BEACH NOURISHMENT AND ITS INFLUENCE ON NEAR-SHORE GRAVEL COMPOSITION ALONG SOUTHERN LAKE MICHIGAN

Coastal structures built over the last 150 years near Michigan City, Indiana, altered longshore current pattern and caused sand accretion on their up-curent side and severe beach erosion on the down-current side. Since the early 1970's more than 1 million tons of sediment has been nourished at Mt. Baldy beach to halt beach erosion.

This study examines the role of beach nourishment in changing gravel composition in near-shore environments along southern Lake Michigan. A sieve fraction of sediment coarser than 8 mm was analyzed for texture and composition from seven sites; an ancient beach, an almost completely eroded nourishment bench, three sites of modern upper-shore environments down-current from the nourished beach, and two sites up-current from the nourished beach.

In the ancient beach the most abundant (87 %) are rounded clastic pebbles with a disc shape. Chert (8%) is more abundant than carbonates (5%) while crystalline pebbles are very rare (<1%). This composition suggests the Michigan Lobe source of sediment. The nourished sediment contains carbonate (38%), clastic (28%), and high amounts of crystalline (20%) and chert (13%) pebbles. Most of the pebbles have angular edges and spherical shape. An almost identical pebble composition is found in the modern upper-shore environment next to the nourished beach. The sample from the modern upper-shore, 7 km down-current from the nourishment, had almost equal amounts of clastic (28%), carbonate (26%), chert (23%), and crystalline (23%) pebbles. The upper-shore sample furthest (10 km) from the nourishment contained clastic (52%), chert (19%), crystalline (17%), and carbonate (13%) pebbles. Mudstones and sandstones in this sample have round edges and a disc shape. The increase in the relative abundance of chert and decrease in carbonate pebbles down-current from the nourishment is attributed to faster weathering of carbonates which are also the main hosts of chert. The two modern upper-shore samples 5 km up-current from the nourished beach contain carbonate (46%), crystalline (26%), clastic (25%), and chert (3%) pebbles, suggesting the influx of sediment from the Saginaw Lobe via St. Joseph River.

Samples from other segments of the modern shoreline, ancient beach ridges, and tributaries that drain into southern Lake Michigan will provide more insight to this research.