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Estuarine, Coastal and Shelf Science 87 (2010) 517–525

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Estuarine, Coastal and Shelf Science

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Storm surge induced flux through multiple tidal passes of Lake Pontchartrainestuary during Hurricanes Gustav and Ike

Chunyan Li a,b,*, Eddie Weeks a, Brian W. Blanchard a

a Department of Oceanography and Coastal Sciences, School of the Coast and Environment, Louisiana State University, Baton Rouge, LA 70803, USAb College of Marine Sciences, Shanghai Ocean University, 999 Hucheng Huan Road, Shanghai 201306, China

a r t i c l e i n f o

Article history:Received 4 April 2009Accepted 3 February 2010Available online 12 February 2010

Keywords:in situ measurementshurricane storm surgestidal passesLake PontchartrainHurricane GustavHurricane Ike

* Corresponding author at: Department of OceanoSchool of the Coast and Environment, Louisiana StLA 70803, USA.

E-mail address: cli@lsu.edu (C. Li).

0272-7714/$ – see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.ecss.2010.02.003

a b s t r a c t

In September 2008, Hurricanes Gustav and Ike generated major storm surges which impacted the LakePontchartrain estuary in Louisiana. This paper presents analyses of in situ measurements acquired duringthese storm events. The main data used in the analyses were from three bottom mounted mooringsequipped with conductivity, temperature, and depth sensors, acoustic Doppler current profilers (ADCPs),and a semi-permanent laterally mounted horizontal acoustic Doppler profiler (ADP). These mooringswere deployed in the three major tidal channels that connect Lake Pontchartrain with the coastal ocean.A process similar to tidal straining was observed: the vertical shear of the horizontal velocity wasnegligible during the inundation stage, but a shear of 0.8 m/s over a less than 5 m water column wasrecorded during the receding stage, 2–3 times the normal tidal oscillations. The surge reached its peak inthe Industrial Canal 1.4–2.1 h before those in the other two channels. The inward flux of water lasted fora shorter time period than that of the outward flux. The inward flux was also observed to have muchsmaller magnitude than the outward flux (w960–1200 vs. 2100–3100 million m3). The imbalance wasbelieved to have been caused by the additional water into Lake Pontchartrain through some small riversand inundation over the land plus rainfall from the hurricanes. The flux through the Industrial Canal was8–12%, while the flux through the other two tidal passes ranged between 17% and 70% of the total, butmostly split roughly half-half of the remaining (w88–92% of the total).

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1. Introduction

In the first two weeks of September 2008, the Louisiana coastwas seriously impacted by two major storm surges generated byHurricanes Gustav and Ike, nearly three years after HurricanesKatrina (e.g. Pardue et al., 2005; Suedel et al., 2007; Stout et al.,2007; Adams et al., 2007) and Rita (e.g. Rego and Li, 2009a,b, inpress) impacted the same areas of the state. The storms wereseparated by about 12 days between their landfalls. Together withwell-known hurricanes such as Andrew (1992) and Katrina (2005),both Gustav and Ike are among the most destructive hurricanesthat made landfall on the U.S. mainland. Gustav started as theseventh tropical depression of 2008 approximately 260 milessoutheast of Haiti on the morning of August 25, 2008. When itmade landfall on the Louisiana coast near Cocodrie and Port

graphy and Coastal Sciences,ate University, Baton Rouge,

ll rights reserved.

Fourchon at 9:30 AM CDT on September 1, it was a Category 2hurricane on the Saffir–Simpson scale with sustained winds of110 mph according to the National Hurricane Center (NWS, 2008a).It then dropped to Category 1 within an hour. Data from the BatonRouge Airport indicated that Gustav had a record high maximumsustained wind speed and second strongest wind gust in thehistory of hurricanes affecting the Baton Rouge area since 1950.

Hurricane Ike started as a tropical disturbance off Africa in lateAugust and developed into a named tropical storm west of the CapeVerde Islands on September 1, 2008. It became a Category 4hurricane on September 4 and was the fourth major hurricane tohit Haiti and Cuba in three weeks (the previous three were Hanna,Fay, and Gustav). After causing tremendous damage there, Ikecrossed the Gulf of Mexico and made landfall at Galveston, Texas,on September 13 at 2:10 AM CDT. At that time, it was a Category 2hurricane with sustained winds of 110 mph and a central pressureof 952 mbar, measured from NOAA’s Doppler weather radar andreconnaissance aircraft, according to the National Hurricane Center(NWS, 2008b).

Because of the severe damages Hurricane Katrina caused to thecity of New Orleans (Pardue et al., 2005), a similar scenario was

C. Li et al. / Estuarine, Coastal and Shelf Science 87 (2010) 517–525518

feared in the beginning of the development of Hurricane Gustav.The city of New Orleans ordered a mandatory evacuation fromAugust 31 and more than 200,000 people from the city and close to2 million from the coastal areas of southern Louisiana evacuated,making it the largest exodus in Louisiana history.

In anticipation of Gustav, and also as part of a project funded bythe National Science Foundation to study the quantification of theflux of water in and out of Lake Pontchartrain (located north of NewOrleans, Fig. 1), we deployed a number of instruments at three tidalchannels leading to the lake and at Port Fourchon 3–4 days prior tothe hurricane’s arrival. These instruments mostly worked wellthroughout Hurricane Gustav and subsequent Hurricane Ike, add-ing a unique and comprehensive dataset recording two stormsurges separated by only about 12 days. Here we present a study ofthe hurricane-induced flux of water through three tidal passes ofthe Lake Pontchartrain estuary. The study on salt flux was pre-sented in Li et al. (2009b). This paper complements the earlierpaper and focuses on the volume flux of seawater.

