Areal Variations in Beach Dimensions in the Apostle Islands of Lake Superior

Abstract

Areal Variations in Beach Dimensions in the Apostle Islands of Lake Superior Wayne N. Engstrom* On a global scale, Davies has shown that areal variations in coastal processes can result in variations in coastal morphology .1 This observation serves as the hypothesis for this study. Areal variations in both wave conditions and beach dimensions, specifically beach height and width, exist in the Apostle Islands of Lake Superior . Stepwise regression procedures are used to identify the relationships which exist between the beach dimensions and certain wave parameters. This paper provides explanations for those relationships , only some of which have been well documented in earlier studies. The present paper also differs from much of the previous work in that the 39 study beaches are located on 23 islands, whereas existing studies are largely concerned with variations in beach dimensions along a single stretch of coast. Study Area The Apostle Islands extend as a group northeastward from the Wisconsin mainland into Lake Superior ( Figure 1 ). A wide variety of wave conditions exists along the beaches of the islands, making the group suitable for this particular study. Most of the islands are composed of sandstone covered with varying thicknesses of glacial * Dr. Engstrom is Associate Professor of Geography at California State University, Fullerton 92634. A version of this paper was presented at the Thirtysixth Annual Meeting of the Association in San Diego in June 1973. I would like to thank John Fraser Hart for his useful comments on the manuscript and Susan Hannes and Debbie Elliott for their cartographic work. 1 J. L. Davies, Geographical Variation in Coastal Development (New York: Hafner Publishing Company, 1973). 33 34 ASSOCIATION OF PACIFIC COAST GEOGRAPHERS deposits and lacustrine clay. One exception is Long Island, the detached portion of a spit. Island beaches are of two basic types, twelve CANADA Jî PERIOR --------1-----------------90 ? 40 ' Gdevils Qn. twin J- ^YORK SAND 17^*1« ^RASPBERRY' /~) ^"fir ? "u AT 225 IS 19 BAYFIELD 5.9 23 i ronwood /manitou 0AK\ 39/§t(?> /basswoo OUTER GULL C^MICI- en C/2 O O I—( > O Z O Ti 1D > OI—I 1— H O W O O£>-ti K m 53 C/3 Source: Calculated by author. * Significant at the 0.05 level. ** The phrase "most important work performing for the indicated period. wave" refers to the specific wave which has the largest wave-work value YEARBOOK · VOLUME 39 · 197745 conducted later by King show that swash bar height increases as wave height increases.15 Bascom has shown that in nature the height of berms on sand beaches is a positive function of the height of the refracted wave which molds the beach.16 Davies notes that the height to which a beach is built depends entirely on the height of the waves which build it.17 Work by Tanner and Stapor has shown that this relationship also applies to beach ridges.18 Beach height appears to increase as wave energy increases because the more vigorous swash generated by such waves is able to travel farther inland, reaching higher and normally unsaturated portions of the beach profile where deposition occurs. Such deposition may take place even during stormy episodes of wave attack despite an accompanying reduction in beach width because of erosion on the lower part of the beach profile.19 Even swash traveling only to the berm crest is capable of promoting the upward growth of the beach through deposition because the crest is seldom saturated and percolation losses are correspondingly great.20 Swash washing over the berm crest cannot return lakeward as erosive backwash and the resulting washover deposits can increase beach elevations as Psuty has reported.21 Mean grain size constitutes one of the remaining independent variables appearing in the final beach height regression equations (Table 1 ). The partial correlation coefficient between mean grain size in phi units and beach height is negative in all instances. This shows that higher beaches are linked with coarser foreshore sediment and lower beaches with finer foreshore sediment. Assuming 15King, p. 321. 16Willard Bascom, Waves and Beaches (Garden City, New York: Doubleday and Company, 1964), p. 199. 17Davies, p. 119. 18Tanner and Stapor, pp. 398-399. 19See, for example, Williams, p. 29...