OCEAN CURRENTS AND CIRCULATION AVARUA - MOTUTOA, RAROTONGA COOK ISLANDS Brendan Holden SOPAC Technical Secretariat April 1992 SOPAC Technical Report 143 Prepared for: South Pacific Applied Geoscience Commission (SOPAC) Coastal and Nearshore Programme, Cook Islands Project: CK.4 [3] TABLE OF CONTENTS Page SUMMARY .......................................................................................................................... 5 ACKNOWLEDGEMENTS .................................................................................................... 6 OBJECTIVES ..................................................................................................................... 7 INTRODUCTION ................................................................................................................. 7 METHODS ............................................................................................................................ 10 RESULTS Wind ...................................................................................................................... 13 Waves ..................................................................................................................... 13 Water Levels ............................................................................................................. 15 Water Temperatures .................................................................................................. 16 Current Data ............................................................................................................ 19 Avarua Top .................................................................................. 19 Avarua Bottom ............................................................................................. 19 Motutoa Top .............................................................................. 20 Motutoa Bottom................................................................................... 21 Motutoa Drogues and Dye ................................... 21 DISCUSSION Currents ....................................................................................................................... 22 Sewer Outfalls ....................................................................................................... 23 CONCLUSIONS .................................................................................................................... 48 RECOMMENDATIONS ........................................................................................................... 48 REFERENCES ........................................................................................................................ 49 APPENDICES A Rarotonga Tide Predictions (January - April 1991) B Notes on Data Analysis Methods (Malcolm Greig) [TR143 - Holden] [4] LIST OF FIGURES Figure Page 1 Rarotonga Site Map ..................................................................... 8 2 Avarua - Motutoa Site Map .............................................................................. 9 3 Current Meter Mooring .................................................................................... 11 4 Drogue : Window blind type ..................................................................... 12 5 Wind Rose : Southern Cook Islands ..................................................... 14 6 Avarua - Water level data ................................................................... 17 7 Avarua - Filtered Water Level Data .................................................................. 18 8 Avarua Top - Current Meter Data ....................................................................... 24 9 Avarua Top - Filtered Non Tidal Data .................................................................25 10 Avarua Top - Tidal Current ........................................................................... 26 11 Avarua Top - Current Rose ....................................................... 27 12 Avarua Top - Progressive Vector ........................................................................... 28 13 Avarua Bottom - Current Meter Data ........................................................... 29 14 Avarua Bottom - Filtered Non Tidal Data .................................................. 30 15 Avarua Bottom - Tidal Current ........................................................................... 31 16 Avarua Bottom - Current Rose .................................................................... 32 17 Avarua Bottom - Progressive Vector ................................................................33 18 Motutoa Top - Current Meter Data.......................................................................... 34 19 Motutoa Top - Filtered Non Tidal Data......................................................................... 35 20 Motutoa Top - Tidal Current ............................................................................. 36 21 Motutoa Top - Current Rose................................................................................................37 22 Motutoa Top - Progressive Vector ....................................................................... 38 23 Motutoa Bottom - Current Meter Data ................................................................ 39 24 Motutoa Bottom - Filtered Non Tidal Data ...................................................... 40 25 Motutoa Bottom - Tidal Current .......................................................................... 41 26 Motutoa Bottom - Current Rose .................................................................... 42 27 Motutoa Bottom - Progressive Vector .............................................................. 43 28 Drogue and Dye Tracks - 15 March 1991 ........................... 44 29 Drogue Tracks - 19 March 1991 ................................ 45 30 Drogue Tracks - 25 March 1991 ................................ 46 31 Current Rose and Resultant Vector Summary ............................................. 47 [TR143 - Holden] SUMMARY In the early 1980s, a sewage collection system with a marine outfall was proposed off Aavarua Harbour. Since there were no current data off Avarua, the Government of the Cook Islands requested SOPAC to undertake Task 91.CK.4d: Physical Oceanography of Avarua - Avatiu and nearshore areas. This task began with the mooring of current meters off Avarua early in 1991. During the course of this field work, it became known that another proposal was recommending a more extensive sewage collection system with an outfall off Motutoa near the airport. Subsequently Task 91 .CK.4d was extended to include Motutoa, and current meter moorings with two Aanderaa current meters (top & bottom) were deployed off both Avarua and Motutoa. In addition, rhodamine dye and drogues were released and tracked off Motutoa. The current meter data off Avarua showed a net southeasterly current (onshore) at the top meter and a net southwesterly current (into the harbour) at the bottom meter. The current meter data off Motutoa showed a net northwesterly current (away from Rarotonga) on both the top and bottom meters. It is recommended that the sewer outfall proposal for Avarua be abandoned because the currents are not suitable to carry away the effluent. It is recommended that the proposal for a sewer outfall off Motutoa be considered further because it is more environmentally safe. [TR143 - Holden] [6] ACKNOWLEDGEMENTS This work was supported by the Canadian International Development Agency (CIDA) and the Government of the Cook Islands. Gratitude is expressed to the many Cook Islands Government personnel who assisted with the field work for this study : Stuart Kingan, SOPAC National Representative; Tony Utanga, Ministry of Internal Affairs; George Cowan, Ministry of Public Works; K. Kamana and T. Short, for use of the Tug boat; R. Story and other personnel of Marine Resources for diving and current meter retrieval; Ata Herman, Sonny Tatuava, Tara Tairea and John Temata for drogue tracking. Data was processed, analysed and plotted by Malcolm Greig of New Zealand Oceanographic Institute (NZOI), DSIR, under contract to SOPAC with financial assistance from the New Zealand government. [TR143 -Holden] [7] OBJECTIVES This work was carried out to fulfill the requirements of the 1991 SOPAC Work Plan, Task 91 .CK.4d: Physical oceanography of Avarua - Avatiu harbours and nearshore areas. The objective of this task was to determine the ocean current regime off Avarua as design and planning data for a proposed sewer outfall. A later proposal for a sewer outfall off Motutoa (airport) required that the task be extended to include this area. This report describes and discusses the ocean current data collected off Avarua and Motutoa in 1991 and the implications of these current data on the location of a sewer outfall. INTRODUCTION Avarua, Avatiu and Motutoa (Lat 21' 12'S, Lon 159' 47'W) are located on the north side of Rarotonga, Cook Islands (Figures 1 & 2). Avarua and Avatiu are adjacent small towns with a total population of about 5000. Both towns have small fresh water streams and gaps in the reef which provide entrances to the small harbours. Motutoa is a small island on a wide section of the reef flat off the airport where there is no gap in the reef. The bottom slope at both sites is 5% from the reef crest out to about the 25 m depth and then increases rapidly to 40% into deep water. The purpose of this report is to describe and discuss the currents and circulation off Avarua and Motutoa on the north side