2. Study site and instrumentation

Lake Pontchartrain is an estuarine lake that has an oval shapewith an average water depth of about 3.7 m. Its major axis is 66 km inthe east–west direction and its minor axis is 40 km in the north–south direction (Fig. 1). It receives limited river discharges (e.g.Sikora and Kjerfve, 1985; Carillo et al., 2001; Haralampides, 2000;

Fig. 1. Study site, hurricane tracks, and mooring locations (circles) where CM is Chef Menteura NOAA NOS station, LP is Lake Pontchartrain, LB is Lake Borgne, MS is the Mississippi STerrebonne Bay, Rig is the Rigolets, IC is the Industrial Canal and ICH is the location of the

Poirrier, 1978; Georgiou and McCorquodale, 2000, 2002) and haslimited communication with the coastal ocean through threenarrow tidal channels. Saltwater intrusion can occur through thesechannels, for example, the Industrial Canal, by tidal and wind drivenmotions (Li et al., 2008). The tide in the area is mainly diurnal witha maximum range of about 0.6 m. Lake Pontchartrain has beena subject of study mainly because of its proximity to the city of NewOrleans. During Hurricane Katrina (August 29, 2005), New Orleanswas flooded by ocean water along the southern shoreline of the LakePontchartrain due to levee failure (e.g. Pardue et al., 2005; Suedelet al., 2007; Stout et al., 2007; Adams et al., 2007).

Among the three tidal channels connecting Lake Pontchartrainwith the coastal ocean (Fig. 1), the Rigolets is a meandering naturalchannel which connects the east end of Lake Pontchartrain withLake Borgne. Lake Borgne in turn is connected with the coastalocean mainly through its northeast end with the western Mis-sissippi Sound. The Mississippi Sound is connected with theChandeleur Sound and Breton Sound. Chef Menteur Pass is anothermeandering natural channel on eastern Lake Pontchartrain that isconnected at the southwest corner of Lake Borgne. The IndustrialCanal is a manmade north–south oriented straight channel, whichis in turn connected with the Intracoastal waterway (ICW) andMississippi River Gulf Outlet (MRGO). The Industrial Canal is con-nected with the Mississippi River through a short canal which isusually blocked by a dam. Therefore, the Industrial Canal has a mostdirect communication with the coastal ocean through the MRGO.

, ICW is the Intracoastal Waterway, MRGO is the Mississippi River Gulf Outlet, NWCL1 isound, CS is the Chandeleur Sound, BS is the Breton Sound, BB is Barataria Bay, TB ishorizontal ADP, AB is Atchafalaya Bay, and BC is Bay Champaign.

C. Li et al. / Estuarine, Coastal and Shelf Science 87 (2010) 517–525 519

In our study, three tripod-shaped bottom platform ‘‘Sea Spiders’’were used in deploying three ADCPs and Seabird Microcat Conduc-tivity–Depth–Temperature sensors (CTD, model SBE37). The SeaSpider is a seafloor platform for ADCPs from Ocean Science,a company for oceanographic tools. It is a tripod with 1.47 m sidelength and 0.53 m in height. We applied 150 pounds of steel weighton each platform for stability during the deployment. Two of theseplatforms were deployed in the Rigolets and Chef Menteur Passchannels on August 27 and the instruments were programmed tostart recording data from 0:00 UTC (GMT), August 28. The thirdplatform was deployed in the Industrial Canal on August 28. Theinstruments were programmed to start recording data from 0:00UTC (GMT), August 29. The sampling intervals were set at 5 min forall the ADCPs and CTDs. The ADCPs used in the Industrial Canal andthe Rigolets had a working frequency of 1228.8 kHz while the ADCPused in Chef Menteur Pass had a working frequency of 614.4 KHz. Thevertical bin size of the ADCPs was 0.25 m. A 1500 kHz Sontek Argo-naut horizontal ADP with a pressure sensor has been deployed at theIndustrial Canal since 2006 with a 15 min sampling interval (Fig.1, itslocation is marked with ‘‘ICH’’, very close to the location marked with‘‘IC’’, here ‘‘H’’ stands for horizontal, Table 1). On August 29, we alsodeployed two YSI CTDs at Bay Champaign, which happened to bevery close (w30 km) to where Hurricane Gustav made landfall.Because of the side lobe effect, the velocity data near the surface arealways distorted. About 10–15% of the total depth on the top layer isusually affected. The near bottom velocity is not recorded because ofa blanking distance. The middle of the first bin for the ADCPs was0.62 m above the transducers in the Industrial Canal and Rigolets,and 0.74 m above in the Chef Menteur Pass. A commonly usedalgorithm (e.g. Li, 2002, 2006; Li et al., 1998, 2004) of linear extrap-olation was implemented to reconstruct the velocity profile at thebottom and on the surface before further analysis was made.

