings were not influenced by temperature or salinity changes.
Near bottom variations of turbidity
North
459
Chonnei
----
Mean
lower
IOW water
7oooo’ 1
Figure 3. General bathymetry three benthic moorings.
of the St Lawrence
upper
estuary,
and location
of the
Field observations Observations were made in the South Channel of the St Lawrence middle estuary between Quebec and the Saguenay (Figure 3). This area lies on the main path of the seaward transport of suspended sediments derived from the St Lawrence drainage basin (d’Anglejan, 1981). The waters flowing north-eastward over the South Channel are less saline and more turbid than those in the North Channel, as they carry a load of particles
660
B. d’dnglejan
~5 R. G. Ingram
originating in the upstream turbidity maximum. Previous work (Muir, 1979a, 6; d’Anglejan & Ingram, 1976) has shown that a distinct two-layer circulation exists in the South Channel, and that the net residual circulation is downstream near the surface and upstream near the bottom. The existence of a significant cross-channel component of the particulate flux was also demonstrated by d’Anglejan & Ingram (1976). The South Channel has an approximate width of 3 to 4 km, with depths lessthan 30 m, and a gentle topography ending into a steep slope over the Laurentian Channel. Most of the South Channel floor is lined with a thin layer of mixed residual gravels and coarse sands on glacial origin, resting over late glacial compact marine clays. No muddy surfaces exist except on isolated silty patches close to a few areas of intertidal mudflats on the south shore. The well-sorted sandy deposits are often reworked into various bedforms by tidal currents, which range between 50 cm s ’ and 100 cm s- ‘. Instrument mooring took place at three stations, shown on Figure 3: The first station, 79-1, was located on the slope of the South Channel (depth: 9 m), facing Baie de Ste Anne, a major embayment along the south shore with large intertidal mudflats (d’Anglejan et al., 1981). Current velocities and light attenuance measurements were obtained over the bottom during 48 h and 13 h, respectively, between 21 and 23 August 1979. The second station, 80-1, was at the centre of the South Channel, a few miles upstream of its opening into the Laurentian Channel. The station depth was 31 m (MLLW) over well-sorted coarse sands reworked into irregular sand waves. Ambient suspended matter concentrations were low ( < 15 mg l- ‘). Local resuspension was minimal, and the turbidity variations were essentially controlled by tidal advection. A continuous record of turbidity, salinity, temperature and current direction was obtained between 1 and 26 June 1980. The third station, 81-1, was at the shelf edge along the south shore, about 4 km due east of station 80-l (water depth: 15 m MLLW), and 0.5 km downstream of the Gros Cacouna deep water harbour entrance on a heterogenous bed of silty sandsand gravels. The observations (velocity, S, T, turbidity) were made in connection with studies of silting rates in the harbour basin (d’Anglejan & Ingram, 1982). A six-day continuous record was obtained between 10 and 18 June 1981. Results Station
79-1
Light attenuation and salinity records at this site are shown on Figure 4. Attenuation measurements were carried out in the continuous mode (12 observations mini). This makes it possible to precisely time the initial appearanceof a turbidity front, marked by a rapid rise in attenuance at 13 : 45 EST, lessthan 2 h after the start of the flood at a depth of 8 m. This increase occurred simultaneously with a sharp salinity rise of over 5%. It took a full hour for the front to migrate across the station, and for the attenuation to return to background values. The velocity vector diagram (Figure 5) showed that there was no significant change in current direction during the event, the velocity pointing upstream along the major axis of the estuary. The polar turbidity plot (Figure 6) provides an additional illustration of the fact that the highest turbidity values within the front were associated with upstream transport. It also indicates a counterclockwise rotation of the turbidity flux during low tide slack. Previous observations have shown that at this phase of the tidal cycle particle laden waters from Baie de Ste Anne are
Near
bottom
variations
Ebb 017” -~
of turbidity
Slack ~~~ +
Flood --.-
661
220” -~
-
^.._.. ~~. -~
-
1,
I -t
45
29
E
40
E ;;
35
27
2 f a
30
25
25
1
20
-/
End,
3 ” ‘I
21
c ::
1 19
IO
17
5
15
I
250’
23
15
Time
Predicted low tide
V Direction,
I
Figure 1979.
