SHORELINE EROSION ON MULINU'U POINT AND RELATED CONSIDERATIONS IN WESTERN

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 Market ...... 16 East of Apia ...... 17

DISCUSSION ...... 19

CONCLUSIONS AND RECOMMENDATIONS ...... 20

REFERENCES ...... 22

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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

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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 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.

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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

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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

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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 and was discontinued nearshore in 1983.

The photo on the cover of the 13th Annual Session Proceeding of CCOP/SOPAC, Was taken in November 1984 and it shows the dredging in progress. It shows that the seawall had failed and soil was being eroded from the roots of the palm trees.

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0

Figure 2. Mulinu'u shoreline during 1970, 1980, and 1990 showing 12.1 metres of erosion from west of the Observatory.

Wave Refraction

Wave refraction during major cyclone conditions was investigated and reported in the 1987 seawall study for Mulinu'u Point (Carter, 1987). Both predredging and dredging bottom profile conditions were investigated. A bathymetry survey was made 16 October 1985. It was found that orthogonal lines (lines perpendicular to the wave crest that separate equal amounts of wave energy) for storm waves from either the northeast, north, or northwest pass to the west of Mulinu'u Point in the predredging condition and converge upon the Point for the post-dredging condition. Dredging the sand from the nearshore area offshore Mulinu'u Point allowed larger waves to approach and erode the shoreline. The refraction diagrams are shown in Carter 1987. The recurrence of large deep water waves outside the reef is shown in Figure 3.

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49 Ho ft 39

29

0 19~ HI (y~s) - 73.S678*LOG(RI)-21.9863

Figure 3. Return interval for given Significant Wave Height estimated from Maximum Wind Speed corrected to overwater at 30 feet.

Shoreline Regression

The position of the shoreline at 10 year intervals is plotted in Figure 2. These data show that rapid shoreline erosion took place between 1970 and 1980. The erosion has continued at a slower rate at the present time, and the sand has returned to the nearshore area. The palm roots are holding the shoreline at 2 metres shoreward of the palms. The maximum extent of erosion appears to be approximately 12.3 metres. The 1990 offset measurements to the vegetation line shown in Figure 2 and the offset distance to the water line are given below (survey between 0900 and 0958 hours, 0.2 m tide at 0930 hours on 1 November 1990):

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Station Vegetation Water line # (m) (m) 15 location of causeway 30 29 40.7 45 27 44.0 60 25.3 39.5 75 27.2 40.0 90 25.5 38.6 105 23 37 135 21 33.5 143.1 19.6 33.0 150 19.5 32.5 165 15.4 29.3 180 11 24.7 195 2.3 22.4

TRIAL REVETMENT

A trial revetment for foreshore protection was constructed adjacent to the West Coast Road on in Western Samoa in 1987 (Bakx, 1987). A 100 metre long rubble mound revetment was placed along the Apia-Falelatai Airport road just east of the village of Faleasiu directly seaward of the sub-village of Tauo'o. Erosion protection for this reach of shoreline was considered to be urgent. The objective for the trial revetment was to demonstrate the feasibility of this method of shore protection to the Public Works Department in Apia.

The design parameters for this revetment given in the project report were as follows:

Tidal range 3.3 ft Height 50 yr return period 5.5 ft Wave setup 1.4 ft Storm surge 1.7 ft Slope ratio 1.5 to 1 Specific weight rock 165 lbs/cf Weight sea water 64 lbs/cf Kd for breaking wave and two layers armour 2.9

The resulting weight for the armour rock was 1,290 lbs. The diameter of this rock, the filter rock, and the bedding rock was 720, 300 to 400, and 40 to 126 millimeters respectively. The construction was completed in 12 working days at a cost of 22,000 Western Samoan Tala (excluding supervision).

