Journal of Coastal Research
809-824
Fort Lauderdale, Florida
Fall 1994
Hydrologic Regimes of Tidal Channel-Salt Marshes
Flow Systems, Fourleague Bay,
Louisiana, U.S.A.
Flora
c. Wangt,
Walter B. Sikora'] and Menglou Wangt
:j:Coastal Ecology Institute
Louisiana State University
Baton Rouge, LA 70803-7503,
U.S.A.
tDepartment of Oceanography
and Coastal Sciences
Louisiana State University
Baton Rouge, LA 70803-7503,
U.S.A.
ABSTRACT
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WANG, F.C.j SIKORA, W.B., and WANG, M., 1994. Hydrologic regimes of tidal channel-salt marshes
flow systems, Fourleague Bay, Louisiana, U.S.A. Journal of Coastal Research, 10(4),809-824. Fort Lauderdale (Florida), ISSN 0749-0208.
A hydrologic study of a meandering channel and its adjacent marshes on the south-central Louisiana
coast (5 km from the Gulf of Mexico) has revealed a flow pattern different from that expected for a
natural tidal channel (bayou). A network of constructed channels, ranging in size from the small trapper's
channels 1.5 meter wide (called trainasses) to 30 meter wide petroleum-well access canals, has altered
the hydrology of the natural bayou and the tidal regime of the adjacent marshes along the upper reaches
of the bayou. Hourly water-level data, recorded at a marsh site in the upper reaches of the bayou, show
that the pattern of marsh inundation is characterized by sporadic flooding interspersed by long draining
periods. The purpose of this study is to interpret the different flow circulation patterns observed in the
bayou during two extensive field trips. These trips were in September and October 1991, during which
strong continuous north and east winds prevailed. Those data are augmented by additional data sets
taken in May and August 1992 and by other field observations showing the effects of man-made canals
which have induced hydrologic changes in the area. These results indicate that the surface flow patterns
in the upper reaches of the bayou have been decoupled from the lower reaches of the bayou because of
the flow interception by man-made canals. The upper reaches of the bayou have then been filled in with
sediments because of the reduction in flow velocity.
ADDITIONAL INDEX WORDS:
Wetland hydrology, surface fiow, tidal channel, salt marshes. Four-
league Bay, Louisiana.
INTRODUCTION
The Louisiana Gulf coast is rapidly subsiding
with subsequent wetland erosion and loss (GAGLIANO et al., 1981). Many factors contribute to and
influence the loss of coastal wetlands. Among these
are the natural processes by which wetlands are
being converted into open water bodies by storms
and hurricanes (TURNER, 1990), by the rise of eustatic sea level (TURNER, 1991), and by the natural
compaction and subsidence of deltaic sediments
(RUSSELL, 1978). Other factors are related to human activities. Localized coastal subsidence is
caused by extraction of oil and natural gas from
subsurface reservoirs (BAUMANN et al., 1984;
SUHAYDA et al., 1993). Large areas of coastal wetlands are also being converted into open water,
by the digging of borrow pits for the construction
of flood protection levees and the dredging of na v-
93027 received 17 July 1993; accepted in revision 20 February 1994.
I,.
igation channels and petroleum-well access canals
(WANG, 1988).
Many coastal basins in south Louisiana are
depressional watersheds. They are primarily river-bay-bayou-lake systems characterized by large
areas of marshes and swamps with slow natural
drainage through meandering bayous (WANG,
1987). These coastal marshes are inundated by
regular diurnal and semi-diurnal tides and irregular tropical and winter storms (CHILDERS and
DAY, 1988). Marsh habitats are dependent upon
the ability to maintain their elevations within local tidal ranges by the sedimentation processes of
vertical accretion to combat the combined effects
of subsidence and sea level rise (DELAUNE et al.,
1990).
This paper examines the flow patterns in a decoupled system which has resulted from the construction of a series of man-made canals connected to a natural bayou and the consequence
of such alterations on adjacent marshes.
810
Wang, Sikora and Wang
........,
LITTLE IILUE HAMMOCK IIA YOU
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Figure 1. Location map of Old Oyster Bayou study area and sampling stations adjacent to Fourleague Bay in south-central Louisiana.
DESCRIPT IO N OF STUDY AREA
The Atchafalaya coastal basin was described as
a sediment-rich and riverine-dominated basin with
a large freshwater inflow (VAN HEERDEN and
ROBERTS, 1978, 1980). Atchafalaya Bay, receiving
a relatively high volume of river discharge and
sediment load from the Mississippi River, has built
a slowly expanding delta at its mouth on the southcentral Louisiana coast since the late 1970's
(ROBERTS et al., 1980). Fourleague Bay, a large
(100 km") and a shallow (2 m average depth) bay,
opens to the eastern side of Atchafalaya Bay via
a 2.5 km wide passage. The lower portion of the
bay empties into the Gulf of Mexico through a
250 meter wide and 8 meter deep tidal inlet, Oyster Bayou (Figure 1).