4. Time
variations
Figure
5. Progressive
of light
(E ST 1 Predicted high tide
attenuation
and salinity
N
23 Aug 1979
‘g:OOE.S.T.
vector
diagram
for station
79- 1.
at station
79-l
on 23 August
662
B. d’Anglejan
& R. G. Ingram
N
5801
Figure 6. Polar turbidity diagram instrument relative scale units.
for station
79-l.
Attenuation
values
given
in the
injected into the South Channel, enhancing the offshore turbidity (d’Anglejan et al., 1981). Earlier surveys have also indicated that within the study area the large estuarine turbidity maximum zone has a downstream limit which exhibits frontal characteristics at the surface (d’Anglejan, 1981). The exact location of the turbidity front changes as a function of tidal motion on a small scale and with freshwater discharge on a larger scale. Turbidity appears to be dissipated by tidal mixing with more transparent and saltier waters in the area of the front. At station 79-1, the earlier flood appearance at depth coinciding with a sharp rise in both salinity and turbidity suggests the existence of a retrograde front which provokes upward movement of the sediment laden (and lighter) waters adjoining the bottom along the frontal interface (Bowman & Iverson, 1978). Lobe-like features as found by Garvine & Monk (1974) or entrainment processes immediately behind the leading edge of the front may account for the double peak observed. A similar sharp peak in turbidity along the bottom at early flood was noted during a previous survey at a nearby 25 m-deep station in the axis of the South Channel (d’Anglejan & Ingram, 1976, Figure 4).
663
Near bottom variations of turbidity
Sprrng
Neap
-7
23 22
“‘I
I
2
3
4
5
6
7
8
9
IO
II
Time
Figure salinity
Station
7. Twenty-six at station 80-l.
day
time
12
13
(E ST),
series
14 June
15 16
17
18 19 20
21
I
1
/
1
22
23
24
2526
27
1980
observations
for
turbidity,
temperature
and
80-l
In June 1980, the benthic water tower was moored in the middle of the South Channel, some 50 km downstream of station 79-l. This station was selected because its coarse sandy bottom rules out local resuspension as a source of particles to the water column. Similar conditions are found over long stretches of the St Lawrence estuary both the North and South Channels, which are lined by reworked morainal deposits and are devoid of recent mud accumulation. Indeed, Coulter counter particle volume spectra from various regions of the middle estuary indicate the dominance of particles below 10 pm mean diameter (Kranck, 1979). In regard to the field experiment, the time release controlling the surface buoy malfunctioned causing the system to remain on site for an extended period. Becauseof limits in the recording capacity, only 26 days (June 1 to 26) of data were obtained. Because of an additional malfunction in the rotor bearings of the current meter, no current speed was obtained. The complete record for the 26 days of observations is shown on Figure 7.
664
B. d’Anglejan
& R. ti. Ingram
27 June
Figure 8. Low frequency 80-l; June 1980.
June
Tlme(EST)
variability
of turbidity,
temperature
and salinity
at station
A seasonal trend, as well as tidal fluctuations, in light attenuation, salinity and temperature are noticeable. The downward trend in turbidity during the month of June is matched by an increase in salinity. The depth of the observations (> 30 m), and the generally low levels of primary production in the surface waters of the upper estuary dismiss the possibility that the trend may have happened as a response to a major plankton bloom slowly decaying in June. So does the marked relationship between attenuation and salinity. This relationship becomes even more evident on comparing the corresponding low frequency variability (Figure 8). The original data were filtered to remove variability with periods between 0 and 25 h to produce these ‘ low pass ’ plots. For a 95”,, confidence level, the correlation coefficient between attenuation and salinity is greater than 0.95. Similar calculations on the original data smoothed to hourly values gave an optimum correlation of 0.64 for a 1 h phase difference. A tidal analysis of the hourly data indicated that the relative amplitudes of the tidal period attenuation fluctuations were in the following order: M2 > 0 I> M4 > K 1. Figure 9(a), shows in more detail the data obtained during the spring tides on 13 and 14 June.
