sea turtle protection by means of coastal engineering...
TRANSCRIPT
Sea Turtle Protection by Means of Coastal Engineering:
Field Study on Sea turtle Behavior, Coastal Processes of a Nesting Beach
and Shore Protection in Kagoshima, Japan-
By
Ryuichiro Nishi Dept. of Ocean Civil Engineering, Kagoshima University
1-21-40 Korimoto, Kagoshima City, Japan. 890-0065
Kazuyoshi Ohmuta
Yakushima Sea Turtle Preservation Association, Kamiyakucho
Kumagegun, Kagoshima Prefecture, Japan
Masanori Ohtomi
Graduate School of Science and Engineering
Kagoshima University, 1-21-40 Korimoto
Kagoshima City, Japan. 890-0065
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ABSTRACT
In April 2000, a new coastal regulations was issued in Japan. The regulation can be summarized
as a management synthesis of shore protection, environment preservation, and accessibility by
local residents. Therefore, hydraulically and mechanically placed beach nourishment to widen
eroding beaches will be frequently applied to create a quasi-natural coastal environment, instead
of the hard structures such as sea walls and mildly sloping revetments heretofore popular in
Japan. However, beach nourishment might alter the ecological environment because of changes
in porosity and compactness as compared to the original beach. In addition, a required beach
width for sea turtle nesting is nearly 30 m and is roughly comparable with an average beach
width in Japan. Coastal engineers and the public know that the width of the beach and number
of sea turtles are decreasing in Japan. Thus, the loggerhead turtle is chosen as a representative
index animal to investigate the preservation of coastal ecosystem and shore protection by beach
nourishment, because the loggerhead turtle needs wide sand beach for landing, nesting, and
hatching. Studies on the nesting of loggerhead turtles, coastal processes, and beach nourishment
were conducted at Inakahama and Maehama Beaches on Yakushima Island, the most significant
nesting area of loggerhead turtles in the Northern Pacific Ocean region. Procedures and results
are summarized in this paper.
Keywords: Sea turtle, loggerhead turtle, coastal process, shoreline change, shore protection,
beach nourishment, environment, Japan
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INTRODUCTION
A new federal coastal law was issued in Japan in April, 2000. This coastal law promotes
“shore protection, preservation of environment, and accessibility to the beach” as key policy,
whereas the old law only concerned shore protection. The new, integrated concept is in a stage
of trial and error. Regarding shore protection and the preservation of coastal environment, sea
turtles become one of the key issues. Sea turtles such as shown in Fig. 1 are an endangered
species and appear on the red list reported by the Japanese Ministry of Environment. For
instance, the number of sea turtle landings has been decreasing in various places in Japan (e.g.
Kamezaki et al. 1994; Ohmuta 1994, 1997a, 1997b, 1999). The decrease in the number of sea
turtles is probably caused by two factors. The first is the loss of wide sandy beaches for which
the nesting and hatching can take place. The second factor is fishing net, which entwines turtles
and can kill them. Sea turtles need a wide sandy beach for nesting. Human beings also prefer
wide sandy beaches. Coastal engineers have been working to protect the beach for human
activity for many years, so knowledge and experience on coastal process and shore protection is
applicable to preservation of turtle beaches, too. Therefore, loggerhead turtle (caretta caretta) is
chosen as an index (representative) animal to apply the coastal engineering knowledge for the
preservation of natural ecosystem, because the loggerhead turtle is a major species in Japan.
Fig. 1 Green turtle and loggerhead
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American coastal engineers have carried out extensive studies on compaction effect of
nourished beaches on the loggerhead turtles especially in Florida, such as by Nelson et al. (1987),
Steinitz et al. (1998), and Davis et al.(1999). Ito (1985) suggested the porosity of a sandy beach
affect an ecosystem under the beach. However Japanese coastal engineers have little knowledge
on the impact of beach nourishment as well as hard coastal structures on sea turtles. Therefore,
studies of loggerhead turtle ecology (behavior), coastal processes of nesting beaches, as well as
the shore protection works were conducted at Inakahama and Maehama Beaches in Yakushima
Island, where the numbers of landings and nestings of loggerhead turtles are the largest in the
Northern Pacific Ocean. In addition, the island was chosen as a world heritage for the 21st
century by UNESCO.
