The Effect of Structures and Lake Level on Bluff and Shore Erosion in Berrien County, Michigan, 1970-1974

摘要

Rates of bluff recession and shoreline change along five 1.6-kilometer beaches located within Berrien County, Michigan, between 1970 and 1974 were measured by use of aerial photos. Annual measurements were made at 30.5-meter intervals, except for two adjacent reaches where biannual measurements were made. The overall average rate of recession for the five reaches was 3.8 meters per year. Average recession rate varied from 2.4 meters per year for a reach with low foredunes to 4.5 meters per year along a reach with a high sandy bluff. The greatest amount of recession resulted from a significant storm occurring 16 to 18 March 1973. Simple regression analysis of the data from both lake level and storm parameters identified storms as the primary cause of recession. However, the data set was too small and at such a unique point in the long-term lake level cycle (the crest of a rising peak) to quantify the effect of lake level. The effect of a 579-meter-long seawall constructed during the study is discussed; the volume of material eroded downdrift of the wall nearly equaled the amount of material removed from the sediment supply by the seawall. The procedures used in analyzing the air photos and their accuracy are described in an Appendix. Guidance is also given for determining the number of measurement points needed per distance along the shore depending on the desired accuracy of the bluff recession rates. (Author)