Gregory W. Stone

Spatiotemporal Patterns and Return Periods of Tropical Storm and Hurricane Strikes from Texas to Maine

Abstract The authors analyze 105 yr (1901–2005) of tropical cyclone strikes at 45 coastal locations from Brownsville, Texas, to Eastport, Maine, with the primary objective of examining spatiotemporal patterns of storm activity. Interpretation of the data suggests that geographically, three focal points for activity are evident: south Florida, the Outer Banks of North Carolina, and the north-central Gulf Coast. Temporally, clusters of hyperactivity are evident in south Florida from the 1920s through the 1950s and then again during the most recent years. North Carolina was a region of enhanced activity in the 1950s and again in the 1990s. A more consistent rate of occurrence was found along the north-central Gulf Coast; the last two years, however, were active in this region. Return periods of tropical storm strength systems or greater range from a frequency of once every 2 yr along the Outer Banks of North Carolina, every three years on average in southeast Texas, southeastern Louisiana, and southern Flori...

Multiple sediment sources and a cellular, non-integrated, longshore drift system : Northwest Florida and southeast Alabama coast, USA

Abstract The morphosedimentary maintenance of a 225 km stretch of coast along the northeast Gulf of Mexico, from Grayton Beach, Florida, to Morgan Point, Alabama, has been interpreted previously within the framework of a unidirectional, integrated, monotonic longshore drift model, with a single headland source of sediment located east of Grayton Beach. Net longshore transport and the granulometry and composition of some 2000 foredune, beach and step samples indicate a cellular net drift system, supplied by three independent sources of sediment. One source is provided by the Pleistocene barrier island complex along Grayton-Mirimar Beach, the second at Pensacola Beach on Santa Rosa Island, and the third is onshore transport across the inner shelf between Pensacola, Florida, and Morgan Point, Alabama. The Pleistocene “headland” and Pensacola Beach supply two cells along Santa Rosa Island, whereas onshore transport from the low gradient inner shelf supplies sediment to three cells along the largely accretional beach-ridge-dominated coast from Pensacola Pass to Morgan Point. Drift cells along this coast experience negligible net sediment exchange. These findings have significant implications for both the late Holocene evolution and the morphodynamic maintenance of this coast.

Hurricane Ivan's Impact along the northern Gulf Of Mexico

Just over a year after the landfall of Hurricane Ivan, scientists have now had an opportunity to evaluate a variety of oceanographic and geologic responses to this storm. Hurricanes Ivan, Katrina,and Rita are among the most powerful hurricanes recently to enter the Gulf of Mexico. Although it weakened from a very powerful Category 5 hurricane to a Category 3 before making landfall along the Alabama coast, Hurricane Ivan devastated the coasts of northwestern Florida and Alabama on 16 September 2004. This article summarizes what researchers have learned about Hurricane Ivan as it moved into the Gulf and made landfall along the northeastern Gulf of Mexico coast. The article focuses on storm meteorology, sea state,shelf circulation, and sediment transport on the shelf and along the coast.

Geomorphologic Evolution of Barrier Islands along the Northern U.S. Gulf of Mexico and Implications for Engineering Design in Barrier Restoration

