Vertical Ship Motion Study for Ambrose Entrance Channel, New York

New York Harbor Pilots have expressed a concern to the New York District NAN about a mound that is approximately 1.5 miles seaward of the new limit of the Ambrose Channel. This mound is probably construction rubble that was placed there when the original Light Tower isolated and protected it somewhat from the main channel. The mound s diameter and depth are similar to the width and depth of the Ambrose Channel offshore reach. Now that the Light Tower is no longer marking this mound, the Pilots are concerned about whether a ship might experience larger or more dramatic ship motions due to possible amplification across the mound relative to similar transits in the offshore reach. The US Army Engineer Research and Development Center ERDC, Coastal and Hydraulics Laboratory CHL, used the numerical models STWAVE and CMS-Wave to evaluate possible amplification effects of a range of wave conditions. The Channel Analysis and Design Evaluation Tool CADET was used to predict vertical ship motions due to wave-induced heave, pitch, and roll. Ship squat estimates calculated with the PIANC and Ankudinov empirical formulas were compared with the Beck, Newman, Tuck BNT squat predictions used in CADET. The net underkeel clearance based on these vertical ship motion components was used in a risk-based method of evaluating transits over the mound compared to similar transits in the main channel. These results were used to select a minimum dredge depth over the mound to insure that inadvertent transits over the mound would not incur any significant differences in ship response and potential grounding relative to similar transits in the offshore reach of the Ambrose Channel.