81 



THE WAIAHOLE TUNNEL PROJECT. 



By G. K. Larrison, Superintendent of Hydrography. 



The largest hydraulic engineering project ever undertaken in 

 the Hawaiian Islands was practically completed when the two 

 headings of the main tunnel of the Waiahole Water Company's 

 great irrigation project on Oahu met on December 22, 1915. 



Why It Was Built. 



The industrial condition which brought this project to com- 

 pletion was, briefly, the necessity for bringing the perennial moun- 

 tain streams from the windward side of Oahu, where there is 

 little need of them, through the rough Koolau mountain range 

 which forms the backbone of the island, to irrigate the broad, 

 rich, semi-arid sugar lands surrounding Pearl Harbor. 



Mother nature who rules so harmoniously when not tampered 

 w^ith by mankind, evidently did not consider the sugar industry 

 in her plans when she arranged her layout of sunshine, rain and 

 soil on the various islands, for she usually put the soil most 

 suited for sugar and the sunshine on one side of the island and 

 the abundant rainfall so necessary for sugar cane growth on the 

 other. 



The discovery of an apparently abundant supply of artesian 

 and ground water on leeward Oahu which could be profitably 

 pumped to serve the fields seemed to solve the problem, until 

 the belief that this water supply was inexhaustible resulted in so 

 many wells being driven that the water not only rapidly de- 

 creased in quantity, but deteriorated in quality. As the fields 

 were extended to higher elevations the cost of pumping became 

 so high and the quality of the water so low that the project of 

 bringing the abundant and pure mountain surface waters from 

 the windward side was developed. 



The plan decided upon was to gather up the mountain streams 

 along the windward side from the Waiahole to the Kahana val- 

 leys and to bring these via a long tunnel through the mountain 

 range to a point where the water could be delivered to the Oahu 

 plantation fields. It was planned not only to use the pure moun- 

 tain water to mix with and rotate with the pumped water which 

 could still be profitably pumped to serve the low lying fields and 

 to eliminate the necessity of costly pumping to the high level 

 fields, but also to bring under cultivation an additional area of 

 about 3800 acres above the old fields. 



The Surface JVater Supply. 



The greatest water supply producing area on Oahu includes 

 the windward valleys of \Vaiahole, \\'aikane, Kahana and Puna- 



