elevations. All the foregoing reference contours fall on the beach face; 

 i.e., 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 ( I G L D ) 

 176.92 



75.80 



^^^-xTr^ 



L_i 



J L 



J I I \ \ I 



90 80 70 60 50 40 30 20 10 -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 



