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Hurricane Rita, 2005: Assessment of a Storm-Induced Geological Event

along the Southwestern Louisiana Coast and Adjacent Interior Marsh

Walter S. Guidroz1, Gregory W. Stone2, and Dane Dartez3

1BP America, Inc., 501 Westlake Park Dr., Houston, TX 77079

2Louisiana State University, Coastal Studies Institute and Department of Oceanography and Coastal Sciences,

Baton Rouge, LA 70803

3Hurricane Center, Louisiana State University, Baton Rouge, LA 70803

ABSTRACT The landfall of Hurricane Rita on September 24, 2005 significantly altered physical

and morphological characteristics along the western Louisiana coast as well as marshes approximately 25 km inland. Storm-surge measurements show super elevated water levels around 15 ft (4.6 m) were measured in the Cameron area to the east. A helicopter overflight of central and southwestern Louisiana, undertaken by the Coastal Studies Institute of Louisiana State University about three weeks after Hurricane Rita’s landfall, revealed major storm-induced sedimentologic phenomena directly attributable to the passage of this Category 3 storm. Overwash deposits (predominantly fine sand) and evidence of excessive storm surge and salt-water intrusion were clearly evident in Cam-eron and Vermilion parishes, Louisiana. These responses became progressively more pronounced farther west, culminating at the point of eyewall impact near Sabine Pass on the border between Texas and Louisiana. Widespread damage to physical infrastruc-ture was also noted within several communities located on or near the coast, particularly in Cameron and Holly Beach, Louisiana. Excessive salinity levels resulting from salt-water intrusion severely impacted local vegetation several kilometers inland and, over the longer term, may exacerbate existing problems linked to coastal erosion through the loss of vegetation that retards coastal erosion and attenuates waves and storm surge dur-ing high-energy events. However, initial post-storm beach recovery was also noted early post-storm landfall. Bar welding to the lower shoreface was evident as sediment was being reworked onshore by waves. Follow-up observations will focus on documenting continued post-storm recovery as well as the potential effects of future tropical cyclones that may impact the area.

INTRODUCTION The coastlines of southwestern Louisiana and southeastern Texas were severely impacted in September 2005

by Hurricane Rita, a Category 3 hurricane that made landfall in the early morning of September 24 in western Cameron Parish, Louisiana (Fig. 1). This intense storm initiated unique sedimentologic responses along the

Guidroz, W. S., G. W. Stone, and D. Dartez, 2006, Hurricane Rita, 2005: Assessment of a storm-induced geological event along the southwestern Louisiana coast and adjacent interior marsh: Gulf Coast Association of Geological Societies Transactions, v. 56, p. 229-239.

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south-central and southwestern Louisiana coast that were documented during a helicopter overflight three weeks after the storm. The purpose of this paper is to provide a qualitative assessment of these effects as well as short-term post-storm recovery after landfall of Hurricane Rita.

Storm History and Sea State Hurricane Rita began as a tropical depression just east of the Turks and Caicos Islands on September 17,

2005 and intensified steadily during the next several days as it moved westward through the Florida Straits. The eye of Hurricane Rita passed approximately 50 mi (80 km) south of Key West, Florida on September 20 and thus missed the U.S. mainland as it approached southern Florida as a Category 2 hurricane on the Saffir-Simpson scale. However, upon entering the Gulf of Mexico, Rita intensified rapidly and attained Category 5 status and a peak sustained wind strength of 175 mph (78 m/sec) within only 24 hrs. The barometric pressure recorded during this phase of the storm reached 897 mbar, the third-lowest ever recorded for a tropical system in the Atlantic Ba-sin at that time. This pressure reading simultaneously displaced to fifth lowest the 902-mbar minimum pressure measurement from Hurricane Katrina recorded only one month earlier (National Hurricane Center, 2005; Knabb et al., 2006). However, both of these measurements were subsequently displaced one level lower in October 2005 by Hurricane Wilma, which yielded the lowest barometric pressure ever recorded to date in the Atlantic Basin at 882 mbar (Pasch et al., 2006).

