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5: The development of groundwater resources on the Miyakojima Islands


Introduction
Outline of the Miyakojima Islands
Geology and the hydrological cycle
Outline of the irrigation project on Miyakojima
The construction and concept of the underground dam
The Construction of the Cut-off Walls
The construction of the intake facilities
The storage situation of the Sunagawa underground dam
Conclusion
Bibliography


Kimio Osuga

Introduction

An estimation of potential and available groundwater resources is very difficult because of their invisibility. In the case of the construction of underground dams that store the groundwater How, we need to consider many technical issues. Since 1990, on the Miyakojima Islands, the Japan Agricultural Land Development Agency (JALDA) has been implementing a project to construct two underground dams that will be the largest in the world. One of the two was completed in November 1993 and the other is now under construction. This report presents the concept of the underground dams on the Miyakojima Islands, the construction methods, and related technical issues.

Outline of the Miyakojima Islands

The Miyakojima Islands are located about 1,900 km south-west of Tokyo, comprising an area of 159 km. The climate is characteristically subtropical with a high annual average temperature of 23C and high humidity of 79 per cent. Though annual rainfall is high, approximately 2,200 mm, it is unpredictable and fluctuating because much of it largely relies on rainy-season rainfall and typhoons. Besides this, all the islands are covered by a highly permeable Lyukyu limestone layer that shapes its flat plateau and has resulted in the absence of rivers on the islands.

The islands, with a population of about 48,000, are divided into four administrative areas, Hirara City, Shimoji Town, Gusukube Town and Ueno Village (fig. 1). The main industry on the islands is agriculture and its development strategy is mixed farming, with sugar cane as the main crop and cattle for meat. In recent years, crop diversification has been increased where, for example, vegetables are grown targeting the pre-season of the main island of Japan, and tropical fruits and tobacco are also grown. Furthermore, the now famous All Japan Triathlon Competition is held on Miyakojima every year in April and, since the inception of direct-flight services from Tokyo and Osaka to the islands, tourism has increased steadily.

Figure 1 Location Map of the Miyakojima Islands

Geology and the hydrological cycle

The Shimajiri mudstone layer of bedrock is an impermeable rock with an average hydraulic conductivity of 2 x 10-6 cm/sec. Over this, the highly permeable Lyukyu limestone layer with a hydraulic conductivity of 3.5 x 10-1 cm/ sec forms the aquifer with a thickness of between 10 and 70 m. The effective porosity of the Lyukyu limestone is estimated at between 10 and 15 per cent. Tectonic movements accompanied by several faults have formed underground valleys, and groundwater flows along these valleys.

Thus, 40 per cent of the abundant rainfall on Miyakojima penetrates underground and quickly flows out into the ocean unused. In addition, the strong subtropical sunshine evaporates as much as 50 per cent of the rainfall. The result is that only 10 per cent of the precipitation can flow along the surface (fig. 2). Using the water resources on Miyakojima has been quite difficult, owing to these natural conditions.

Figure 2 The Hydrological Cycle on Miyakojma and the Overall Average in Japan

Before construction of the drinking-water supply system in the 1960s, obtaining drinking water was hard work for the women and children, owing to the above natural conditions. Agriculture, the main industry on the islands, still experiences frequent drought.

Outline of the irrigation project on Miyakojima

In order to reduce the burden of drought and to modernize agricultural management, an irrigation project has been implemented since 1987. The area benefiting from the project is 8,400 ha, which occupies half of the total surface area of the islands and about 90 per cent of the arable land. The total project cost was estimated at 89 billion in 1986.

The irrigation project is divided into the following three parts (table 1):

1. The construction of two underground dams, named Sunagawa and Fukuzato, and intake facilities (a number of wells with submergible motor pumps) installed in the storage area of those dams, which will be implemented by the JALDA Project;
2. The construction of farm ponds and the main pipelines, which will be implemented by the Japanese National Irrigation Project;
3. The construction of irrigation facilities on the farm and the field consolidation, which will be implemented by the Prefectural and Local Government Project.

The water sources for the project depend on two underground dams, one experimental underground dam (Minafuku) and one natural groundwater basin (Nakahara). The total amount of developed groundwater resources in this project will be 24 million cubic metres (mcm), which constitutes 40 per cent of the total unused groundwater returning to the ocean per annum, an estimated 60 mcm (table 2).