STUDY AREAS
The length of coastal line in Kagoshima Prefecture (see Fig. 2) is nearly 2,600 km, which is
the third longest in Japan. There are 59 cities and towns along the coast of which 39 are known
for loggerhead turtle landing. In fact, there are many more remote islands and towns for which
no measurement and observation have been conducted; thus these locations are not included in
the statistics. Turtle season is a period from May to September (nesting; May to July and
hatching; July to September). From 1994 through 1998, the total numbers of loggerhead landing
in Kagoshima Prefecture are 4,127 (42); 3,299 (9); 3,412 (38); 3139 (60) and 3,023 (101), with
the number of green turtles (chelonia mydas) given in brackets. It is noted that the number of
turtle landing corresponds to only the number of female turtles. Half the total number of turtle
landing in Kagoshima Prefecture originate from Inakahama and Maehama Beaches in
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Yakushima-Island (2076, 1721, 1920, 1486, and 1333 for the years between 1994 and 1998).
Therefore, both beaches were chosen as study areas.
Fig. 2 Location of Yakushima Island.
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Inakahama and Maehama Beaches
Figs. 3 and 4 are overviews of Inakahama and Maehama Beaches. Inakahama Beach is
extends 975 m and opens to the northwest. Maehama Beach is located at the mouth of Nagata
River and extends 1,050 m. Directions of longshore sediment transport in summer and winter
seasons are to the northeast and the southwest, respectively. Typical seasonal profile changes
such as erosion in summer(July to September) and accretion in winter(December to Feburary)
occur. Therefore, southwest side of the beach is eroded more during the summer season, and
tends to create a beach scarp. Shoreline recession on the southwest side of both beaches causes
the removal of turtle eggs by wave action. In addition, beach scarps interfere with landing of
loggerhead turtles on the backshore if the scarp is higher than about 0.8 to 1.0 m. Figs. 5 and 6
show beach and scarp scarps for which the turtles are able and not able to climb up, respectively.
Green turtles can climb up slightly higher beach scarp than loggerhead turtles. Inakahama is a
natural sandy beach except the area of a stone sea wall on the northeast side of the beach. In
contrast, southern half of Maehama Beach is protected by a sea wall. A jetty is also located on
its northern terminus. Part of the sand spit on north side of Maehama Beach is protected by
armor blocks and artificial dune-type nourishment.
Fig. 3 Aerial photograph of Inakahama Beach. (Supported by Kagoshima Prefecture Government)
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Beach Profile
Eight representative transects were surveyed along the two beaches, as shown in Fig. 7. The
datum is set to be the low tide level at the day of measurement in Figs. 8 and 9, giving the cross-
shore profiles at Inakahama and Maehama Beaches, respectively. Field surveys were conducted
in September 1999. Therefore, these profiles show the topography created by the northern
Fig. 4 Aerial photograph of Maehama beach.
Fig. 5 Passage of the landing on a
beach scarp which is able to climb up.
Fig. 6 Passage of the landing on a beach
scarp which is not able to climb up.
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longshore sediment transport and offshore sediment transport during the typhoon season(August
to October). A trough geometry between offshore distances of 25 and 40 m on Transect 1 shows
a meander of the small river running into the beach.
Fig. 7 Study area in Yakushima Island ( right);
Fig. 8 Beach profiles in Inakahama beach (1999).
