Dispersion of Waves From Hurricane Gilbert and Their Intermittent Reception on the Alabama Coast

Abstract

ABSTRACT The U.S. Army Corps of Engineers is monitoring waves, currents, and bottom response at several test berms (mounds or bars built with dredged material) in the open ocean. Two of the most energetic episodes monitored thus far occurred in September 1988 off the Alabama coast. The frost high-wave episode coincided with the nearby landfall of Hurricane Florence. Beginning 5 days later, largewaves built again out of the south but there was no storm in the Gulf of Mexico. Over the next 4 days, the waves built, diminished and rebuilt again out of the south. Because of wave refraction, the local wave direction did not point to remote areas of wave generation. However, the rate at which energy shifted toward higher frequencies and the regional weather do confirm the second episode began with the arival of waves from the Caribbean (south of the Yucatan Straits) and ended with the arrival of waves from a remote part of the Gulf of Mexico. An intervening period of lower energy coincided with the passage of Hurricane Gilbert over the Yucatan Peninsula. Waves from Gilbert took 30 to 60 hours to reach the Alabama coast. The intensity of both the distant and nearby storms provided rare opportunities to record seafloor effects of waves with unusually long wave periods for the Gulf of Mexico. INTRODUCTION As part of continuing optimization of economic and environment aspects of the Nation?s dredging program, the U.S. Army Corps of Engineers has constructed and is monitoring experiment submerged berms at several sites on the open seafloor. The purpose is to evaluate beneficial uses for dredged material formerly placed far offshore. Uses under consideration include: reducing wave damageto the lee of the structures, contributing additional material to the coastal sand supply, diverting near-bed transport away from offshore sinks, reducing dredging costs, and improving fisheries? habitat. The practicality of such uses hinges on mound stability. The Corps is developing predictive models of long-term fate based on actual measured responses to winds, waves, and currents. This paper discusses several episodes of energetic waves an bottom velocities at an instrumented site offshore of the Alabama coast. Hurricane waves caused two episodes of intense bottom velocities, Hurricane Florence made landfall about 160h Southwestof the berms. Although a second hurricane, Gilbert, never came within 1,000 km, it caused similarly intense velocities just above the seafloor. The purpose of this paper is to describe the meteorological conditions creating these two unusual situations and to document the wave conditions themselves. Waves and bottom velocities from these episodes significantly expand the information available on hurricane-driven nearshore processes and is a first for berm monitoring. REG1ONAL SETTING The coastal climate of Alabama is characterized by mild winters with cold fronts arriving every week to 10 days. Approaching fronts bring southerly onshore winds end high water levels. After passage of a front, northerly (offshore) winds depress the water level and flatten the seas. Summers are warm and humid with wind-intense, but short-lived thunderstorms.