EROSION MECHANICS OF A CARBONATE- TOMBOLO BEACH IN MIYAKOJIMA ISLAND, OKINAWA PREFECTURE, JAPAN.

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1 EROSION MECHANICS OF A CARBONATE- TOMBOLO BEACH IN MIYAKOJIMA ISLAND, OKINAWA PREFECTURE, JAPAN. Ryuichiro NISHI 1, Takaaki UDA 2, Akio KIKUCHI 3 and Kou FURUIKE 4 1) Associate Prof. Dept. of Ocean Civil Eng., Kagoshima University; 2) Exective Director for Research Affairs, National Institute for Land and Infrastructure Management. 3) President of Coastal Techno, Co., Ltd., 4) Research Engineer, Coastal Engineering Laboratory Co., Ltd.

2 EROSION MECHANICS OF A CARBONATE-TOMBOLO BEACH Contents of this study; 1. Background 2. Study area 3. Long-term coastal processes 4. Estimation of longshore sediment transport rate 5. Long-term tidal level fluctuation 6. Numerical modeling 7. Conclusions

3 1. Background and motivation 1. The beach had been eroded since the beginning of 1990 s especially from 1996.

4 1. Background and motivation 2. Maehama Beach in Miyakojima Island is one of the most popular natural beach in Okinawa Area, thus the beach should be protected. 3. Nakaza et al.(1996) pointed out that the main reason of erosion is the westward deviation of typhoon course, but such erosion report was not documented in 1960 s when the courses was westward. 4. A construction of Kurima Causeway could cause the beach erosion? 5. What is the cause of erosion and is the shore protection work?

5 EROSION MECHANICS OF A CARBONATE-TOMBOLO BEACH Aerial photographs of this slide: HP: Japan Coast Guard 11 th Region Simoji Island 2. Study area Miyakojima Island, Okinawa Prefecture, Japan Maehama Beach Study Area HP:Okinawa Prefecture Kurima Island

6 EROSION MECHANICS OF A CARBONATE-TOMBOLO BEACH 0m 500m 1,000m N Kurima Island Jan Kurima Causeway Photograph Maehama Beach Maehama Harbour Miyakojima Island

7 EROSION MECHANICS OF A CARBONATE-TOMBOLO BEACH IN MIYAKOJIMA ISLAND, OKINAWA PREFECTURE, JAPAN. SE-side Maehana Hrabour Tokyu resort Hotel Size of the carbonate-tombolo beach is nearly 1,600m long and 200m wide. Zoom-up of the study area (Yonaha Maehama Beach)

8 3. Long-term coastal processes by aerial photographs SE Dec NW Oct March 1972 Jan Underwater dune Nov The southeast side of the tombolo beach was eroded by the typhoon waves approached from southeast. In contrast, eroded material was accumulated in the northwest side of the tombolo and was deposited over the reef flat in the form of underwater dunes in 1995.

9 Dec March, 1972 Nov., 1977 Oct., 1986 Jan., 1995 Dec., 1996(Survey data) Coastal structure Board walk & structure Vegetation boundary(1995) Time history of shoreline position based on aerial photographs (m) 3. Long-term coastal processes by aerial photographs

10 Ref. (Dec. 1962) March, 1972 Nov Oct Jan Dec (Survey) Groin Groin Groin Maehama harbour Groin Sept. 1997(Survey) Change in shoreline position (m) (Reference is set to be the position in 1962)

11 Change in the shoreward boundary of vegetation since 1962 Kurima Causeway Groin Shoreline Dec Jan Coastal structure Vege. boundary (1962) Kurima harbour N

12 Time history of the position of shoreline and vegetation (at A-A ) Shoreline position Vegetation boundary Maehama Harbour construction Time delay Time response? (1986) Kurima Causeway open Damage of coastal forest Significant correlation Board walk position Board sea wall

13 Time history of the position of shoreline and vegetation (at B-B ) Shoreline position Vegetation boundary Maehama Harbour construction Relatively weak correlation Construction of Kuruma causeway completed Board walk

14 Cumulative grain size distribution of the sediments along the shoreline Deposition area Erosion area

15 4. Estimation of a longshore sediment transport rate ( based on surveyed profile data) (Integration of the conservation equation) Cross-shore profile data is provided by Okinawa Prefecture Government Office