3. Data analysis

3.1. Meteorology data

Meteorological data from NOAA’s National Ocean ServiceStation NWCL1 (8761927) located at 30.027� N, 90.113� W are usedin this study. This station is located at the central south shore of theLake Pontchartrain (Fig. 1). The barometer is 4.3 m above mean sealevel. The anemometer is at 11.9 m above the ground. The airpressure recorded at that station shows a minimum at 16:00 UTCSeptember 1 (the 245th day of 2008, Fig. 2a). Since the center ofGustav made landfall near the Port Fourchon area at 14:30 UTC, theminimum air pressure occurred 1.5 h later at Lake Pontchartrain.The maximum wind speed recorded at NWCL1 shows a sharp peakat about the same time of the lowest air pressure (w990 hPa) andthe maximum sustained wind speed is close to 25 m/s (or w50knots, Fig. 2b). In comparison, the second hurricane (Ike) has onlya minor dip in pressure at this site about 11 days after the passing ofthe first. The maximum sustained wind speed is also much smaller– only w15 m/s. This is because the distance of Ike to Lake

Table 1Information of deployment during Hurricane Gustav.

Station Latitude (�N) Longitude (�W)

Rigolets 30.1742 89.6882Chef Menteur Pass 30.0844 89.7914Industrial Canal 1 30.0051 90.0255Industrial Canal 2 30.0312 90.0342Bay Champaign 1 29.1189 90.1791Bay Champaign 2 29.1121 90.1826Bay Champaign 3 29.1136 90.1850

Pontchartrain is much farther than that of Gustav (Fig. 1). HurricaneGustav started with north and northeasterly winds at the NWCL1site which lasted for more than two days. The wind then switchedto southeasterly and southerly for about 4 days, during which themagnitude of the wind decreased almost steadily (Fig. 2c), eventhough the strongest hurricane winds only lasted for a few hours.The second hurricane started with weaker and shorter durationnortheasterly winds and then relatively strong southerly winds forw3 days followed by northerly winds for another 3 days (Fig. 2d).

3.2. Movement of mooring platforms and associated errors inpressure

Pressure data indicated about 23 ‘‘sharp’’ jumps, correspond-ing to w0.10–2.5 m depth changes over 5 min (sampling interval)in the Rigolets (or at a rate of 1.2–30.0 m/h), and 15 ‘‘sharp’’jumps, corresponding to w0.15–0.70 m over 5 min in the ChefMenteur Pass (or at a rate of 1.8–8.4 m/h) during the peak ofGustav’s surge, suggesting abrupt movements of the Sea Spidersunder extreme flow conditions. With 3953 total records of datafrom the Rigolets up to September 10, the 23 jumps form onlya very small fraction of the total record (0.58% of the total record).The data from the Sea Spider in the Rigolets recorded afterredeployment on September 10 were downloaded on October 8,and did not need any correction as the platform did not moveduring Hurricane Ike. We found that the platform deployed in theRigolets had moved by about 50 m downstream from where itwas originally deployed. This distance exemplified the violentforce of the storm surge since the Sea Spider was designed asa low profile platform and is very stable under normal conditions.The platform deployed in the Chef Menteur Pass also appeared tohave moved for a shorter distance for about 20 m. The platformdeployed in the Industrial Canal however stayed at its originallocation without a movement.

3.3. Correction of water level

With the exception of the jumps, the maximum rate ofincrease of the surge from Hurricane Gustav was about 0.35 m/h,much smaller than the rate of the sharp jumps. The maximumvelocity reached 2.3–2.5 m/s during the storm surge according tothe current meter data. The velocity from the Industrial Canal wassmaller. The recorded pressures from the Rigolets and ChefMenteur Pass were a combination of the storm surge and thedepth changes caused by the movement of the platform due tothe swift currents of the storm surge. To correct these errors, weidentified the times of the sharp pressure changes, took out thosechanges, and replaced them with a value consistent with theslope of the adjacent observations without the jumps. By doingso, the resultant storm surge curves from both the Rigolets andChef Menteur Pass were recovered. After correcting the pressuredata from the mooring platforms deployed in the Rigolets andChef Menteur Pass, we further aligned the pressure from each

Start time (UTC) End time (UTC) Dt (min)

0:00, August 28, 2008 14:35, Oct. 6 50:00, August 28, 2008 14:35, Sept. 22 50:00, August 29, 2008 16:40, Sept. 22 5Since 2006 Continuing 1518:46, August 29, 2008 16:17, Sept. 6 118:19, August 29, 2008 1:15, Sept. 8 122:15, September. 10, 2008 14:30, Sept. 19 15

Fig. 2. Meteorological data from NOAA NOS station NWCL1. (a) Air pressure; (b) Wind speed; (c) Wind vector during Hurricane Gustav; and (d) Wind vector during Hurricane Ike.

C. Li et al. / Estuarine, Coastal and Shelf Science 87 (2010) 517–525520

station by finding the mean sea level from each pressure timeseries during times of tidal oscillations without the stormsurges, that is by a ‘‘de-mean’’ operation (subtraction of the meanvalue). This way, we successfully put them on an apparentlycorrect level of reference that is consistent with all datasets. Itshould also be noted that the timings of the peaks are notaffected by the movement of the platforms and the correctionprocedure. The final results produce a time series in agreementwith data from nearby NOAA water level measurements. Thisoperation is not without error because the pressure changesinduced by the movements of the two platforms cannot be 100%removed. The alignment of the pressure data will provideroughly correct depth data for the calculation of the flux. Thecorrection is much larger than the error and thus needed. Ananalysis of the errors introduced by this procedure is discussed inthe Appendix.

