Outline of North Dam Construction Office
Summary of Main Land of Okinawa
History of Water Resources Development
Condition & Demand of Water
Elements Chart of Dams
Water Resources Development in Okinawa
Okukubi Dam is the world first trapezoidal CSG Dam
New Technology and Environment
New Technologies for Dam Construction
Trapezoidal CSG dams (Okukubi Dam and Taiho Side Dam stream engineering)
As part of the stream engineering for the Taiho Side Dam, the North Dam Construction Office built the world’s first structure designed according to trapezoidal CSG dam theory. We are employing the trapezoidal CSG dam format for the world’s first main dam of its type at the Okukubi Dam currently being built.
The First Use of Extended Layer Construction Method in a Subtropical Zone (Taiho Dam)
In recent years, rationalized construction methods such as the planar RCD method or extended layer construction method have come to replace columnar block construction for the construction of concrete gravity dams.
With the planar method, dump trucks are used to haul slumping concrete so that a wide block can be placed at a single time, but it’s necessary to consider how to reduce thermal stress and ensure workability. Portland fly ash concrete containing cement fly ash is an effective solution to both of these issues, but in subtropical zones, there’s a lot of evapotranspiration due to heat and humidity, and a number of issues had to be resolved.
The North Dam Construction Office considered the climate characteristics of the subtropical zone and conducted tests of various mix proportions. By using the extended layer construction method and fly ash produced cheaply in Okinawa for the concrete mix proportion, it was possible to achieve streamlined construction and lower costs.
Development of New(Bellows-type)selective Intake Device(Taiho dam)
We also reduced construction, operation and maintenance costs by replacing the round, steel gates of the multistage selective intake devices that are used on many conventional dams with a bellows structure made of stainless steel rings covered with sheets of rubberized fabric using many low cost, lightweight and corrosion resistant rubber parts.
Joint Research
We solicited proposals for new technology in order to make use of the high technology available in the private sector and conducted joint research into a promising bellows-type selective intake device.
Environmental Conservation Measures
The planning, design and engineering work for dam construction aims to reduce the impact on the environment as far as possible.
The conservation measures are designed as follows.
1. Avoidance measures
Avoid impact to the environment by modifying the project plan.
2. Reduction measures
Restrictions were placed on areas where direct impact on the environment was unavoidable.
3. Compensatory measures
Priority is given to avoidance and reduction measures, but if necessary, a similar environment is created in a different location to compensate as far as possible.
The Okukubi River Natural Environment
The mangrove forests along the flow of Okukubi consist primarily of Kandelia candel and Bruguiera gymnorrhiza. The estuary is covered with Rhizophora mucronata and Lumnitzera racemosa, rarely found in Okinawa Island.Small creatures such as crab, shrimp, and shellfish inhabit the banks near the forest attracting wild bird predators. Visitors can see a diversity of wildlife here.


Avoidance Measures
-Conservation of Swamp Environments by Bridging-

We avoided affecting the swamp environment by bridging the embankment of the road diversion. We also avoided alteration of the surrounding forest.


Reduction Measures
Transplantation of Valuable Plants
Valuable plants growing in areas scheduled for engineering work and submersion were transplanted to cathment areas with similar environments.

Temporary Enclosure of the Work Site
Work sites where many vehicles such as dump trucks are operating are closed off with cloth sheeting to prevent small animals from the surrounding forest from entering.

Rapid Forestation of Road Slopes
To promote the prompt recovery of the forest, seeds of the original trees are distributed using the seed spraying method, or saplings are planted.

Fishways are provided for the aquatic organisms whose up and downstream movement would otherwise be obstructed by the dam body.


Compensatory Measures
Artificial Nesting Trees for Birds
Trees that had to be cut down during work on the dam were set up as artificial nesting trees for birds. They have been used as roosts by ruddy kingfishers and other birds, and the Okinawa woodpecker, designated a special national treasure, has been confirmed to nest and produce young in them.

Alternative Habitats
Artificial caves have been built for bats, and honeycomb board is provided as a home for small birds and insects.

Animal Shelter
Trees felled during the dam engineering work were stacked in various places around the river basin as homes and hiding places for insects and small animals. Mice and many types of insects have been confirmed to be living here.


Monitoring the Site (environmental monitors)
Environmental monitors monitor the project area and work site, and conduct surveys of valuable plants and animals, making recommendations for improvements and advising the contractors of points to note.

Effective Utilization of Felled Trees
Trees cut in the course of the engineering work including branches and leaves, trunks, and roots are put through a chipper and are used as material for slope planting and preventing soil runoff and as fertilizer. In this way felled trees are recycled and waste is reduced.