Coastal development is classified into the phases of overall planning, facility designing, work execution, and facility management. To promote coastal conservation while maintaining symbiosis with nature, it is essential to consider the natural life forms during these phases of coastal development.
The table below lists the considerations in the respective phases of coastal development for sea turtle, which is a typical natural life form inhabiting sand beaches.
| Phase |
Item |
Countermeasure |
Knowledge, case example |
| Overall plan |
Objective of sand beach conservation
|
Backbeach should be secured.
|
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| Selection of the types of wave absorbing facilities
|
Offshore wave absorbing facilities rather than wave dissipating works, as well as artificial reefs and headlands rather than detached breakwaters, should be selected.
|
Impact of wave dissipating works and detached breakwaters
|
| Facility designing |
Alignment and configuration of levees and revetments |
To be determined in consideration of securing sufficient sand beach width |
In case sufficient sand beach width cannot be secured, gently sloping dykes should not be constructed. |
| Body length, opening width, constituting materials, and crest height of offshore wave absorbing facilities |
To be determined so as not to affect the landing of sea turtles |
A case example of setting the crest water depth of an artificial reef at 70 cm or deeper as a standard for assessing the impact on the landing behavior of sea turtles |
| Sand beach width |
Sufficient width for spawning should be secured. |
Spawning rate is high when the backbeach width is around 30 m or wider, distance from the water's edge is 20 m or longer, and the sand beach width is 20 m or wider. |
| Sand beach gradient |
To be determined so as not to affect the landing and spawning of sea turtles |
1/20 gradient or more gentle, etc. |
| Deposits |
To be arranged so as not to affect the landing and spawning of sea turtles |
Grain size of around 0.2 to 2.0 mm, 60% or more of sand grains of around 1 mm diameter, and sand with high softness |
| Sand layer thickness |
To be determined so as not to affect the spawning of sea turtles |
50 cm or thicker, 70 cm or thicker, or 100 cm or thicker |
| Execution |
Timing |
The spawning period should be avoided, and the time immediately before spawning should also be avoided so that beach scarp will not be created when conducting artificial beach nourishment. |
Technical Manual of the US Army Corps of Engineers, etc. |
| Compaction of artificial beach nourishment sites |
To be conducted without affecting the spawning of sea turtles |
If the beach sand is excessively compacted, sea turtles cannot easily dig holes for spawning. |
| Management |
Beach usage restrictions |
To be implemented for protecting the landing and spawning of sea turtles |
Restriction on the use of vehicles at Enshunada beach in Hamamatsu, etc. |
| Lighting |
To be devised without affecting the landing and spawning of sea turtles |
If the illuminance is 0.5 lux or lower, larval sea turtles do not present phototaxis. Planting for light interception is also helpful. |
| Protection activities |
Cooperation with protection activity groups, and provision of information, should be implemented. |
|
In regard to coastal vegetation which is also a typical natural life form inhabiting sand beaches as with sea turtle, aerial photos were taken over the Kashimanada and Kujukuri coasts, and based on an analysis of these photos, the sand beach width required for inhabitation was examined. As shown in the figure below, it was clarified that a sand beach at least 20 m wide is required for plants to inhabit the gently sloping coasts facing the Pacific Ocean even in the autumn to winter period, and that the sand beach width of 100 m should be a yardstick when designing a sand beach where vegetation can exist with a probability of 50% or more.
These results were reflected in the "Guidelines for Promoting Coastal Development in Symbiosis with Nature," supervised by the Seacoast Office, Land Conservation Division, River Bureau, Ministry of Land, Infrastructure and Transport (MLIT), issued by the National Association of Sea Coast in 2003.