Old Oyster Bayou, a relatively small meandering channel, opens into Fourleague Bay at its
mouth and runs from west to east for the lower
two-thirds of its length and from north to south
for the upper one-third before opening into Old
Oyster Bayou Lake (Figure 1). This system was
chosen as a study site within this coastal basin by
the U.S. Geological Survey multi-year, multi-discipline, and large-scale wetland project with which
the present study was coordinated. The surrounding area is flanked by apparently healthy salt and
brackish marshes.
Old Oyster Bayou is intersected by a series of
man-made canals. These canals were constructed
during the period from 1960 to 1981. The largest
and oldest petroleum access canal, designated as
"Camp Canal" in this study, runs from west to
east; a second canal runs north to Old Oyster
Bayou; a third canal runs south to Old Oyster
Bayou Lake and beyond (Figure 1). These canals
intersect at the point of the original Camp Canal
well site (abandoned dry hole) and are treated as
one canal with a north fork and a south fork.
Because of the relative remoteness of the area,
boat traffic in the canals and bayou is light. It
Journal of Coastal Research, Vol. 10, No.4, 1994
Hydrologic Regimes in Coastal Marshes
mainly consists of a few crab fishermen tending
over one hundred crab traps daily.
In addition, two narrow channels are connected
to Old Oyster Bayou at the bend where the bayou
turns from west-east to north-south (Figure 1).
These channels were developed from trainasses,
narrow canals about 1.5 meters wide, dug by fur
trappers using either hand implements or power
excavators, for the purpose of maneuvering small
lightweight boats called pirogues (DAVIS, 1976).
These trainasses allowed the trappers access to
their trap lines. If left undammed, these pirogue
trails often developed into large watercourses.
DAVIS (1976) recounted an instance near Barataria Bay (about 100 km east of Fourleague Bay)
in which an individual trapper dug a 1 meter wide
trainasse using a pirogue paddle which, in the
span of fifty years, became a 60 meter wide and
3 meter deep canal.
The tidal regime at Old Oyster Bayou is a mixeddiurnal tide (MARMER, 1954). It is the resultant
tide from a combination of a diurnal and semidiurnal tidal signals of similar tidal ranges. The
mixed-diurnal tide has two highs and two lows
that are unequal. The greatest tidal inequality is
exhibited in the low tides (MARMER, 1954), with
two lowtides, a high-low tide and a much lower
low-low tide for most days of the month.
METHODS AND MATERIALS
Channel Flow Measurements
Originally, four stations (8-1,8-2, S-3, and 8-4,
Figure 1) along Old Oyster Bayou were chosen for
intensive field measurements. Two field trips were
made to the study area in September and October
1991. Twelve people participated in the hourly
sampling of current velocities, water temperature
and salinity, and suspended sediment concentration at four stations. The schedules were designed
such that two teams, consisting of two persons
each in a small boat, shuttled between Station S-l
to Station 8-2 and Station S-3 to Station 8-4 for
an 8- hour shift.
Prior to each field trip, tidal heights at Eugene
Island (24 km west of our sampling stations) were
predicted according to the methods described by
the National Oceanographic and Atmospheric
Administration (NOAA, 1991). Eugene Island is
designated as a "subordinate station" where short
periods of tidal records exist. Tidal predictions
are estimated from Galveston, Texas, a "reference
station" where the longer periods of tidal records
exist. Observed hourly tidal stages for Atchafa-
811
laya Bay near Eugene Island were obtained from
the U.S. Army Corps of Engineers, New Orleans
District. Temporary staff gauges were set near
Stations 8-1 and S-4, and relative water levels
were recorded hourly.
Hourly measurements of flow velocity at each
station were taken simultaneously at 0.5 m increments from 20 em below the surface to near the
channel bottom at a depth which varied from 2.0
to 2.5 meters. Current velocities were measured
using two Montedoro Whitney PVM-2A current
meters. Hourly water samples were collected concurrently at mid-depth with Lamotte water samplers (Model JT -1). The water samples were kept
in clean bottles and stored in ice coolers for analysis of suspended sediment concentrations in the
laboratory.
Channel Cross Sections
Additional monitoring stations were established in the man-made watercourses to aid in the
determination of the water-flow relationships of
these channels to the natural bayou. Three Camp
Canal stations, Station S-5 in the western portion
at the location of a barge-mounted camp, 8-6 in
the north fork, and 8-7 in the south fork, were
selected. Stations 8-6 and 8-7 were approximately
equidistant from the canal bifurcation. Two trainasse stations, Station T -1 near bayou Station S-3,
and T-2 closer to bayou Station S-4, were also
established (Figure I).
