SAND-BYPASS AND SHORE EROSION, BRIDGMAN, MICHIGAN

C.N. Johnson, L.W. Hiipakka

Abstract


The objectives of this paper are two-fold: a. Demonstrate by means of a well-documented full-scale case history in the Great Lakes that beach nourishment can mitigate shore damage due to a littoral barrier; b. Detail a methodology for analysis of the effectiveness of mitigation measures which lead to conclusions on the needed frequency of nearshore soundings as a tool for monitoring. In 1970 a temporary harbor was installed by private interests along the southeastern Lake Michigan shoreline near Bridgman, Michigan, (Figure 1). The harbor was necessary to protect floating plant involved in building a large privately-owned electric generating facility. The harbor was constructed of steel sheet pile and extended lakeward about 125 meters from the water's-edge to a depth about 3 meters below low water datum (LWD)2. Net littoral transport is about 75,000 cubic meters per year southward. The updrift and downdrift lakebed and bluff materials consist of sand. To ensure that this littoral barrier would not have an adverse effect on adjacent properties it was necessary for the Corps of Engineers to impose stringent requirements for issuance of a Federal permit for the installation. Bypass of at least 75,000 cubic meters per year of sand was required to mitigate potential erosion of downdrift beaches due to interruption of littoral transport. The permit also required intensive monitoring of the shoreline to ensure that any adverse effects of the harbor would be promptly detected and remedied. The monitoring consisted of: a. Monthly 1:3000 scale aerial photographs of the shoreline sixteen kilometers north (updrift) to sixteen kilometers south (downdrift) ro\-fF tthheo htla'i rVtbt-lof\rv>. b. Monumenting and thrice-yearly measurement of backshore-nearshore profiles spaced 150 meters apart, for a distance of 2450 meters north and 2450 meters south of the harbor, extending about 900 meters lakeward to about -8 meters LWD (Figure 2). The nearshore soundings were made by acoustic sounder every 30 meters along each profile. The survey boat was located along the profile by triangulation. Backshore profiles were provided by photogrammetric mapping.

Keywords


shore erosion; sand bypass system; Bridgman, MI

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