Ground water impounded by dikes in the Koolau Range is a major source of water for the island of Oahu, Hawaii, and many tunnels have been bored into the range to develop it.
All water-development tunnels, except Waihee tunnel, have depleted storage in the rocks they penetrate and are now discharging at rates that are but fractions of the rates possible at full storage. Rocks above the floor of the water-development part of Waihee tunnel have never been completely dewatered, and storage can be manipulated by regulating outflow. Thus, storage for this tunnel can be increased during periods of low demand and discharged at high rates during periods of high demand.
A measure of the rate of drainage or depletion of storage is the recession constant b in the recession-curve equation Qt=Q0e-b t. The higher the value of b, the faster water can be drawn from storage or returned to storage through artificial recharge. Mathematical analysis of the flow-recession curve of Waihee tunnel shows that (1) its recession constant is 0.00401, (2) net storage (exclusive of recharge) is 2,200 million gallons (6,800 acre-feet), and (8) initial discharge from full storage would be about 19 million gallons per day.
Analysis of flow-recession curves for Waiahole ditch tunnel (main bore) and Haiku tunnel shows that these tunnels have drainage characteristics that are similar to those of Waihee tunnel.
The composite recession constant computed for the four tunnels north of Waiahole is about one-third as large as that computed for the Waiahole ditch tunnel
(main bore) and the tunnels to the south. The difference is due to an abrupt change in spacing of dikes north of Waiahole. At and south of Waiahole Stream, dikes are spaced tens or hundreds of feet apart; north of Waiahole, they are spaced inches or a few feet apart.
Storage could be restored by bulkheading at the controlling dike or dikes after an analysis is made of the flow-recession curve for each tunnel. Such analyses will show which tunnels are best for storage restoration. The reciprocal of the recession constant is the time, in days, it would take to drain about two-thirds of the storage. The shorter this time is the better storage may be manipulated to meet seasonal peak demands for water.