Coastal Circulation and Sediment Dynamics in Maunalua Bay, Oahu, Hawaii

Introduction

High-resolution measurements of waves, currents, water levels, temperature, salinity and turbidity were made in Maunalua Bay, southern Oahu, Hawaii, during the 2008–2009 winter to better understand coastal circulation, water-column properties, and sediment dynamics during a range of conditions (trade winds, kona storms, relaxation of trade winds, and south swells). A series of bottom-mounted instrument packages were deployed in water depths of 20 m or less to collect long-term, high-resolution measurements of waves, currents, water levels, temperature, salinity, and turbidity. These data were supplemented with a series of profiles through the water column to characterize the vertical and spatial variability in water-column properties within the bay. These measurements support the ongoing process studies being done as part of the U.S. Geological Survey (USGS) Coastal and Marine Geology Program’s Pacific Coral Reef Project; the ultimate goal of these studies is to better understand the transport mechanisms of sediment, larvae, pollutants, and other particles in coral reef settings.

Project Objectives

The objective of this study was to understand the temporal variations in currents, waves, tides, temperature, salinity and turbidity within a coral-lined embayment that receives periodic discharges of freshwater and sediment from multiple terrestrial sources in the Maunalua Bay. Instrument packages were deployed for a three-month period during the 2008–2009 winter and a series of vertical profiles were collected in November 2008, and again in February 2009, to characterize water-column properties within the bay. Measurements of flow and water-column properties in Maunalua Bay provided insight into the potential fate of terrestrial sediment, nutrient, or contaminant delivered to the marine environment and coral larval transport within the embayment. Such data are useful for providing baseline information for future watershed decisions and for establishing guidelines for the U.S. Coral Reef Task Force’s (USCRTF) Hawaiian Local Action Strategy to address Land-Based Pollution (LAS-LBP) threats to coral reefs adjacent to the urbanized watersheds of Manualua Bay.

Study Area

Maunalua Bay is on the south side of Oahu, Hawaii, and is approximately 10 km long and 3 km wide. The bay is flanked by two large, dormant craters: Koko Head to the east and Diamond Head to the west. Rainfall in the watersheds that drain into Maunalua Bay ranges from more than 200 cm/year at the top of the Ko‘olau Range that borders the northwestern part of the bay to less than 70 cm/year to the east at Koko Head. Seven major channels flow into the bay, and all but one have been altered by engineering structures.

First posted October 25, 2010