1

LANDFALL OF TWO TROPICAL SYSTEMS SEVEN DAYS APART ALONG SOUTHCENTRAL LOUISIANA, USA

Gregory W. Stone1,2 Alex Sheremet1,2 X. Zhang1 Qing He1 Baozhu Liu1 Brandon Strong3

Abstract: In 2002 and for the first time in recorded history, coastal Louisiana was struck by two tropical cyclones only seven days apart. Tropical Storm Isidore made landfall along the Caminada Moreau Headland on September 26 and Hurricane Lili came ashore near Marsh Island on October 3. Both storms were unique in many ways; Isidore although loosely organized in the Gulf, developed sea state that affected beaches all along the northern Gulf of Mexico (GOM). The strength of Lili generated waves in excess of ~14 m while a Category 4 storm in the central GOM. The storms' trajectories brought both close to several ocean observing stations (WAVCIS---Wave-Current-Surge Information System) that subsequently provided comprehensive and unique data sets; one station (CSI 3) was located in the eye wall of Lili as the hurricane approached landfall. Here we present selected aspects of the wave and surge evolution with a focus on Hurricane Lili. The morphological impacts along the coast are summarized. One particularly interesting phenomenon observed during Hurricane Lili, was the important role played by the muddy inner shelf off western Louisiana in dissipating wave energy and inhibiting the development of the storm surge field on the shelf and along the coast. Theoretical estimates of surge significantly over predict surge when data are compared to in situ observations obtained from the WAVCIS array. INTRODUCTION Historical records suggest that the southeast of the U.S. experiences among the greatest number of tropical cyclone landfalls around the globe with the Gulf of Mexico experiencing a large number of them (Figure 1). In a recent study that focused on the frequency of tropical systems impacting the Louisiana coast, evidence was presented showing that along with Key West, Florida, south-central Louisiana ranked the highest over a one hundred year period (1900-2000) in frequency of strikes of major storms (category 3 and above) for an area extending from Texas to North Carolina (Muller and Stone, 2001). Several recent scientific studies have underscored the significance of tropical and extratropical storms in driving coastal erosion along the Louisiana coast (Stone and Finkl, 1995; Stone et. al., 1993; 1995; 1997; 1999; 2003; Muller and Stone, 2001). Winter storms in addition to tropical cyclones have frequently impacted the coast and more often than not, resulted in severe overwashing and breaching of the barrier and mainland systems. This is largely due to the fact that the beach and dune system elevation is exceptionally low, typically less than ~3 m above sea level. Thus, the cumulative impact of these events over time is a gradual but significant decline in the physical stability of this coast. The combination of storms, sea level rise, subsidence and a reduction in sediment supply to the coast has resulted in rapid deterioration of the barrier islands in Louisiana. -------------------------------------- 1) Coastal Studies Institute, 312 Howe- Russell Geoscience Complex, Louisiana State University, Baton Rouge, LA 70803, USA, gagreg@lsu.edu, ashere1@lsu.edu, 2) Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA, zhang2@lsu.edu, bliu2@lsu.edu, qinghe@sklec.ecnu.edu.cn. 3) RD Instruments, San Diego, CA. 9855 Business Park Ave., San Diego, CA, USA 92131 Bstrong@rdinstruments.com

2

Figure 1. Upper: An example of the frequency of landfalls of hurricanes in the Atlantic basin over a one hundred year period during the 20 th century. Lower: Example of the spatial distribution of hurricane trajectories and landfalls for the same time period in the Gulf of Mexico (data obtained from the National Hurricane Center).

The islands are particularly vulnerable to breaching during storms and post-storm recovery is generally not accomplished, or slow between storm events. Reviews of the importance of hurricanes and tropical storms on coastal Louisiana can be found in (Penland et al, 1989; Stone et al, 1993; 1996; 1997; 1999; 2003). Recent work also shows that with the gradual demise of barrier islands along south-central Louisiana, wave energy conditions in the bays are increasing with time (Stone and McBride, 1998). This phenomenon is apparent during fair-weather wave

