WRRCSR No.10:18:91 Impoundment of Stream Flow in West Loch, Pearl Harbor, Oahu, Hawaii: A Feasibility Study

Abstract

This study investigated the feasibility of capturing and storing fresh stream water flowing into West Loch, an estuarine, enclosed inland embayment of Pearl Harbor, Oahu, Hawaii, that is accessible to the sea through a narrow channel. The concept is to create a sea-level reservoir in the loch by, in some manner, displacing the salt water with fresh water. Three possible methods for accomplishing this were examined. The first two involved damming the embayment at a desired point on the seaward side by using a conventional impermeable core earth dam or a fabridam or rubber dam, for which their technology is well developed and operational. The third method involved using a flexible impermeable membrane structure to separate the fresh water from the salt water. Thus, the salt water is displaced by the freshwater-filled container without a rigid barrier. This membrane technology is insufficiently developed to use operationally at this time. Six alternative dam sites having reservoirs of varying capacities were examined and analyzed as possible locations for the enclosure. The farthest downstream site has the largest storage capacity not only because of the reservoir's larger surface area, but also because the bottom becomes progressively shallower upstream. Major conclusions were (1) no dam site has a clear advantage because the sites storing the most water are the deeper areas used by the Navy, while the smaller shallower sites cannot store significant amounts, (2) there is sufficient stream inflow to annually fill even the largest reservoir, and (3) environmentally, considerable impact will occur within the reservoir as salinity decreases to freshwater levels.