Case studyShelf sediment transport during hurricanes Katrina and Rita
Introduction
Episodic events, such as river floods and hurricanes, happen over a relatively short period of time (hours to weeks), but can greatly modify the sedimentary record. However, our coastal scientific community has rather limited ability to predict the characteristics of strata produced by even well-observed modern-day events (Corbett et al., 2014). This is probably due to either the lack of field measurements or the damage of optical and acoustic sensors during energetic and extreme events.
It is well known that hurricanes drive energetic winds and generate currents and waves, both propagating toward the land. When the hurricanes reach continental shelves, seabed sediment is suspended to water column, causing the erosion on seabed. The seabed elevation difference between pre-hurricane level and the “deepest cut” is thus defined as “maximum erosional depth” (Fig. 1A), which is also called “bed scour” (Keen and Glenn, 2002). After making landfalls, hurricanes dissipate and sediment settles back to seabed or in coastal estuaries and wetlands. The seabed elevation difference between the deepest cut and post-hurricane level is thus defined as “post-hurricane deposit” (Fig. 1A and B). Net erosion/deposition is defined as the difference between pre- and post-hurricane levels. During the passage of hurricanes, some regions may experience erosion only (Fig. 1C), whereas other areas like costal bays/wetlands may have deposition only (Fig. 1D). Turner et al. (2006) reported that the major source of mineral sediment to Louisiana coastal marshes is from hurricanes, not river floods. Törnqvist et al. (2007), however, believed that there were an over-estimation of hurricane return periods and a lack of erosion measurements in the study of Turner et al. (2006). By now our coastal scientific community has not reached a consensus on sediment dynamics during hurricanes. No matter if rivers or hurricanes play a more important role in wetland sedimentation, it is the net erosion/deposition that actually impacts sediment budget, not the post-hurricane deposition, as shown in Fig. 1. Unfortunately it is well known that the erosional process is challenging to measure in sedimentary system. In some environments erosion can be measured using either the comparisons among repeated field surveys (e.g., Goff et al., 2010) or the numerical models (e.g., Xu et al., 2011).
In this study we focus on Louisiana continental shelf sediment transport during hurricanes. Hurricanes strike the Louisiana coast approximately once every three years, normally between May and November (Neumann et al., 1993). Based on an overview of hurricane impacts, Stone et al. (1997) found that hurricanes cause chronic erosion to Louisiana barrier systems, but sometime also generate considerable deposition in Louisiana’s marshes and bays. They also reported that Hurricanes Audrey (1957) and Andrew (1992) formed 0.70 and 0.16 m of mixed organic and inorganic debris in Louisiana marsh areas, respectively. Goni et al. (2006) collected sediment samples in the inner shelf southwest of Atchafalaya Bay after Hurricane Lili (2002) and identified a storm layer up to 0.2 m thick; they also found fining-upward deposits to be composed of silty clays with a sandy basal layer. Keen and Glenn (2002) predicted bed scour on the continental shelf during Hurricane Andrew (1992) and simulated storm sedimentation on the sandy Ship Shoal (Fig. 2) of Louisiana shelf; they found that the bottom boundary layer was wave-dominated and bed scour was primarily by resuspension. Goff et al. (2010) found offshore sediment transport during Hurricane Ike (2008) and believed that shoreface sands appear to have been incised by the storm and advected offshore by the strong storm-surge ebb currents.
In this study we focus on Hurricanes Katrina and Rita, both of which happened in the year 2005. The economic and environmental damages caused by Katrina were over $40 billion of insured losses (Knabb et al., 2006). Hurricane Katrina formed as a tropical depression on August 23, 2005 over the Bahamas Islands. After passing the southern tip of Florida, it moved westward into the Gulf of Mexico where it gained strength to become a category five hurricane in August 28. Then it swept northward and made landfall on August 29 over the Mississippi River Delta (MRD) as a category four hurricane (http://www.csc.noaa.gov; Fig. 2). About three weeks later, Hurricane Rita formed as a tropical depression in the Caribbean Sea and headed westward into the Gulf of Mexico. On September 21 it became a category five hurricane and moved northwest toward the western Louisiana coastline, where it made landfall on September 24 at the Texas–Louisiana border as a category three hurricane (Fig. 2).
