Tutuila, American Samoa coastal resource inventory sites: reconnaissance shoreline geology

Richmond, Bruce

Tutuila, American Samoa, is a high volcanic island with a highly embayed and irregular coastline. A narrow fringing reef borders most of the coast. As part of this study, 47 sites along the coast of Tutuila were examined using a combination of brief field visits (where accessible), aerial reconnaissance overflight, and reference to selected maps, vertical aerial photographs, and publications. This report contains a brief review of some of the relevant geologic parameters, descriptions of the 7 research areas and individual 47 sites, and a discussion of the “natural” coast. Coastal plains, composed of a mixture of terrigenous and marine sediment, occupy the heads of most embayments. Headlands of volcanic rocks, often forming steep cliffs, separate athe embayments. Beaches, wetlands, alluvial fans, stream valleys, and mangrove forests are the main physiographic components of the coastal plains. Depositional beach morphological features include pocket beaches, barrier spits, tombolos, and small flood deltas at stream mouths. Somewhat atypical environments on Tutuila include: a) the deep harbor of Pago Pago, b) the shallow lagoon, barrier reef, and extended barrier spit in the Pala Lagoon/Coconut Point area, c) recent basalt flows forming a low cliffed coastline with marginal fringing reef from Fogagogo to Sogi, and, d) the extensive coral rubble coastal plan of Aunu’u island. The coast of Tutuila has been shaped by volcanic activity, changes in relative sea level, reef growth, erosion of the hinterland, and more recently by storms and human activities. Tutuila lies within the easterly tradewind zone where winds, and hence waves, are consistently high along east- and southeast-facing coasts. Long distance swell waves and tropical storms are less common but can affect any part of the Tutuila coastline. Waves and wave-generated currents are responsible for transporting reef detritus landward resulting in beach deposits dominated by carbonate sand. Approximately 20% of the Tutuila coast is fronted by beaches. Where villages, buildings, or roads have been threatened by erosion, these beaches are bordered by some type of engineering structure such as revetments (sloping structure), seawalls (vertical structure), or loosely placed rip-rap. This shoreline armoring has caused a severe reduction in the amount of beached coastline available for traditional uses. While engineering structures may stabilize the position of the shoreline they often result in the loss of “normal” recreational beach shoreline. Although the proliferation of shoreline engineering structures may be necessary to protect investments, it has led to a decline in the nature state of Tutuila beaches.

Tutuila, American Samoa coastal resource inventory sites: reconnaissance shoreline geology

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