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The site was visited in November 1990. At that time the top of the revetment was well below road level. A photograph taken about 5 February 1990 or three days after Cyclone Ofa is shown in Figure 4. The road had been repaired at that time. Observers said that about one-half of the road had been washed away. As the top of the original revetment was finished to about one foot above road grade, it appeared that all of the top row of armour rock was carried away during the cyclone. All of the backfill was gone. The engineer doing the damage assessment to foreshore protection and coastal roads after the cyclone indicated that the revetment had been over topped by the waves, and that the plunging breakers onto the road surface had eroded the road behind the revetment (Byrne, 1990).

As shown in Figure 4, much of the rock revetment survived the waves. Some of the armour rock was scattered seaward of the revetment. That rock was almost covered with sand when viewed in November 1990. The original construction photograph in the project report shows that about 3 meters horizontally between the top of the revetment and the edge of the road had yet to be backfilled. The construction drawing show that about a metre of that backfill was to be armour rock. All of that rock was missing in the post storm photograph.

Figure 4. Trial revetment damaged during Cyclone Ora.

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A map of the area shows that the fringing reef forms a convex projection from the coastline offshore of the trial revetment. The shape of the reef suggests that a north to north west wave would be focused upon the shore where the trial revetment was located. A wave refraction analysis should be made at this site to determine if waves larger than the design wave could reach the revetment along this section of coast. The depth of water over the reef flat will control the size of unbroken waves to reach the foreshore area. If the wave setup and/or the storm surge was higher than estimated due to the shape of the reef, a wave larger than the design wave could break directly upon the top of revetment and backfill. Such wave action would cause the serious erosion that occurred. The weight of rock used in the revetment would be adequate for the design wave.

The revetment did not appear to influence the degree of erosion. The unprotected reach of road Immediately to the west of the revetment was eroded about the same as the road section protected by the revetment.

As indicated above, it would be wise to review this condition in considerable detail with respect to wave setup and storm surge as well as possible focusing of the waves upon the site from wave refraction before passing judgment upon the adequacy of the revetment. The wave that eroded the road was probably significantly larger than the design wave. However, the provision for overtopping and splash does not appear adequate for the site. A low-lying coast road exposed directly to the lagoon with no more than a narrow shoulder between the road and the revetment, could be expected to have a lot of overtopping and splash onto the road.

Further along the shoreline, the rock revetment protecting the causeway section near the Malua Theological College was undamaged by Cyclone Ofa. A photograph of this revetment (Figure 5) shows the small rock used in the structure. However, it is located on the flanks of a headland which is protected by an extensive rocky point. The wave would refract a significant amount before reaching the structure. The fringing reef in this area forms a concave line seaward of the lagoon which would cause the wave energy to be spread over a greater reach of shoreline. Both of these conditions will reduce the wave energy per unit length of beach. Much small waves would be expected at this location than along the shoreline at the trial revetment site.

It appears that the wave that reached the trial revetment was a wave larger than its design wave. The waves probably broke directly on top of the structure, and when it failed, they probably broke over the top of the damaged structure directly onto the backfill and road. If the revetment had been higher with a wider rock back fill, it may have survived Ofa. However, more

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Figure 5. Revetment at Malua.

rock would have been required to construct such a structure. A step type revetment would have forced the waves to break a few meters more seaward.

OTHER EROSION AREAS

Mulinu'u Peninsula

The north side of the Mulinu'u peninsula exposed to the lagoon was protected by a rubble seawall that has now failed. This seawall was constructed about 1962 without a proper bedding or filter layer. The footing and backfill have been eroded, the rock has settled, and wave now attack the shoreline (Figure 6). The shore should be surveyed, and the critical areas should be identified for maintenance. It would be desirable to remove the present rock and place proper bedding and filter layers. The present armour could then be replaced on the face of the filter layer to make a sound seawall. A suitable design is described by Carter (1987). However, if no funds are available to rebuild the seawall then some maintenance should be done or the erosion will become serious.

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Figure 6. Failed seawall on Mulinu'u Peninsula.

A minimum effort would be to place coarse fill behind the seawall, and additional armour rock on top of the seawall to build it up to about 0.5 metre above the natural grade. The backfill for splash protection should extend a distance of at least 2 metres landward of the seawall. Several years of protection would be provided with this minimum effort.

