-2 



nourishment volumes from dredging increased to 74,000 m /year, with an 

 additional 14,000 m^/year of coarse sand trucked from inland borrow sites, for 

 a total of 88,000 m^/year. From 1991 to 1995, combined dredged and trucked 

 volumes average only 41,000 m /year, a substantial reduction from the annual 

 volume delivered to the feeder beach between 1976 and 1991. 



Discussion of Cohesive Shores 



Sandy shores are generally distinguished by an inexhaustible local supply of 

 beach sediment. In contrast, a shore is defined as cohesive when a cohesive 

 sediment substratum (such as glacial till, glaciolacustrine deposits, soft rock or 

 other consolidated deposits) occupies the dominant role in the change of the 

 shoreline shape (i.e., through erosion). In other words, underneath any cohe- 

 sionless deposit (i.e., sand and gravel) there is an erodible surface which plays 

 the most important role in determining how these shorelines erode, and ulti- 

 mately, how they evolve. A cohesive shore erodes and recedes because of the 

 permanent removal and loss of the cohesive sediment (both from the bluff and 

 the lake bed). The sand cover may come and go (depending on the season, 

 water level, and storm activity), but the erosion of the cohesive layer is irre- 

 versible. The characteristics of cohesive shores are discussed in more detail in 

 Parson, Morang, and Nairn (1996). 



The critical point to understand is that shoreline recession, and the associ- 

 ated problems of undermining of shore-based structures, could not continue 

 without the ongoing downcutting of the nearshore lake bed. The long-term 

 average rate at which the bluff or shoreline recedes on a cohesive shore must 

 be governed by the rate at which the nearshore profile is eroded or downcut. 



Where there are downdrift erosion problems related to the interception of 

 sand at an updrift barrier on a cohesive shore, downdrift mitigation efforts 

 such as beach nourishment must be carefully assessed, since the sand can act 

 as either protective cover or as an abrasive agent (contributing to erosion) 

 depending on the quantity and type of sediment. 



Chapter 2 Background 



