SHORELINE EROSION ON MULINU'U POINT AND RELATED CONSIDERATIONS IN WESTERN SAMOA Ralf Carter SOPAC Technical Secretariat April 1991 SOPAC Technical Report 118 Prepared for: South Pacific Applied Geoscience Commission(SOPAC) Coastal and Nearshore Programme, Western Samoa Project: WS.5 Contributed by: ESCAP/UNDP ProjectRAS/86/125, Development of South Pacific Institutional Capability in Marine Minerals &Technology (CCOP/SOPAC) [3] TABLE OF CONTENTS Page SUMMARY .............................................................................................................. 5 ACKNOWLEDGEMENTS ........................................................................................... 6 OBJECTIVES .......................................................................................................... 7 INTRODUCTION ....................................................................................................... 7 STUDYMETHODS ................................................................................................... 7 EROSION ON MULINU'U POINT Dredging impacts ................................................................................... 9 Wave refraction ....................................................................................... 10 Shoreline regression ................................................................................. 11 TRIAL REVETMENT ........................................................................................ 12 OTHER EROSION AREAS Mulinu'u Penisula ........................................................................... 15 West of Apia Market .................................................................................. 16 East of Apia ........................................................................................... 17 DISCUSSION .................................................................................................... 19 CONCLUSIONS AND RECOMMENDATIONS ................................................ 20 REFERENCES ....................................................................................................... 22 [TR118 - Carter] [4] LIST OF FIGURES Figure Page 1 Study site location at the Apia Observatory on Mulinu'u Point .... 8 2 Mulinu'u shoreline during 1970, 1980, and 1990 showing 12.1 metres of erosion from west of the Observatory ..................... 10 3 Recurrence of cyclone waves .................................................... 11 4 Trial revetment damaged during Ofa ......................................... 13 5 Revetment at Malua ................................................................. 15 6 Failed Seawall on Mulinu'u Pennisula................................................... 16 7 Sand on Laulii Beach ................................................................ 17 8 Rocks mark former shoreline at Leusoalii ................................. 18 9 Loss of shoreline at Lautuanu'u ................................................. 19 [TR118 - Carter] [5] SUMMARY This report presents a detailed review of shoreline erosion along the west end of Mulinu'u Point, and gives recommendations regarding seawall design at that location. The remaining shoreline exposed along Mulinu'u Peninsula is discussed and recommendations are made regarding maintenance of shore protection works. Particular attention is given to the vertical seawall just seaward of the Tusitala Hotel. The trial revetment constructed at Faleasiu in 1987 is reviewed with respect to its failure during Cyclone Ofa in 1990. Recommendations are made regarding foreshore protection for locations behind a lagoon, reef Rat and fringing reef. The need for wave setup, storm surge, and wave refraction analyses is indicated, in addition to the usual selection of the design wave for all significant protection works. The need to consider possible focusing of waves onto structures and the effect of overtopping and splash erosion from behind low profile coastal protection works is emphasised. The need is discussed for a master plan for Apia for the design of long term coastal development that results in changes to the shoreline. [TR118 - Carter] [6] ACKNOWLEDGEMENTS The major contributor to this study was ESCAP/UNDP Project RAS/86/125, Development of South Pacific Institutional Capability in Marine Minerals & Technology (CCOP/SOPAC). The study was supported and assisted by the Government of Western Samoa through the Department of Agriculture, Forests and Fisheries. The study was directed by the Marine Scientist, Ralf Carter with the assistance of Brendan Holden from the SOPAC staff. Others participating or contributing to the study included the following: Dr Kilifoti Eteuati, Secretary to Government Mr Faatoia Malele, Apia Observatory Mr Maselino Mataafa, Apia