ATCHAFALAYA RIVER

Wind surge and saltwater intrusion in AtchafalayaBay during onshore winds prior to cold front passage

Chunyan Li • Harry Roberts • Gregory

W. Stone • Eddie Weeks • Yixin Luo

Published online: 7 October 2010

� Springer Science+Business Media B.V. 2010

Abstract Cold front passages are largely responsi-

ble for accretion along the Chenier Plain, west of the

Atchafalaya River as well as many processes impact-

ing the overall health and functioning of the coastal

bays and wetlands. The associated water setup and set

down during a frontal passage, when wind quickly

switches from generally south to north, has significant

implications for Louisiana’s bays, coastlines, larval

transport, fishery, and oyster resources. The Atchafa-

laya River discharges up to 30% of the freshwater

from the Mississippi River which results in an almost

entirely fresh Atchafalaya Bay in spring. A one-

month deployment of two tripods equipped with

multiple sensors was made in the central Atchafalaya

Bay near the Wax Lake Delta for the study of the

impact of cold front passages on saltwater flux into

the bay between March 20 and April 19, 2006. It was

found that two episodes of saltwater intrusion

occurred during the 1-month deployment. These

events had rapid and transient increases of salinity

of approximately 2 PSU. These saltwater intrusion

events occurred after a high water slack and lasted for

0.5 to 2 h, respectively. High tide appears to be a

necessary condition for the saltwater intrusion. This

‘‘high tide’’, however, can be a combination of the

astronomical tides and wind-induced surge. Strong

southerly wind prior to a cold front passage can be an

important additional forcing to initiate and/or

strengthen saltwater intrusion. It is estimated that

roughly 50% of the observed setup is due to local

wind stress, 25% due to wave setup, and 25% due to

low atmospheric pressure during a cold front passage.

The Coriolis-induced setup is found to be negligible

in the current study.

Keywords Wind-induced setup � Saltwater

intrusion � Atchafalaya Bay � Wax Lake � Cold front

Introduction

Cold fronts or winter storms are worldwide weather

systems that impact mostly the mid-latitude regions.

The definition of a cold front is the leading edge of a

cooler and drier mass of air moving against a lighter,

damper, and warmer mass of air (Hsu, 1988). Across

a cold front, there is often a significant shear of wind

Guest editors: M. Ford & J.A. Nyman / Interactions among

rivers, floodplains, and coastal areas examined at the

Atchafalaya River

C. Li (&) � H. Roberts � G. W. Stone � E. Weeks �Y. Luo

Department of Oceanography and Coastal Sciences,

Coastal Studies Institute, Louisiana State University,

331 Howe-Russell Geocomplex, Baton Rouge, LA, USA

e-mail: cli@lsu.edu

C. Li

College of Marine Sciences, Shanghai Ocean University,

Shanghai, China

123

Hydrobiologia (2011) 658:27–39

DOI 10.1007/s10750-010-0467-5

velocity, a sharp drop in pressure (low pressure at the

front) and a rapid decrease of air temperature into

the colder air mass. A typical cold front system in the

northern hemisphere usually moves from the north-

ern quadrants toward the south or southeast direc-

tion. On the ground, the front can be delineated with

a line more or less in the east–west direction in

most cases, advancing southward or southeastward

(Hsu, 1988).

The Louisiana coast has a coastline oriented

roughly east to west. As a result, at a given location

on the Louisiana coast, a cold front is often preceded

by strong onshore winds from the south which

produce high waves and water setup (surge) on the

coast. The water setup is followed by a rapid change

in wind direction: wind changes to that from the

northern quadrants that cause the water to set down,

and the related transport of water, offshore. This

oscillation can potentially impact the ecosystem with

significant results. For example, the onshore wind can

bring in situ saltwater into bays and onto the wetland.

The subsequent set down of the water level can drain

shallow bays and dry out some of the shallow water

areas. The hydrodynamics influences the larval

transport and fish population by strong advection.