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When a loggerhead turtle encounters a river, it does a U-turn often, because the temperature
of fresh water coming from the mountain is usually lower than the temperature of seawater
which is normally higher than 25 deg during the turtle season. The turtle then approaches the
back-beach for nesting. However, some turtles cross the river. Transect 2 is located in front of
the sea wall which protects a small hotel. There is a 40-m-wide beach, which seems to be
sufficient for landing and nesting of turtles, but light from the hotel is interference. A large
number of the sea turtles land and nest in the area of Transect 3 where a dry beach and dune are
well developed. A dry (back-) beach extends from 10 to 40m. Vegetation such as sea oats and
beach grass grows in an area between 40 and 60 m where the dune slope merges to the beach.
Bush and coastal forests grow just behind the vegetation area. A few turtles lay eggs in the bush
Fig. 9 Beach profiles in Maehama beach
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zone. Transect 4 shows that there is a narrow dry back-beach from 10 to 30 m, and the beach is
backed by bluffs, hence not suitable for climbing by turtles. The shoreline around the Transect 4
recedes because of the northern longshore sediment transport during summer season with
incident typhoons; thus beach scarps are often created. Therefore, it has been suggested that
turtle eggs in this region should be transplanted to the central and north part of the beach as much
as possible to prevent their washout to sea during a storm.
Transect 8 was located around the middle of the spit along the Nagata River mouth in
Maehama Beach. It is seen that a dry back-beach exists from 5 to 40 m, and an artificial dune is
set behind it. The artificial dune is composed of material dredged from the river mouth once a
year. Nagata River runs through behind the nourished section toward northern terminal beach.
The beach and artificial dune appear to have sufficient width and height for nesting and hatching;
however, a few typhoons have inundated the artificial dune and created an overwash fan inside
the river. Transect 7 represents a cross-shore profile between the sea wall area and the sand spit
area. Vegetation grows and covers the upper part of the beach in the neighboring area of sea
wall. This area seems to be relatively stable, because the beach is sheltered by the sea wall
against typical SSW incident waves caused by typhoons.
Transects 5 and 6 are located in the sea wall area. It is seen that the width of the dry back-
beach is nearly 15 to 23 m. In addition, there is a beach scarp higher than 1 m on Transect 5.
The beach scarp significantly interferes with sea turtle landing. Loggerhead turtles were capable
of climbing the beach scarp if the scarp height is low, and some hit the sea wall several times
while looking for nesting area. These turtles laid their eggs in front of the sea wall, which were
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then vulnerable to swash during severe wave conditions. Additional beach nourishment as a
point source was made farther north of Transect 6.
CHARACTER OF SEDIMENT
Character of the Sediment on a Beach
Sediment is supplied mainly from the abrasion of rocks around the river basin and by sea
cliff erosion. The rock around the study area is granite. Fig. 10 shows the mineral content of the
beach sediment. The island is isolated and raised from the sea bottom, and the ocean waves
arrive directly to the beach without significant dissipation. Therefore, fine sediment is lacking
on the beach. Sediment samples were taken at six locations on Inakahama Beach and at four
locations on Maehama beaches, and the grain-size distributions are shown in Fig. 11. The
medium grain size is in a range of 1.2 to 1.3 mm, except for samples taken at the boundary of the
beach and dune. As seen in the figure, the sediments are well sorted.
Fig. 10 Mineral contents of beach
0.1 10
10
20
30
40
50
60
70
80
90
100
No.1 No.2 No.3 No.4 No.5 No.6 No.7 No.8 No.9 No.10
Cum
ulat
ive
dist
ribut
ion
by w
eigh
t (%
)
Grain size (mm)
Fig. 11 Grain size distribution of beach sand.
