The engineering feasibility of a tidal siphon, and alternative to the jetty hardened coastal inlet

The focus of this research was to determine the feasibility in function, design and cost of an alternative to the jetty hardened coastal inlet. There are four main objectives of this research: (1) To research the immediate reasons for coastal inlet closures specifically as a function of inlet hydraulics and coastal dynamics; (2) To identify and quantify the parameters necessary to facilitate various tidal exchanges as a function of the design of the proposed alternative (i.e. length and cross section of sub-oceanbed conduit, and conduit material) and coastal dynamics (i.e. tidal prism, overland runoff, longshore sediment transport; (3) To determine the shallowest depth of the inlet/outfall structure without being in water so shallow that wave action would destroy the structure or carry significant sediment into the siphon; and (4) To determine the design alternative performance and use this information to review the feasibility of construction from the point of view of cost and identified physical benefits. Collection of field data was required to verify the published information and facilitate the design parameters for the tidal siphon. Data collected included: offshore wave height and length; turbidity as a function of elevation from the oceanbed; tidal range in the lagoon and tidal lag as a function of offshore tide; inlet velocity as a function of tidal phase; and shoaling as a function of time One of the main concerns for the design of the offshore tidal siphon (OSTS) is the location of the ocean side intake/outlet structure. In this analysis 2100 feet of conduit appeared reasonable (1600 feet offshore in 36 feet of water). A muted tidal prism must be acceptable as the alternative for tide full exchange using the OSTS design. A tidal prism of approximately one-third the calculated stable tidal prism was chosen. The construction cost of the proposed inlet alternative was estimated based on the deign parameters at $4.5 million and compared with the known cost of the jetty hardened inlet for the same study area at $13.1 million. The site that was used as the sample lagoon is a recently constructed tidal inlet at the Batiquitos Lagoon in Carlsbad, California. (Abstract shortened by UMI.