RIASSUNTO
Abstract
Almost without exception, the small island nations of the Pacific are facing the need to accommodate growing populations in a global environment that imposes significant changes in how people live. Roads, airports, public buildings, durable dwellings, hotels, and shore protection are all part of the considerable infra structure without which contemporary lifestyles cannot be sustained. Most of these structures are built with concrete and island people who have traditionally relied on their beaches to supply sands and aggregates for local construction are finding the source materials in short supply. The island of Tongatapu in the Kingdom of Tonga has for some years been affected by degradation of beaches and hinterland caused by the removal and sale of beach sand. It was proposed that an alternative source of sand be sought offshore and surveys by the regional South Pacific Applied Geoscience Commission located several deposits in the lagoonal area to the North of the island. The potential for ecovery of these sands and the likely environmental impacts were presented in an Environmental Impact Analysis prepared for the government of the Kingdom.
The action studied in the EIA was to replace the mining of sand beaches on the island of Tongatapu with the mining of sand from Basin A, an offshore deposit located in the lagoon approximately 10 km north of Nuku'alofa in 15 m water depth. Review of available reserves indicated as much as 2.6 million m3 of sand, sufficient for 180 years at present rates of usage. A production of 15,000 m3/yr was assumed, using a 4"" submerged suction pump, mounted on a 25 meter motorized barge with a capacity of 75 m3. The dredge would operate on site for three hours each working day and discharge to a stockpile at the wharf in Nuku'alofa. Public hearings disclosed a number of concerns including anchoring scars on nearby coral heads; visual effects and operational noise affecting nearby tourist resorts; the social effects of displacing workers presently engaged in beach mining; and possible but minor effects on commercial, subsistence, and sports fisheries in the vicinity ofthe operation. Environmental effects requiring specific analysis were limited to dispersion of fine sediment from the dredge overflow and possible effects on nearby bottom fish and some corals. The most far reaching effect appeared to be the recovery of the beaches from which sand is presently being removed.
The effects of the overflow plume on the dredge may be reduced by using an anti-turbidity overflow system (ATOS) on the dredge hopper and discharging near the bottom. Monitoring would be required to confirm the presence on the bottom of fine sediment from the discharge plume; and monitoring of fish catches or sightings would be necessary to determine any effects on the fisheries, or on prime dive sites. Noise from the operations may be monitored at appropriate intervals. It may also be appropriate to monitor for unauthorized beach mining,