Fig. 3. Time series of water surface elevation at the Rigolets and

4. Results and discussion

The surface elevations from all three ADCPs were smoothed twiceby a 5-point (or 20-min) moving average. Fig. 3 shows the compar-ison of data from the Rigolets and the Industrial Canal. It should benoted that each Seabird CTD deployed with the ADCP on the sameSea Spider platform also recorded pressure time series. They are notshown here as they are almost identical with those from the ADCP onthe same platform. The 1500 KHz Sontek horizontal Argonaut ADPalso provided pressure data. They are all consistent and thus notincluded in the figures for clarity. The fact that the corrected pressuretime series are consistent with that from the Industrial Canal,especially after Ike’s influence was dissipated, suggests that thecorrection procedure as outlined above is reliable, since the Indus-trial Canal platform and horizontal Argonaut ADP did not moveduring the storm surge.

Industrial Canal, showing the overall consistency of the two.

Fig. 4. Time series of along channel velocity from a horizontal ADP and time series ofsurface elevation from a YSI CTD at the Industrial Canal during Gustav.

C. Li et al. / Estuarine, Coastal and Shelf Science 87 (2010) 517–525 521

4.1. Maximum surface height due to the storm surges

The maximum surface height from the Rigolets was always thehighest among the three stations, although the surface height fromthe Industrial Canal was the highest during a period before themaximum peak. More specifically, the surface height from theRigolets reached w2.5 m above its pre-storm mean level, indicatinga storm surge (the original data subtract the predicted tidalelevation) of greater than 2 m there. The maximum height at theIndustrial Canal was about 0.4 m lower than that from the Rigolets.The height from Chef Menteur Pass was the lowest of the three,reaching only about 1.4 m maximum. After day 246 (September 2,UTC), the surface height from all three stations tended to merge.This further indicates that the error correction procedure doesappear to have removed the jumps reasonably well. Otherwise, thesurface elevation data after the storm would not have a similarmean value. The increase of surface height due to Hurricane Ike hada similar response, and the water surface had the highest maximumvalue recorded at the Rigolets, followed by that from the IndustrialCanal and then that from the Chef Menteur Pass. Note that duringHurricane Ike, there was no movement of any of the platforms. Thefact that the elevation from Chef Menteur Pass was the lowest maybe because this site is further away from the coastal ocean andwhen the storm surge propagated to this site from the open ocean,it had to go through Lake Borgne, which is very shallow and thebottom friction may have caused more attenuation to the surge.

4.2. Duration and timing of peak elevation

The high water induced by Hurricane Gustav lasted for about 3days which included a sharp increase reaching the peak, and anequally sharp decrease for a short duration (w0.25 day) followed bya prolonged relatively gradual decrease afterwards (Fig. 3). Thesurface height for the second hurricane lasted for about 4 days witha relatively symmetric change so that the rate of increase and rateof decrease were about the same. The asymmetric behavior of thefirst storm surge may be caused by the close distance of the firsthurricane (Gustav) and the rapid dissipation after its landing.Hurricane Gustav made landfall at Cocodrie, Louisiana whileHurricane Ike made landfall in Texas (Fig. 1). The second hurricanewas much farther away and therefore only the remote effect of thewind (Garvine, 1985) and pressure contributed to the much lowersurges in Lake Pontchartrain.

The timings for the surface height peak caused by the firsthurricane at the Industrial Canal, Rigolets, and Chef Menteur Passwere at 245.73, 245.755, and 245.79 (day in 2008), or at about17.52 h, 18.12 h, and 18.96 h on September 1, respectively. Thismakes the time lag between the surface height peaks in IndustrialCanal and Chef Menteur Pass to be about 1.4 h. The timings for thesurface height peak caused by the second hurricane at the Indus-trial Canal, Rigolets, and Chef Menteur Pass were at 256.63, 256.72,and 256.68 (day in 2008), or at about 15.12 h,17.28 h, and 16.32 h onSeptember 12, respectively. This makes the time lag between thepeaks in Industrial Canal and the Rigolets to be about 2.1 h. TheIndustrial Canal always had the earliest peak, at least 1.4 h earlierthan the other stations, indicating that the storm surge propagatedfaster into the Industrial Canal, probably because of the easiercommunication through the manmade straight canals for shallowwater waves (i.e. the storm surge waves, but not the non-hydrostatic short surface waves).

4.3. Storm induced velocity

All the acoustic Doppler current profilers worked flawlessly andrecorded high quality data. However, the velocity from the ADCP

