David A. Pepper

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.