Author(s): Andrew M. Tuthill; Leonard J. Zabilansky
Linked Author(s): Andrew M. Tuthill
Keywords: No Keywords
Abstract: The Soo Locks, in Sault Ste. Marie, Michigan, connect Lake Superior to the St. Marys River and Lakes Huron and Michigan. The U. S. Anny Corps of Engineers and the Canadian Department ofTransportation operate the locks. In the spring of 1992, as a result of requests from the shipping industry, the Corps opened the locks on 21 March, eleven days earlier than the traditional opening date of I April. This study assessed the effects of that early season navigation on shorelines, near-shore vegetation and sediment along the St. Marys River, south of the locks. The study focused on the effects of ship-induced ice movement, bottom water currents, and sediment transport in areas where the navigation channels border wetlands. Of concern was potential damage to emergent plants by ice movement and sedimentation of wetland areas. General conditions of the ice cover and breakup processes were also monitored during the observation period. Three observation sites located offshore of wetland areas were chosen to represent worst case situations in terms of potential damage to the wetlands and emergent vegetation by vessel passage. Vessel-induced changes in water surface elevation (drawdown and surge), near bottom current velocities and sediment motion were measured at these sites for a wide range of vessel sizes and speeds as well as a variety of ice conditions. As in past studies, ship speed was found to be the single most important factor influencing vessel effects on the shoreline. For a vessel of a certain size and draft, passing a specific site, there appears to be a threshold speed above which significant drawdown and surge, near-bed velocity and sediment motion occur. The results of this study provide guidance for the threshold speeds of the various vessel types passing specific observation sites. Ship draft, distance from the navigation channel and shoreline geometry were also critical parameters. An active shore crack isolated shorefast and grounded ice from vessel induced and naturally caused ice movement on the channelward side. In all cases observed, wetlands lay on the shoreward side of the active crack system. Depending on the river bed material present at a site, near bottom water velocities in excess of I to 1.5 ft/s caused dislocation of fine sediment, temporarily obscuring the bed. Shoreward transport of this sediment into wetland areas was blocked, however, by the zone of grounded ice. Viewed in the context of average year-round turbidity levels, the increase associated with vessel passages during the early opening season is relatively small.
Year: 1994