Observatory Mr Iosefa Siny, Apia Observatory Mr Tanu Tuia, Apia Observatory Mr R.A. Peacocke, Central Bank Project Mr R. Gordon, Engineer Mr M. Tuimaleali'ifano, Citizen Apia Mr Seumanutafa A. Tiavolo, Acting Director, Lands & Surveys Mr Petelo Ioane, Chief Photogrammetrist Mr Guy Ewards, Project Cartographer, Lands & Environment Mr N.V. Hawkins, Chief Civil Engineer, Public Works Mr W. Patterson, Public Works Department [TR118 - Carter] [7] OBJECTIVES The objectives of this study were to document the erosion rate along the western end of Mulinu'u Point, define coastal protection required for this site, and recommend further work for the exposed seaward side of Mulinu'u Peninsula, and shoreline near Luatuanuu east of Apia. This study was undertaken as part of the SOPAC Coastal and Nearshore Programme and contributes to Western Samoa country project WS.5 (Baseline Studies of Inshore Areas to assist with Coastal Zone Management). The study fulfills the requirements of Task 90.WS.05 - Coastal erosion on the seaward side of Mulinu'u Peninsula, requested by the Government of Western Samoa. INTRODUCTION The Government of Western Samoa requested an assessment of shoreline erosion about Mulinu'u Peninsula. Several studies have been made (Carter, 1987; Carter, 1989; and Carter, 1990) that relate to this shoreline erosion problem. During 1970 to 1980 sand and gravel was mined to a depth of about 8 metres (chart datum) within a few metres of the shoreline. The shoreline began to erode and a study recommended that dredging be restricted to a distance of approximately 300 metres from the eastern end of Mulinu'u Pt. A revetment using armor rock weighing up to 3.6 metric ton was suggested due to the increased water depth up to the shoreline and the possibility of having a harbour development in the reef lagoon area west of the point. The dredging nearshore has now been discontinued, and the along-shore transport has returned the sand bottom. Only small waves now approach the shoreline in the study area, see Figure 1. It is assumed that in the future, dredging nearshore will not occur. The recent experience with Cyclone Ofa and changes in nearshore dredging practice have shown that it is now safe to recommend a seawall constructed of much smaller rock at this location. STUDY METHODS The changing location of the shoreline was established from aerial photographs taken in 1970 and 1980. A small building at the Observatory and the large tomb of Tuimaleali'ifano are prominent landmarks in the photographs and were used to establish an offset line for measuring [TR118 - Carter] [9] the ground distance to the vegetation line and to the step at the toe of the beach. This same line was established by survey at the site in 1990. Aerial photographs were taken in 1990, but the required scale was not readily available, so the distance between the northwest corner of the building foundation and the southwest fence corner at the tomb was measured. This distance (143.1 metres) was used to calibrate the scale for the early photos. The perpendicular distance from the line to the vegetation line at the beach, starting from the tomb, was measured every 15 metres. The tomb had been moved from a location further west sometime around 1959. One palm tree, number 31 is located directly on the offset line between station 60 and 75. The corner of the building is at station 143.1. The three shorelines were plotted to scale for comparison (Figure 2). EROSION ON MULINU'U POINT Dredging Impacts The dredging site was visited in September 1982. At that time the original seawall was intact. Overtopping had occurred at its southern end, and some sand was eroded from behind the seawall. The northeast swell approaching the fringing reef was refracted around the northwest projection of the seawall on Mulinu'u Point, and the swell was breaking along the west end of Mulinu'u Point. The original revetment structure was constructed without a filter. The dredging operation was reviewed with Mr Fuimiaono Lautasi of the Public Works Department. He said that in 1979-1980 some 100 five cubic yard truck loads of screened sand per week were removed from the bay. This required dredging of 260,000 cubic yards of bulk aggregate to yield 52,000 cubic yards of screened sand in two years of dredging. A commercial 1.5 cubic yard dragline and the government owned 3/4 cubic yard dragline were used to dredge the sand. In 1979 a private operation dredged 19,300 cubic yards of material. An additional 219,700 cubic yards (168,000 cum) of material was dredged to construct a causeway for further dredging. At that time it was estimated that approximately 15 percent of the available aggregate from some 67 hectares had been removed. Dredging continued at a slower rate between 1980 to 1990