|Freshwater Resources in Arid Lands (UNU, 1997, 94 pages)|
|5: The development of groundwater resources on the Miyakojima Islands|
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.
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.