3

conditions and during storms when lower frequency waves propagate through wider inlets and breaches along the barrier system. TROPICAL CYCLONES ISIDORE AND LILI The 2002 Hurricane Season proved to be one of the most active seasons for the State of Louisiana in more than 100 years. Four storms – Bertha, Hanna, Isidore and Lili – made landfall along the Louisiana coast, the greatest number of storms to strike the Louisiana coast in a single season in recorded history. Bertha was the first landfalling tropical cyclone to strike the coast since Danny in 1997. Isidore and Lili (Figure 2), however, had the most significant impact on the coast. Upon crossing Cuba and entering the Gulf of Mexico on 20 Sep 2002, Isidore became the Gulf’s first hurricane of the 2002 Atlantic Hurricane Season. The cyclone tracked westward towards the Yucatan Peninsula, and meandered over that landmass for two days (Figure 3). Although more than 800 km south of the U.S. central Gulf Coast, the tropical cyclone already was having an impact on coastal Louisiana. The cyclone was interacting with a ridge of high pressure over the continental U.S. and generating a Gulf-wide pressure-gradient, and the resulting northeasterly and easterly winds began increasing water levels along the southeast Louisiana coast.

Figure 2. Satellite images of cyclones Isidore (upper) and Lili (lower) in the GOM.

4

Figure 3. Storm tracks for cyclones Isidore and Lili.

Isidore began its long-anticipated northward advance on 24 Sep. Satellite imagery suggests that a steady intrusion of “dry” air on the southwestern and southern flanks of the system was at least partly responsible for the lack of significant strengthening. The National Hurricane Center estimates indicate peak sustained winds ranged from 25-28 ms-1 (50-55 kts) as Isidore crossed the GOM. Data indicate that the center of the storm passed directly over NDBC Buoy 42041 (27.50?N, 90.50?W) at approximately 0000 UTC on 26 Sep. Peak sustained winds recorded at Buoy 42041 exceeded 17 ms-1 (33 kt), but briefly dropped to less than 1 ms-1 while the center of the system was over the buoy. Tropical-storm force winds began arriving along the Louisiana coast at approximately 0000 UTC on 26 Sep. Wind gusts exceeded 26 ms-1 (> 50 kt) near Grand Isle, with sustained winds in excess of 17 ms-1 (> 33 kt) extending along the storm’s path as the system made landfall. Lili originated from a tropical wave that moved over the tropical Atlantic Ocean from the west coast of Africa on September 16. On the 21stt the system was upgraded to a tropical depression. For the remainder of the month, Lili underwent phases of strengthening and weakening but on October 1 the center of the hurricane moved over the western mainland of Cuba, with wind speeds as high as 47 ms-1 (90 kt) gradually accelerating its forward speed to approximately 8 ms-1 (15 knots). Lily turned northward and made landfall along the western Louisiana coast on the October 3, with an estimated 42 ms-1 (80 kt) maximum wind speed. However, between Cuba and Louisiana, Lili intensified to 65 ms-1 (125 kt) earlier that dayd over the north-central Gulf of Mexico and then rapidly weakened during the 13 hours until landfall. Lili was absorbed by an extratropical low on October 4 while moving northeastward near the Tennessee/Arkansas border. Lili was the first hurricane to make landfall in the United States since Irene hit Florida in 1999.

90°0'0"W 80°0'0"W 70°0'0"W 60°0'0"W 50°0'0"W

10°0

'0"N

20°0

'0"N

30°0

'0"N

Atlantic Ocean

Pacific Ocean

Gulf of Mexico

South America

LiliIsidore

^NDBC 42001

5

HYDRODYNAMIC RESPONSE Four stations comprising a portion of Louisiana State University’s WAVCIS array were ideally located to capture unique hydrodynamic (and meteorological) data. As shown in Figure 4, Isidore generally paralleled the north-south array (CSI 11, 5 and 6) offshore to NDBC 42041. CSI 3 was located in the eye wall of Hurricane Lili near

Figure 4. Image showing Isidore and Lili storm tracks and location of WAVCIS array.