deployed in the Industrial Canal turned out to be affected bya nearby bridge structure – because of concerns of busy boat trafficin a narrow channel, the mooring was put too close to the bank neara bridge structure. As a result, the flow velocity was small (w0.5 m/s) although it had all the characteristics of the rest of the data witha much reduced magnitude. The horizontal ADP deployed at themouth of the Industrial Canal, however, captured the flow throughthe main channel. We therefore used the ADP data for the analysisof the flux through the Industrial Canal. It should be noted that thisADP only recorded an averaged velocity within w15 m of the sensorand no velocity profile across the channel was measured. As shownin Fig. 4, the ADP recorded strong north component of tidal velocityoscillating between �1 and 1 m/s prior to the storm surge (Murty,1984). The water level showed a gradual ramp-up for 2–3 daysbefore the hurricane’s arrival, indicating a result of long wavegenerated by the far-away strong storm. The storm surge causedthe northward velocity to remain positive for almost 2 days duringwhich the ebb tidal currents were suppressed. The maximumvelocity at Industrial Canal during Gustav was about 2.2 m/s. It isinteresting to see that during the receding stage, there was a 2 dayprolonged strong ebb currents with maximum currents at1.5–1.7 m/s lasting for more than 1.5 days (between days 246.7 and248.2). The tidal oscillation of the current velocity and waterelevation were affected for at least 4 days during Gustav pluscontinued dissipation at a much less magnitude (not shown forclarity in Fig. 4). Fig. 5 shows time series of along channel velocity inthe Rigolets at various vertical distances above the bottom. In theIndustrial Canal, the along channel flow is roughly in the north–south direction, while in the Rigolets it is roughly in the east–westdirection. All the velocity values presented here were those aftera rotation to get the along channel flow component. The negativepeak current velocity reached more than 2 m/s at 5.37 m above thebottom. Again the tidal oscillation was interrupted for close to 4days. The asymmetric feature of the storm surge (shorter floodstage and longer receding stage) is quite common, e.g. as thatshown by a numerical model in Rego and Li (2009b).

An interesting feature of the time series is that, during the floodstage, the vertical shear of the velocity was almost non-existent,while during the receding stage, there was a relatively strongvertical shear of the along channel horizontal velocity which wasabout 0.8 m/s of velocity difference between 0.62 and 5.37 m abovethe bottom or within a 4.75 m water column. This is about 2–3

0.62 M1.62 M2.87 M4.12 M5.37 M

242 244 246 248 250 252 254

2

-1

0

1

Days

Ve

lo

city

(m

/s

)

ebb stage

flood stage

Aug. 29 Sept. 2 Sept. 6 Sept. 10

Fig. 5. Time series of along channel velocity at different depths along the watercolumn at the Rigolets during Gustav.

Fig. 6. Vertical profiles of the along channel velocity (m/s) during a 25-day periodcovering both Gustav and Ike. (a) Color contours of the time series of velocity profiles(m/s) with a 1-h 6th order Butterworth low pass filter at the Rigolets; (b) Colorcontours of the of velocity profiles (m/s) with a 40-h 6th order Butterworth low passfilter at the Rigolets.

Fig. 7. Total transport of water through the three tidal passes.

C. Li et al. / Estuarine, Coastal and Shelf Science 87 (2010) 517–525522

times of the vertical difference of horizontal velocity during normaltidal oscillations. The difference is similar to ‘‘tidal straining’’(Simpson et al., 1990, 2005; Rippeth et al., 2001; Li et al., 2009a) and‘‘wind straining’’ (Li et al., 2008) such that the receding stage mayhave a relatively larger vertical gradient of salinity. This cannot beverified as our moorings only measured near bottom salinity.

Fig. 6 shows the vertical profiles of the along channel velocity inthe Rigolets. It should be noted that positive means flood into thelake. For better visual effect, we applied the 6th order Butterworth1-h low pass filter for the raw data to show main features of thetidal and subtidal variations (Fig. 6a). For a better view of the lowfrequency variations, we also applied a 6th order Butterworth 40-hlow pass filter (Fig. 6b). It is apparent that the flood stage wasalways shorter than the receding stage of the storm surge regard-less whether one looks at the raw data (the 1-h low pass filtereddata is a close representation of the raw data) or the 40-h low passfiltered data.

4.4. Integrated water volume flux

From the two bottom mounted ADCPs in the Rigolets and ChefMenteur Pass we integrated vertically and multiplied by the width(433, and 272 m for the Rigolets and Chef Menteur Pass, respec-tively) to get the total flux through these two channels (Fig. 7,Li et al., 2009b). From the horizontal ADP mounted at the mouth ofthe Industrial Canal, we averaged laterally and multiplied by thewidth (60 m at the location) and depth (10 m) to get the flux (Fig. 7).We then calculated the total inward volume flux of water duringflood stage and receding stage of each of the two storm surges forall three channels. We also estimated the time periods of theseflood and receding stages. Finally, the percentage of flux through allthree channels for each stage was calculated (Table 2). DuringGustav, the volume fluxes into (>0) Lake Pontchartrain through theRigolets, Chef Menteur Pass, and Industrial Canal were estimated tobe 342, 529, and 92 million m3 (or tons in weight), respectively.These represented about 35.5%, 54.9%, and 9.6% of the total for thethree channels, over a time period of roughly 2.2, 2.1, and 2.7 days,respectively. The total inward flux over this less than 3-day floodperiod amounted to about 963 million m3. The receding stage wasmuch longer (all exceeding 8 days). The Rigolets and Chef MenteurPass had volume fluxes of about 45% and 46% of the total. Theremaining w9% of the flux was through Industrial Canal. Appar-ently, the net volume flux was not balanced (net ¼ �2200

Table 2Water volume flux in and out through the three inlets during Hurricanes (Positive: inward flux; Unit: 106 m3).