Although cold fronts are less severe compared to

hurricanes and the related wind surge is much less

than a major storm surge, they are frequent in winter

and spring: the time interval between fronts is

approximately bout 3–7 days (Roberts et al., 1989)

which makes a total of 20–30 events between each

winter and the following spring. As an example, we

analyzed water level time series and the timing of

cold front passages measured at Port Fourchon

(29�6.8420N, 90�11.0190W) from Oct 2006 to April

2007. Using a 6th order Butterworth 40-h lowpass

filter (Butterworth, 1930) we identified 24 high-

amplitude low-frequency variations in water level

caused by cold front events during this period, and

found that each cold front passage appeared to be

associated with a peak setup (or surge) before set

down of water levels. In Louisiana, the cold front

events have impacts particularly important to shell-

fish (mostly oysters) production and harvesting (S.A.

Hsu, personal communication). This impact occurs

because of the low gradient in bathymetry of the

shallow water of the bays which makes the shellfish

vulnerable to the exposure of municipal discharges at

low water during the cold front passage when mixing

and dispersion of polluted water becomes less

efficient because of low water level after the frontal

passage.

Cold fronts can cause significant inertial oscilla-

tions at frequencies close to those of diurnal tides in

offshore waters near the coast along the northern Gulf

of Mexico (Daddio et al., 1978; Jarosz et al., 2007).

During a cold front event, a given location often

experiences a wind vector variation over time in a

clockwise fashion, indicating that the atmospheric

flow has an inertial oscillation component as well.

This inertial oscillation component in the wind will

promote and strengthen inertial oscillations in the

water.

The change in wind direction during frontal

passages was found to have a significant impact on

fecal coliform levels in Mississippi Sound (Ufnar

et al., 2006). Fish kills were reported in unpolluted

flood plain lakes of Brazil during cold-front events

because of special chemical conditions of the lakes

with H2S (Brinkmann & Santos, 1973). Cold front-

induced reduction in water temperature may also

trigger the fall reproductive pulse of a tropical

macroalga (Morrison, 1984). The postlarval brown

shrimp recruitment was also found to be affected by

cold front events as the wind changes direction, and

as the temperature and salinity also change magni-

tudes with the weather conditions and water level

oscillations (Rogers et al., 1993). The impact of cold

front passages can also include the distribution,

abundance, and diversity of nekton (Castellanos &

Rozas, 2001). It appears evident that the entire

ecosystem can be altered by these winter storms

(Gallucci & Netto, 2004).

Cold fronts can generate significant subtidal (low

frequency) oscillations of water levels and transport

that are particularly important for the exchange of

water and flushing of bays and estuaries (Chuang &

Swenson, 1981; Swenson & Chuang, 1983; Denes &

Caffrey, 1988). The cold front-induced water level

change can be on the order of 1 m and associated

flow velocity can reach more than 0.5 m/s. The

increase in wind speed during cold fronts was found

to increase sediment flux and residual currents on an

intertidal flat in San Francisco Bay (Talke and Stacey,

28 Hydrobiologia (2011) 658:27–39

123

2008). As a result of the dynamical process inherent

in cold front passages over the northern Gulf of

Mexico, a large turbid coastal plume can form

extending 180 km alongshore and 75 km offshore

off the coastal Louisiana (Walker & Hammack,

2000). The geological consequence can thus be

significant: as discussed in Roberts et al. (1989) in

that the contrasting forcing and dynamical processes

inherently associated with pre- and post-frontal stages

are integrally related to short- and long-term changes

(year and decadal time-scales) of the Chenier Plain

coast along southwest Louisiana. A separate study

(Reed, 1989) concluded that such dynamics were also

important in marsh morphodynamic evolution. The

effect of southerly winds and subsequent wave and

water level setup prior to a cold front passage

typically causes sediment transport onto the marsh,

often resulting in aggradation. During post-frontal

(northerly) winds, water level set-down results in

desiccation of sediment. However, resuspension of

sediment and deposition on the marsh surface can be

accomplished when sediment is reworked from a

shallow nearshore source in these low-energy envi-

ronments, during a subsequent pre-frontal event.

Long-term marsh accretion by cold front events is

thus possible.