landfall. Plots of time evolution of wave spectra at CSI 5 (Figure 5A) bear the distinct signature of Isidore (longer duration - 5 days, less energetic associated wave field) and Lili (2 days, strong event). The two peaks, at approximately 0.8 Hz, separated in time by some 3 days, correspond to the two phases in Isidore’s evolution, separated by the decrease in intensity during the time the storm spent over the Yucatan Peninsula. Lili entered the Gulf approximately 6 days later with a rapid forward speed and intensifying quickly. As a result, its wave system was more energetic but had a shorter duration. The bimodal spectral distribution at Terrebonne Bay (CSI 11, Figure 5B) is likely due to swell propagating into the bay through Cat Island Pass (located between CSI 11 and 5) and strong local generation of high frequency waves (0.25-0.3 Hz) by wind. Dissipation in the long wave band (frequency less than 0.2 Hz) was likely due to strong refractional scattering through Cat Island Pass and bottom friction. At this location, locally generated short waves (frequency > 0.2 Hz) dominate the spectrum. The difference in strengths between the two systems is illustrated also by the stronger seas associated with Hurricane Lili. TS Isidore followed a general south-north path which took it over 5 observation stations from the central GOM (42001) north to Terrebonne

ÊÚ

"8"8

"8

"8

#

#

#

#

#

#

#

#

#

#

#

#

NDBC42041

CSI 11CSI 05

CSI 03

CSI 06

09/26/00Z

09/26 /03Z

09/26/06Z

09/26/09Z

09/26/12Z

10/03/03Z

10/03/05Z

10/03/07Z

10/03/09Z

10/03 /11Z

10/03/13Z

10/03/15Z

Hurricane LiLi

T S Is

i dor

e

UTM 83 ZONE 15, DEPTH IN METERS (FEET)

GULF OF MEXICO

5 2 5 0 0 0

5 2 5 0 0 0

6 0 0 0 0 0

6 0 0 0 0 0

6 7 5 0 0 0

6 7 5 0 0 0

7 5 0 0 0 0

7 5 0 0 0 0

8 2 5 0 0 0

8 2 5 0 0 03000000

3075000

3150000

3225000

3300000

3300000

3225000

3150000

3075000

3000000

30 0 30 60 Kilometers

N

1000 (3280)500 (1640)200 (656)

100 (328)

50 (164)

2000 (6560)1500 (4920)

10 (33)

20 0 20 40 Miles

CSI,LSU, 20 02.8

6

Bay, over a distance of >400 km. A time series of significant wave height during Isidore’s passage is presented in Figure 6A. At the peak of the storm, deep water stations recorded waves with significant heights in excess of 6 m. Very energetic wave conditions (5 m significant wave height) were observed closer to the coast (CSI 6, near 20 m isobath). Figure 6B shows a similar plot for H. Lili as measured at CSI 3. The very steep

Figure 5. (A) Spectral evolution during Isidore and Lili at CSI 5: (B) Spectral evolution for same storms at CSI 11 in Terrebonne Bay.

A

B

7

ramping and decay of wave height at CSI 3, agrees with previous observations during winter storms where strong attenuation of both swell and sea have been observed along the west Louisiana shelf which is characterized by cohesive sediment (Sheremet and Stone, this volume). SPECTRAL EVOLUTION The evolution of each storm emphasizing the bimodal characteristics is presented in Figure 7 and is based on ADCP measurements taken at CSI 3. Hurricane Lili

SIGNIFICANT WAVE HEIGHT DURING T.S. ISIDORE

0

1

2

3

4

5

6

7

9/21/02 9/22/02 9/23/02 9/24/02 9/25/02 9/26/02

DATE

WA

VE

HE

IGH

T (M

)

NDBC 42001 NDBC 42041 CSI 06 CSI 05 CSI 11

SIGNIFICANT WAVE HEIGHT DURING HURRICANE LILI

0

2

4

6

8

10

12

9/30/02 10/1/02 10/2/02 10/3/02 10/4/02

DATE

WA

VE

HE

IGH

T (M

)

NDBC 42001 CSI 03

Figure 6 (A). Time series of Hs (m) at 5 locations from the central GOM to Terrebonne Bay along the path of TS Isidore (left). (B) Time series of Hs (m) from the central GOM to CSI 3 along the path of H Lili.

