David A. Horn1, Bernard Laval1, Jörg Imberger1
and Angelos N. Findikakis2
1Centre for Water Research, The University of Western Australia, Crawley
W.A., Australia.
2Bechtel Systems & Infrastructre Inc., San Francisco, California, U.S.A.
Correspondence: Dr David Horn, Centre for Water Research, The University of Western Australia, 35 Stirling Highway, Crawley W.A., Australia, 6009
Telephone: +61-8-93801684; Fax: +61-8-93801015; E-mail: horn@cwr.uwa.edu.au.
Abstract: A major field program was carried out in Lake Maracaibo as part of a study for the environmental remediation of the system. The field program was designed to provide an understanding of the major physical processes that affect the water quality of Lake Maracaibo and to provide sufficient data for the calibration and validation of numerical models. Fieldwork was carried out during two separate field campaigns in 1998-99, timed to correspond with the wet and dry seasons of the region. Measurements included: current velocities; conductivity and temperature time-series at fixed depths; fine- and micro-scale conductivity and temperature profiles; wind speed and direction; and water level. The field data collected during the field program confirmed the following hydrodynamic features: (a) a predominantly semi-diurnal tide throughout Maracaibo Strait and Tablazo Bay with an anti-node in Tablazo Bay, (b) a tidally modulated saline underflow extending well into Maracaibo Strait, and (c) a dome-shaped, saline hypolimnion and a large-scale anti-clockwise circulation in Lake Maracaibo. The data also indicated a possible internal hydraulic control near Zapara Mouth. The field program did not observe any saline underflow south of the strait and into the lake.
Keywords: Lake Maracaibo, field study, exchange flow, lake hydrodynamics
This is one of a series of five papers describing a recent study on the environmental remediation of Lake Maracaibo, Venezuela (for an overview of the study and a description of the Lake Maracaibo system see Findikakis et al., 2001). This paper describes the field program to collect hydrodynamic data in the Maracaibo region and presents results from one of two field campaigns – the 1998 wet season campaign. The hydrodynamic field program presented below was complemented by a simultaneous intensive water quality field sampling program over 1998-99.
Considerable historical data exists on the flow and stratification in these various regions, but these data merely suggest a set of dynamics. The data were all collected for a diverse set of programs over nearly a 40-year period and as such form a valuable resource, especially for long term trends of salinity (eg. Redfield, 1958, 1961; Parra Pardi, 1980; Molines, et al. 1989; Bautista et al., 1997). However, these data are not coherent enough, nor were they collected with particular hypotheses in mind, to allow detailed dynamical interpretations in any of the key hydrodynamic processes that determine the salinity distribution in Lake Maracaibo or to validate the numerical models of these processes.
The field program was thus designed to serve several purposes: (a) to provide an understanding of the major processes that affect the salinization, stratification and renewal of hypolimnetic water of Lake Maracaibo, (b) to provide sufficient data for the calibration and validation of numerical models, and (c) to provide the modeling groups sufficient data on boundary and initial conditions. Fieldwork was carried out during two separate field campaigns in the Maracaibo region. The two campaigns where nominally timed to correspond with the wet and dry seasons of the region. In this paper we present some results from the wet season campaign carried out in October-December 1998.
The field program focussed on the following key dynamical regimes:
Transfer across the mouth between the Gulf of Venezuela and The Bay of Tablazo
Formation of stratification in The Bay of Tablazo
Hydraulic controls in the Strait of Maracaibo and the influence of the contractions and sills
Plunging of the salt water underflow into Lake Maracaibo
Formation, maintenance and stability of the salt hypolimnion in Lake Maracaibo
The present field work confirmed the findings of earlier studies that tides within the Maracaibo System are predominantly semi-diurnal, and of decreasing amplitude from Tablazo Bay to Lake Maracaibo (Redfield, 1961; Molines et al, 1989). A semi-diurnal anti-node in Tablazo Bay was identified by tidal amplitudes being in phase at the north and south ends of Tablazo Bay, while the tidal currents converge and diverge on Tablazo Bay (Figure 2). Maximum flow velocities have an amplitude of about 1.5 m/s at Zapara and Cañonera Mouths, and about 1m/s in the Maracaibo Strait. Mean, depth-averaged northward velocities are about 0.10 m/s at Zapara and Cañonera Mouths, and about 0.13 m/s at the Maracaibo Strait entrance to Tablazo Bay.