Near
bottom
-3
I 0:oo
variations
I 500
I IO:00
665
of turbidity
I 15:oo
I 20:oo
I I:00
I 6100
I3 June
I II:00
I 16:OO
I 21:00
-~.
14 June Tlme(EST)
Figure spring dCjdt,
9. (a) Details of the turbidity (attenuation) tide (13 and 14 June 1980) at station 80-l. the local time rate of change of the turbidity.
fluctuations, C, during 2 days (b) Corresponding variations
of of
The effect of the diurnal inequality on the attenuation levels is clearly indicated. On comparing neap and spring tide conditions one finds an increase in the amplitude of both turbidity and salinity fluctuations during the latter. This may result from the increased energy available for resuspension in the source areas and/or the much larger tidal excursion length which may allow for the advection of turbid waters over the site. There is also weak evidence of a flattening of the seasonal downward trend in turbidity during spring tides which may result from higher levels of mixing. The dominance of particles below 10 pm in average size and the absence of local resuspension as discussed before was confirmed by Coulter counter particle size measurements at station 80-l. These measurements indicated that the particle concentrations were less than 10 mg l- t and that the range of particle size remained uniform over a tidal cycle, ruling out any potential effect of flocculation on attenuation levels. Such observations suggest that most particles in suspension have long settling times and are conservative elements in the water mass. Since the distribution of these particles is mainly governed by advection and diffusion processes within the water column, it could (in theory) be adequately described by means of the general conservation equation. Particle distribution models based on this equation have been previously applied to estuarine studies (Schubel & Okubo, 1972).
666
B. d’Anglejan
& R. G. Ingram
From high density continuous data, such as we have collected, the instantaneous value of the local time rate of change of the turbidity C, dC/dt, can be obtained providing one essential term of the conservative equation. Figure 9(b) gives the fluctuations of dC/dt at station 80-l for two days during the spring tides of 13 and 14 June. An examination of Figures 7 and 9(a) confirms the importance of advection in determining particle concentration, since maximum values in C (dC/dt = 0) were found at the end of the ebb, near low tide slack, when the velocity approaches zero. Larger values of dC/dt were recorded during flood and ebb, when particle transport is most efficient [u(dC/dt) increasing] (Schubel et al., 1977). The concentration peak near low tide slack water is a general feature of the particle distribution observed everywhere in the estuary (d’Anglejan & Ingram, 1976; Silverberg & Sundby, 1980). The rapid fluctuations of dC/dt seen in Figure 9(b) also illustrate the importance of frontal migrations over the station, emphasizing the impossibility of estimating the net advective flux of suspended matter in large estuaries from discrete hourly observations. The resolution of dC/dt would improve considerably with higher frequency sampling, as was obtained at station 79- 1. The background trends in both attenuation and salinity during the month long mooring at station 80- 1 are considered to reflect the reduction in freshwater discharge during June, provoking a progressive upstream penetration of saltier, clearer waters from the lower estuary. Evidence for this was found when comparing the Lowpass curves shown on Figure 8 with the daily discharge measurements made in the St Lawrence river 250 km upstream of station 80-l at La Salle (Quebec) during the spring freshet (Water Survey of Canada, Department of the Environment, unpublished data). A plot of the hydrograph for April, May and June is shown in Figure 10. It indicates that the last and most pronounced decrease in the St Lawrence discharge at La Salle occurred during a few days at the end of April. To estimate the time required for this disturbance to propagate downstream and reach the Gros Cacouna cross-section, we used the mean river flow velocities computed by Forrester (1972) for successive segments of the lower river and estuary. The travel time thus calculated is about 45 days. We further note in Figure 8 the major drop towards lower attenuation and higher salinities around 16 June, which is from 45 days to 2 months after the sharp decrease in discharge at La Salle. This suggests a relationship between the two variables. The seasonal decrease in attenuation during June is large (-SO”,,) compared with the most extreme tidal fluctuations observed during the month. In