Ground-water reservoirs impounded by volcanic dikes receive a substantial part of the total recharge to ground water on the island of Oahu because they generally underlie the rainiest areas. They accumulate the infiltration from rainfall, store it temporarily, and steadily leak it to abutting basal reservoirs or to streams cutting into them. The dike reservoirs have high hydraulic heads and are mostly isolated from saline water.
The most important and productive of the dike-impounded reservoirs occur in an area of about 135 square miles in the main fissure zone of the Koolau volcano where the top of the dike-impounded water reaches an altitude of at least 1,000 feet. Water is impounded and stored both above and below sea level. The water stored above sea level in the area of about 135 square miles has been roughly estimated at 560 billion gallons by using a mean water level of 400 feet and a mean specific yield of 0.05. In comparison, the water stored above sea level in reservoirs with a mean water level of 300 feet and mean specific yield of 0.03 underlying a dike-intruded area of about 53 square miles in the Waianae Range has been roughly estimated at 100 billion gallons. Storage below sea level is indeterminable, owing to uncertainties in the ability of the rock to store water resulting from increasing dike density and decreasing porosity.
Total leakage from storage in the WaLanae Range has not been estimated because underflow is difficult to determine. Much of the surface leakage, about 4 Mgal/d in the upper parts of Waianae, Makaha, and Lualualei Valleys, has been diverted by tunnels. Hence, supplies available, other than surface leakage, cannot be estimated from the discharge end of the hydrologic cycle. Infiltration in the Waianae Range to dike-intruded reservoirs in the upper part of the valleys on the west (leeward) side has been estimated at about 20 Mgal/d, and on the east (windward) side, at about 10 Mgal/d. The available supply has been estimated at about 15 Mgal/d from the infiltration on the leeward side, of which about 4 Mgal/d is now being developed. No estimate has been made for the available supply on the windward side. Dike-intruded reservoirs at shallow depths west (lee side) of the crest are in upper Makaha, Waianae, and Lualualei Valleys. They are at moderate depths in upper Haleanu and in lower Kaukonahua Gulches on the east (windward) side.
Flow hydraulics in dike tunnels is also discussed.
|Title||Evaluation of major dike-impounded ground-water reservoirs, Island of Oahu, with a section on flow hydraulics in dike tunnels in Hawaii|
|Authors||Kiyoshi J. Takasaki, John F. Mink|
|Publication Subtype||USGS Numbered Series|
|Series Title||Open-File Report|
|Record Source||USGS Publications Warehouse|