A brackish seaward flowing surface layer, was observed to exit Zapara Mouth and enter the Gulf of Venezuela as a shallow, laterally spreading, tidally modulated, buoyant plume. F-probe salinity data (Figure 3, the north end of Zapara Mouth is at ~11.025°N), show the outflow of Maracaibo water into the Gulf of Venezuela during diffluence, when tidal currents diverge from Tablazo Bay, (panel b) and subsequently during confluence of currents to Tablazo Bay (panels c-e). During the diffluent phase the outflow water is seen to extend as a thin (less than 1 m deep) surface layer from the north end of the eastward breakwater at Zapara Mouth, 25 km into the Gulf. During the confluent phase on the following day, the northern extent of the surface plume was found to be ~10 km further south. The interface between the plume and deeper water was less defined and fresher than previously.
The interface between the fresher surface water and deeper water slopes down to the south from the north end of the breakwater at Zapara Mouth to the south end of Tablazo Bay. This tilting of the pycnocline suggests an internal hydraulic control by the contraction at Zapara Mouth. The stratification in the shipping channel changes between the ebb and flood phases of the tide indicating that the tides likely have a significant influence on the net exchange through the Zapara Mouth.
East west F-probe transects show that away from the shipping channel, Tablazo Bay is vertically well mixed. There was horizontal salinity gradient across Tablazo Bay, with higher salinity in the vicinity of Cañonera Mouth. Semi-diurnal inflow phase lags between the three entrances to Tablazo Bay (Zapara and Cañonera Mouths to the north and Maracaibo Strait to the south) could generate a secondary circulation in Tablazo Bay. Measurements on an incoming tide showed that the water entering Boca Cañonera was unstratified with a salinity of 26 psu. This high salinity combined with a maximum channel depth of 6 m and maximum measured tidal currents of 1.4 m/s, indicates that Boca Cañonera may allow a significant quantity of salt to enter Tablazo Bay.
F-probe data show that hydraulic control is more likely to occur in the Zapara Mouth region than in Maracaibo Strait. Observations show that a salt wedge penetrates into the Strait of Maracaibo. Considerable longitudinal freshening of the salt wedge was observed in the Maracaibo Strait. The near-bottom salinity at the north end of the strait reached a maximum of ~22 psu, while it was always observed to decrease to ~4 psu within 30 km of the north end of the strait.
The salt wedge was observed to propagate north south along the shipping channel at tidal and sub-tidal frequencies. The southernmost extent during the wet season campaign was just south of the Maracaibo City (ie. not far enough to plunge into the Lake). The semi-diurnal tidal excursion of the salt wedge was observed to be ~3.5 km. At sub-tidal frequencies, the salt wedge was seen to have moved northward ~15 km in eight days (November 6-14), and then move southward a similar distance in the subsequent four days.
This displacement does not appear to be related to the spring neap cycle of the tides. The large salt wedge excursion at sub-tidal frequencies is important because it suggests that there is a physical process, or combination of processes, not necessarily related to the tides, that controls the location of the salt wedge in the strait. A possible mechanism is a longitudinal barotropic pressure gradient between the Gulf of Venezuela and Lake Maracaibo. Rainfall in the catchment does not seem to have a large effect at time-scales of days or weeks. However, the wind set-up in the Gulf of Venezuela and in Lake Maracaibo could induce a significant longitudinal barotropic pressure gradient at these time scales.
Three weeks of low-passed salinity data in Maracaibo Strait in November 1998 show sub-tidal variations of ~14 psu (Figure 4, top panel). From shortly after the beginning of the signal to Julian Day 316 there is a decrease of salinity of ~14 psu, with a corresponding water level decrease of ~25 cm at Zapara Mouth. Thereafter, the salinity at MS10 slowly increases as the water level in the lake decreases, and the water level at Zapara varies little. This clearly demonstrates a relation between the southward intrusion of the salt wedge and barotropic pressure gradient between the Gulf of Venezuela and Maracaibo Strait. Relatively high water in the Gulf of Venezuela will tend to push the salt wedge southward which leads to a corresponding increase in bottom salinity in the Maracaibo Strait.