In this article, we discuss the hydrodynamic

processes influenced by cold front passages recorded

by a suite of instruments fixed on two tripods outside

of the Wax Lake and Atchafalaya River Deltas,

deployed between March 26 and April 19, 2006. At

these locations, water is usually fresh at least during

spring flood periods when the saline coastal water

seldom reaches the mouths of the deltas’ distributar-

ies. During strong southerly winds, particularly prior

to a frontal passage, however, saltwater was recorded

to reach the site of deployment. Saltwater intrusion is

of particular interest as salt can be used as a

conservative tracer to quantify the mixing of fresh

and salt water, thereby providing useful validation of

a numerical hydrodynamic model before it is applied

to sediment transport and other biogeochemical

processes. Understanding the saltwater intrusion

process and the related setup and set down of the

water level during a frontal passage is important for

the evaluation of the impact of the frequent winter

weather events to the Louisiana’s bays, coastlines,

and oyster resources.

Materials and methods

Sampling method

The study area is at the Wax Lake Delta Outlet within

the Atchafalaya Bay (Fig. 1). This is an area of interest

for the last few decades because of the rapid evolution

and formation of new deltas around the mouths of the

Atchafalaya River and Wax Lake. This emerging delta

area makes it a unique natural laboratory for multidis-

ciplinary studies (e.g., Roberts et al., 1980, 1989, 1997;

Allison et al., 2000; Neill & Allison, 2005). This study

site is within the Louisiana’s Shellfish Harvest Area

#24, an area with a total of about 1,500 km2, covering

the Atchafalaya Bay and adjacent wetlands (http://

www.dhh.louisiana.gov/maps.asp).

Two stainless steel tripods were designed on which

multiple sensors were mounted for shallow water

deployment. The deployment was made between

March 26 and April 19, 2006. The first tripod was

deployed between the Atchafalaya River Delta and

the Wax Lake Delta. The second tripod was deployed

outside of the mouth of a distributary, south-central

Wax Lake Delta. The coordinates of the deployments

are (29�26.4260N, 91�27.0250W) and (29�27.6710N,

91�23.6190W), indicated by Sontek and EasyQ in

Fig. 1, respectively. The first tripod (Sontek, Fig. 1)

was configured with a Sontek acoustic Doppler

profiler (ADP), a Seabird Electronics temperature

and salinity sensor (model SBE 16plus), a Wetlab

FLNTU-SB Fluorometer, three Wadars temperature

sensors at various levels in the water column. An

acoustic pinger was used for retrieval of the tripod.

The second tripod (EasyQ, Fig. 1) was configured

with a Nortek EasyQ horizontal acoustic Doppler

current profiler, a custom built Tattletale instrumen-

tation housing with a Paroscientific pressure sensor

for wave measurements, and two Wadars for temper-

ature measurements. The Sontek deployment site had

a water depth of *2 m while that at the EasyQ site

2.4 m, at the time of deployment.

The sampling interval for the Sontek ADP was set

at 0.5 s; while for the SBE 16plus and Nortek, it was

10 min; for the Wetlab it was 2 min; and for all the

Wadar temperature sensors, the sampling interval

was 1 min. The different sampling intervals were

selected based on the maximum capabilities of each

instrument as well as battery life considerations for

Hydrobiologia (2011) 658:27–39 29

123

the same length of deployment. The ADP data had a

high sampling frequency and required near 1 GB of

computer storage to facilitate these data files. A

summary of the instrument setups and total number

of data returns is provided in Table 1. In addition to

the instruments mounted on the two tripods, we also

used the wind data from three CSI WAVCIS

(http://www.wavcis.lsu.edu/) stations (CSI 3, CSI 5,

and CSI 6, Fig. 1) located South of Vermilion Bay

(29�26.470N, -92�03.680W), South of Terrebonne

Bay (29�03.20N, -90�320W), and on a Chevron Plat-

form, ST-52B (28�520N, -90�290W), respectively.

River discharge data were obtained from USGS

website. The data were recorded at the Wax Lake

Outlet at Calumet (USGS 07381590) (29�4105200N,

91�2202200W) and Lower Atchafalaya River at Mor-

gan City, LA (USGS 07381600) (29�4200900N,

91�1200700W). The latter is farther downstream and

is thus closer to the total discharge at the mouth.