Channel cross-sectional profiles were measured
using a portable fathometer (Raytheon Fathometer Model DE-719) from a small boat. A rope,
marked and numbered at 4-meter intervals, was
stretched taut across the bayou. During the measurement' the fathometer operator pressed a button at each 4-meter mark and the stylus drew a
vertical mark across the depth tracing on the chart.
The traced profiles were then reconstructed to
scale on graph paper for true cross-sectional dimension (Figure 2). The cross-sectional areas were
calculated directly from the reconstructed profiles
using a digitizer (Numonics Model 2400 Digitablet). The channel cross sections at Stations T-I
and T-2 were measured manually using a horizontal rope marked at 2-meter intervals; the depths
were read directly from a tape measure lead line.
At Station S-2, the profile was also measured
manually to compare with the results from recording fathometer. Both methods agreed closely.
For the computation of water flux, the crosssectional area was divided into a number of 0.5
Journal of Coastal Research, Vol. 10, No.4, 1994
Wang, Sikora and Wang
812
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Figure 2. Channel cross sections scanned by Raytheon Fathometer and measured manually (a) natural channels, and (b) manmade canals.
Journal of Coastal Research, Vol. 10, No.4, 1994
Hydrologic Regimes in Coastal Marshes
m layers. The total water discharge was computed
as the sum of the water flux in each layer. Average
flow velocity was calculated by dividing total discharge by the respective cross-sectional area. Positive values indicate flood tides, whereas negative
values indicate ebb tides.
Wind Data
Wind speed and direction, recorded in Fourleague Bay near Blue Hammock Bayou about 4
km north of Old Oyster Bayou every three hours,
were obtained from the Coastal Studies Institute,
Louisiana State University, beginning September
1991. Long-term wind data, recorded hourly on
Bayou Lafourche at the Golden Meadow floodgate, approximately 80 km east of the study area,
were obtained from the U.S. Army Corps of Engineers, New Orleans District. These data provided information used to examine the response
of water levels and flow circulations to surface
wind forcing in this shallow estuary.
Water Levels
To understand short-term marsh sedimentation processes and the response of marsh surface
flow depth to tidal levels and prevailing winds,
continuous hourly measurement of water levels
in-situ is essential. These records provide an estimate of the probability of marsh inundation
subject to various weather conditions on a given
day, critical for the investigation of the marsh
surface water depth and patterns of surface flows.
Two Fisher & Porter recording, punched-tape
water-level gauges were installed to delineate the
patterns of marsh inundation (WANG et al., 1992).
Gauge A was installed at Old Oyster Bayou marsh
site near Station 8-4, about 22.5 meters in the
marsh from the west bank of Old Oyster Bayou
in September, 1991. Gauge B was installed in open
water near Station 8-1, about 10 meters from the
south bank of the bayou in October, 1991. Both
gauges were set to record water levels at one-hour
intervals. Beginning in November 1991, hourly
water levels were recorded simultaneously at
Gauge A and Gauge B. Unfortunately, these established gauges were seriously damaged by Hurricane Andrew in August, 1992. Gauge A was repaired and reinstalled, but Gauge B was swept
away and destroyed.
Sediment Concentration Laboratory Analysis
Water samples were analyzed in the laboratory
for total suspended sediment concentrations by a
813
modification of the Environmental Protection
Agency Method 160 (EPA, 1979) for residual analysis. Whatman microfiber filters (GF/C, diameter
= 4.7 em, pore size = 1.2 ~m) were used to filter
the water samples. Filters were weighed immediately before use. A 200-ml well-mixed water
sample was filtered through the fiberglass filter,
and the residue retained on the filter was dried
for 24 hours at 100°C and reweighed. The dried
filters with residue were then ignited at 500 °C for
24 hours to burn off the organic matter. The burned
filters were reweighed. The total suspended, inorganic, and organic sediment concentrations were
determined (WANG et al., 1993). The sediment
flux passing each sampling station was computed
as the product of the total water discharge and
the total suspended sediment concentration at
each station.
Historical Aerial Photographs
An aerial photograph of the study area taken
in 1953 by the U.S. Department of Agriculture is
shown in Plate La. Trainasse T-2 was present, but
the connection of Trainasse T -1 in the bayou was
too small to show in this photograph. A high altitude aerial photograph of the same area in 1985
was obtained from the National Aeronautics and
Space Administration (Plate 1b). In this photograph, Trainasse T-1 was enlarged somewhat, but
Trainasse T-2 was larger than Trainasse T-2. This
photography also shows the well access canals
constructed in 1960 and later.
