Physical Model Study of Wave Action in New Thomsen Harbor, Sitka, Alaska

A 1-to-75 scale physical model of Sitka, Alaska, encompassing portions of the Western Channel, the region protected by the three breakwaters, New Thomsen Harbor, and the Sitka and Japonski Island shorelines, was constructed at the modeling facilities of the U.S. Army Engineer Research and Development Center's Coastal and Hydraulics Laboratory. The primary objectives of the physical model study were to (1) establish the cause for wave action within the harbor causing vertical motion of the floating docks and (2) investigate potential engineering alternatives to reduce wave action within the harbor to acceptable levels. A total of 179 tests were conducted in the Sitka physical model during four time periods between the completion of the model in September 2005 and February 2007. Several hypotheses explaining increased wave action in New Thomsen Harbor were tested. Of these hypotheses, wave focusing by local bathymetry near New Thomsen Harbor appeared to be the most plausible; incident waves interacting with waves reflected by the shoreline at high water were a possible contributor for longer period waves. Large, short-period waves from the northwest could cause high waves in the harbor, but only when the wind blows hard from that direction. Distance between the rubble-mound breakwaters and harbor is also adequate to generate sizable short-period waves within the harbor. The short-wave energy could excite a harmonic frequency of the dock system resulting in adverse motions. At present the floating dock harmonics are unknown. Closing one or more gaps between adjacent breakwaters and/or breakwaters and the shoreline reduced wave heights in New Thomsen Harbor. Leaving only one gap open when waves came from the southwest reduced wave heights by about half. Closing only one gap while leaving the rest open did not create an appreciable wave height reduction when waves came from the southwest.