Data analysis method

All but the turbidity data from our deployment were

successfully obtained. The turbidity data went over

Fig. 1 Study area.

a Louisiana coast;

b Atchafalaya Bay and

mooring sites

Table 1 Instruments and sensors used in the Wax Lake deployment

Tripod Instrument Distance to the

ocean floor (m)

Data start time (UTC) Sampling

interval (s)

Number of

data

Sontek Sontek 0.89 3/25/2006 19:00 0.5 3921371

Seabird 0.973 3/25/2006 22:10 600 3581

Wet labs 0.78 3/25/2006 19:20 120 53515

Wadar (1) 0.275 3/25/2006 19:19 60 44769

Wadar (2) 0.812 3/25/2006 19:16 60 44776

Wadar (3) 1.157 3/25/2006 19:26 60 44677

EasyQ Nortek 1.23 3/25/2006 23:00 600 4720

Wadar (1) 0.17 3/20/2006 19:19 60 55929

Wadar (2) 1.19 3/20/2006 20:22 60 55954

30 Hydrobiologia (2011) 658:27–39

123

the limit due to an incorrect setup. The water

temperature data did not show much variation at

different positions along the vertical water column.

Therefore, there was no obvious vertical thermal

stratification during the observations. The time series

data obtained from both tripods were lowpass filtered

by a 6-order Butterworth filter with a cutoff fre-

quency of 0.6 CPD (cycle per day) or equivalently, a

40-h filter. This filter has the advantage of a close to

ideal passing band of almost 100% energy recovery

below the cutoff frequency, a relatively narrow and

monotonic transition band, and an almost 100%

cutoff beyond the cutoff frequency. This filter is also

referred to as the ‘‘maximum flat magnitude’’ filter

because of its property preserving the passband

energy. Figure 2 shows the response function of this

filter in the frequency domain. The horizontal axis is

frequency in cycles per day (CPD). The ‘‘cutoff

frequency’’ is indicated by the vertical line at

0.6 CPD. It can be seen that for oscillations with

frequencies higher than the diurnal tide (which is the

dominant component along the Louisiana coast), or

f [ 1 CPD, the curve approaches zero as the

frequency increases. This tendency makes the filter

very efficient in extracting the tidal signal while

retaining the low frequency cold front-induced oscil-

lations with frequencies lower than 0.5 CPD (or

periods longer than 48 h). With a proper selection of

the order and cutoff frequency, the Butterworth filter

is used successfully by physical oceanographers

(Emery & Thompson, 2004). We also conducted

the power spectrum analysis for the energy distribu-

tion at various frequencies.

Results

Wind vectors from all three WAVCIS stations show

very similar patterns (Fig. 3). While most of the data

obtained from the 1 month deployment had winds

from the southern quadrants, there were 4–5 times

when the wind suddenly switched to those coming

from the northern quadrants and are indications of

cold front passage of the study site. This was verified

by examining the air temperature, atmospheric

pressure, and weather maps. Prior to these northerly

winds, the southerly winds increased significantly.

The winds also showed clockwise rotation, similar to

inertial oscillation.

The river discharge ranged between 1,600 and

4,600 m3/s and was decreasing before April 16 and

increased after April 16. Two short-term peaks

occurred after the frontal passage before April 9

and before April 23, respectively (Fig. 4). The USGS

discharge data obtained from station 07381600 is

farther downstream of 07381590 and thus has larger

values but with the same lowpass-filtered trend.

Water temperature (Fig. 5a) had daily/tidal varia-

tions and an almost steady increasing trend from

approximately 14�C in the beginning to approxi-

mately 22�C at the end of the deployment. There was,

however, an obvious decrease of lowpass-filtered

water temperature before April 9. The lowpass-

filtered drop was greater than 4�C. In addition, before

the rapid temperature drop, it appeared to have

increased first by approximately 2�C within 2 days.

This change of water temperature is most likely

associated with the cold front passage as the prefrontal

southerly winds blow over warmer air and the

postfrontal northerly winds are usually much colder.

The pressure sensor from the EasyQ horizontal

acoustic Doppler current profiler recorded daily water

level oscillations and the low frequency variations

(Fig. 5b). They clearly demonstrate the wind surge or

setup of water level prior to the frontal passage in

addition to a rapid set down immediately after frontal

passage. Set down ended on April 9 after which there

was almost a full rebound of the lowpass-filtered

water level. This rebound was then followed by a

Fig. 2 The frequency response of the 6th order Butterworth

lowpass filter with a cutoff frequency at 0.6 CPD (Cycle Per

Day) used in the study

Hydrobiologia (2011) 658:27–39 31

123

decrease of the mean water level to an average state.