Table 1 Planned Infrastructure Facilities of Irrigation Project

Administrative organization Planned infrastructure facilities Quantity Costa (million $)
JALDA Underground dam (incl intakes) 2 181
Government of Japan Irrigation canal (km) 135.1  
  Water compressing facility 8  
  Farm pond 7  
  Water management facility 1 145
Local government On-farm irrigation facility (ha) 8,394  
  Land consolidation (ha) 3,140 267
Total     593

a. Estimated costs based on 1986 prices ($1 = 150).

Table 2 Water Resources Development Plan

Underground basina Sunagawa Nakahara Fukuzato Minafuku
Underground dam Sunagawa: main dam (Natural basin) Fukuzato: main dam, 2 sub-dams Minafuku Dam
Basin area (km) 7.2   12.4 1.2
Reservoir area (km) 4.89   7.0 0.9
Total capacity (mcm) 9.5   10.5 0.7
Effective capacity (mcm) 6.8   7.6 0.4
Water utilization capacity (mcm) 8.8 3.6 11.0 0.6

a. mcm = million cubic metres.

The construction and concept of the underground dam

Widely fluctuating groundwater levels throughout the year and salt-water infiltration along the coastline heavily constrain the use of groundwater. If we could control the groundwater level at a constant level, the available groundwater resources would increase enormously. The idea mentioned above has led to the underground dam project, which dams up groundwater flow within an aquifer so that a great amount of water which up until now returned to the ocean unused can now be used for irrigation (figs. 3 and 4).

The underground dam cannot be constructed just anywhere. The following appropriate conditions are necessary:

• An aquifer with high effective porosity, sufficient thickness needs and great areal extent;
• An impermeable bedrock layer under the aquifer;
• Sufficient groundwater inflow to the underground area;
• An underground valley where an underground barrier can be built;
• Land-use practices that do not contribute to groundwater contamination.

Figure 3 Location of the Groundwater Basins and Underground Dams

Figure 4 The Concept of the Underground Dam

Compared with conventional dams, underground dams have the following advantages:

• Since water is stored underground, submergence of houses and land can be avoided, and thus the land above the underground dam can be utilized as it was prior to the construction of the dam;

• Potential disasters caused by collapses of the barrier (cut-off wall) can be excluded;

• The construction cost of the cut-off wall can be lowered by thinning it, provided that it satisfies the necessary permeability requirement;

• The dam's life span can be semi-permanent because of the absence of the accumulation of sediments.
However, there are also the following disadvantages:

• Accurate estimation of the reserve volume of groundwater is very difficult;

• Construction control, such as control of the completed work and quality control of the cut-off walls, needs careful consideration because of its invisibility;

• The cost of the operation and maintenance of the intake facilities may be more expensive than that of conventional dams.

The Construction of the Cut-off Walls

The site of the cut-off walls was decided from the following points of view:

• The site needs to be located at the most downstream point to the extent that the necessary water volume can be stored;

• The site needs to be located at the narrowest point of the underground valley to reduce the construction cost;

• The site needs to be located at a place where no obstacles can impede construction;

• The site should not be situated over ground containing caverns;

• The depth of the cut-off walls should be less than 70 metres from the surface, taking into consideration the drilling capacity of machines.

As a construction method for the cut-off walls, the consecutive pillar wall using the in-site churning method is adopted in most parts, taking into consideration the geological conditions, the working efficiency, and cost performance. Many different machines are used in this method to make the best use of each machine's capabilities in constructing the cut-off walls. The procedures are as follows:

1. From the surface, augers with a casing are used to make holes down to the crest of the dam;

2. For places deeper than the crest of the dam, powerful augers with a single shaft are used to predrill into the hard limestone layer, making the remaining work easier;

3. Augers with three shafts are used in constructing the cut-off wall. This wall consists of pillars made of liquid cement and crushed limestone;

4. To form the shape of the crest of the dam, another machine with a rectangular-shaped bucket is used.

Figure 5 The Construction Procedures of the Cut-off Wall

A wall as deep as 65 metres from the surface was constructed as the Sunagawa Underground Dam by using this method (fig. 5). The depth, the continuity, and the quality are the most important aspects for the construction of the wall. A computer-aided system has been introduced to manage the construction. The data on the drilling depth, the verticality, and the amount of liquid cement injected were displayed in the control office where operators could issue very detailed and efficient directions.