RESULTS AND DISCUSSION
Man-Made and Natural Channels
The general patterns of hydraulic geometry for
unaltered tidal-marsh streams are that progressively larger cross-sectional areas exist over relatively short distances downstream from the head
toward the mouth, and that tidal channels change
more rapidly in width than depth in relation to
downstream distance compared to non tidal
streams (MYRICK and LEOPOLD, 1963). In a natural tidal bayou, the cross section at the mouth
is typically the largest; each cross section upstream is progressively smaller as a function of
the progressively decreasing volume of tidal water
that must flow through the channeL This pattern
has been found in tidal-marsh streams in widely
varying geographic areas in Virginia (MYRICK and
LEOPOLD, 1963), in Massachusetts (REDFIELD,
1965), in California (PESTRONG, 1965), and in
Louisiana (WANG, 1990).
Journal of Coastal Research, Vol. 10, No.4, 1994
814
Wang , Sikora and Wang
P late 1. Historical aerial photographs of Old Oyster Bayou showing the locations of Trainasses T -1 and T -2 in (top) 1953 (obtained
from the U.S. Department of Agriculture, # CDC-6K-50, May 27, 1953) and (bottom) 1985 (obtained from the National Aeronautics
and Space Admi nistration, Flight 86-032, Frame #1450, December 6, 1985).
Journal of Coastal Research, Vol. 10, No.4, 1994
Hydrologic Regimes in Coastal Marshes
In Old Oyster Bayou, unlike the expected pattern from an unaltered tidal bayou, the crosssectional areas at Stations S-l, 8-2, and S-3 are
nearly equal (around 50 m", Figure 2a), while the
cross-sectional area at Station S-4 is less than half
(25 m"). This suggests that the water flow pattern
in Old Oyster Bayou has been altered. From field
observations and the shape of the cross section,
the data further suggest that Station 8-1 with soft
mud bottom has probably been filled in slightly,
and Station 8-2 with hard oyster shell bottom has
probably retained its original cross-sectional area.
It is of particular interest to note that the combined cross-sectional area oftrainasses at Stations
T-1 and T-2 (Figure 2h) is comparable to that of
the natural bayou; thus, water flow at least equal
to that of the present bayou could be accommodated through the cross-sectional area of trainasse
at Station T -1 alone, which is actually greater
than the cross section at Station S-4 (Figure 2a).
In addition to natural and man-made canals, a
sequence of oil wells (W -A, W-B, W -C, and W -D,
Figure 1) were drilled in the study area. Canals
to these wells were dredged within one month
prior to the start of oil well drilling. Drilling dates
for the oil wells W-A, W-B, W-C, and W-D were
started on March 26, 1960, January 14, 1961,
March 2, 1973, and April 24, 1981, respectively
(from computerized well files, Louisiana Department of Conservation). These wells were all dry
holes (no oil or gas). The canal at oil well W -C is
no longer blocked at its terminus. Thus, the end
of the south fork of Camp Canal is open, having
a cross-sectional area of 26 m", providing water
flow through W -C to the south to Little Hellhole
Bayou (Figure 1).
September 1991 Data Results
Figure 3a shows the predicted and observed
tidal heights (NGVD) at Eugene Island, Atchafalaya Bay during the September 20-21, 1991,
sampling period (from 0100 hours September 20
to 0800 hours September 21, 1991). The observed
tidal pattern closely resembled the patterns of
predicted tide, except that the range of observed
tide was smaller than the range of the prediction,
with the levels of both the high tides and low tides
being lower than predicted. The reduced tidal
range (from 55 em to 35 ern) produced much weaker tidal forcing to the system than predicted.
The current velocities measured at all the bayou stations were relatively low (Figure 3b), but
separated into two flow groups, 8-1 with S-2 and
815
S-3 with S-4. From the patterns of flow velocity,
it can be seen that Stations S-1 and S-2 closely
resembled each other; Stations S-3 and S-4 behaved somewhat similarly. The peak velocities,
however, differed significantly, being three times
greater at Stations S-1 and S-2 (± 60 em/sec) than
those at Stations S-3 and S-4 (± 20 em/sec). At
Station 8-1, the fastest ebb flow (-95 em/sec)
occurred at 2000 hours September 20. These observations indicate that the hydrologic regime of
Old Oyster Bayou, bisected by the north fork of
Camp Canal, has been modified; the resulting flow
patterns and current velocities on the west side
of the intersection differ significantly from those
on the east side.
October 1991 Data Results
During the October 24-26, 1991, sampling period (from 2000 hours October 24 to 0800 hours
October 26, 1991), the observed high-high, lowhigh, and high-low tides were lower than predicted, but the low-low tide was nearly the same
as predicted (Figure 4a). The larger tidal range
(60 em) in October 1991 provided much stronger
tidal forcing to the system than in September
1991.