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Grain Size Distribution of Nourished Sediment
Beach nourishment has been conducted at both of the beaches, as shown in Photograph 12,
by using material dredged from the Nagata River mouth. The material is replaced in the back-
beach of terminal areas of both beaches. The beach nourishment material is subjected to swash
wave attack only during the severe storms (typhoon), because it is placed beyond the average
swash wave height of ordinal waves. Therefore, the fine portion of the nourished material is
dispersed to the offshore only during storms. The beach quality sand sustains in the longshore
sediment transport system and contributes to the sand budget. The grain-size distributions of the
nourishment sediment is shown in Fig. 13, which shows that the distributions of nourished
material are wider compared to the existing beach sand, containing both finer and coarser
materials.
ECOLOGICAL STUDY OF LOGGERHEAD TURTLE
Ecological studies on landing and nesting of the loggerhead turtles have been conducted at
Inakahama and Maehama Beaches since 1985 and 1990, respectively. The basic statistics on the
Fig. 12 Point-source terminal nourishment.
Fig. 13 Grain size distribution of nourished material.
0.1 1 100
10
20
30
40
50
60
70
80
90
100
No.1' No.2' No.3' No.4' No.5' No.6' No.7' No.8' No.9' No.10'
Cum
ulat
ive
dist
ribut
ion
by w
eigh
t (%
)
Grain size (mm)
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landing and nesting as well as sea turtle behavior are given here, and the reader may refer the
annual reports of the Yakushima Sea Turtle Preservation Association, for instance (1994, 1997,
1999) for detail. The beach access was experimentally limited to visitors at Inakahama Beach,
while a group of local communities in the Maehama area conducted commercial observation
tours to Maehama Beach in 1999. The number of landing and nesting decreased because of
disturbance by human activity by the visitors and local guides. Thus, some of the loggerhead
turtles moved to Inakahama Beach for the nesting in 1999. A detailed analysis including the
passage of the loggerhead turtle on the beaches and the effect of structures is currently being
conducted. Therefore, human impact on sea turtle landing and nesting in Maehama Beach can
be estimated.
Landing, Nesting, and Hatching
Observation and measurement of the number of the landing and nesting are conducted on
Inakahama and Maehama Beaches every night during turtle season. Landing, nesting and
hatching of loggerhead turtles, as well as the eggs, are shown in Figs. 14 through 17. Visitors
and researchers should be aware of the influence of light and sound to the landing and nesting of
loggerhead turtle. Loggerhead turtles move about 20 to 30 m prior to nesting, then excavate the
sand surface by digging to a depth of 0.6 to 0.8 m. A turtle lays nearly 100 to 130 eggs per
nesting. The eggs are breathing under a sandy beach, so a wide dry beach is necessary to keep
them under incubation until hatching. The turtle eggs incubate 45 to 75 days. Newly hatched
turtles move together toward the sand surface for 7 to 10 days and pause for a while near the
sand surface by night. Because baby turtles tend to be attracted by light, in a natural
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environment they move toward the sea because the sea is often brighter than the hinterland.
Artificial light from the shore can cause baby turtles to move inland, where they might be
entangled by roots of vegetation and bushes, and finally dying if the inland is brighter than the
sea.
A daytime field observation is conducted as a check of the data taken in the previous night.
The number of the eggs is also measured at several nesting spots. In addition, the number of
hatchlings is measured in daytime as well as occasional measurement in the night. Table 1 and 2
show the numbers of landing and nesting at Inakahama and Maehama Beaches. Numbers in
Fig. 14 Loggerhead turtles. Fig. 15 The passage of landing and nesting.
Fig. 16 Loggerhead turtle eggs
(normal and abnormal)
Fig. 17 Baby turtles returning to the sea.
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square brackets correspond to green turtles, and numbers in parentheses correspond to abnormal
eggs. The nesting percentage is based on the total data for both of loggerhead and green turtles.
The total landing and the total nesting numbers for the two beaches are 1,012 and 603,
respectively in 1999. The number of landing and nesting drastically decreased by 254 and 99,
respectively, compared to the data for 1998. Seventeen green turtles landed and eleven of them
nested at both beaches in 1999. However, the numbers decreased by 22 and 4 compared to 1998.