The water level increased again before April 22,

although to a lesser degree (Fig. 3). The subsequent

set down was smaller in magnitude as well. There

was again a rebound of the water level afterwards.

The entire cycle of setup, set down, rebound, and

damping to a ‘‘normal’’ level, formed two peaks with

the second peak slightly lower than the first (Fig. 5b).

The power spectrum of the water level shows

a few important peaks (Fig. 6). There are two

contrasting peaks at near the semi-diurnal tidal

frequencies with periods of 12.48 and 11.91 h.,

respectively. Note that the M2 and K2 tides have

periods of 12.42 and 11.97 h., respectively, which are

very close to the two peaks identified in the water

level signal. There are two comparable peaks at near

the diurnal tidal frequencies with periods of 25.36

and 23.83 h., respectively. Note also that the O1 and

K1 tides have periods of 25.82 and 23.93 h, respec-

tively, close to the peaks identified in the signal.

There are also a few smaller peaks at lower frequen-

cies with periods of 6.5, 4.7, and 3.6 days, respec-

tively (Fig. 6). These latter oscillations fall within the

energy band of weather-induced motion and are

consistent with that caused by cold fronts given that

theses frontal systems have periods between 3 and

7 days (Roberts et al., 1989; Walker & Hammack,

2000).

Air temperature from the CSI stations clearly

captured the cold front passages. Prior to the

deployment, there was a strong cold front passage

indicated by a rapid drop in temperature by as much

as 16�C. The frontal passage before April 9 had a

temperature drop of 11�C. The low pass-filtered

values are smoother and had less dramatic changes

but still showed similar trends (Fig. 7a). The

Fig. 3 Wind vectors from

three different offshore

stations as shown in Fig. 1.

The arrows indicate the air

flow velocity or the

direction that the air particle

is going to

Fig. 4 River discharge from two different USGS stations

(07381600 and 07381590) along the Atchafalaya River

32 Hydrobiologia (2011) 658:27–39

123

spectrum of the air temperature had similar energy

bands but with a much reduced fraction of energy for

semi-diurnal oscillations. The diurnal frequency of

the air temperature is almost exactly the same as the

larger of the two diurnal frequencies seen in the water

level signal, similar to the semi-diurnal oscillations

(Fig. 7b). For convenience of comparison, Fig. 7b

has included the same lines for the frequency peaks

identified in the Power Spectrum of the water level.

Associated with the wind surge or setup in water

level during an onshore wind and high tide, episodes

of saltwater intrusion were recorded during our

deployment. The salinity of water at the site was

mostly close to zero, as a consequence of freshwater

from the Atchafalaya River and Wax Lake outlet.

During a number of occasions, however, salinity

spikes with a maximum of 2.5 PSU were recorded at

high tides (Fig. 8), indicating saltwater intrusion,

when the onshore wind had been strengthened. The

first major spike occurred during the strong onshore

wind prior to the cold front passage on April 8 after a

few days of continued strong southerly winds. The

spike was short-lived, only lasting for less than half

an hour. It should also be noted that since the

sampling interval (10 min) was close to the short

time period of the spike (*30 min), the actual

maximum salinity value was most likely larger than

what was recorded (2.5 PSU). The second event of

saltwater intrusion occurred after the strengthening of

onshore wind (Fig. 3) at high tide before April 16

(Fig. 8). This onshore wind, however, was not

associated with a cold front. In contrast, the persistent

and strong onshore wind lasted for a week or so

which resulted in a steady increase in air temperature

(Fig. 7a). Following each saltwater intrusion event,

there appeared to be a damped oscillation which

pushed the saline water back and forth within a

course of less than a day, shown by the smaller

salinity peaks after each major spike (Fig. 8). In

addition, prior to the second event, there were two

very small spikes in the previous 2 days during high

tides, indicating the effect of tidal oscillation (Fig. 8).

Since, the locations of the deployment are where

water is primarily fresh, the tidal oscillation is usually

not reflected in the salinity data unless there is an

onshore wind resulting in saltwater intrusion.