Abstract Aspects of northern Gulf of Mexico (NGOM) (Louisiana, Mississippi, Alabama, and Florida panhandle) processes and barrier islands that are pertinent to their geomorphologic response are contrasted with the broader knowledge base summarized by Schwartz (1973) and Leatherman (1979, 1985). Salient findings from studies documenting the short-term (storm-induced; timescales of hours, days, and weeks) and long-term (timescales of years, decades, and centuries) response of barrier island systems in the NGOM are synthesized into a conceptual model. The conceptual model illustrates the hypothetical evolution of three barrier island morphologies as they evolve through a typical Category 1–2 hurricane, including poststorm recovery (days to weeks) and long-term evolution (years to decades). Primary factors in barrier island geomorphologic response to storms, regardless of location, are the elevation of the island relative to storm (surge plus setup) elevation, and duration of the storm. Unique aspects of the NGOM barrier islands, compared with knowledge summarized for other barrier types, include (1) storm paths, wind speed, and large bays that create the potential for both Gulf and bayshore erosion and (2) in Louisiana and Mississippi, the potential for loading of the underlying substrate by the barrier island, which, through time, increases consolidation, relative sea level rise, overwash, morphologic change, and migration. We recommend that design of large-scale beach restoration projects incorporate the potential for (1) time-dependent consolidation of the underlying sediment due to project loading and future migration, (2) Gulf and bayshore erosion and overwash, and (3) eolian transport toward the Gulf from north winds.

A new depositional model for the buried 4000 yr BP New Orleans barrier: implications for sea-level fluctuations and onshore transport from a nearshore shelf source

Abstract The Holocene New Orleans Barrier Complex, now buried by the St. Bernard delta of the Mississippi River, provides an excellent example of barrier deposition fed by a nearshore sediment source. This reworking and onshore transport was initiated by a sudden change in the shelf equilibrium profile caused by a sea-level fall about 4100 yr BP. Here we present a new model of barrier formation which does not invoke an Shepard-type Holocene sea-level curve nor the supply of sediment from a longshore source. The Holocene New Orleans Barrier Complex consists of fine-grained, locally cross-bedded, quartz sand that contains Ophiomorpha nodosa burrows and disarticulated mollusks, primarily marine, buried beneath up to 4 m of silty mud of the St. Bernard Lobe. This barrier island and shoal deposit overlies interbedded, fine-grained sand and mud containing marine mollusks, some articulated, that is interpreted to be a nearshore shelf deposit. Its deposition took place between 5500 and 4200 yr BP (14C), based on individual dates on seven articulated and seven disarticulated shells. The barrier formation is effectively limited to a several-hundred-year window approximately 4000 yr BP by the 3800 yr BP Rangia sp. shells from the immediately overlying St. Bernard Lobe delta-plain deposits and the buried 3900–3500 yr BP Linsley archeological site, situated on a more gulfward distributary levee. In this paper we present a new depositional model on the New Orleans Barrier. The barrier complex contains an abundance of large mollusk shells that have been reworked to the extent that 14 individual shells yield a 2500-year range, 6000–3500 yr BP. An older, nearby source is required. The current model of a spit/shoal complex migrating westward from an eroding eastern Pleistocene headland probably cannot account for the deposition of large reworked shells given the effects of abrasion and selective size sorting over approximately 50 km of longshore transport. Furthermore, this model demands transport rates of millions of cubic meters per year for the present northern Gulf coast which are at least an order of magnitude higher than its highest known rates. We postulate a nearby shell and sand source that is subjacent and offshore rather than adjacent and littoral. We propose it to be the underlying nearshore shelf deposit that could be mobilized by a brief fall of sea level and carried landward. The barrier complex and the uppermost nearshore shelf deposit have markedly different net deposition rates. The upper 25 cm of the nearshore shelf deposit were deposited over 800 years, based on ages of articulated marine pelecypods. A 20-km-long, 3-km-wide and 4-m-thick segment of the barrier complex was deposited in less than 300 years. A 10-cm-thick lag pavement of bryozoan- and oyster-encrusted mollusk shells that comprises the nearshore shelf deposit beneath the northern edge of the barrier complex is evidence for an essentially zero net deposition rate. The current interpretation of a conformable, progradational relationship between these two units is rejected in favor of a disconformable contact. The hiatus across this disconformity must be less than the several-hundred-year duration of barrier complex deposition. The positioning of the shallower-water barrier complex disconformably over the deeper-water nearshore shelf deposits indicates a sea-level fall.

