Conclusion
8
CONCLUSION
During the late 1980’s, early 1990’s Tomales Bay (California) was studied
through the “Biogeochemical Reactions In Estuaries” (BRIE) project of the “Land
Margins Ecosystems Research” (LMER) programme of the U.S. National Science
Foundation.
BRIE included research on terrigenous inputs, air-sea exchange,
sediment-water exchange, ocean-bay exchange and internal cycling in estuaries.
Tomales Bay was selected as the study site as it was perceived as being
hydrographically and morphologically simple. There was well-documented freshwater inflow from two primary sources along the estuary and the water quality was
considered good as the land-use patterns are stable and the watershed has not been
urbanised.
Tomales Bay estuary is situated along the west coast of the U.S.A. at 38° 13' 50"
N, 122° 58' 30" W. The estuary runs along the San Andreas fault, extending southeast
from the southern end of the open embayment of Bodega Bay. Data from the estuary
were collected from 1987 to 1995. While the biogeochemical cycling within the
estuary has been well documented in previous publications, water circulation and
ocean-bay exchange has received limited previous attention.
This thesis is focused on the water circulation that drives the observed water
column structure in Tomales Bay, describing its variability, both temporal and spatial.
In doing this, a comprehensive description of the estuarine circulation has been
provided, including tidal, event scale, seasonal and inter-annual variability.
An
important feature of this region (and many mid-latitude west-coast regions) is wind
driven coastal upwelling, affecting both the adjacent coastline and the estuary. Due to
the offshore surface transport driven by equatorward winds, cold, nutrient-rich water
is upwelled along the open coast. Northern California experiences spring-summer
upwelling with strong and persistent winds resulting in the inundation of the coast and
bays with cold upwelled water (e.g., Bodega Bay). However, within inner Tomales
Bay the water remains warm and a strong thermal gradient is observed between the
outer and inner estuary, with cold water advected into the outer estuary tidally.
Significant subtidal intrusions were observed a few times each summer, with colder
waters penetrating halfway into the mid/inner regions. These intrusions are expected
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Conclusion
to be key events in nutrient delivery to the summer estuary. The Delft3D model was
used to simulate this intrusion effect and then test the response of the estuary, during
these events, to different parameters i.e. tide, solar heating, wind, depth and ocean
temperature.
In order to address the water circulation, the first research question was proposed:
What controls the observed variability in circulation in Tomales Bay and over what
temporal and spatial scales?
To achieve the above research aim, the following were examined:
 tidal circulation within Tomales Bay and
 circulation ‘patterns’, due to fresh-water inflow, solar heating and evaporation and
wind driven upwelling, and their spatial and temporal variability.
Morphologically, Tomales Bay is a long and narrow estuary with an even
narrower mouth that restricts exchange with the open ocean.
experiences a mixed tide.
Tomales Bay
The estuary is situated in a microtidal region but is
subjected to mesotidal conditions over some spring tides.
The tide drives the
circulation in the outer estuary but tidal circulation is weak in the middle and inner
regions. Analysis of the data shows that although the tidal influence is dominant in
the outer region, its effect might be modulated by seasonal signals. Maximum tidal
velocities are up to 2 ms-1 at the mouth but are weaker over neap tides. Within the
first 6 km these velocities decrease by  90 %, where the narrow channel opens into a
wider ‘bay’ area. The effect of the tide in the outer region is to completely vertically
mix the water such that stratification seldom persists for longer than 6 - 12 hours.
The relative importance of other mechanisms increases with increasing distance
from the mouth. Winter fresh-water inflow enters Tomales Bay from two sources,
primarily from Lagunitas Creek at the head of the estuary but also from Walker Creek
 5 km from the mouth. The fresh-water flows seawards as a surface layer, stratifying
the water column. The strength and persistence of the stratification depends on the
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Conclusion
intensity and duration of the fresh-water inflow. The estuary rapidly (< 1 day) returns
to initial conditions after low inflow events. After continuous or high inflow events,
the fresh-water signal is retained in the sustained stratification of the middle and inner
regions. The outer region seldom stratifies for longer than 12 – 24 hours. The effect
of the fresh-water inflow, other than stratifying the water column, is the development
of a longitudinal salinity gradient. This increases the flushing of the estuarine water
and increases exchange between the middle and outer estuarine regions. Mixing
exchange between the outer region and middle and inner regions increases as the
stream inflow increases due to the increased longitudinal density gradient,
stratification and associated estuarine circulation.
Estuarine conditions change dramatically from the cool, wet winter conditions
to the warm, dry summer months.
Increased solar radiation increases the water
temperature in spring and summer, creating a longitudinal temperature gradient within
the estuary.
Estuarine salinity also increases during spring and summer, due to
decreased inflow and increased evaporation. The increase in salinity of the inner
estuary results in a decrease in the longitudinal salinity gradient and consequently
density gradient. The salinity of the inner estuary reaches marine values by early
summer but continues to rise throughout summer and the estuary is typically
hypersaline in late summer. However, temperature compensation of the density signal
occurs and a temperature maximum matches the salinity maximum and the density of
the estuary water rarely exceeds that of the ocean. Longitudinal exchange within the
estuary is at a minimum during this period as the density gradient is near zero, except
near the mouth where the thermal gradient is strong. Flushing times are  3 months.