B

A

8

generated long waves (12-18s) which moved out radially as they dispersed with swell waves arriving first along the western Louisiana coast. Because the wavelength was long, the swell refracted normal to the shelf bathymetry and by the time it reached CSI 3 it was predominantly south or shore normal. This refraction can be seen in the wave direction of the swell and as shown in Figure 8, the higher frequency end of the swell formed a tail

Figure 7. Time series of spectra shows the spectral evolution of Isidore and Lili. Sept. 18 - Oct 3, 2002 measured at CSI 3. that tends toward the southeast. Also evident in Figure 7 are the high frequency sea waves propagating from the northeast. The storm rotated counter clockwise and as the leading edge of the hurricane approached the coast, a wave spectrum some 90 degrees to the radius of the storm developed. As the eye of the hurricane approached CSI 3, the wind speed increased and sea state increased for the offshore waves. The opposing direction for the wind-sea and the swell leaves a conspicuous gap in the wave height spectrum, at frequencies just above the swell. Offshore wind, opposing onshore swell waves, attenuated higher frequency swell (0.1-0.2 Hz) as the offshore spectrum built in the opposite direction (0.2-0.5 Hz). Figure 9 demonstrates the changing direction of the wind-sea. As the eye of the storm approached CSI 3, the radius of curvature decreased and the wind speed increased. The wind-sea rotated from northeast to southeast as the eye wall passed over and then to southwest as it moved toward landfall. As the eye wall passed both sea and swell spectra merged. Isidore showed the same rotation of wind-sea direction, but in the opposite direction since the eye passed more than 100 km to the east of CSI 3. CURRENT AND SURGE EVOLUTION A time series of significant wave height, water level and mean current velocity are presented in Figure 10. All three are seen to be out of phase. Wave energy (top, green) increased first because the spectrum included both local wind-sea and swell that was generated earlier in the storm. The advancing storm created storm surge (middle, blue) where the pressure of the storm forced water out before it. The storm surge

9

presented in Figure 10 (bottom, red) is shown with onshore current magnitudes on the order of >1 m/sec (3-4 knots). These currents peaked just a few hours before the maximum water level was reached (middle, light blue).

Figure 8. Swell and sea generated by the same storm in opposing directions.

Figure 9. Wave/wind direction vectors point 90 degrees counter-clockwise to the path between the eye and the measurement.

CSI 3

10

Figure 10. Time series of significant wave height, water level and mean current velocity for hurricane Lili. Water elevations measured on the shelf (CSI 3 and 5) and in Terrebonne Bay (CSI 11) are presented in Figure 11. During Isidore, surge levels approximated 0.5 m on the inner shelf and adjacent bay at the CSI 5 and 11 location. Setdown of approximately 0.5 m occurred at CSI 5 in western Louisiana where coastal waters responded to offshore winds as the system made landfall to the east. Lili generated surge levels of ~1.5 m at CSI 3, approximately double that measured at CSI 5 to the east. North of CSI 5 in Terrebonne Bay, however, surge levels of ~1.2 m were measured. Ongoing work in which various surge models are being skill assessed suggests that some numerical models are over estimating surge along this portion of coast by between 3-6 fold. MORPHOLOGICAL IMPACTS Examples of the morphological impacts associated with Isidore and Lili are presented in Figure 12. The barrier islands in southcentral Louisiana underwent significant breaching and erosion during both events, even though Lili’s track was located well in excess of 100 km to their west. Erosion in the range of 60-70 m along the Gulf facing beach was widespread along these barriers (Shea Penland, pers. com.). Although the track of Lili put CSI 3, located in western Louisiana, in its eastern portion of the eyewall, significant wave heights were essentially the same as those measured along the south-central barrier coast (between 2.25-2.5 m) located to the east. An important controlling factor is the damping effect of fluid muds that characterize the shelf off western Louisiana. This is clearly apparent on comparing the various hydrodynamic data presented earlier and is equally important during winter storms (Sheremet and Stone, this volume). CONCLUSIONS The Louisiana coast was impacted by four tropical cyclones during the hurricane season of 2002. Isidore and Lili had a significant impact on much of the coast and a comprehensive suite of hydrodynamic and meteorological data were captured from the WAVCIS array. Impacts along the coast were significantly influenced by fluid mud on the west Louisiana shelf, where a considerable amount of wave damping was observed. Several post-storm studies funded by the Louisiana Department of Natural resources are

11

Figure 11. Water level during both storms measured on the inner shelf (CSI 3 and 5) and

in Terrebonne Bay (CSI 11).

12

Figure 12 (A) Overwash near the location of Lilil’s landfall. (B) Breaching along Raccoon Island due to Isidore and Lili. (C) Pre- (2001) cyclone impact at Raccoon Island breakwaters. (D) Post-cyclone landfall at Raccoon Island breakwaters.

