The Impact of Typhoon Pamela (1976) on Guam's Coral Reefs and Beaches

Located on a main typhoon corridor, Guam receives approximately one tropical cyclone per year. Typhoon Pamela, Guam's third most intense typhoon of this century, generated 8-meter waves, but these had little direct effect on Guam's coral reefs, even on the exposed northern and eastern sides of the island. Damage to the reefs was isolated and in the form of breakage due to extraneous material being worked over the reef by the surf and surge. These findings are contrasted with reports of typhoon-induced, large-scale reef destruction, mostly from areas off the major storm tracks. Guam's reef formations have developed in a way that enables them to withstand intense wave assault. Pamela caused significant modification of Guam's northern and eastern beaches, however. Most vegetation was removed to an elevation of 3 to 4 meters above mean lower low water, and the beach profiles were reduced from pretyphoon 8°_5° slopes to 3°_5° slopes through the transport of sand seaward. The first stage of recovery is the retreat and steepening of the lower beach. Longshore transport of sand during the typhoon yielded net erosion or deposition of up to 25 m3 per meter of beach face. The maximum height of the wave surges along the coast was linearly related to the width of reef flat and beach traversed. A I-meter drop in maximum surge height per 115 meters of distance traversed with an initial potential head of 9 meters is indicated. TYPHOON PAMELA PASSED DIRECTLY OVER the island of Guam (13° N, 144° E) on 21 and 22 May 1976, devastati.ng the trees, crops, and buildings with estimated maximum sustained winds of 220 to 270 km/hr. The island experienced 18 hr of typhoon-force winds in excess of 115 km/hr and 6 hr in excess of 185 km/hr (100 knots). Pamela moved over Guam in a northwest direction at 13 km/hr. During the first half of the storm, due to its cyclonic circulation, the winds came from the northeast (wind direction 050-070), the usual direction from which storm winds are received (according to Fleet Weather Central! Joint Typhoon Warning Center). 1 Manuscript accepted 30 January_I 978. __ _ ->Scripps Institution of Oceanography, A-008, University of California, San Diego, La Jolla, California 92093. The order of authorship was determined by the toss of a coin. Koslow was responsible for the reef survey (part I) and Ogg for the beach survey (Part 2). Guam is located within a major typhoon corridor of the Caroline-Mariana-Philippine region. Over the period of the last 30 years, Guam has received an average of approximately one cyclone per year, 13 of which have generated typhoon-force winds. Typhoon Pamela was rated as the third most intense typhoon to strike Guam this century; Karen (1962) is rated of equivalent destruction, and the most intense typhoon was in 1900. Catastrophic typhoons have been described as a major geomorphologic agent for tropical islands and reefs. The associated high waves can sweep accumulated reef debris and coral blocks from the outer reef slope and deposit it on the reef flat as rubble bars (Maragos, . Baines,-and-Bevel"idge -l-9'7-J) and-b€aGh-mm-parts and ridges (Blumenstock 1961, McKee 1961). The storm deposition of rubble is an important island-building process on atolls, as shown by the accumulation of successive