During the field trip in October 1991, the flow
patterns at Stations S-1 and S-2 were again similar to one another. Flow patterns at Stations 8-3
and S-4 differed from one another as well as from
8-1 and S-2 (Figure 4b). During the first 12 hour
sampling period, a current meter was malfunctioning, resulting in low readings of flow velocities
at Stations S-1 and 8-2. At Station S-3 considerable flood flow occurred from 0400 hours until
1300 hours October 25, becoming ebb flow an hour
later at 1400 hours when the fastest ebb flow at
this station was observed. Flow at S-l and 8-2
was also ebbing strongly at this time. However,
strongly contrasting current velocities at Station
8-4 were observed, being generally low and oscillating from flood to ebb during the period from
0600 hours October 25 to 0000 hours October 26.
Average current velocities (positive sign indicating flood flow and negative sign indicating ebb
flow) recorded at Stations S-1 and S-2 were much
greater than those at Stations 8-3 and 8-4. The
peak velocities reached ± 160 ern/sec at Stations
S-1 and S-2, four times greater than ±40 em/sec
at Station S-3, and six times greater than ± 25
em/sec at Station 8-4. All stations exhibited flood
velocities at least as strong or stronger than ebb
velocities.
Journal of Coastal Research, Vol. 10, No.4, 1994
Wang, Sikora and Wang
816
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Figure 3. September 20-21, 1991, data sets (0100 September 20-0800 September 21, 1991): (a) predicted and observed tidal heights
(NGVD) at Eugene Island, and (b) average flow velocities measured at Stations S-I, 8-2, 8-3, and S-4.
Journal of Coastal Research, Vol. 10, No.4, 1994
817
Hydrologic Regimes in Coastal Marshes
Oct 24-28. 1991
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Figure4. October 24-26,1991, data sets (2000 October 24-0800 October 26,1991): (a) predicted and observed tidal heights (NGVD)
at Eugene Island, and (b) average flow velocities measured at Stations 8-1, S-2, S-3, and S-4.
Journal of Coastal Research, Vol. 10, No.4, 1994
818
Wang, Sikora and Wang
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Figure 5. Wind data recorded at Golden Meadow (solid line) and Blue Hammock Bayou (broken line) during (a) September 2021, 1991, and (b) October 24-26, 1991, sampling periods.
From both September and October 1991 data
sets, he effect of hydrologic alterations on flow
regime in the study area is apparent. It is noted
that the flow circulation patterns at Stations S-3
and S-4 are decoupled from Stations 8-1 and 8-2.
This is largely due to the flow interception by the
north fork of Camp Canal (Figure 1) and to a
lesser extent by the longer flow path of the south
fork of Camp Canal through Old Oyster Bayou
Lake to Station S-7. From the October data set,
it also appears that Station S-4 is being decoupled
from Station S-3 as evidenced by almost 18 hours
of oscillating (nearly slack flow at Station S-4 between 0600 hours October 25 and 0000 hours October 26).
Effect of Winds on Flow Regimes
Wind speeds and directions recorded in Fourleague Bay near Blue Hammock Bayou and at
Golden Meadow, about 4 km north and 80 km
east of our study area, during September 20-21
and October 24-26, 1991, sampling periods, are
presented in Figure 5. The directions of wind recorded at both remote and local areas were in
concert (Figure 5). During the September sampling period, steady winds were from the north,
while in October, east to southeast winds prevailed. The wind speeds were much lower in the
local area (1 to 2 m/sec) as compared to the remote
area (2 to 8 m/sec),
Steady winds from these directions represent
the frontal overrunning stage and the coastal return stage of cold front passages (MULLER, 1977).
The pre- and post-frontal events cause the wind
direction to change from predominantly south to
north in southern Louisiana (MULLER et al., 1991).
Cold fronts passed through the study area one
day (September 19, 1991) and three days before
(October 21, 1991) the September and October
1991 field trips, respectively.
Garvine (1985) showed that the remote effects
of the wind acting on the adjacent coastal ocean
were greater than the local surface stress acting
on the estuary itself. In Chesapeake Bay and the
Delaware Estuary, Garvine (1985) found that such
remote effects were transmitted to the estuary by
the impact on its mouth of sea level change induced by the onshore component of coastal Ekman transport when the wind parallels the coast.
In the Gulf of Mexico which is characterized by
low amplitude tides, Smith (1977) found that along
the Texas coast meteorologically induced water
Journal of Coastal Research, Vol. 10, No.4, 1994
Hydrologic Regimes in Coastal Marshes
exchanges between the estuary and the Gulf were
an order of magnitude more important than the
diurnal tidal exchanges; two orders of magnitude
more important than the semi-diurnal tidal exchanges. Smith (1977) concluded that these
weather induced exchanges of water, accompanied by concurrent water level variations, are the
result of windstress forcing water directly onshore
or offshore, or Ekman transport associated with
the longshore component of the quasi-steady
winds.
