. Changes in rates of shore retreat : Lake Michigan, 1967-76. Coast changes; Beach erosion. elevations. All the foregoing reference contours fall on the beach face; , intersect the profile between the berm and the first longshore trough (Fig. 10). 184 1831- 182 181 3. 180 I 179 o ^ 178 LlJ 177 176 175 174 Reference Elevations (IG L D ) ^^^-xTr^ L_i J L J I I \ \ I 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 Distance from Base (m) Figure 10. A fairly typical inner profile. Note the various reference ele- vations (International Great Lakes Datum - IGLD) at which contour migrat
. Changes in rates of shore retreat : Lake Michigan, 1967-76. Coast changes; Beach erosion. elevations. All the foregoing reference contours fall on the beach face; , intersect the profile between the berm and the first longshore trough (Fig. 10). 184 1831- 182 181 3. 180 I 179 o ^ 178 LlJ 177 176 175 174 Reference Elevations (IG L D ) ^^^-xTr^ L_i J L J I I \ \ I 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 Distance from Base (m) Figure 10. A fairly typical inner profile. Note the various reference ele- vations (International Great Lakes Datum - IGLD) at which contour migration was measured to determine rates of beach face recession plotted in Figure 9. 3., Encroachment. Encroachment refers to the loss in shore width due directly to sub- mergence. Given only the initial profile, the encroachment which would result from a subsidence of Az is exactly ,Az x cot °: ( where °: is the slope of the profile between initial and final mean water elevations). This simple calculation may be sufficient to indicate the extent of po- tential flooding problems along low-lying coasts. The same approach has also been used in the scientific literature to estimate long-term effects of sea level rise, but this is a severe oversimplification because en- croachment by the sea is only one aspect of shore retreat. Submergence will usually increase erosion rates causing extensive shore recession which contributes to further shore retreat. Between 1969 and 1975, a period of persistent submergence on Lake Michigan, the overall retreat of the shore exceeded the encroachment by a factor of 5 (the total 24. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hands, Edward B; Coastal Engineering Research Center (U. S. ). Fort Belvoir, Va. : U. S. Army, Corps of Engineers, Coastal Engineering Research Center ; S
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