Marine Monitoring for Small-Scale Beach Replenishment at Maalaea, Maui, Hawaii

摘要

Marine and coastal environmental monitoring was carried out on a monthly basis from October 2002 to September 2003 along approximately 300 m of shoreline in Maalaea, South Maui. The purpose was to establish a baseline of physical, chemical, and biological conditions from which comparisons could be made during and after beach replenishment to determine the extent of environmental impacts from this activity. The small-scale beach replenishment project consisted of the addition of 3,000 cubic yards of inland dune sand with less than 1 percent fine particles at the number 200 sieve. The sand was placed at the Kanai A Nalu condominiums over the period of June 2-6, 2003. Water quality monitoring was conducted on a daily basis during the sand placement, or construction period, and three times within the five days following the completion of sand placement. Monthly monitoring resumed one month after the sand placement, for three months. Of the water quality parameters monitored, turbidity was the only parameter that appeared to be affected by the sand placement, increasing by up to 21.63 nepha-lometric turbidity unit (NTU) over pre-construc-tion levels. On average, turbidity levels increased by 2.22 NTU, and returned to pre-construction levels within five days following the completion of construction. Beach profiles revealed increases in the seaward extent of the beach in the month following construction that did not greatly exceed the seaward extent seven months prior to sand placement. The beach face experienced an average inflation of 0 m to approximately 0.5 m above levels of the previous year, and up to 1 m above pre-nourishment levels. Video analysis revealed that the bottom composition prior to sand placement consisted mainly of sand and algae; after nourishment, sand coverage increased and algae coverage decreased. Fish counts revealed an increase in fish following beach nourishment; however this may have been influenced by the improved visibility following the sand placement. Ghost crabs, which had disappeared entirely before the beach replenishment due to the lack of sand, were well established within three months following sand placement. Sediment trap analysis was inconclusive due to frequent damage to the sediment traps by high surf events.