Multiple event-response studies have been performed in wetland, estuary and shelf areas after Hurricanes Katrina and Rita. Turner et al. (2006), for example, reported that more than 131 Million Tons (Mt) of post-hurricane sediment deposited in coastal wetlands when Hurricanes Katrina and Rita crossed the Louisiana coast; they identified several cm thick of sediment deposition on wetlands. Based on further data analyses, Tweel and Turner (2012) reported that sediment deposition on coastal wetlands was 68 and 48 Mt from Hurricanes Katrina and Rita, respectively. Using gain size, X-radiographs, and gamma-density data for sediment cores, Keen et al. (2006) found that Hurricane Katrina deposited a storm bed east of landfall on the Louisiana shelf with a maximum observed thickness of 0.58 m, which thinned to approximately 0.1 m about 200 km west of landfall; they also reported that the fining-upward bed is similar to event beds observed in both ancient and modern sedimentary environments. Based on a collaborative multi-institution rapid-response effort, Walsh et al. (2006) collected bathymetric and sediment core data in the MRD and reported the evidence of mud flow activities near the Mississippi subaqueous delta after two hurricanes. Goni et al. (2007) studied radionuclides, x-radiographs and stable isotopes by analyzing 1-cm thick slices in post-hurricane Katrina/Rita deposits and found that the post-hurricane layers was predominantly local sediments mobilized by the intense wave activity during the storms; they also believed that post-hurricane deposit thicknesses ranged from centimeter to decimeter levels (Fig. 3).
Section snippets
Motivations and objectives
Despite the above extensive research efforts, 3-D hurricane sediment transport modeling studies are rather limited for Louisiana shelf. Surprisingly the only 3-D hurricane sediment transport modeling work published so far for Louisiana shelf has been Keen and Glenn (2002), which focused on regional sediment transport at the sandy Ship Shoal during Hurricane Andrew. Xu et al. (2011) developed a 3-D hydrodynamic-sediment transport model for the Texas–Louisiana shelf using the Regional Ocean
Methods
The study of Xu et al. (2011) was focused on fluvial sediment dispersal, but in this study we focus on seabed sediment resuspension under intense and rapidly changing (hourly) hurricane environments. Details of modeling methods are described later, but major improvements since our 2011 study are summarized here: (1) The wind field was changed from spatially-uniform to spatially-variable to better represent the hurricane conditions, such as the eyes of hurricanes; (2) Open ocean boundary
Winds, waves and discharge
In August 2005, the passage of Hurricane Katrina brought wind speeds that exceeded 20 m/s and waves that were 15 m tall (Fig. 5A and B). In August and September 2005 water and sediment discharge from the Mississippi and Atchafalaya Rivers were relatively low (Fig. 5C and D). Modeled wind speeds were compared with the values measured at both NDBC buoys and WAVCIS stations. The model reproduced wind speeds recorded at the four stations, and correctly represented the spatial structure; Stations
Discussion
This section synthesizes our results by evaluating the relationship between spatial patterns of sedimentation and storm track, summarizing the computed sediment budget, and suggesting how the sensitivity and uncertainty analysis may be of use in future studies. Additionally, we relate our study to some previous modeling efforts from the literature to evaluate hurricane-driven sediment transport.
Conclusions
Below are the major findings and conclusions in this study:
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Our coupled hydrodynamic – sediment transport model successfully represented extreme storm conditions, such as 50 m/s wind speeds and 25 m tall waves. The model estimated peak nearbed wave orbital velocities of 3 m/s that produced wave–current bed shear stresses of 50 Pa and over 1.5 m of seabed scour. The CSTMS sediment transport routine developed by Warner et al. (2008) proved to be robust to represent extreme and rapidly changing
Acknowledgments
J.P. Walsh and D. Reide Corbett of East Carolina University provided post-hurricane deposit data which were published in Goni et al. (2007), facilitating the comparison between the modeled and observed hurricane deposit. Paul Mattern (Dalhousie University) gave advice of uncertainty analysis and helped with polynomial chaos approximation. Martinho Marta-Almeida (Texas A&M University) helped with the access of boundary and climatology files used in the ROMS model. Chunyan Li of Louisiana State
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