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Prevention of shoaling at Little Lake Harbor, Michigan : hydraulic model investigation

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Date

1982-07

Authors

Seabergh, William C.
McCoy, James W. (James Wesley), 1946-

Journal Title

Journal ISSN

Volume Title

Publisher

Hydraulics Laboratory (U.S.)
Engineer Research and Development Center (U.S.)

Abstract

Description

Technical Report
Abstract: Little Lake Harbor, Michigan, was constructed as a harbor of refuge for small craft on the south shore of Lake Superior. However, the existing breakwaters have not prevented a heavy influx of sediment into the entrance channel and shoaling makes navigation difficult and dangerous. A 1:75-scale (undistorted) hydraulic model, reproducing the harbor, breakwater structures, entrance channel, adjacent shoreline for 1 mile both east and west of the harbor, and underwater contours out to -30 ft, was used to investigate various plans proposed to minimize or eliminate channel shoaling. The model simulated wind waves, wave-generated currents, seiche action, seiche-generated currents, and the movement of sediment (by the use of a crushed coal tracer). From an analysis of prototype data on water-level variation in Lake Superior and the harbor basin, it was determined that seiche oscillations with periods near the inlet-bay Helmholtz period occurred frequently. These oscillations could generate velocities as high as 4.5 fps in the entrance channel, with a median velocity of 0.62 fps. Proposed measures to reduce channel shoaling were primarily concerned with the addition of a new structure on the east side of the entrance channel and/or extensions of the existing breakwaters. From model test results it was concluded that: (A.) When a plan extended through the existing intermediate bar which bypasses the harbor entrance, sediment tracer shoaled in the entrance channel. (B.) If either the west or east breakwater protruded farther lakeward than the other, heavy channel shoaling usually occurred. (C.) Of the plans tested, the Plan 8B configuration (a 570-ft-long, detached, dogleg breakwater on the east side of the existing entrance) was the optimum in minimizing sediment tracer movement into the channel from both the east and west at the least cost. (D.) The gap between the new breakwater structure of Plan 8B and the shore would close due to sediment movement from the east, and the addition of a caisson to the lakeward terminus did not adversely impact shoaling patterns. (E.) Plan 8B provided a straight-in approach for boat traffic into the protected area between the breakwaters. (F.) Plan 8B reduced seiche oscillations in the harbor and velocities in the entrance (for the 0.5-hr period, 0.6 ft-seiche) when compared with the base conditions. NOTE: This file is large. Allow your browser several minutes to download the file.

Keywords

Harbors, Harbor structures, Hydraulic structures, Hydraulic models, Little Lake Harbor, Michigan, Shoaling, Sediment transport, Sedimentation, Deposition, Design, Shore protection

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