Studying the importance of hurricanes to the northern Gulf of Mexico coast

A pilot study was recently begun dealing with the impacts and post-storm adjustment of barrier islands to severe storms along the northern Gulf of Mexico. The study, funded by the National Science Foundation, may lead to a more comprehensive understanding of coastal morphodynamics and longer term evolution of the coast in that area. Study of the recent impacts and post-storm adjustment of the area to Hurricane Georges is playing a very important role in enhancing our comprehension of the significance of these high-energy events in coastal dynamics. A few hours before dawn on September 28, 1998, Hurricane Georges made landfall near Biloxi along the Mississippi Gulf Coast (Figure 1) as a strong Category 2 system (as defined by the Saffir/Simpson scale: Simpson, 1974). Waves off the Mississippi/Louisiana coast exceeded 10 m in height and storm surge varied between 2–3 m. The entire stretch of coast from the modern Mississippi delta to the Florida Panhandle, a distance in excess of 200 km, was severely impacted by the hurricane through overwash and breaching of the barrier islands, and erosion of the distal ends of the sub-deltas and major distributaries comprising the Birdsfoot delta in Louisiana. Near the landfall zone, the Category 2 storm impacted the coastline with estimated sustained winds of over 45 ms−1, and rainfall between 300 and 400 mm in coastal Mississippi. Georges was the sixth storm to impact this stretch of coast since 1995, the most severe being Hurricane Opal, at its strongest a powerful Category 4 system that made landfall east of Pensacola Beach, Florida, as a marginal Category 3 hurricane [see Eos article by Stone et al., 1996; Lawrence et al., 1998]. The cumulative impact of storms during this period of intense hurricane activity has altered significantly the morphology of the Northwest Florida coast. A review of historical photographs dating back to the early 1900s suggests that the entire coast is now in quite probably the most degraded morphological condition of the 20th century.

Ship Shoal as a prospective borrow site for barrier island restoration, coastal south-central Louisiana, Usa: Numerical wave modeling and field measurements of hydrodynamics and sediment transport

Abstract Ship Shoal, a transgressive sand body located at the 10 m isobath off south-central Louisiana, is deemed a potential sand source for restoration along the rapidly eroding Isles Dernieres barrier chain and possibly other sites in Louisiana. Through numerical wave modeling we evaluate the potential response of mining Ship Shoal on the wave field. During severe and strong storms, waves break seaward of the western flank of Ship Shoal. Therefore, removal of Ship Shoal (approximately 1.1 billion m3) causes a maximum increase of the significant wave height by 90%–100% and 40%–50% over the shoal and directly adjacent to the lee of the complex for two strong storm scenarios. During weak storms and fair weather conditions, waves do not break over Ship Shoal. The degree of increase in significant wave height due to shoal removal is considerably smaller, only 10%–20% on the west part of the shoal. Within the context of increasing nearshore wave energy levels, removal of the shoal is not significant enough to cause increased erosion along the Isles Dernieres. Wave approach direction exerts significant control on the wave climate leeward of Ship Shoal for stronger storms, but not weak storms or fairweather. Instrumentation deployed at the shoal allowed comparison of measured wave heights with numerically derived wave heights using STWAVE. Correlation coefficients are high in virtually all comparisons indicating the capability of the model to simulate wave behavior satisfactorily at the shoal. Directional waves, currents and sediment transport were measured during winter storms associated with frontal passages using three bottom-mounted arrays deployed on the seaward and landward sides of Ship Shoal (November, 1998–January, 1999). Episodic increases in wave height, mean and oscillatory current speed, shear velocity, and sediment transport rates, associated with recurrent cold front passages, were measured. Dissipation mechanisms included both breaking and bottom friction due to variable depths across the shoal crest and variable wave amplitudes during storms and fair-weather. Arctic surge fronts were associated with southerly storm waves, and southwesterly to westerly currents and sediment transport. Migrating cyclonic fronts generated northerly swell that transformed into southerly sea, and currents and sediment transport that were southeasterly overall. Waves were 36% higher and 9% longer on the seaward side of the shoal, whereas mean currents were 10% stronger landward, where they were directed onshore, in contrast to the offshore site, where seaward currents predominated. Sediment transport initiated by cold fronts was generally directed southeasterly to south-westerly at the offshore site, and southerly to westerly at the nearshore site. The data suggest that both cold fronts and the shoal, exert significant influences on regional hydrodynamics and sediment transport.