Wind and wave conditions during Hurricane Rita were measured by a series of buoys maintained by the National Data Buoy Center. While still at sea in the Gulf of Mexico, the eye of Rita passed approximately 2 mi (3 km) southeast of Buoy 42001, the data buoy closest to the path of the storm (Fig. 1). A barometric pressure of 926 mbar was recorded at 6:00 p.m. local time (2300 Zulu or Coordinated University Time) on September 23 (about 8.5 hrs prior to landfall). The maximum significant wave height measurement was 38 ft (11.6 m); sus-tained winds were measured at 100 mph (45 m/sec) and maximum gusts were measured at 136 mph (61 m/sec), both approximately 7 hrs prior to landfall. As Rita neared the coast, C-MAN station SRST2, located near Sabine Pass on the upper Texas coast (Fig. 1), recorded a barometric pressure of 951 mbars at 1:00 a.m. on September 24 (approximately one hr prior to landfall). Central pressure fell considerably as the storm approached the north-central Gulf Coast by September 24.

Hurricane Rita weakened as it approached the north-central coast of the Gulf of Mexico on September 23, eventually becoming a Category 3 hurricane approaching landfall. The hurricane turned northwesterly relative to

Figure 1. Track of Hurricane Rita and location of NDBC buoy 42001 and C_MAN station SRST2 (map courtesy of National Hurricane Center, 2005).

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Hurricane Rita, 2005: Assessment of a Storm-Induced Geological Event along the Louisiana Coast and Adjacent Interior

its previous westward track through the Gulf and moved ashore between Sabine Pass and Johnson’s Bayou early on the morning of September 24 with sustained winds of 120 mph (54 m/sec). Concurrent with its onshore arri-val and wind field, Rita initiated a substantial storm surge east of its center that reached a maximum of 18-20 ft (5-6 m). Rita then weakened rapidly as it continued northward through east Texas and western Louisiana, even-tually turning northeastward and merging with an advancing cold front on September 26, 2005.

Field observations of high water marks along the coast and inland for several miles were obtained by the Louisiana State University (LSU) and the Federal Emergency Management Agency (FEMA). Both data sets are generally in good agreement and are presented in Figures 2 and 3 (URS Group, Inc., 2006). Maximum water levels associated with Rita were between 14 and 15 ft (4-4.5 m) at Cameron and slightly farther to the east (all elevations were measured relative to NAVD88). Inland along the margins of Calcasieu Lake, storm surge was significantly attenuated, and high water marks ranged between 3 and 10 ft (1 and 3 m). To the east as far as Ver-million Bay, storm surges remained significant and high water marks of up to 12 ft (3.7 m) were measured. Nu-merical models (SLOSH and AdCirc) were run in hindcast mode and consistently underpredicted surge levels at the coast while overpredicting surge levels inland. The latter may be a direct function of the inability of these models to account for differences in bottom friction between water and marsh.

Overflight Route The flight route along which post-hurricane coastal conditions were observed extended along practically the

entire south-central and southwestern Louisiana coastlines and interior marshlands (Fig. 1). The coastal flight path began near the Atchafalaya and Wax Lake delta system, extended westward toward Marsh Island and Ver-milion Bay, and concluded at Sabine Pass in Cameron Parish, Louisiana. An interior marshland flight pass was then made that extended eastward from Sabine Pass and continued through Cameron, Calcasieu and Vermilion Parishes. Digital photos were taken throughout the trip as was a constant stream of video data that provided a continuous record of the entire trip. Photos were calibrated to a GPS to ensure accurate positioning of each pic-ture so that sedimentologic and morphologic changes detected in potential follow-up trips could be accurately calibrated to those from this original flight.

Figure 2. Louisiana State University (LSU) high water mark field observations, Hurricane Rita, south-western Louisiana.

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RESULTS

Several unique phenomena were observed during the overflight. Massive overwash deposits were observed

on the backside of many beaches along the coast. These became progressively more pronounced farther west and culminated at the point of eyewall impact near Sabine Pass. In addition, excessive storm surge and salt-water intrusion also became more evident farther west as indicated by the loss of vegetation in inland marshes as well as the catastrophic infrastructure and property loss in locations such as Holly Beach and Cameron. Scour effects and sediment re-distribution patterns were also noted, particularly in association with several series of rock jetties located just offshore from the southwestern Louisiana coast. Despite the destructive nature of the phenomena observed, the initial phases of post-storm recovery were also detected, primarily through bar welding to the lower shoreface as sediment was being reworked from offshore by post-storm waves.