This changes as estuarine temperatures decrease due to surface cooling in autumn.
With autumn cooling, maximum density conditions within the inner region occur that
may lead to inverse conditions (inverse conditions do not develop every season) and
inverse circulation may occur over short periods of time.
Tomales Bay is situated along a coastline dominated by wind-driven upwelling
during summer. The upwelled water is moved into the estuary with the flood tide and
thereafter may intrude landwards as a bottom density current. To examine the effect
of coastal upwelling on Tomales Bay the second research question was proposed:
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Conclusion
How does wind-driven coastal upwelling influence the circulation and stratification
within the estuary?
The above question was examined by determining:
 what processes control cold, upwelled ocean-water intrusions into Tomales Bay.
Spring and summer winds are typically upwelling favourable (northwesterly)
and consequently cold water is brought to the surface of the water column and fills
Bodega Bay. The cold, upwelled water enters Tomales Bay with the flood tide and
may subduct beneath the warmer, less dense estuarine water at the landward extent of
the tidal excursion - on occasion, persisting through the subsequent ebb tide. A
density current may then form and the cold water intrudes further into the estuary as a
thermally stratified bottom layer.
Several of these subtidal cold-water intrusions
were observed in Tomales Bay during 1992-93.
Density current intrusions into
Tomales Bay are different to those commonly studied in estuaries. Firstly, the density
current is due to temperature structure while the salinity of the estuarine water is very
close to that of the ocean due to low inflow and high evaporation during summer. The
longitudinal salinity gradient is very weak. In contrast, there is a strong longitudinal
temperature gradient in the outer estuary, between the cold ocean water and the
warmer estuarine water. This drives the intrusions. Secondly, the plunging of the
cold, ocean water does not occur near the mouth but at the landward extent of the tidal
excursion, which is  6 km from the mouth where the estuarine water is no longer
pushed back by the inflow but the inflowing water rather plunges beneath the less
dense estuarine water. This is because, at a distance of 6 km from the mouth the
cross-sectional area of the channel increases markedly, further reducing the tidal
inflow velocities. These intrusions of seawater persist for variable lengths of time and
may greatly affect the circulation and stratification of the estuary during spring and
summer.
In order to determine the sensitivity or dependence of these intrusions to a
range of physical parameters including tide, solar heating, wind, depth and ocean
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Conclusion
water temperature, the Delft3D model was applied to Tomales Bay to answer the
following questions:
 Can these cold, ocean water intrusions into Tomales Bay be adequately modelled
using the Delft3D model?
 Can the model show the influence of input parameters: tide, solar heating, wind,
stream inflow, depth and ocean temperature on the estuarine circulation and
stratification during these cold, ocean water intrusion events?
The Delft3D model successfully reproduced the intrusion events and showed
that the persistence and removal of the density currents are affected by other physical
parameters, to varying degrees. The model showed that there are two pre-requisites
for a density current intrusion event. Firstly, a tidal excursion of  6 km is required,
indicating that intrusion events will not occur where the tidal excursion is < 6 km i.e.
typical neap tidal phases (the tidal excursion sometimes reaches 6 km over neap tide
but this is not common).
The formation of a density current is thus typically
associated with spring tide, or either side of spring tide conditions. Secondly, cold
ocean temperatures at the plunging area ( 6 km) are required. The formation of a
density current is thus associated with coastal wind-driven upwelling as the cold,
upwelled water is required to develop a longitudinal density gradient that will drive a
density current. This limits the intrusions to occurring during spring and summer
when northwesterly winds drive the offshore upwelling.
The sensitivity analysis showed that solar heating, wind, fresh-water inflow
and depth all had varying degrees of effect on the formation, persistence and removal
of the density current signal in the middle region of the estuary.
Increased solar heating strengthened the vertical stratification in the middle region,
reducing vertical mixing. The enhanced stratification and reduced vertical mixing
resulted in strengthening of the density current, indicating that intrusion events are
likely to be stronger during summer, when the seasonal insolation is at its peak. The
absence of solar heating resulted in cooler estuarine temperatures and thus a
weakening of the longitudinal density gradient required to drive the density current,
indicating that density currents are likely to be weaker in spring.
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Conclusion
Winds have a significant impact on the intrusions in that a sufficient wind speed
vertically mixes the water column, breaking down the vertical stratification. With
very weak or no vertical stratification, the wind is able to completely mix the water
column and any cold water already present or intruding into the middle estuary will be
mixed throughout the water column, resulting in any density current intrusion being
destroyed. The middle and inner regions are shallow and a continuous wind speed of
> 6ms-1 was observed to break down a typical density current through vertical mixing
of the water column, even though the vertical mixing was not complete. A wind
speed of > 15 ms-1 was required to completely mix the water column. Winds also
affect the timing of the intrusions as a density current can only develop in the absence
of wind mixing i.e. under calm or no wind conditions.