16 EROSION MECHANICS OF A CARBONATE-TOMBOLO BEACH IN MIYAKOJIMA ISLAND, OKINAWA PREFECTURE, JAPAN. Estimated annual longshore sediment transport rate June 1989 to Nov (Mainly T9413) March 1996 to Aug. 1996(T9609) Sept to Dec. 1996(T9621)

17 5. Long term Tidal level fluctuation 0.3m 7.3m 2.0m 0.1m 0.5m 5.6m 1.2m 0.2m W Typhoon related Wind rose N S E Hirara Port Datumn ( ) NHHW MHWS 5.1m 0.1m 1.7m Coral reef 0.9m 2.8m 0.4m 3.1m 1m 2.4m Tide Hirara Port MSL MLWS 0.6m 2.8m 1.2m 0.6m 1.6m 1.7m DL

18 During a typhoon in Sep During a beach recovery in Feb Storm surge

19 SE 260m Maehama Beach(SE side) Maehama Harbour construction Coastal forest damaged Kurima Causeway construction Change in the shoreline position and the vegetation boundary 140 Board sea wall position Nearly 20 years cycle Annual mean water Hirara Port level and two years moving average 105 Hirara Port Annual highest water level and two years moving average

20 Typhoons July

21 EROSION MECHANICS OF A CARBONATE-TOMBOLO BEACH IN MIYAKOJIMA ISLAND (5 years interval) Reference; Dictionary of Typhoons

22 Beach erosion was reported by Newspaper Typhoon Other cause Tide(m) Wave data at Hirara Port (1996 to 1997) is provided by Okinawa Prefecture H 1/3

23 Typhoon and tide condition at Hirara port in 1990 s

24 6. Numerical simulation; shoreline change modeling Continuity equation of bed materias Longshore sediment transport rate(ozasa and Blampton) Wave modeling; Isobe

25 Simulation set-up Water level D.L. +2.6m (MWL+1.5m) Wave height H01/3=2.5m Wave period = 8.0sec Wave direction N35 N Initial shore line (October 1986, SE side of the beach was the widest) Probability distribution of significant wave K1=0.77 K2=1.62 height (Hirara Port) Ds=5.0m DX=20m Dt=0.1hr Simulation period = seven days

26 Mean water level condition Storm surge condition m m

27 Zoom-up of the wave field during the high water level condition

28 Shoreline change under storm sure condition Initial shoreline(1986) Simulated shoreline

29 EROSION MECHANICS OF A CARBONATE-TOMBOLO BEACH IN MIYAKOJIMA ISLAND, OKINAWA PREFECTURE, JAPAN. Initial shoreline After one storm After two storms After three storms After four storms After five storms (GENESIS application)

30 Conclusion 1; The major conclusions are as follows; (1) Maehama Beach has been migrated alternatively either SE or NW directions. This migration causes the beach erosion and accretion alternatively at the study area. In general, the beach seems to be an accretional stage. (2) The migration period of the beach is nearly 20 years. (3) The changes in shoreline and vegetation boundary shows reasonably good correlation. (4) The dry beach width tends to wider during the accretion period and to narrower during the erosion period. (5) The major erosion in the southeastern side of tombolo beach was nearly 50 m and eroded material enhanced the shoreline advance nearly 60 m in the northwestern part of the beach in 1990 s. In addition, some material was deposited over the northern reef flat as the underwater dunes. (6) Maximum longshore sediment transport rate in the northwest direction is estimated as much as 7,000m 3 /year in1990 s.

31 Conclusion 2; The major conclusions are as follows; (7) Beach erosion of SE side of Maehama beach had occurred in the high mean water level periods. (8) When a typhoon moves the westward of Miyakojima Island, beach tends to be damaged more such as reported by Nakaza et. al(1996). (9)Due to the news-database and tide record, beach erosion was mainly caused by the elevated mean water level, even though the causeway construction could cause the secondary effect. (10) A board sea wall construction was a reasonably sound shore protection method at this study site, because it may has less negative impact to the beach recovery compared to other concrete hard structure. (11) The best shore protection method at this site might be a set-back as much as a hundred meter, because the shoreline position naturally tends to fluctuate in long term.

32 Time for Q/A? Thank you for your attention!!

33 Beach recovery in Feb