Overwash deposits were observed in western Vermilion Parish and continued westward through most of Cameron Parish. These overwash deposits were typically located approximately 30-50 ft (9-15 m) behind the immediate coastline and displayed widths of approximately 40 ft (12 m; Fig. 4). In many cases, ripple beds were detected on top of overwash deposits, indicating significant wave-induced currents. In other cases, overwash deposits were seen as distinct lobes in tandem with distributary patterns that appeared to consist of relatively finer-grained, muddier material (Fig. 5). However, overwash was not detected continuously along the coast. This fact may stem from either the lack of an initial, sand-sized offshore sediment supply or from localized varia-tions in coastline morphology (Figs. 6 and 7). In some cases, distinct incision events were detected across a broad mud apron along the immediate shoreline and seemed to indicate evidence of return flow of water and sedi-ment to the Gulf of Mexico. These dendritic-shaped patterns continued from the shoreline southward to a hinge point just offshore where they appeared to bifurcate and form subaqueous channels directed offshore (Figs. 6 and 7). Although significant onshore sediment movement occurred during Hurricane Rita, this evidence indicates that steady, post-storm sediment and water movement had been taking place in the offshore direction.

Hurricane Rita generated a significant storm surge that resulted in substantial salt water intrusion, much of which occurred as much as 25 km inland along southwestern Louisiana. Evidence for this intrusion was noted through vegetation loss (primarily through marsh grass that had been killed off through saline seawater intoler-ance) as well as through debris fields that had been transported by these super-elevated water levels. In particu-lar, several large shrimp boats had been dragged onshore by the storm surge and then stranded several hundred

Figure 3. Federal Emergency Management Agency (FEMA) high water mark field observations, Hur-ricane Rita, southwestern Louisiana.

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Figure 4. Overwash deposits, eastern Cameron Parish, Louisiana. Inset shows ripple beds on over-wash deposits.

Figure 5. Lobate sand overwash deposits, western Vermilion Parish, Louisiana.

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Figure 6. Incision and return flow into Gulf of Mexico, western Cameron Parish, Louisiana. Note (1) incision into mud apron just offshore, and (2) subaqueous channel bifurcation concurrent with water and sediment return flow into Gulf of Mexico.

Figure 7. Incision and return flow into Gulf of Mexico, western Cameron Parish, Louisiana. Note (1) incision into mud apron just offshore, and (2) subaqueous channel bifurcation concurrent with wa-ter and sediment return flow into Gulf of Mexico.

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yards (meters) inland once storm waters receded into the Gulf of Mexico. Scour marks associated with these surge-driven shrimp boats were oriented east-southeast to west-northwest, which was in agreement with the pri-mary maximum wind and surge field experienced during the storm (Fig. 8).

Numerous cases of catastrophic infrastructure and property loss were also noted, particularly near the towns of Holly Beach and Cameron, Louisiana. Holly Beach, in particular, bore the brunt of the excessive storm surge and wave effects during Hurricane Rita, with surge elevations estimated up to 18 ft (5 m). Buildings at Holly Beach consisted of summer camps constructed on pilings designed to allow unimpeded surge inundation during a relatively minor storm event. However, the surge associated with Hurricane Rita effectively destroyed every structure in Holly Beach such that only occasional pilings were left remaining (Fig. 9). Ponded water, remaining from the surge event three weeks earlier and trapped from re-entering the Gulf by localized topographic relief, was also clearly evident. However, west of eyewall impact along the upper Texas coast, storm surge levels were much lower compared to areas east of eyewall impact in southwestern Louisiana due to the counter-clockwise circulation associated with Rita (D. B. King, 2006, personal communication).

An interesting observation noted during the overflight was the interaction of sediment dispersal patterns as-sociated with a series of rock breakwaters located just offshore the coastline near Holly Beach, Louisiana. Where breakwaters were located close to shore, the resulting tombolo located behind each structure appeared to have been partially scoured in an east-southeast to west-northwest direction, which also was coincident with the ex-pected primary wind and surge field during Hurricane Rita (Figs. 10 and 11). In addition, small overwash depos-its were observed behind the scour features. Directly offshore of Holly Beach, an entire series of lobate deposits were oriented in a similar direction behind the structures, further reflecting the primary direction of surge-wave current (Fig. 12).

Evidence of post-storm beach recovery was detected despite the short time interval between the storm and the overflight (approximately three weeks). Bar welding to the lower shoreface was evident as sediment was being reworked from offshore by post-storm wave activity (Fig. 13). A series of parallel lineaments were de-tected in western Cameron Parish, particularly near Sabine Pass, in which bar welding had already begun to take place progressively farther away from the shoreline. Additional evidence for offshore bar welding was evident offshore from Holly Beach, where a similar series of lineaments was noted (Fig. 14).

Figure 8. Stranded shrimp boat, Cameron, Louisiana. Note drag marks oriented east-southeast to west-northwest (orange arrow) in tandem with primary wind and surge field. Boat has been dragged approximately 2,500 ft (760 m) inland.

