Structural safety of an ocean outfall against hurricane damage

Abstract

Ocean outfall pipelines, made of cast iron with concrete coating, are used to carry secondary-treated effluent which has almost the same density as freshwater. The pipelines, which are laid in trenches backfilled with sand and rock boulders, are often insufficiently protected in the event of a major storm. This paper presents a study on the prediction of the wave/wave-induced hydrodynamic forces and erosion that would result from a hurricane, and evaluates the potential for damage to the structure due to a Category 4 hurricane, i.e. a hurricane (measured on a scale of 1–5) with a wind velocity between 131 and 155 m.p.h. and storm surges of 13–18 ft above normal, which may lead to major beach erosion. The investigation includes liquefaction analysis, evaluation of scour due to wave breaking, vertical and horizontal stability analyses, and recommendations for a concrete flexible revetment scour protection system. Both fully exposed and partially buried pipelines are considered in the stability analysis. A detailed discussion of the main elements of a scour protection system, including the revetment underlayer, cellular units, tendons, and pin anchors is presented, with typical illustrations. Further study is needed to include possible pipeline breakout from the seafloor which results from fluctuating lift forces caused by an induced gap at the pipe-soil interface.