The construction of the intake facilities

The other important facility of the underground dam is the water intake that pumps up the stored groundwater. Many wells are installed over a large area of the basin for effectively collecting the groundwater. The elevation of the bedrock, the space between the wells, and the permeability of the aquifer are examined carefully in order to decide on the arrangement of the wells. The permeability of Lyukyu limestone is not constant. As a result of a careful examination and analysis of the data on geology and pumping tests, it has become clear that the permeability of the Lyukyu limestone on the Miyakojima Islands is mostly affected by the content of the clay derived from near the surface layer. Based on this fact, a finite element method model of an aquifer was built up and then the intake possibility in each of the planned wells was calculated by using this model.

A submergible motor pump with the diameter of 125 mm and with a water intake of 0.023 m/sec was designed. These intake wells are located directly under the facility road for easy maintenance. The water-intake facilities are controlled by the pumping station. The groundwater is pumped up from the wells to the farm ponds on the hills through pipelines and then distributed to the agricultural areas (fig. 6).

Figure 6 Cross-section, from Upstream to Downstream, of the Underground Dam (GOJ, Government of Japan; JALDA, Japan Agricultural Land Development Agency)

The storage situation of the Sunagawa underground dam

The groundwater level of the Sunagawa underground dam started to rise immediately after the completion of the cut-off wall in November 1993 and reached the full water level of 31.0 m on 30 September 1995. Since then it has remained at overflowing level, around 31.7 m (fig. 7). We have not seen any partial fluctuation of the groundwater level at upstream or downstream sites along the cutoff wall, either before or after reaching the full water level. Thus, the facts mentioned above verify that the impermeability of the cut-off wall is secured and so also is the permeability of the crest of the wall.

Conclusion

It is now possible to irrigate 914 ha using the water stored in the Sunagawa Dam (as of March 1996). This area will increase gradually according to the increase in the area of land consolidation and of irrigation facilities installed.

Construction of underground dams applying the same methods used on the Miyakojima Islands has started on the main island of Okinawa and on Kikaijima Island in Kagoshima Prefecture. Building underground dams has thus been verified as an effective way of water-resources development on these remote islands or in areas with no surface water. This method is also effective in arid lands, from the point of view that it can reduce high evaporation and store the fluctuating rainfall.

Figure 7 Changes in Groundwater Levels in the Sunagawa Dam over Time

Bibliography

Ministry of Agriculture, Forestry, and Fisheries. 1993. Technical Guidelines for the Planning and Designing of Underground Dams (in Japanese), 2 6. Natural Resource Division, Planning Department, Agricultural Structure Improvement Bureau.

Mori, K., M. Asano, and T. Shirakawa. 1996. Lithology and Permeability of Lyukyu Limestone in Sunagawa Subsurface Dam in Miyakojima (in Japanese). The Japan Geology Association.

Nagata, J. 1991. How are Underground Dams constructed? (in Japanese). Report of lecture meeting in autumn 1991. Japanese Society of Groundwater.

Nagata, J., S. Nagata, and M. Okamoto. 1991. Hydrogeology and Underground Dam Project on Miyakojima Islands (in Japanese). Report of lecture meeting in autumn 1991. Japanese Society of Groundwater.

Nilsson, A. 1988. Groundwater Dams for Small-scale Water Supply. Intermediate Technology Publications, London.

Okinawa Development Bureau. 1987. Basic Design Report on Miyako Irrigation Project (in Japanese).

Okinawa Development Bureau. 1983. Groundwater in Okinawa (in Japanese), 4, 5, 811, 67. Agriculture, Forestry and Fisheries Department.

Okinawa Prefecture. 1995. Miyako Outlook (in Japanese), 2-4, 31-32. Miyako Branch Office.

Yamada, T. 1993. Construction of Underground Dams on Miyakojima (in Japanese). The Dam Digest, No. 580, pp. 81-95. The Japan Dam Foundation.


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