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Figure 9. Storm surge and wave effects, Holly Beach, Louisiana. Note infrastructure damage and evi-dence of ponded water three weeks after passage of Hurricane Rita.

Since 1901, at least 55 tropical storms and hurricanes have made landfall within Louisiana, implying a long-term average of one storm about every other year (Stone et al., 1997). However, storm frequency along the cen-tral and southwestern Louisiana coasts is nearly twice that of eastern Louisiana with a peak in activity usually occurring in the month of September. The relatively high frequency of storms along this section of the Gulf Coast therefore exerts a cumulative effect on and can significantly alter coastal geomorphology from year-to-year. However, post-storm adjustments to these effects also play a critical role. As an example, post-storm re-covery has been previously documented along several Louisiana barrier islands after Hurricanes Juan (1985), Danny (1985) and Gilbert (1988) (Stone et al., 1999). Although post-storm recovery depends upon the degree of damage and the availability of sediment within the coastal system, more investigation is needed to further quan-tify the full effects. This overflight was only the initial investigation into documenting destruction and recovery effects; follow-up observations are required to address adequately the pace and efficiency of future beach recov-ery.

CONCLUSIONS The observations noted in the aftermath of Hurricane Rita demonstrate both the destructive nature of severe

storms as well as post-storm recovery events prevalent within the coastal environment. These observations will serve as the basis for future investigations that will document what is expected to be continued post-storm recov-ery. However, recovery may be interrupted, and possibly delayed, by the effects of future tropical cyclones that impact the north-central and northwestern Gulf of Mexico coasts.

ACKNOWLEDGMENTS The authors would like to thank BP America, Inc. for underwriting costs associated with this overflight. We

also thank the Coastal Studies Institute of Louisiana State University for their support in arranging overflight

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Figure 10. Sediment re-distribution patterns adjacent to rock jetty, Vermilion Parish, Louisiana. Note (1) scour behind the breakwater coincident with primary wind and current fields, and (2) sand over-wash deposits behind areas of scour.

Figure 11. Sediment re-distribution patterns adjacent to rubble around breakwaters, Vermilion Par-ish, Louisiana. Note (1) the orientation of remaining subaqueous sediment coincident with primary wind and current fields, and (2) the return flow of sediment and water into the Gulf of Mexico.

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Figure 12. Sediment dispersal patterns behind a series of rock breakwaters, Holly Beach, Louisiana. Note the orientation of sediment coincident with primary wind and current fields behind each struc-ture.

Figure 13. Bar welding as evidence of post-storm recovery, western Cameron Parish, Louisiana. Note lineaments parallel to coast that represent active bar welding processes.

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Figure 14. Bar welding as evidence of post-storm recovery, Holly Beach, Louisiana.

logistics. Xiongping Zhang and Felix Jose of the Coastal Studies Institute acquired digital photo images and video data during the flight that were used in assembling this paper. The authors would also like to thank Don Davis, Director of the Louisiana Oil Spill Research and Applied Development Program, for sharing his observa-tions of Hurricane Rita’s effects on the Louisiana chenier plain.

REFERENCES CITED Knabb, R. D., D. P. Brown and J. R. Rhome, 2006, Tropical cyclone report: Hurricane Rita, 18-26 September 2005:

National Hurricane Center, Miami, Florida, 33 p. National Hurricane Center, 2005, Monthly tropical weather summary, September 2005: Miami, Florida, 3 p. Pasch, R. J., E. S. Blake, H. D. Cobb, III, and D. P. Roberts, 2006, Tropical cyclone report: Hurricane Wilma, 15-25

October 2005: National Hurricane Center, Miami, Florida, 25 p. Stone, G. W., J. M. Grymes, III, J. R. Dingler, and D. A. Pepper, 1997, Overview and significance of hurricanes on the

Louisiana coast, U.S.A.: Journal of Coastal Research, v. 13, no. 3, p. 656-669. Stone, G. W., P. Wang, D. A. Pepper, J. M Grymes, III, H. H. Roberts, X. Zhang, S. A. Hsu and O. K. Huh, 1999,

Studying the importance of hurricanes to the northern Gulf of Mexico coast: Eos (Transactions, American Geo-physical Union), v. 80, p. 301-305.

URS Group, Inc., 2006, Hurricane Rita rapid response: Louisiana coastal & riverine high water mark collection: Fed-

eral Emergency Management Agency, Hazard Mitigation Technical Assistance Program, Contract No. EMW-2000-CO-0247, Task Orders 445 & 450, Gaithersburg, Maryland, 42 p.

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