During the study periods, the steady north wind
in September decreased both the high tides and
the low tides as compared to the predicted tides.
The high tides were decreased to a greater extent
resulting in a smaller tidal range (Figure 3a). In
October, east winds moderately depressed the high
tides and the high-low tide, but the height of the
low-low tide was not affected (Figure 4a). Under
north wind conditions, slower flow velocities can
be expected to prevail. The flow velocities in October were two to three times greater than in September (Figures 3b and 4b). In October, the direction of the wind from the east was the same
as the orientation of the axis of the lower twothirds of Old Oyster Bayou, which had the effect
of intensifying the decoupling of S-4 and the upper one-third of the bayou from the lower reach.
Influence of Tides on Local Drainage
In this study area, marshes are subject to microtidal fluctuations and irregular floodings. The
diurnal tidal ranges are small, varying from 30 to
60 em during spring tides and 10 to 20 em during
neap tides. The daily flooding and ebbing of diurnal tides and resulting circulation within channels are fairly predictable (WANG, 1988). However, the flow patterns over tidal marshes adjacent
to bayous are less predictable.
Sampling periods were scheduled during spring
tides for each field trip. Tidal heights at Eugene
Island (a "subordinate station") were estimated
from the "reference station" at Galveston, Texas
(NOAA, 1991). It appears, however, that Old Oyster Bayou is on the boundary of two separate tidal
systems. The one to the east is based on another
"reference station" at Pensacola, Florida, and the
one to the west is based on Galveston, Texas.
Tides at these reference stations differ not only
in time and stages, but also in overall tidal signatures, with Galveston exhibiting much greater
separation and thus expression of mixed-diurnal
tides than Pensacola.
819
To examine the influence of two separate tidal
systems on local drainage patterns, another "subordinate station" at Wine Island, Louisiana (60
km east of the study area) was used. The predicted
tidal heights at Eugene Island and Wine Island
during September and October 1991 sampling periods are shown in Figure 6. There is a 1 to 2 hour
phase lead at Eugene Island compared with Wine
Island. The low tides of mixed and diurnal tides
are nearly equal. The high tides of the diurnal
tide (Pensacola based) are much lower than either
of the high tides or the high-low tides of mixed
tide (Galveston based), resulting in a smaller tidal
range at Wine Island than at Eugene Island (Figure 6). This means that a hydraulic gradient exists
toward the drainage area east of Old Oyster Bayou, creating a favorable condition for water to flow
through trainasse T -2 draining toward Fiddlers
Lake, a phenomenon frequently observed during
field trips.
Patterns of Marsh Inundation
Figures 7a and 7b show 3-month water level
records (from October 12, 1991, to January 18,
1992) obtained at a marsh site (Gauge A near
Station S-4) and in open water (Gauge B near
Station S-l), respectively. In the figures, the water
levels are shown relative to the marsh surface,
which is preset to a zero reading as a datum plane
for reference. During prolonged flooding periods,
starting on October 21 until November 2nd, the
marsh surface was continuously flooded for 12
days (Figure 7a). During this period, the peaks of
marsh water levels were proportional to the peaks
of open water levels (Figure 7b).
During the month of December and January,
marshes were rarely inundated. The patterns of
marsh inundation were characterized by only sporadic flooding interspersed by long draining periods. Under such conditions, marsh surface-water levels were low; the marsh peaks were greatly
reduced as compared to the open water peaks. It
is noted that on December 2nd and January 14th,
the peaks (arrows, Figure 7) in open water (3540 em) were nearly as high as the peaks in November (Figure 7b); but the marsh peaks (10-15
em) in December and January were much lower
than in November (Figure 7a).
From these water-level records, it is apparent
that local drainage patterns play an important
role in water storage at the marsh site and in the
adjacent area. There appears to be a threshold
effect which is dependent upon both the water
Journal of Coastal Research, Vol. 10, No.4, 1994
820
Wang, Sikora and Wang
80
Tld.l Helg"t (em)
(a)
Sept19-21. 1991
70
60
60
"
/
40
/
,/
30
/,-,
-,
"
/
<,
/
\
-,
/
/
<,
/
/
/
~O
/
10
..
0
12
100
20
16
0
8
4
12 16 20 0
Tim. (t'lo\Jrl)
TJdal Helgt'lt (em)
8
12
1e 20
(b)
Oct.24-26. 1991
80
eo
/'
,/~,,\
\
/
40
-
,
....
,I
/
/
I
/
I
I
I
/
I
20
I
I
/
0
-20
12
16
20
0
.