Fig. 5 a Water temperature time series (thin solid line) with

the lowpass-filtered values (thick solid line). b Water level time

series (thin solid line) with the lowpass-filtered values (thicksolid line)

Fig. 6 Power density spectrum of the surface elevation. The

thin solid lines indicate the peaks of frequencies corresponding

to periods at 6.5, 4.7, 3.6 days, and 25.36, 23.83, 12.48, and

11.91 h, respectively

Hydrobiologia (2011) 658:27–39 33

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Discussion

Contributions from Various Mechanisms

Wind-induced water setup/surge includes the follow-

ing mechanisms: (1) wind stress effect, (2) wave

effect, (3) barometric pressure effect, and (4) Coriolis

(earth rotation) effect. Here we estimate each of these

four components for the present dataset. We discuss

the first event of the cold front passage before April 9.

The wind stress-induced setup is basically a quasi-

equilibrium balance between the wind stress and the

pressure gradient force produced by the surface slope.

This balance can be quantified by (e.g., Murty, 1984)

ofox� ss � sb

qghð1Þ

in which f is the surface water level related to mean

sea level; x is the distance in the direction of the wind

(positive onshore for convenience); ss and sb are the

surface and bottom shear stress, respectively; q, g,

h are the water density, gravitational acceleration,

and water depth, respectively. The surface shear

stress is from wind and is expressed as

ss ¼ qaCDW2 ð2Þ

where qa is the air density, W is wind speed, and CD

is the surface drag coefficient ranging between

1.3 9 10-3 and 3.4 9 10-3 (Dean & Dalrymple,

2002). Here we use 1.5 9 10-3 as a general practice.

For the first event with the cold front passage, the

strong southerly (onshore) wind is *10 m/s. The

bottom stress sb is expressed in a similar fashion

sb ¼ qCDV2 ð3ÞHere q is the water density, CD is the bottom drag

coefficient, and V is the current velocity, which is less

than 0.1 m/s as observed by the acoustic current

meters (Fig. 9). We use the same drag coefficient for

the bottom as a first order approximation.

The distance from the mouth of Wax Lake to the

outside of the Atchafalaya Bay is approximately

20 km. Assuming that the southerly wind-induced

setup has a linear slope within the bay, we then have

from Eq. 1 that

Fig. 7 a Air temperature time series (thin solid line) with the

lowpass-filtered values (thick solid line). b Power density

spectrum of the air temperature. The thin solid lines are exactly

the same as those in Fig. 6 showing the peaks at 6.5, 4.7,

3.6 days, and 25.36, 23.83, 12.48, and 11.91 h in period,

respectively

Fig. 8 Time series of salinity (thick solid line), and surface

elevation (thin solid line)

34 Hydrobiologia (2011) 658:27–39

123

fwind �sS � sb

qghL ð4Þ

The value for g and q are 9.8 m/s2 and 1,020 kg/m3,

respectively. The water depth is approximately 1.5 m.

Using L * 20 km, we estimate that

fwind � 0:18m ð5ÞThe wave-induced setup is estimated to be (Guza

& Thornton, 1981)

fwave � 0:17 Hs ð6Þ

in which Hs is the significant wave height. For the

present study, using the first event as an example, the

wave height is shown in Fig. 10a. The mean Hs prior

to frontal passage was *0.5 m. This yields an

estimate of wave-induced setup to be

fwave � 0:08 m ð7ÞThe well-known barometric pressure-induced

setup is such that a 1 millibar drop in air pressure

(compared to the mean pressure outside of the

weather system) is equivalent to an approximate

0.01 m rise in water level. Specifically (Dean &

Dalrymple, 2002),

fpressure � 1:04� 10�2Dp ð8Þ

in which Dp is the pressure drop in millibars. In our

case, the first event had a drop of lowpass-filtered air

pressure (Fig. 10b) of a approximately 9 millibars,

which yields

fpressure � 0:09 m ð9Þ

The Coriolis effect-induced setup is estimated by

ofox� � fv

gð10Þ

in which f is the Coriolis parameter, or the local

vertical planetary vorticity, determined by the lati-

tude: f = 2 X sin(h), where X is the angular speed of

the earth rotation which equals 7.29 9 10-5/s. v is

the east velocity component. Equation 10 gives

fCoriolis � �fv

gL ð11Þ

This yields

fCoriolis � � 0:01 m ð12ÞThe plus and minus signs are used here to indicate

that the sign of this contribution depends on whether

Fig. 9 Time series of flow velocity from the EasyQ horizontal

ADCP (thin solid line) and its lowpass-filtered values (thicksolid line)