A B

C D

13

currently underway to quantify further the impacts of both storms and the degree of post-storm adjustment that is taking place along the coast. ACKNOWLEDGEMENTS The authors acknowledge the following current sponsors of the WAVCIS program: Louisiana Department of Natural Resources, Contract #169704190; Federal Emergency Management Agency, Contract #2529-01-01; National Oceanic Atmospheric Administration #NA160C2938; Louisiana Applied Oil Spill Research and Development Program, #169704196. ChevronTexcao is acknowledged for infrastructure and logistical support. REFERENCES Penland, S., Suter, J.R., Sallenger, A.H., Williams, S.J., McBride, R.A. Westphal, K.E., Reimer, P.D. and Jaffe, B.E., 1989. Morphodynamic Signature of the 1985 Hurricane Impacts on the Northern Gulf of Mexico. Proc. Sisixth Symposium on Coastal and Ocean Management (ASCE July 11-14, 1989, Charleston, South Carolina), pp. 4220-4233. Muller, R.A. and Stone, G.W., 2001. A Climatology of Tropical Storms and Hurricane Strikes to Enhance Vulnerability Predicition for the Southeast U.S. Coast. Journal of Coastal Research, 17, 4, 949-956. McBride, R. A., Penland, Shea, Hiland, M. W., Williams, S. J., Westphal, K. A. Jaffe, B. E., and Sallenger, A. H., Jr., 1992, Analysis of barrier shoreline changes in Louisiana from 1853 to 1989, in Williams, S. J., Penland, Shea, and Sallenger, A. H., Jr., eds., Louisiana barrier island erosion study-atlas of barrier shoreline changes in Louisiana form 1853 to 1989: U.S. Geological Survey Miscellaneous Investigations Series I-2150-A, p. 36-97. Sheremet, A. and Stone, G.W., 2003. Wave Dissipation due to Heterogeneous Sediments on the Inner Louisiana Shelf. Proc. Coastal Sediments’03 (this volume). Stone, G.W. and Finkl, C.W. (eds.), 1995. Impacts of Hurricane Andrew on the Coastal Zones of Florida and Louisiana: 22-26 August, 1992. Coastal Education and Research Foundation, Fort Lauderdale, Fl., 364 pp. Stone, G. W., Grymes, J., Steyer, K., Underwood, S., Robbins, K., and Muller, R. A., 1993. A Chronologic Overview of Climatological and Hydrological Aspects Associated with Hurricane Andrew and its Morphological Effects Along the Louisiana Coast, U.S.A. Shore and Beach. 61(2):2-12.

Stone, G. W., Xu, J.P. and Zhang, X.P, 1995. Estimation of the Wave Field During Hurricane Andrew and Morphological Impacts along the Louisiana Coast, in Impacts of Hurricane Andrew on the Coastal Zones of Florida and Louisiana: August 22-26, 1992. G. W. Stone and C. W. Finkl (eds.). Journal Coastal Research Special Issue 21, 234-253.

14

Stone, G.W., Grymes, J.M., Armbruster, C.A. and Huh, O.K., 1996 Overview and Impacts of Hurricane Opal on the Florida Coast." EOS, Transactions of the American Geophysical Union, 77551-553. Stone, G.W., Grymes, J.M., Dingler, J.W. and Pepper, D.A., 1997. Overview and Significance of Hurricanes on the Louisiana Coast, USA. Journal of CoastalResearch, 13, 3, 656-669.

Stone, G.W. and McBride, R.A., 1998. Louisiana barrier Islands and their Importance in Wetland Protection: Forecasting Shoreline Change and Subsequent Response of Wave Climate. Journal of Coastal Research, 14, 3, 900-916. Stone, G.W., Wang, P., Pepper, D.A., Grymes, J.M., Roberts, H.H., Zhang, X.P., Hsu, S.A. and Huh, O.K., 1999. Researchers Begin to Unravel the Significance of Hurricanes on the Northern Gulf of Mexico Coast. EOS, Transactions of the American Geophysical Union,80, 27, 301-305.

Stone, G.W., Sheremet, A., Zhang, X., Walker, N.D., Grymes, J.M., Huh, O.K., Hsu, S.A., Blanchard, B., Bentley, S.J., 2003. A Decade of Tropical Cyclones in one Hurricane Season; Impacts of Isidore and Lili on the Louisiana Coast. (in prep.)