West of Apia Market

The vertical seawall just west of the bus parking area at the Apia market seaward of the Tusatala Hotel is under serious wave attack. The source of non-organic sand to this area has been severed by landfill development to the east. There is a breach or low area in the fringing reef on the seaward side of the lagoon at this point, and the wave energy reaching this seawall is significantly greater than that which reaches the shoreline further to the west. If maintenance is not provided, the road will be damaged by the wave action within a relatively short period of time.

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With the building development on the filled land adjacent to Apia Harbour, there may be a need to expand the bus parking area at the market, particularly if some of the area on the filled land is no longer available for bus use. This would be an opportunity to construct a seawall from the northwest corner of the present bus parking at the market to the notch in the shoreline at the west end of the vertical seawall seaward of the Tusitala Hotel. The area could be filled and provide additional bus parking or other use, and it would be a suitable buffer zone for the road against wave damage. Filled land at this site should not be used for a building site. A master plan for downtown Apia should be developed to identify where the future shoreline should be placed. Maximum use should be made of the available resources for repair of this seawall section.

Coastal Erosion at Luatuanu'u, east of Apia

Erosion along this reach of coast line was investigated in 1989 (Carter, 1989), and a study plan was proposed. Since that review was made, Cyclone Ora struck and caused serious wave damage to the area where the beach sand had been removed by mining or other causes. The beach at Laulii, just west of Leusoalii and Luatuanu'u has a good supply of sand (Figure 7). Sand mining is not allowed at Laulii and very little or no wave damage occurred there during

Figure 7. Sand on Laulii Beach.

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Cyclone Ora. The reef flat is wider at Laulii, and a wave refraction analysis of the two areas would be expected to show that more concentrated wave energy reaches the beach at Luatuanu'u than at Laulii.

A significant amount of coastal erosion occurred at Leusoalii, where the precyclone coast line was marked by large rock near the water edge (Figure 8). The seawall had failed prior to the cyclone, but a wide strip of land was lost along with a row of coconut palms during the cyclone. However, the mining of sand at Lautuanu'u has probably contributed greatly to the loss of sand along that beach and to the resulting wave damage in the foreshore area (Figure 9).

A detailed assessment is being made by the road consultant for Public Works Department, and this report is expected to make some recommendations regarding the protection of the foreshore area at Lautuanu'u.

Figure 8. Rocks mark former shoreline at Leusoalii.

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Figure 9. Loss of shoreline at Lautuanu'u.

DISCUSSION

With respect to future coastal protection about Apia and Mulinu'u Penisula, it is essential that a master plan of the Apia area be developed that takes account of future shorelines. With the growth of Apia, many utilities, facilities, and buildings are being constructed. They will all interact and compete for space. Much detailed planning is essential to ensure practical, cost-effective infrastructure. This included roads, parking, support services, port facilities, and industrial development.

Coastal protection is a consideration for all future development about Apia. As coastal protection facilities generally have a long design life and may require a significant fraction of available resources during their construction, it is imperative that some master planning be done before much development has occurred, so appropriate and safe coastal protection will be designed.

Wave refraction can focus or spread the wave energy along a reach of beach. The wave climate along the shoreline varies greatly with different depth, width, and shape of the fringing

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reef and reef flat due to wave refraction. Thus, the coastal protection requirements will vary greatly for each reach of shoreline. It is essential that a wave refraction analysis similar to the one done for the 1987 Mulinu'u Point study (Carter, 1987) be done for any important coastal protection works. If such analyses had been done for design of the trial revetment at Faleasiu, it is possible that a larger structure would have been selected.

It should be noted that the road at Faleasiu had been repaired and was in use within three days following Cyclone Ofa. Rebuilding the road for some areas may be more practical than providing an adequate but expensive foreshore protection which may be necessary only a few times in a century. Each area must be judged by its individual needs. The west end of Mulinu'u Point should probably have a relatively modest protection, say one designed for a 30 year wave at that location. The seawall along the road to Apia from the Observatory should have protection for a much larger wave as that road may be the only way to evacuate people from Mulinu'u Peninsula during a storm.