8
12 16 20 0
Tim. (houri)
4
8
12
16
20
Figure 6, Predicted tidal heights at Eugene Island (solid line) and Wine Island (broken line) during (a) September 19-21, 1991,
and (b) October 24-26, 1991, sampling periods.
level at Station S-1 and the water storage in the
drainage area of trainasse T -2. It further suggests
that Station S-4 is decoupled from Station 8-1
during prolonged draining periods (September 2021, 1991). Under prolonged flooding periods (October 24-26, 1991), after the water level reaches
a threshold value in the trainasse T -1 drainage
area, the adjacent marshes will remain inundated.
Overland flow across the marsh surface will then
appear.
Additional Field Observations in May and
August 1992
To identify the pattern of the canal-bayoutrainasse loop flow, two additional trips were car-
ried out in May and August 1992 (WANG et al.,
1992). In an effort to compare the similarity or
dissimilarity of flow patterns among sampling stations, two pairs of stations were chosen to be sampled simultaneously. An additional station, Station S-8, located just south of Camp Canal was
selected (Figure 1). Station S-8 (6 meters deep,
270 meters wide) is situated at the head of a tidal
inlet (Oyster Bayou) through which Fourleague
Bay empties into the Gulf of Mexico. Stations 8-8
and S-5 and Stations S-2 and T -1 were paired
during the May 18-20, 1992, sampling trip. The
cross-sectional averaged flow velocities at these
stations are plotted in Figure Sla), which shows
that flow velocities have similar patterns among
-lournal of Coastal Research, Vol. 10, No.4, 1994
821
Hydrologic Regimes in Coastal Marshes
stations and that the maximum flow velocities
during flood tides at each station are nearly equal.
However, Station 8-5 had much greater flow velocity during ebb tides. Apparently, the water flow
through T -1 reached nearly the same speed as the
flow through the man-made Camp Canal at S-5
(Figure 8a).
During the August 12-1~3, 1992, sampling trip,
simultaneous measurements at Stations S-5 and
8-1 were conducted. Figure 8(b) displays the computed water and sediment fluxes during the sampling periods. The results indicated that, during
a full tidal cycle, the water flow and sediment flux
in and out of Station S-l were nearly balanced,
while at Station S-5 they were not. At Station 8-5,
net water and sediment fluxes were flowing out
from Camp Canal to Oyster Bayou near Fourleague Bay.
Camp Canal, which drains water from both the
north and south forks, shunts water from the lower reaches of Old Oyster Bayou as well as from
the upper portion of Old Oyster Bayou Lake, Furthermore, the combined effect of the trainasses
and well access canals appear to be the isolation
of that segmen t of Old Oyster Bayou between T-1
and Old Oyster Bayou Lake. Camp Canal may
also be functioning as a sediment drain, siphoning
off suspended sediment which otherwise might
settle in the marshes adjacent to Old Oyster Bayou.
In summary, the results of these additional field
observations substantiate the hypothesis that
trainasses are shunting water flow from Old Oyster Bayou before the water gets to Station S-4.
This further indicates that Old Oyster Bayou Lake
has been decoupled from Old Oyster Bayou and
is now flooded and drained by the south fork of
Camp Canal (Figure 1).
CONCLUSIONS
The following conclusions are made based upon
the results of manually-measured and machinerecorded data sets obtained during a 2-year (19911992) field investigation, augmented with the existing historical aerial photographs, for study of
the hydrologic regimes of a tidal channel-salt
marsh flow system near Fourleague Bay in southcentral Louisiana:
(1) Old Oyster Bayou, a natural tidal bayou
opening into Fourleague Bay at its mouth, is intersected by a series of man-made canals and
trainasses (Figure 1), resulting in an altered hydrologic regime in the area.
(em)
(a)
40 -
-20
-40
_ _
-----L..---L........I.~
OlNov'91
01 Dec'91
oIJan'92
0lNov'91
OlDec'91
OlJan'92
_
(em)
40
20
-20
-40
Figure 7. Continuous water levels, relative to marsh surface,
recorded from water gauges installed: (a) at Old Oyster Bayou
marsh site, and (b) in open water at Old Oyster Bayou study
site from November 1991 to January 1992.
(2) Camp Canal, a petroleum access canal with
its extended north and south forks (Figure 1), has
a cross-sectional area comparable to that of the
natural bayou (Figure 2). The flow circulation patterns in the lower reach of Old Oyster Bayou are
different from the upper reach (Figures 3b and
4b). Such flow decoupling is largely due to the
flow interception by the north fork of Camp Canal. The water and sediment fluxes transported
through the man-made Camp Canal (via Station
S-5) are larger than through the natural Old Oyster Bayou (via Station S-l, Figure 8b).
(3) The cross-sectional areas of trainasses, in
particular T-l, were gradually enlarged from a
small trapper's channel, around 1.5 meters wide,
to their present sizes, 14 meters wide (Plate 1,
Figure 2). Trainasse T -1 cross-sectional area (24
m") is larger than that at Station S-4 (23 m"), and
it is shunting water from Old Oyster Bayou before
water gets to Station S-4 (Figure Sa), decoupling
Station S-4 from Station 8-3 (Figures 3b and 4b).
