Pontoon Launch Channel Analysis, Design and Performance

Abstract

New concrete pontoons for the SR520 Bridge Replacement Project in Seattle, Washington, were constructed at a new casting basin facility located in Aberdeen, WA, as a design-build project by Kiewit Corporation. Upon completion, the pontoons were towed from the casting basin into the nearby Chehalis River and Grays Harbor estuary via a 620-foot-long launch channel. The launch channel was constructed in December 2011 and the first completed pontoons were towed from the casting basin in July 2012; pontoon production will continue. This paper presents numerical modeling analysis, engineering design, and performance of the launch channel, including analysis of hydrodynamic forces on pontoons due to currents and passing vessel effects in the adjacent deep draft federal navigation channel. Due to the location of the launch channel and casting basin in the estuarine portion of the Chehalis River, the site is subject to processes including combined effects of tides, wind, waves, river discharge and vessel effects. All these factors were considered in designing the channel and estimating forces on the pontoons. Geomorphic analysis and three-dimensional numerical modeling were the basis for determining that the channel would initially experience a period of relatively more rapid sedimentation followed by more gradual sedimentation rates. Such rates of sedimentation were incorporated into the dredging plan as advance maintenance to minimize maintenance dredging frequency. The launch channel side slopes, composed of native soft silty sediments, are subject to erosion and scour primarily due to propeller wash from tugs operating in the channel. Numerical modeling of waves, currents, and propwash was performed to optimize the extents and type of armoring required. Based upon this analysis and coordination with professional tug operators, the final design achieved a significant reduction of rock armoring while maintaining protection for the channel slopes against scour. A unique rock slope key design was developed to transition from armored to soft unarmored slopes. Recommendations for turning dolphin design and tug operations were based on numerical modeling of effects of riverine and tidal currents on the pontoons at various positions in the channel and at various flows. Forces, moments, and pontoon motions were also developed to account for the potential effects of passing vessels on rafts of multiple completed pontoons temporarily moored along the navigation channel. These were provided to naval architects for design of mooring systems. Ongoing monitoring of the channel bathymetry indicates that the actual sedimentation rates were slightly lower than predicted. Side slope protection and towing forces appear adequate for continued cycling of pontoon construction into the future.