CONCLUSIONS AND RECOMMENDATIONS

The conclusions reached in this study are based upon information given in this report and from several other studies that deal with the Same general area and topic. These conclusions are expected to be modified as more information is developed for the area, and as needs change with time.

1. A detailed master plan for Apia that includes the future shoreline should be prepared before further development takes place. This will allow long term coastal protection to be designed that is compatible with planned future development.

2. Shoreline erosion at Mulinu'u Point was greatest when the water depth was increased by dredging in the nearshore region. A little more than 12 metres of land was eroded over 20 years. Erosion is still occurring at this location, and a modest revetment should be constructed.

3. The entire exposed shoreline about Mulinu'u Peninsula should be surveyed and erosion protection provided as necessary. In some instances, modest erosion protection is indicated as repair costs following major cyclones can be less than the cost of adequate protection facilities.

4. A seawall could be constructed along the 1970 shore alignment on the west end of

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Mulinu'u Point. The area shoreward of the seawall should be backfilled to the original grade to provide additional recreation area.

5. The seawall near the Apia market and seaward of the Tusitala Hotel requires a significant amount of repair or maintenance work. Consideration should be given to some land reclamation at this site to provide a land buffer zone seaward of the road. Additional land at this location could be of great value.

6. A coastal protection plan for the area east of Apia should include consideration of the findings of the road consultant report being prepared for the Department of Public Works.

7. All beach mining should be controlled for areas where coastal erosion is of concern. To protect foreshore development, no aggregate should be removed from the reef flat area. Dredging should not be permitted within 300 metres of Mulinu'u Point.

8. Wave setup and storm surge analysis should be used in the selection of the design wave for all significant coastal protection works. When reef flats are present, the wave setup and storm surge can be significant during cyclones and much larger waves may reach the shoreline than during usual weather conditions. Wave refraction analysis should be used for selection of design criteria for all important foreshore protection facilities. Along the coast where severe wave action and a significant storm surge is anticipated, the design of a seawall or revetment must include due consideration to overtopping and erosion from behind the structure. A rubble mount revetment on a 2 to 1 or greater slope is preferred over a vertical wall for all areas where a beach is to be maintained. All seawall structures should include adequate bedding and filter layers.

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REFERENCES

Bakx, M.A. 1987. Consultancy on shore protection, Western Samoa. UNDP/ILO and Government of Western Samoa, Project No. INT 84/002, WS 86/012, Part I, Test, May 1987. Byrne, T. 1990. Cyclone Ofa February 1990 - damage assessment - foreshore protection - coastal road system - Upolu and Savai’i. Report for Government of Western Samoa and Australian International Development Assistance Bureau. Riedel & Byrne Consulting Engineers Pty. Ltd of Walcott Street, Mount Lawley, Western Australia 6052, April 1990.

Carter, R. 1987. Design of the seawall for Mulinu'u Point, Apia, Western Samoa. CCOP/SOPAC Technical Report 78. WS.5, 15 May 1987.

Carter, R. 1990. Probability and recurrence of tropical cyclones in Western Samoa. SOPAC Technical Report 106. WS.5, WS.13. August 1990.

Carter, R. 1989. Coastal erosion at Luatuanu'u on Upolu, Western Samoa. DRAFT CCOP/SOPAC Preliminary Report 14. 18 July 1989.

Carter, R. 1989. Coastal erosion on the seaward side of Mulinu'u peninsula were cultural landmarks are being destroyed by wave erosion of the beach sand. DRAFT CCOP/SOPAC Preliminary Report 15. 15 September 1989.

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1. The views expressed in this report are those of the Author and do not necessarily reflect those of the United Nations.

2. Mention of any firm or license process does not imply endorsement by the United Nations.

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