One method of estimating the proportion of shore-eroded material that will 

 be lost is to use the textural characteristics of the active beach as a guide. 

 If the newly eroded deposits have a size distribution identical to that of the 

 sediment in the active zone prior to the water level change, then only insig- 

 nificant amounts will be lost through selective transport processes and 

 R. ~ 1. If a part of the eroded material is finer than the overlying native 

 beach it may be carried far offshore in suspension where it does not contrib- 

 ute to the building of a new profile. In which case R^ > 1 and additional 

 shore erosion must compensate for the loss. Thus, the situation is similar to 

 the problem of calculating the overfill ratio for a beach nourishment pro- 

 ject. Hobson (1977) explains how to compute R^ to evaluate the suitability 

 of borrow material. The same procedures apply here except the "borrow mate- 

 rial" characteristics must be based on a composite sample of the eroding 

 section of the shore, i.e., the upper beach in the case of increases in lake 

 level since it is supplying sediment to the lower part of the adjusting pro- 

 file. If the water level declines the lower part of the responding profile is 

 eroded to supply material to prograde the upper profile. In this case the 

 "native material" characteristics must be based on a composite sample of the 

 lower profile (i.e., the zone of offshore erosion). In either case the native 

 material characteristics must be based on a composite sample of the entire 

 responding profile from the limit of wave uprush to the point of profile 

 closure. 



If the engineer concludes, without specific textural data, that all of the 



shore-eroded material will remain in the zone of profile adjustment, then 



R. = 1. If the engineer estimates by other methods that only P percent of 



the eroded sediment will remain in the active zone then R^ = 100/P. 



Equation (1) with R. = 1 was applied to sea level rise on Florida beaches 

 by Bruun (1962), and in this context is often referred to now as Bruun's rule. 

 It is not so much a rule in any formed sense, as it is a statement of a fairly 

 simple concept based on assumptions which had been used by many early coastal 

 geomorphologists. However, explicit applications of the concept prior to 

 Bruun (1962) are unknown. Although references to the concept are frequent, it 

 is still rarely used for predictive purposes. 



2. Difficulties in Applying the Sediment Balance Approach . 



Given the long-term effect of rising sea levels throughout most of the 

 Northern Hemisphere, it may be wondered why the sediment balance approach 

 (Bruun's rule) has not been more widely applied. The following difficulties 

 have been encountered with this approach: 



(a) Skepticism as to the adequacy of an equilibrium model for 

 explaining short-term dynamic changes; 



(b) difficulty in determining R^ or the pereentage of sediment 

 lost from the active zone; 



(c) problems of establishing a realistic closure depth below 

 which water level changes have no effect on profile stability; 



26 



