Hydraulics of Great Lakes Inlets

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Hydraulics of Great Lakes Inlets

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Seelig, William N. / Sorensen, Robert M.

Reversing currents in inlets on the Great Lakes are generated primarily by long wave seiching modes of the lakes rather than by the astronomical tide. Field measurements were conducted in 1974-75 at nine harbors on the Great Lakes to: (a) Investigate the nature of long wave excitation and the generating mechanism for significant inlet velocities, (b) establish techniques for predicting inlet-bay system response, and (c) develop base data for future planning and design studies. Data collected include continuous harbor water level measurements at all sites, inlet velocity measurements at the primary site (Pentwater, Michigan), and channel hydrographic surveys at the sites where more recent data were needed. Available historic water level and velocity data for some of the harbor sites were also used. Amplified harbor oscillations and generation of the highest inlet velocities are caused by the Helmholtz resonance mode which has a period of 0.6 to S hours for the inlet-bay systems studied. A recently developed, simple numerical model is shown to be effective in predicting inlet-bay response over the range of excitation periods encountered. A finite-difference form of the continuity equation is shown to adequately predict inlet velocities if high-quality bay water level records are available. Selected data from the study sites are presented to demonstrate the hydraulic response of the inlet-bay systems and the applicability of the prediction schemes. Examples to demonstrate use of the concepts and techniques developed in the study are applied to the design of a new inlet channel and to the modification of an existing channel.