(4) Wind speed and direction, in particular wind
direction, have direct impact on tidal forcing to
the shallow-water environment (Figure 5). North
Journal of Coastal Research, Vol. 10, No.4, 1994
Wang, Sikora and Wang
822
(a)
Ave Vel (emIl)
May 18-20. 1992
10,....-----------------------------,
--....- S-8
8-5
~ 8-2
--Q- T-1
40
20
Ol-----+---------+~-------I------~fr---------i
-20
-40
- 60
I--"-.............--..............t--'I'-...........+--'-............+-'I-""'--Io..+-'- ...............................+o-i-"""""""'-+-'-............--+-t-............-+--'...........-+-'...........lo...+-"........."'"'+-~
17
(b)
21
8
13
5
17 21
1
Tim. (hour.)
Sf
13
17
~1
Aug 12-13, 1992
Q (m3/a)
30
-s~
----- 1-1
20
10
-10
-20
- 30
\---l--+-....L....+-L....f--J:::+-...Io........Ir---L-+-.....I.-t----t'---+..........+-~---'-""'1-"""""'-~-t----"t-~-----r­
11
13
15 17
18
21 23
11
1 3
5
7
Tim. (houri)
13
15
17
18
S.d FILodkg/.)
2r-----------------------------~
-1
-2
-3
-4
~_+..........-+-.....o....-+-~~___t_
11
13
15
17
18
..........._t_....a..-"t"_~-........_+_ .........._+_...I....-+___oi..............L._+_""""'--_+_....... ~____+_~
21 23
1
3
1
7
8
11
13
11
17
18
Tim. (houri)
Figure 8. (a) Averaged flow velocities measured at Stations 8-8, S-5, 8-2, and T-l during May 18-20, 1992, sampling trip, and (b)
computed water discharge and sediment flux at Stations S-5 and 8-1 during August 12-13, 1992, sampling trip.
Journal of Coastal Research, Vol. 10, No.4, 1994
Hydrologic Regimes in Coastal Marshes
winds greatly reduce the tidal range and slow down
the flow velocity in the bayou as compared to the
wind from the east (Figures 3a and 4a).
(5) Old Oyster Bayou is situated at the boundary of two separate tidal systems (Figure 6). The
differences in tidal phase and tidal amplitude provide a hydraulic gradient toward the drainage area
east of Old Oyster Bayou and promote water flow
through trainasse T -2 and drain farther to the
east to Fiddlers Lake (Figure 1).
(6) Marsh inundation is characterized by sporadic flooding interspersed by long draining periods (Figure 7). The patterns of local drainage
play an important role in water storage at the
marsh site and in the adjacent area. Under prolonged flooding periods, after the water level
reaches a threshold value in the trainasse T -2
drainage area, the marshes adjacent to Old Oyster
Bayou will be inundated.
(7) In summary, the north fork of Camp Canal
decouples Stations S-3 and S-4 from Stations S-l
and 8-2. Trainasse T -2 is draining the area east
of Station S-4 and decoupling Station S-4 from
8-3. The south fork of Camp Canal (via Station
8-7) is draining Old Oyster Bayou Lake and the
area to the south of Station S-4. These combined
results are that water flow past Station S-4 is
drastically reduced, and gradually, S-4 will be filled
in with sediment due to decreased flow velocity.
Eventually, S-4 will be nearly closed off as a result
of the induced hydrologic changes in the area. The
results of this study show that the hydrologic alteration has affected surface flow regime in Old
Oyster Bayou by short-circuiting the flow of head
waters and the surface overland flow across the
adjacent marshes.
ACKNOWLEDGEMENTS
Funding for this study was provided, in part, by
the Department of the Interior, U.S. Geological Survey, under the Contract Number 14-08-0001-23413,
and through the Louisiana Water Resources Research Institute under the Grant Number 127-905175. Wind data sets were kindly provided by Dr.
S.A. Hsu, Professor, Coastal Studies Institute,
Louisiana State University, Additional wind and
observed tidal height data sets were kindly provided by Mr. Cecil Soileau, Chief Hydrologist,
New Orleans District, U.S. Army Corps of Engineers. Field work was carried out with the help
of graduate students from the Department of
Oceanography and Coastal Sciences. The authors
thank Mr. Calvin LeLeaux. He graciously provid-
823
ed the use of his camp during many sampling trips
throughout this study. The authors also thank
Mr. Erick Swenson for his help in refurbishing
the water-level gauges and in processing data
tapes. The constructive comments and helpful
suggestions of two anonymous reviewers are
greatly appreciated.
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