Fig. 10 a Time series of the significant wave height from CSI

3 (Fig. 1). b Time series of the atmospheric pressure CSI 3

(Fig. 1)

Hydrobiologia (2011) 658:27–39 35

123

the east–west component of the flow is positive or

negative. For the other contributions to the setup,

when the wind is onshore and the pressure is lower

than surrounding conditions, they are all positive. In

summary, the total water setup contributed by the

above four factors adds up to:

f ¼ fwind þ fwave þ fpressure þ fCoriolis� 0:18þ 0:08

þ 0:09� 0:01� 0:35 m

The last term contributed by the Coriolis effect is

negligible to the total setup. This result is consistent

with the observations: as shown in Fig. 5b, that the

setup in water level prior to the large drop of the

water level before April 9 was *0.30 m, obtained

from the lowpass-filtered results. The difference

between the calculated total surge value of 0.35 m

and the observed value of 0.30 m is well within the

error of estimate as the approach used here is overly

simplified which neglected the dynamics, advection,

and local acceleration. It nevertheless gives a rea-

sonable order of magnitude estimate that apparently

captures the main factors.

This simple estimate suggests that the most

important factor in producing the observed wind

surge prior to the cold front passage is the wind stress

(50%). The effects of wave setup and barometric

pressure are comparable to each other (*25% each).

The Coriolis effect appears insignificant in this case.

It should also be noted that in our calculation of

different contributions to the water level setup, we

did not include the effect of river discharge. River

discharge variation is not considered in this case

because of two reasons: (1) The largest river

discharge variation is seasonal, and the cold front

time scale is only 3–10 days. Because of this

disparity in time scales, the river discharge effect

will not influence the wind-induced setup. Rather, the

river discharge-induced water level change can be

simply linearly superimposed on that caused by wind.

(2) The discharge-induced water level change in the

ocean is very limited. Of course, the river discharge

variation induced changes in water level inside the

river is more pronounced but our focus is in the bay.

The discharge variation changes the suspended

sediment, etc. but it does not change the bay water

level significantly. This is demonstrated in our recent

analysis of more water level records in the river and

in the Louisiana bays. It shows that the river

discharge does impact the water level significantly

in the river but had much smaller signature through-

out the bays along the coastal Louisiana.

Significance of study

The Louisiana continental shelf is broad and shallow

with a gentle slope. The coastal area is covered by

shallow bays with low profile barrier islands, low-

lying wetlands and coastal plains. With this setting, it

is an area vulnerable to hurricane and winter storm

surges. While hurricanes can cause severe damages

as we have witnessed in recent years by Hurricanes

Katrina (e.g., Pardue et al., 2005; Adams et al., 2007;

Stout et al., 2007; Suedel et al., 2007), Rita (e.g.,

Rego & Li, 2009a, b, 2010a, b), Gustav, and Ike

(Li et al., 2009, 2010), the effect of winter storms

(cold fronts) cannot be neglected because of the much

higher frequency of occurrence and its cumulative

effects (e.g., Roberts et al., 1987; Moeller, et al.,

1993; Walker & Hammack, 2000; Pepper & Stone,

2004; Feng & Li, 2010). The hydrodynamics can

significantly influence the sediment transport (e.g.,

Cundy et al., 2007) and wetland stability (e.g.,

Valdemoro et al., 2007). The wind-driven oscillation

of the shallow bays can also have a significant impact

to coastal, wetland, and marine life. Many econom-

ically important fish and natan invertebrate species

inhabit in the estuaries of the region during part of

their life cycle. The estuaries and bays provide

critical habitats for foraging, refuge, and reproduction

(Kneib, 1997; Cattrijsse & Hampel, 2006). Spawning

grounds, however, are sometimes offshore, and larvae

must be transported across shelf to estuarine nursery

grounds (Miller et al., 1984, 1985; Shaw et al., 1985,

1988; Boehlert & Mundy, 1988; Miller, 1988;

Lyczkowski-Shultz et al., 1990). Successful estuarine

recruitment of larvae is affected by many physical

factors operating on different scales of tide and

particularly of wind-driven flows (Garcia, 1983;

Zimmerman & Minello, 1984; Childers et al., 1990).

