DESIGN CONSIDERATIONS FOR THE SAND ISLAND AND BARBERS POINT OUTFALLS

Abstract

The design and construction of a major ocean outfall and diffuser system for disposal of wastewater effluents is a complex process involving an interplay of requirements originating from various disciplines. These include, among others, considerations of physical oceanography, mixing and dispersion, treatment processes, regulatory requirements, marine geology, economics and construction. The recently completed Sand Island Outfall and the newly designed Barbers Point Outfall are both on the southern coast of the island of Oahu, Hawaii, and are designed for treated sewage effluents from the densely populated portion of the City and County of Honolulu. In this paper, some design considerations of these outfalls will be examined. The emphasis in this paper is on the hydrodynamics, although other design aspects are also discussed briefly. The design of a large submarine outfall must consider two basic objectives. First, the design must assure the physical integrity of the structure when subjected to the sometimes violent environment. Second, the mixing, dispersion and transport of the effluent must be such that environmental degradation is acceptably small and that water quality requirements are met. In addition, the design process also includes considerations of economics, overall system compatibility, and construction. The Sand Island Outfall was constructed in 1975-1976 while construction of the Barbers Point Outfall began in 1976. The cost of the Sand Island outfall was 13.7 million.