The Rigolets Chef Menteur Pass Industrial canal Total Net Mean flux rate (m3/s) Net flux rate (m3/s)

Gustav flux in 342 (35.5%) 529 (54.9%) 92 (9.6%) 963 �2200 5109 �2418Time lasted (days) 2.2 2.1 2.7Gustav flux out �1451 (45.9%) �1440 (45.5%) �272 (8.6%) �3163 �4446Time lasted 8.1 8.4 8.1Ike flux in 561 (46.6%) 492 (40.9%) 150 (12.5%) 1203 �944 5398 �984Time lasted 2.7 2.4 2.8Ike flux out �376 (17.5%) �1505 (70.1%) �266 (12.4%) �2147 �4024Time lasted 8.1 5.4 11.8

C. Li et al. / Estuarine, Coastal and Shelf Science 87 (2010) 517–525 523

million m3, negative is outward). The 7th column of Table 2 is themean volume flux rate which was obtained by distributing the totalinward volume flux over the flood time period (weighted averagebased on the length of time and the volume flux). It resulted to5109 m3/s for the flood and�4440 m3/s for the receding stages. Thelast column was calculated by considering the entire length (w10days), and the total net flux (342 � 1451 ¼ �1109 million m3) fora net flux rate (NFR):

NFR ¼ Net volume fluxtimeflood þ timerecede

This last column is useful in providing an overall average: the�2418 m3/s for Hurricane Gustav induced flow is the outwarddischarge rate that would be required to explain the total watervolume budget for the entire greater than 10 day period. The totalarea of Lake Pontchartrain is about 1630 km2. With an averagewater depth of 3.7 m, the total volume of the lake is roughly 6 km3

(Haralampides, 2000; White et al., 2009). The magnitude of fluxthrough the tidal passes can thus be put in the context of theflushing rate of the lake. For instance, with the net flux rate of2418 m3/s, the entire lake water would be replaced within 28 days;while at the rate of 984 m3/s, it would take 70 days. River dischargeinto the lake is largely variable but generally small. For instance,

Fig. 8. Potential error introduced by the second order derivative when the water elevation vavalue and the slope value). The vertical bars show the actual times when the mooring apSeptember 1, 2008 (Day 245 in 2008 was September 1).

discharge from the West Pearl River can reach w150 m3/s occa-sionally, an order of magnitude smaller than the deficit (2418 m3/sas shown in Table 2). Of course, during hurricane conditions, weanticipate larger discharge. The effect of discharge to the overallwater budget is not discussed in this paper but it can be a subject offuture study. For Hurricane Ike, results were somewhat different.The inward volume flux through the Rigolets was similar inmagnitude to that through Chef Menteur Pass. The magnitude andpercentage of inward flux through the Industrial Canal were bothlarger when compared to Gustav (12.5% vs. 9.6%). The outwardfluxes through the Rigolets and Chef Menteur Pass however weremuch different (70% through Chef Menteur alone!), while thepercentage through the IC (12.4%) remained similar to the floodstage. Again, the total inward flux was smaller than the totaloutward flux, with an imbalance of�944 million m3 (6th column ofTable 2). The weighted average mean flux rates were similar toGustav. The net flux rate of �984 m3/s over the entire flood andreceding stages was much smaller than that of Gustav.

The imbalance of total net volume flux is likely due to 1) therainfall from the hurricane and river discharge through a few smallrivers in the north of Lake Pontchartrain and through Lake Maur-epas to the west which can add to the inward flow; and 2) theinward fluxes into the lake due to inundation of land during theflood stage, which were not included in our calculations. This

lue is approximated by the Taylor series expansion to the first order (using the functionparently moved with a jump in depth values and where corrections were made on

C. Li et al. / Estuarine, Coastal and Shelf Science 87 (2010) 517–525524

inundation apparently occurred during Gustav but might not haveoccurred or at least occurred with much less magnitude duringHurricane Ike. Therefore, the net flux for Hurricane Ike, �944million m3, was more likely a combination of rainfall and riverdischarge while that for Gustav, �2200 million m3, probably hada significant contribution from inundation over land.

5. Summary

In this paper we presented results from multiple mooringinstruments at three tidal passes of Lake Pontchartrain and calcu-lated storm surge induced water volume flux during HurricanesGustav and Ike in September 2008, separated by 12 days. While theinward and outward flux through each pass varied, we haveobserved a few common features. The flooding stage of the surgeslasted 2–3 days while the receding stage was much longer –exceeding 8 days for the surge influence to be dissipated. HurricaneGustav induced inward flux of water volume was 963 million m3

through the three passes, with less than 10% through IndustrialCanal, 55% through Chef Menteur Pass, and the remaining throughthe Rigolets. The outward flux (3163 million m3) was more thanthree times the inward flux, with roughly the same percentage asthe inward flux (963 million m3) through the Industrial Canal andnearly equivalent fluxes through the Rigolets and Chef Menteur.

The inward flux of water volume induced by Hurricane Ike was1203 million m3 through the three passes, with 12.5% throughIndustrial Canal, 46.6% through the Rigolets, and 40.9% throughChef Menteur Pass. The outward flux (�2147 million m3) was abouttwice the inward flux, with roughly the same fraction (12.4%) goingthrough the Industrial Canal but 70% through Chef Menteur Passand only 17.5% through the Rigolets (Table 2). The imbalance of theinward and outward fluxes was probably caused by a combinationof rainfall, river discharge, and most importantly, additional inputdue to inundation over land. The water coming into Lake Pontch-artrain over land during the storm surge was apparent from thewater level data but the total amount cannot be estimated. Thesedata can be very useful in validation of numerical models forsimulations of storm-induced inundations into the Lake Pontchar-train system and adjacent areas (such as the impact to the city ofNew Orleans) and the application of such models to study therelated response of the ecosystem.