Researchers study impact of Hurricane Opal on Florida coast

On October 4, 1995, over 2000 km of coast-line stretching from southwest Florida to Louisiana was struck by storm-generated waves as Hurricane Opal moved northward across the Gulf of Mexico toward landfall east of Pensacola Beach, Florida (Figure 1). Approximately 12 hours before landfall on October 4, Opal neared category 5 strength (measured on the Saffir/Simpson scale) with sustained wind speeds of over 65 m s−1. Storm surge levels of ∼5 m were estimated across the Northwest Florida shelf by the National Hurricane Center (NHC), resulting in the overwash of most of Santa Rosa Island, the most extensively affected section of coast in the Gulf.

The Importance of Cyclogenesis on the Short-Term Evolution of Gulf Coast Barriers

ABSTRACT Over the past several years, research has been conducted to determine the importance of cyclones on the morphodynamics and morphological maintenance of barrier islands along the northern Gulf of Mexico. Data are presented indicating the complexity and highly variable response of a 12 km stretch of barrier along the Florida panhandle to cold front passage across the northern Gulf, in addition to the impacts of tropical storms, hurricanes and winter storms in the Gulf of Mexico. Two temporal scales are used to evaluate short-term response and adjustment to these events, i.e., months and a near five year record of sediment volume change, the combination of which have resulted in a time series of change for the subaerial and nearshore environments. These data incorporate the morphological signature of six tropical storms/hurricanes and approximately 100 frontal passages. Three important conclusions have been reached based on these data: (1) low-lying barrier islands, which are vulnerable to overwash, can conserve mass during catastrophic hurricanes (e.g., Hurricane Opal, a strong category 4 hurricane near landfall); (2) less severe hurricanes can promote rapid dune and berm aggradation and contribute sediment to the entire barrier system; and (3) cold fronts play a critical role in the post-storm adjustment of the barrier by deflating the subaerial portion of the overwash terrace and eroding its marginal lobe. The data presented also indicate that barrier systems along the northern Gulf do not necessarily enter an immediate post-storm recovery phase after catastrophic hurricanes, even though they are nested in sediment-rich nearshore environments. It is anticipated that these findings will have important implications for better understanding the longer-term evolution of barrier systems in mid-latitude, micro-tidal settings elsewhere around the globe.

Numerical Simulation of Typhoon Wind Forcing in the Korean Seas Using a Spectral Wave Model

Abstract This study investigates the application of the wave model (WAM) to simulate the generation and propagation of typhoon waves in Korean seas. The model solves the energy balance equation for wave growth based on wind energy input, and simulates spatial and time evolution of wave spectra. Although WAM has been extensively validated and used in various global and regional wave forecasts, its application to the simulation of typhoon waves has not been investigated thoroughly. Crucial to the application of WAM in typhoon wave modeling is the specification of accurate wind input data and adequate resolution of the wind structure. This study compares and analyzes two different wind fields for the same event, viz., simulated wind field from a storm model and blended global QSCAT/NCEP winds. The simulation experiment was run for 4 days as Typhoon Olga (1999) approached the west coast of Korea. The results are compared with shallow water buoy observations as the typhoon was approaching landfall. The simulated significant wave heights in the open ocean were approximately 8.6 m, which gradually decreased as the typhoon approached shore. Olga was an intense typhoon, and its compact wind structure provides a unique test case to examine the required directional, frequency, and spatial resolution in WAM for modeling typhoon generated waves.