Five and three individual green turtles were recognized in 1998 and 1999, respectively.
Table 1 Number of landings and nestings on Inakahama Beach. Month No. of
landings
No. of
nestings
No. of inspected
eggs
No. of turtles
inspected with eggs
Percentage
of nestings
4 2[0] 2[0] 232 2 100.0
5 158[0] 102[0] 4430(9) 38 64.6
6 273[1] 185[1] 7318(57) 64 67.9
7 158[7] 115[5] 3716(42) 35 72.7
8 2[3] 1[3] 101 1 80.0
Total 593[11] 405[9] 15797(108) 140 68.5
Table 2 Number of landings and nestings on Maehama Beach.
Month No. of
landings
No. of
nestings
No. of inspected
eggs
No. of turtles
inspected with eggs
Percentage
of nestings
4 10[0] 3[0] 107 1 30.0
5 139[0] 53[0] 1745(1) 15 38.1
6 150[1] 70[0] 2459(15) 21 46.4
7 118[2] 70[0] 1211(4) 13 58.3
8 2[1] 2[0] 0 0 66.7
11 0[1] 0[1] 0 0 100.0
12 0[1] 0[1] 0 0 100.0
Total 419[6] 198[2] 5522(20) 50 47.1
Five hundred ninety three loggerhead turtles and eleven green turtles landed at Inakahama
Beach. Four hundred five loggerhead turtle and nine green turtle nested in 1999. The
loggerhead turtle landings increased by 14, whereas the number of green turtle landings
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decreased by 7 at Inakahama Beach in 1999. Four hundred nineteen loggerhead turtles and six
green turtles landed at the Maehama Beach, and one hundred ninety eight loggerhead turtle and
two green turtle nested on the beach in 1999. The numbers of landing loggerhead and green
turtles decreased by 268 and 15, respectively. The number of green turtle nesting drastically
decreased at Maehama Beach in 1999. The number of landing at Maehama was usually larger
than that in Inakahama Beach except for year 1990 when the observation started in Maehama
Beach. However, there were 179 more landings on Inakahama Beach than at Maehama Beach in
1999.
CONCLUSION
Join ecological and coastal processes studies of sea turtles and their environment was made at
Inakahama and Maehama Beaches. Seasonal shoreline change and generation of beach scarps,
which coastal engineer can probably predict and manage, significantly influence the sea turtle
landing, nesting, and hatching. Therefore, the coastal engineers can work efficiently for the
protection of sea turtles.
Both beaches consist of the sand originating from granite. The sediment is well sorted over
the width of beaches, and the medium grain size is in the range of 1.2 to 1.3mm. The amount of
fine sand was very limited. Typical seasonal shoreline and profile change was observed.
Northeastern sediment transport occurs by typhoons in summer and southwestern sediment
transport by winter storms. The nesting and hatching in the southwestern part of both beaches
tends to be more washed away to the sea compared to the middle and northeastern part of both
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beaches. Therefore, it is recommended to transplant the eggs in southwestern part of the beaches
into the central part of the beaches as much as possible.
Minimum beach width for turtle nesting is nearly 30 m, and both beaches satisfy the
minimum beach width for the landing and nesting in general. In addition, the middle parts of
both beaches are stable and are suitable for turtle landing and nesting. Green turtles can climb up
slightly higher beach scarps (0.8 to 1.0 m) as compared to loggerhead turtles. A mildly sloping
dune is necessary for hatchlings to avoid overwash by high waves, because the study beaches are
open to the sea. Most of the backshore on southwest side of the beaches tended to be eroded and
contained scarps during typhoons.
Nourished material has been placed in the area between a dune and backshore where only
storm swash waves can reach; thus the finer component of the nourished material is dispersed
only during times of high waves and high tide. Thus, point source backshore nourishment has
less potential to impact the ecosystem compared to nourishment material placed directly on the
berm and beach face.