Figure 11, the attenuation data are plotted on a polar diagram, giving turbidity levels as a function of flow direction. Such plots are useful to identify locally important sources of fine particles. At station 80-1, the highest attenuation values are for southwesterly flow or from the general direction of the nearby Ile Verte mud flats (Figure 3). These findings suggest this area as a possible point source of suspended particulate matter in the lower South Channel. Further work is needed to substantiate this speculation. The polar plot, using 22.5. sector units, indicates that the turbidity flux is mostly from the north with 33”,, of the recorded values between 157.5” and 202.5 . Station 81-l The third mooring of the benthic tower to be discussed took place between 11 June and 15 July 1981 (Figure 3). Attenuation was measured at 15 min intervals in bursts of 12 samples over 1 min. Velocity was recorded every 10 min. Because of a failure in the rechargeable batteries energizing the transmissometer, the 6.5 day record shown 11 to 17
Near
bottom
variations
667
of turbidity
80-I
I M
I J
I J
I A
Figure 10. Hydrograph for the St Lawrence 1980 observed at LaSalle, 250 km upstream
river for the spring of station 80-l.
and summer
months
of
N 360.
Figure 11. Polar turbidity diagram instrument relative scale units.
for station
80-1, attenuation
values
given
in the
Neap
Spring
I i i b”J’
/I\: I )JL 14
13 Time
(E.S.T.),
Figure 12. Five-day time series of turbidity, direction at station 81-1; 12-17 June 1981.
15 June
16
I7
1981
temperature,
salinity,
current
speed and
June was the only portion of the entire record which includes both turbidity and current meter data. Fortunately, the record covers a full interval between neap and spring tide. In Figure 12, semi-diurnal fluctuations in turbidity are present, but not as apparent as at the earlier stations because of high-frequency noise related to wave turbulence and day-to-day changes in wind conditions. However, no simple relationship between daily wind velocities (recorded at Rivitre-du-Loup) and bottom attenuation levels could be found. On comparing the hourly averaged salinity and turbidity time series, the former led the latter in most cases. Highest values of attenuation often occurred near low tide slack, when the flow was generally downstream. In contrast to station 80-1, data at this site indicate that the longshore transport downstream controls the movement of suspended matter along the shallow southern shelf. Further evidence for this can be found in Landsat imagery in which a stream of particle-rich waters is generally found adjacent to the South shore off Riviere du Loup. Furthermore, a previous nephelometric survey of the bottom waters between Riviere du Loup and Gros Cacouna indicated the downstream propagation of a turbid plume on the shelf (d’Anglejan & Ingram, 1982). An important local source of turbidity in the region may be the Riviere du Loup with peak instant discharges above 100 m3 s-i (Figure 3). This small river carries a significant suspended load into the estuary, particularly following brief intense summer storms. Figure 13 shows the effects of a sudden rise in the river discharge around 25 June on bottom salinity values at station 81-1. Although attenuation data were not obtained,
Near
bottom
variation
669
of turbidity
Spring
60 k
14 Time
(E.s.T.),
16 June
1981
the sharp decrease in salinity probably led to a rise in turbidity because of the inverse correlation generally found between these variables. The polar frequency diagram (not shown) showed a higher dispersion of the attenuation data than at the previous station, but with a strong mode to the northeast (23”,,). Progressive vector diagrams constructed from the current meter data at 81-1 show a definite eastward trend, with a net velocity around 7 cm s I. These findings indicate a net turbidity flux towards the entrance of the Gros Cacouna deep water basin, superimposed over the general downstream drift alongshore. This flux is thought to play an important role towards the silting of the harbour basin (d’Anglejan & Ingram, 1982). A simple computation of the bottom shear stress (r) at station 81-1 was made by applying the quadratic relation, ‘t = A x u”, where A is the friction coefficient (= 3 x 10e3) and U2 is the mean of squared bottom velocity at 1 m above the interface (Sternberg, 1972). Figure 14 shows the contrast in the r values between neap and spring conditions. However, no simple relationship between r and the near bottom attenuation levels recorded between 11 to 17 June was found, suggesting no local resuspension effects on the water column turbidity.