Fresh-water inflow had no impact on the density current intrusions. The simulated
fresh-water inflow may also have been too low to have an effect and it is likely that a
summer release of flood conditions would influence the water column structure and
thus influence the density current intrusion.
Even though the estuary is considered to be in a steady state with respect to depth,
further infilling of the estuary, for example, through sedimentation from riverine
input, would inhibit the formation of density currents. A decrease in depth results in a
decrease in the cross-sectional area through which the tidal volume must enter or
leave the estuary and thus an increase in tidal velocities occur. The cold water is then
either vertically mixed throughout the water column or flushed out of the estuary with
the increased ebb tide velocities and no density current forms.
Any mechanism that results in the inhibition of density current formation i.e. wind
mixing, reduced tidal range, warmer ocean temperatures is likely to be detrimental to
the currently “pristine” status of the system as these summer intrusions are the only
source of ‘new’ water that enters the estuary during the long summer residence
periods.
Not only tidal and event scale signals are important to the circulation within
Tomales Bay and exchange with the ocean. Larger scale forces, both seasonal and
inter-annual, also play a role in driving the circulation within Tomales Bay. The
seasonal signal is governed by the Mediterranean-climate that Tomales Bay
experiences i.e. warm, dry summers and cool, wet winters. An increase in solar
heating (and evaporation) through spring and summer causes an increase in the
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Conclusion
salinity, temperature and density of the estuary. Combined with limited (very low)
fresh-water inflow, the estuary is typically warm, with a longitudinal temperature
gradient evident from the mouth to the head, salty and vertically mixed. Flushing of
estuarine water is consequently of the order of months. A decrease in solar heating
through autumn and winter leads to a decrease in temperature throughout the estuary
and eliminates the spring, summer longitudinal temperature gradient. Increased freshwater inflow during this time decreases the salinity and density of the estuary, causing
longitudinal salinity and density gradients to develop. The fresh-water inflow also
causes vertical stratification of the water column as the fresh-water flows seawards as
a less dense layer above the denser ambient estuarine water. Flushing of the estuary
increases by approximately an order of magnitude during this period. The seasonal
effects are more pronounced at the head of the estuary and diminished towards the
mouth.
Inter-annual variability is closely linked to the climate of the region, such that
any climate changes would alter the current state of the system.
Inter-annual
fluctuations in water temperature and salinity were observed although there were no
obvious increasing or decreasing trends. These fluctuations are due to variability in
the climate and resulted in a strengthening or weakening of the expression of the
seasonal signal. For example, an unusually warm summer may result in increased
hypersalinity, increased inverse conditions and increased inverse circulation, thus
increasing exchange within the estuary. Increased estuarine temperatures may also
result in strengthened density current intrusions, discussed above.
Inter-annual
variability was also observed in the intensity of upwelling. This affects both the
timing and intensity of density current intrusions. A change in the timing of freshwater inflow, for example late winter rainfall, may result in extended autumn
hypersaline (and possibly inverse conditions), lengthening the summer residence time,
where early rainfall will have the opposite effect. The most likely impacts on the
system would be the extent of anthropogenic control over the system, specifically, the
controlled release of fresh-water into the system during the dry summer periods and
the sediment loading that shallows the system. Controlled dam release during the dry
period will increase the flushing during the dry period but also alter the typical
circulation during that time.
The current circulation will also be altered if a
shallowing and shortening of the estuarine basin occurs due to sediment infilling. The
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Conclusion
potential for this, however, is small as Tomales Bay land use patterns are stable and
further development is unlikely through government control of land use planning and
zoning ordinances and the protection of the estuary under the Gulf of Farallones
National Marine Sanctuary (USA).
The primary research focus of this thesis was twofold. Firstly, the analyses
resolved the observed variability with respect to Tomales Bay circulation by
examining the tidal circulation, water exchange (between the estuary and ocean and
within the estuary) and by describing the circulation patterns governed by tidal
effects, wind effects, solar heating and evaporation and fresh-water inflow. The
variability in the circulation was determined on both a spatial scale, where the
observed circulation differed along the longitudinal axis of the estuary from the mouth
to the head, and on a temporal scale, where the hourly (tidal), daily (event), monthly
(seasonal) and yearly (inter-annual) variability was explored.
Secondly, the analyses determined the effect of wind-driven coastal upwelling on the
estuary, through both observations and modelling of the conditions that result in the
formation of density currents of cold, upwelled water in the estuary, what effect the
density current has on the water column structure and what effect other physical
parameters have on the density currents.
This thesis thus contributes to the ocean-bay exchange section of the
LMER/BRIE project. This thesis also positively contributes to the applicability of 3D
modelling as a tool for understanding estuarine processes and highlights the success
of the model used in resolving high resolution / fine scale processes in systems
experiencing vertical and horizontal stratification.
The benefit of this is that
predictive capabilities for the management of these systems increases with the
development of 3D modelling where it is no longer financially viable to collect
extensive data sets from each unique estuary but targeted data collection will suffice.
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