Acknowledgment

The information about Hurricanes Gustav and Ike was obtainedfrom the US National Weather Service. The research was supported byNSF (OCE-0554674 and DEB-0833225), and two NOAA grants,NA06NPS4780197 for NGoMEX funded to LUMCON and LSU, andNA06OAR4320264-06111039 to the Northern Gulf Institute byNOAA’s Office of Ocean and Atmospheric Research, U.S. Departmentof Commerce and Shell (http://www.ngi.lsu.edu/), and througha contract, NNS05AA95C, by Louisiana Board of Regents. We wouldlike to particularly thank Field Technicians Charlie Sibley, ChrisCleaver, and Darren Depew of the Coastal Studies Institute, LSU for thedeployment and retrieval of the instruments. We thank two anony-mous reviewers who provided critical comments and constructivesuggestions that helped the improvement of the manuscript.

Appendix. Error estimate

As we mentioned in the text, the mooring platforms deployed inthe Rigolets and Chef Menteur Pass moved during Hurricane Gus-tav due to the strong currents within the two tidal channels. Weused an algorithm to correct the depth change to reconstruct thestorm surge time series. It is necessary to have an estimate of the

error introduced by the correction algorithm. Here we providesome analysis and discussion of this error.

Recall that the time interval of the measurements was 5 min.Using data from the Rigolets as an example, there were 23 pointswithin the time series during Hurricane Gustav, when the platformapparently moved. Excluding these ‘‘bad’’ data points, the averageddepth change and the associated standard deviation over the 5 mintime interval were estimated to be 23.4 and 24.6 mm, respectively.If we use the standard deviation as the maximum error, the 23corrections would result to a maximum error of 0.57 m. Althoughthis is possible, it is most likely an overestimate. This is becausewithin the 23 points, the correction may be larger or smaller thanthe actual water level change over the 5 min interval. The positiveand negative errors will cancel each other to a certain extent andreduce the overall error.

This can be further justified by the following. In the correction,we used the slope of the surge to estimate the water level value atthe point where there was a bad data point. This is equivalent tousing the Taylor series expansion:

f ðt1Þ ¼ f ðt0Þ þ f 0ðt0Þðt1 � t0Þ þ f 00ðt0Þðt1 � t0Þ2=2

þ o�ðt1 � t0Þ3

�

in which t0 and t1 are adjacent time instances when measurementswere made in our application and t1 � t0 ¼ dt ¼ 0.0035 days(5 min). In our application, t0 and t1 are also the times with andwithout a valid data point, respectively. In the analysis, we used theadjacent ‘‘correct’’ slope of the data before the jump in value tocorrect the value at time equals t1 when there was a jump in depthvalue. Therefore, we are using the first two terms of the aboveequation to approximate the depth value at t1. The error of thisoperation is thus of the order of

Eðt1Þ ¼ f 00ðt0Þðt1 � t0Þ2=2:

here f(t) is the depth time series. Since the data points havecertain random nature, the derivatives of the depth time serieshave large errors. We therefore used a 5-point smoothing function(a moving average) to smooth the time series first before calcu-lating the first and second order time derivatives, from which E(t1)can be calculated as shown by Fig. 8. It is apparent that each of thecorrection operations resulted to an error of a few millimeters andcertainly all less than 0.01 m and the maximum would be muchless than 0.23 m even if all errors have the same sign (so nocancellations of positive and negative errors). A further verifica-tion of this is the fact that the water level time series from theRigolets and Chef Menteur Pass all matched that of the IndustrialCanal after the Hurricane Ike induced surge dissipated (Fig. 3).Based on these analysis and reasoning, we conclude that ouralgorithm of correction applied to the ‘‘bad data’’ due to apparentmovement of the platforms have reasonably small errors (lessthan 0.23 m with the worst case scenario).

References

Adams, C., Witt, E.C., Wang, J., Shaver, D.K., Summers, D., Filali-Meknassi, Y., Shi, H.,Luna, R., Anderson, N., 2007. Chemical quality of depositional sediments andassociated soils in New Orleans and the Louisiana Peninsula following Hurri-cane Katrina. Environmental Science and Technology 41, 3437–3443.

Carillo, A.R., Berger, C.R., Sarruff, M.S., Thibodeaux, B.J., 2001. Salinity Changes in thePontchartrain Basin Estuary, Resulting from Mississippi River Gulf Outlet PartialClosure Plans, 2001. U.S. Army Corps of Engineers. ERDC/CHL TR-01–14.

Garvine, R.W., 1985. A simple model of estuarine subtidal fluctuations forced bylocal and remote wind stress. Journal of Geophysical Research 90 (C6),11945–11948.

Georgiou, I.Y., McCorquodale, J.A., 2000. Salinity stratification from a navigationcanal in a shallow lake. In: Laurence, G.A., Pieters, R., Yonemitsu, N. (Eds.),Stratified Flows, vol. 2. IAHR, pp. 859–864.

C. Li et al. / Estuarine, Coastal and Shelf Science 87 (2010) 517–525 525

Georgiou, I., McCorquodale, J.A., 2002. Stratification and circulation in LakePontchartrain. In: Spaulding, Malcom L., Butler, Lee H. (Eds.), Estuarine andCoastal Modeling. ASCE, New York, pp. 875–887.