670
B. d’ Auglejan
& R. G. Ingram
June
July TimeCEST)
Figure 13. Relation between the daily discharge of Riviere-du-Loup monitored at station 81-l; 7 June to 14 July 1981.
7.0
and the variables
1
Spring I
Neop I
Neap I
Spring I
Neop I
6.0
01
I 8
12
I6
20
24
28
2
6
IO
14
I8
July
June Ttme
(E ST)
Figure 14. Near bottom shear stress at station 81-1 calculated from the current velocity measurements, June and July 1981. Values are smoothed over a tidal cycle from 6 phase lag per cycle.
Near
bottom
Summary
variation
671
of turbidity
and conclusions
Deployment of, a benthic tower, recording light attenuation (turbidity), temperature, salinity and velocity at three different sites in the St Lawrence estuary has demonstrated the importance of various phenomena on the turbidity regime. In addition to semi-diurnal fluctuations of turbidity, the influence of fronts, river runoff (both the St Lawrence and smaller tributaries), and local particle sources has been documented. The area sampled, downstream of the turbidity maximum zone, shows a dominance of advection over local resuspension in determining the medium time scale variations in turbidity. Another important finding in this study was the observation that high frequency changes in turbidity occur over this area of the St Lawrence. Therefore, previous studies employing hourly or half-hourly sampling of the turbidity field were subject to a serious aliasing problem. The extended time series taken at station 80-l indicates the importance of river runoff to the long-term variations in turbidity. A major shift to lower turbidity values occurred during the month of June in the downstream part of the survey area. Closer to shore, the importance of local sources of turbidity, such as Baie de Ste Anne, Riviere du Loup and the Ile Verte mud flats, has been shown. In regard to tributary effects, consonant (but opposing) changes of both attenuation and salinity were observed. The influence of Baie de Ste Anne and the Ile Verte mudflats on offshore bottom turbidity was found to result from the local circulation patterns emphasizing the importance of cross-channel effects in particle flux through large estuaries such as the St Lawrence (d’Anglejan & Ingram, 1976). In summary, a self-recording benthic tower has been shown to be of great value in any study of turbidity distribution in a large estuarine system. Burst sampling of the turbidity field shows importance high frequency variability. The complexity of the various features influencing the local bottom turbidity values in the St Lawrence estuary amply demonstrates the need for high frequency sampling of attenuation, velocity and salinity over long periods at a number of stations. Traditional profiling techniques using low frequency sampling were found to be deficient in both describing the variability and allowing for a rigorous analysis of the causes responsible for the observed changes.
Acknowledgements We are particularly grateful to Jean-Pierre Savard who helped us throughout the various phases of this study, and to Paul Peltola for technical assistance. This work was supported financially by a Strategic Grant of the NSERC of Canada to both authors, and by grants from the Ministry of Education of the Province of Quebec (FCAC Programme) to GIROQ (Groupe Interuniversitaire de Recherches Oceanographiques du Qukbec). References d’Anglejan, B. 1981 On the advection of turbidity in the Saint Lawrence estuary. Estuaries d’Anglejan, B. & Ingram, R. G. 1976 Time-depth variations in tidal flux of suspended Lawrence estuary. Estuarine and Coastal Marine Science 4,401-416. d’Anglejan, B. & Ingram, R. G. 1982 Investigation of natural sediment and dredge spoil vicinity of Gros Cacouna harbour (St Lawrence Estuary). Department of Supply & Unpublished manuscript. 50 pp. d’Anglejan, B., Ingram, R. G. & Savard, J. I’. 1981 Suspended sediment exchanges Lawrence estuary and a coastal embayment. Marine Geology 40,85-100.
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the St
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& R. G. Ingrum
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