During neither the wet season nor the dry season field programs was the salt wedge observed to travel far enough to plunge into the lake. Although such saline underflow events were not expected during the wet season, the absence of such events during the dry season field program supports the hypothesis that these events are sporadic and dependent on a number of coincident conditions such as favourable winds, tides and barotropic pressure gradients.
Figure 5 presents ADCP data from the three lake stations, LM10, LM20 and LM30. The data confirm the existence of a counter-clockwise circulation of 20-30 cm/s. Velocity at the southern end of the lake, station LM30, is slower than in the north. However, the flow is still in a direction consistent with a general counter-clockwise circulation.
The downward curvature of isopycnals in the main basin of Lake Maracaibo with consequent higher surface salinity near the lake center, first documented by Redfield (1958), is evident in Figure 6 presenting the F-probe salinity data. The internal Rossby radius of deformation based on the measured stratification was about 12.5 km, indicating that rotation is important in the internal dynamics, which supports the hypothesis that the doming of the hypolimnion is predominantly due to a geostrophic balance with the cyclonic circulation (Redfield, 1958; Parra-Pardi, 1983; de Bautista et al, 1997).
Figure 7 presents data from the bottom-mounted CT sensor at station LM30. The record shows a salinity of ~12.5 psu with two freshening events where the salinity decreased to ~8.2 psu on 1998 November 24 (Julian day 327) and ~8.2 psu on 1998 November 28 (Julian day 331). These events are likely due to the passage of internal waves propagating on the halocline. Although there has been no previous documentation of internal waves or seiches in Lake Maracaibo linear two-layer theory predicts that the measured fluctuations could be due to internal Kelvin waves.
The field data collected during the 1998/99 field program confirmed the following hydrodynamic features of the Maracaibo system: (a) a predominantly semi-diurnal tide throughout the Maracaibo Strait, and Tablazo Bay with an apparent anti-node in Tablazo Bay, (b) a tidally modulated saline underflow extending well into Maracaibo Strait, and (c) a dome-shaped, saline hypolimnion and a large-scale anti-clockwise circulation in Lake Maracaibo. The data also indicates a possible internal hydraulic control near Zapara Mouth. The field program did not observe any saline underflow south of the strait and into the lake during either the wet or dry season.
Acknowledgements
The work presented in this paper was part of the Integral Study for the Environmental Restoration of Lake Maracaibo, conducted for PDVSA by Bechtel International. Luis Delgado and Nelson Corrie managed the study for PDVSA. We acknowledge the contributions made to the project by all members of the project team, including the PVDSA Technical Committee. The authors are grateful to the employees of Incostas who provided field logistics and technical support. We especially thank Juan Font, Juan V. Font, and Roberto Font of Incostas who managed the field deployment. This paper is CWR reference ED 1146.1 DH.
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Fig.1 Depth averaged tidal water level and velocity at ADCP stations M10, M20, and MS10
Fig.3 A comparison of low-frequency (2 day cut-off) T, S, and water level data; 1998. Top panel: S and T from bottom-mounted CT sensor at station MS10. Bottom panel: Measured water level data at Zapara tide station, and MS35 and MS20 ADCP stations. Julian day 300 represents 27/10/98 and 335 represents 1/12/98.
Fig.4 Time averaged velocities from three bottom-mounted ADCPs within Lake Mararcaibo from the wet season deployment, 21/11/98 to 26/11/98. Each arrow at a particular station represents a depth bin. The deeper bins have slower velocities.
Fig.5 F-Probe salinity data in Lake Maracaibo; a) TE 24/11/98; b) TF 24/11/98; c) TE 25/11/98; d) TF 25/11/98.
Fig.6 Salinity and temperature time series from bottom mounted CT sensor at station LM30, Lake Maracaibo South. Julian day 326 represents 22/11/98 and 333 represents 29/11/98.