COMPARISON OF SEDIMENT BUDGETS OF BLUFF/BEACH/NEARSHORE ENVIRONMENTS NEAR TWO RIVERS, WISCONSIN, ON LAKE MICHIGAN, AND AT PAINESVILLE, OHIO, ON LAKE ERIE

The volume of sand and gravel derived from bluff and lakebed erosion was compared to the volume of sand and gravel stored in the beach and nearshore. At Two Rivers, the 30-foot-high bluff consists of 10 ft of Two Rivers diamicton (~31% >2 mm fraction) overlying 10 ft of interbedded beach gravel and lacustrine sand, silt, and clay (~20% >2 mm fraction) overlying 10 ft of Haven diamicton (~19% >2 mm fraction). At Painesville, the 55-foot-high bluff contains 8 ft of sand and fine gravel (~90% >2 mm fraction) overlying 47 ft of Ashtabula diamicton (~20% >2 mm fraction). High-resolution bathymetric and side scan sonar surveys indicate the nearshore consists of thin, patchy deposits of sand and gravel over cohesive clay at Two Rivers and cohesive clay and shale at Painesville.

To calculate the volume of sediment contributed by erosion of the bluff, beach, and nearshore, parallel retreat of the cross-shore profile was assumed. Sand and gravel derived from bluff recession (2.5 ft/yr) at Two Rivers is ~17.5 ft3/yr per linear foot of shoreline. At Painesville, where the bluff is higher and recession rates are higher (5.3 ft/yr), the volume is ~88 ft3/yr/ft. Coarse-grained sediment derived from nearshore erosion is about 4.6 ft3/yr/ft at Two Rivers and about 20 ft3/yr/ft at Painesville. Thus, the total volume of sand and gravel contributed at Two Rivers by erosion of the bluff and nearshore is about 22 ft3/yr/ft and about 108 ft3/yr/ft at Painesville.

Although sand thickness on the beach and in the nearshore is not well constrained, estimates for Two Rivers suggest the volume of sand and gravel stored in the beach and nearshore averages about 760 ft3/ft of shore at Two Rivers and is at most 965 ft3/ft of shore (probably much less) at Painesville. These volumes are 36 times and 9 times the volumes produced annually through erosion at Two Rivers and Painesville, respectively. Despite greater bluff height and higher erosion rates, the volume of sand and gravel stored in the beach and nearshore at Painesville is only ~200 ft3/ft more than at Two Rivers, suggesting that the Painesville system is very starved for sand. This type of analysis may aid in understanding the impacts of shore protection structures and other past and present modifications to the bluff/beach/nearshore system.