Haralampides, K., 2000. A study of the hydrodynamics and salinity regimes of thelake Pontchartrain system. Ph.D. dissertation, Univ. of New Orleans, 219 pp.

Li, C., 2002. Axial convergence fronts in a barotropic tidal inlet – sand shoal inlet,VA. Continental Shelf Research 22, 2633–2653.

Li, C., 2006. Modeling of bathymetry locked residual eddies in well-mixed tidalchannels with arbitrary depth variations. Journal of Physical Oceanography 36,1974–1993.

Li, C., Blanton, J.O., Chen, C., 2004. Mapping of tide and tidal flows using vesseltowed ADCP. Journal of Geophysical Research (Oceans) 109, C04002.doi:10.1029/2003JC001992.

Li, C., Valle-Levison, A., Wong, K., Lwiza, K., 1998. Separating baroclinic flow fromtidally induced barotropic flow in estuaries. Journal of Geophysical Research(Oceans) 103, 10405–10417.

Li, C., Walker, N., Hou, A., Georgiou, I., Roberts, H., Laws, E., McCorquodale, J.A.,Weeks, E., Li, X., Crochet, J., 2008. Circular plumes in Lake Pontchartrain estuaryunder wind straining. Estuarine, Coastal and Shelf Science 80, 161–172.

Li, C., Swenson, E., Weeks, E., White, J., 2009a. Asymmetric tidal straining across aninlet: lateral inversion and variability over a tidal cycle. Estuarine, Coastal andShelf Science 85, 651–660.

Li, C., Weeks, E., Rego, J., 2009b. In situ measurements of saltwater flux through tidalpasses of Lake Pontchartrain estuary by Hurricanes Gustav and Ike in September2008. Geophysical Research Letters 36, L19609. doi:10.1029/2009GL039802.

Murty, T.S., 1984. Storm surges: meteorological ocean tides. Canadian Bulletin ofFisheries and Aquatic Sciences 212 (Ottawa: Canadian Government PublishingCentre, 1984), ix þ 897 pp.

National Weather Service (NWS), 2008a. Hurricane Gustav Summary Availableonline at. http://www.srh.noaa.gov/lix/html/Gustav08/summary.htm.

National Weather Service (NWS), 2008b. Hurricane Ike Summary Available onlineat. http://www.srh.noaa.gov/lix/html/Ike08/summary.htm.

Pardue, J.H., Moe, W.M., McInnis, D., Thibodeaux, L.J., Valsaraj, K.T., Maciasz, E., VanHeerden, I., Korevec, N., Yuan, Q.Z., 2005. Chemical and microbiologicalparameters in New Orleans floodwater following Hurricane Katrina. Environ-mental Science and Technology 39, 8591–8599.

Poirrier, M.A., 1978. Studies of salinity stratification in southern Lake Pontchartrainnear the inner harbor navigation canal. The Proceedings of the LouisianaNational Academy of Sciences XLI, 26–35.

Rego, J., Li, C., 2009a. Numerical experiments and an overlooked parameter in stormsurge forecasting: the forward speed of a hurricane. Geophysical ResearchLetters 36, L07609.

Rego, J., Li, C., 2009b. On the receding of storm surge along Louisiana’s low-lyingcoast. Journal of Coastal Research SI 56 (2), 1045–1049.

Rego, J., Li, C. Nonlinear terms in storm surge predictions: effect of tide and shelfgeometry with case study from Hurricane Rita. Journal of Geophysical Research,in press.

Rippeth, Tom P., Fisher, Neil R., Simpson, J.H., 2001. The cycle of turbulent dissi-pation in the presence of tidal straining. Journal of Physical Oceanography 31,2458–2471.

Sikora, W.B., Kjerfve, B., 1985. Factors influencing the salinity regime of LakePontchartrain, Louisiana, a shallow coastal lagoon: analysis of long-term dataset. Estuaries 8, 170–180.

Simpson, J.H., Brown, J., Matthews, J., Allen, G., 1990. Tidal straining, densitycurrents, and stirring in the control of estuarine stratification. Estuaries 13,125–132.

Simpson, J.H., Williams, E., Brasseur, L.H., Brubaker, J.M., 2005. The impact of tidalstraining on the cycle of turbulence in a partially stratified estuary. ContinentalShelf Research 25, 51–64.

Stout, S.A., Liu, B., Millner, G.C., Hamlin, D., Healey, E., 2007. Use of chemicalfingerprinting to establish the presence of spilled crude oil in a residential areafollowing Hurricane Katrina, St. Bernard Parish, Louisiana. EnvironmentalScience and Technology 41, 7242–7251.

Suedel, B.C., Stevens, J.A., Kennedy, A.J., Brasfield, S.M., Ray, G.L., 2007. Environ-mental consequences of water pumped from Greater New Orleans followingHurricane Katrina: chemical, toxicological, and infaunal analysis. Environ-mental Science and Technology 41, 2594–2601.

White, J.R., Fulweiler, R.W., Li, C.Y., Bargu, S., Walker, N., Twilley, R.R., Green, S.E.,2009. The Mississippi River flood of 2008 – observations of a largefreshwater diversion on physical, chemical and biological characteristics ofa shallow, estuarine lake. Environmental Science and Technology 43,5599–5604.