The number of turtle nestings at Inakahama Beach was higher than at Maehama Beach
where the sea wall was set and where the beach was open to the public at time during the turtle
season of 1999. Even though the landing and nesting were affected by the sea wall in Maehama
Beach, it was concluded that the main reason for the decrease in turtle landings and nestings was
the public access and human activity on Maehama Beach. The human disturbance such as light
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and sound by visitors must be carefully controlled in the turtle season. Thus, environmental
education of local residents and visitors was a strong recommendation of this study.
Research on the turtle behavior around coastal structures is continuing. Results will be
presented in subsequent papers.
ACKNOWLEDGEMENTS
Special acknowledgement goes to the members of Yakushima Sea Turtle Preservation
Association for their efforts in supporting high-quality measurements. The authors would like to
express thanks to Dr. Nicholas C. Kraus for proofreading this manuscript and to Mr. Terazono
and Mr. Yoshimizu for providing the Fig. 2.
REFERENCES
Davis, R. A., Jr., FitzGerald, M. V., and Terry, J. 1999. Turtle nesting on adjacent nourished
beaches with different construction styles: Pinellas County, Florida, Journal of Coastal
Research, 15(1), 111-120.
Kamezaki, N. 1994. Is the number od sea turtles decreasing ?, In Sea Turtle Protection and It’s
Future, Edited by The Japanese Sea Turtle Association, 28-75.
Ito, T. 1985. Ecosystem under the sandy beach -Introduction of the ecosystem in porous sand.
Kaimei L.T.D, 241 pp (in Japanese).
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Nelson, D. A., Mauck, K., and Fletemeyer, J. 1987. Physical effects of beach nourishment on
sea turtle nesting, Delray Beach, Florida, Technical report EL-87-15, US Army Corps of
Engineers, 56 pp.
Ohmuta, K. 1994. Annual report of sea turtles in Yakushima Island. Yakushima Sea Turtle
Association, 24 pp (in Japanese).
Ohmuta, K. 1997a. Annual report of sea turtles in Yakushima Island, Yakushima Sea Turtle
Association, 32 pp (in Japanese).
Ohmuta, K. 1997b. Passage of sea turtle, Institute of Ocean Engineering L.T.D, 237 pp (in
Japanese).
Ohmuta, K. 1999. Annual report of sea turtles in Yakushima Island, Yakushima Sea Turtle
Association, 40 pp (in Japanese).
Steinitz, M. J., Salmon, M., and Wyneken, J. 1998. Beach renourishment and loggerhead turtle
reproduction: A seven year study at Jupiter Island, Florida. Journal of Coastal Research,
14(3), 1,000-1,013.
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List of tables
Table 1 Number of landing and nesting in Inakahama beach.
Table 2 Number of landing and nesting in Maehama beach.
List of figures
Fig. 1 Green turtle and loggerhead turtle.
Fig. 2 Location of Yakushima Island.
Fig. 3 Aerial photograph of Inakahama Beach.
(Supported by Kagoshima Prefecture Government)
Fig. 4 Aerial photograph of Maehama beach.
Fig. 5 Passage of the landing on a beach scarp which is able to climb up.
Fig. 6 Passage of the landing on a beach scarp which is not able to climb up.
Fig. 7 Study area in Yakushima Island ( right; Inakahama beach, Left; Maehama beach).
Fig. 8 Beach profiles in Inakahama beach (1999).
Fig. 9 Beach profiles in Maehama beach (1999).
Fig. 10 Mineral contents of beach sand.
Fig. 11 Grain size distribution of beach sand.
Fig. 12 Point-source terminal nourishment
Fig. 13 Grain size distribution of nourished material.
Fig. 14 Loggerhead turtles
Fig. 15 The passage of landing and nesting.
Fig. 16 Loggerhead turtle eggs (normal and abnormal)
Fig. 17 Baby turtles returning to the sea