The cold front-induced bay oscillation and saltwater

intrusion can have an impact to not only the nektons

but also potentially wetland plants. A severe saltwater

intrusion can put excessive stress on the marsh

community and even cause mortality which will in

turn affect the carbon sequestration (Delaune et al.,

1978, 1983a, b; DeLaune & Smith, 1984; Nyman

et al., 1995; J. White, personal communications) and

36 Hydrobiologia (2011) 658:27–39

123

eventually cause the wetland to lose its carbon storage

capacity and hundreds of years of carbon storage, and

release excessive carbon into the environment to add

more green house gases into the atmosphere.

In addition, coastal Louisiana is rich in oil and gas

and has a heavy industry presence. As we have seen

from the ongoing biggest oil spill in history following

the explosion and sinking of the Deepwater Horizon

oil platform on April 20, 2010, Louisiana coast is

facing a serious problem of contamination by the

massive oil and dispersant applied at the spill site. As

the oil gets to the shore and coastal wetland, wind-

induced transport plays an important role in the

extent of the affected area.

Because of these reasons, the studies of storm-

induced oscillation and flushing of the bays are

important. This study is one of the first attempts in

recording and quantifying saltwater intrusion events

in this region. It also provides analysis of the

mechanisms. It emphasizes the most important factor:

the wind stress. At the same time, we have deter-

mined that the effect of atmospheric pressure and

waves are all important, although secondary, com-

paring with the wind stress. One of the studies that

we have been conducting related to this study is the

use of the Finite Volume Coastal Ocean Model

(FVCOM) for this region. The data obtained are very

valuable for model validation.

Conclusions

Mooring observations of 1-month in time using

multiple instruments at the mouth of the Wax Lake

Delta recorded daily tidal oscillations and wind-

induced setups and set downs and associated saltwater

intrusion events. The 1-month deployment was rela-

tively short. The successful deployment and retrieval

of the tripods was, however, not trivial in this specific

area. This is because the area is characterized by a

nascent delta, accretional coastline and aggradational

mid-upper shoreface. Although a relatively short-time

series, the data enabled a reasonably detailed analysis

of the impacts of cold front events on the oscillations

of the bays and saltwater intrusion. Power spectrum

analysis identified two semidiurnal frequencies

close to those of M2 and K2 tides and two diurnal

frequencies close to O1 and K1 tides. The total energy

of the diurnal tides is twice that of the semi-diurnal

tides. In addition, lower frequencies were identified

within the range of cold front frequencies. The

onshore wind-induced setups prior to a cold front

passage caused saltwater intrusion during high water

levels. The salinity increase associated with this event

was, however, very short in duration. Following each

saltwater intrusion event, there appeared to be minor

oscillations of salinity within 1 day. The wind-

induced setup was followed by a set down, a rebound

with a reduced magnitude of the original setup, and

what appeared to be a viscous damping afterwards. By

examining the factors contributing to the setups, we

have found that the wind stress-induced surge consti-

tutes approximately half of the total, while the

atmospheric pressure and wave-induced setups con-

tribute some 25% each. The effect of the Coriolis

force was found to be very small and can be neglected

in this small bay.

Acknowledgments This project is supported under an award

NA06NPS4780197 by NOAA NGoMex and NA06OAR432

0264 06111039 to the Northern Gulf Institute by NOAA’s

Office of Ocean and Atmospheric Research, U.S. Department of

Commerce and Shell (http://www.ngi.lsu.edu/). The stainless

steel tripods for the instruments were fabricated by personnel in

the Coastal Studies Institute’s Field Support Group (CSI FSG).

The deployment, retrieval and initial data quality control were

all conducted by the technicians of CSI FSG. Zhixuan Feng

helped with some of the wind data processing and data

organization.

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