•7 



have a volume of less than 100 m /m in these cases. Under these conditions, 

 the underlying glacial till is either only thinly covered (i.e., with beach and bar 

 thickness of less than 1 m) or entirely exposed. In other words, the tiU is 

 frequently exposed over the entire profile to conditions of active downcutting. 

 In these situations, it is not clear that the impoundment of sand in an updrift 

 fdlet beach, and the deprivation of this sand from the downdrift beaches and 

 lake bed wiU have any measurable impact on the rate of lake bed downcutting 

 and the associated rate of shoreline recession. This hypothesis was 

 successfully applied in the Port Burwell (north central shore of Lake Erie) 

 litigation case where the Government of Canada successfully defended against 

 a $30-miUion claim which held that the harbor structures at Port Burwell had 

 caused accelerated recession for 40 km of downdrift cohesive shore (see 

 Philpott (1986)). 



The opposite extreme consists of a situation where the glacial tiU under- 

 neath the sand cover is rarely, if ever, exposed in the natural condition (prior 

 to the construction of harbor jetties). This situation has been documented for 

 the niinois shoreline north of Chicago by Shabica and Pransclike (1994). In 

 this case, the interception and impoundment of alongshore sediment by large 

 shore-perpendicular structures has resulted in a reduction of sand cover from 

 over 500 m^/m to less than 200 rn^/rn in places. In this case, the reduced sand 

 cover resulting from the impoundment at the shore-perpendicular structures 

 results in accelerated shoreline recession along the downdrift shore. Beach 

 nourishment is required in these cases, not only to reinstate the historic 

 sediment supply rate, but also to replenish the sand cover to its historic level. 

 The latter requirement may be achieved through augmenting the sand cover 

 volume to its natural level (this may not be practical or realistic owing to the 

 large volumes required). Otherwise, the requirement may be relaxed if the 

 effectiveness of the protective characteristics of the overlying sand cover can 

 be augmented. The protectiveness of the sand cover could be improved 

 through the provision of sediment which is coarser than the natural or native 

 sediment Specific grain size requirements should be determined based on the 

 profUe shape, properties of the underlying till, wave exposure, and sediment 

 transport characteristics (both alongshore and crossshore). 



A special condition of cohesive shore which may be relatively common 

 relates to cases where the natural profile shape is convex instead of concave 

 (see Stewart and Pope (1993)). Gray and Wilkinson (1979) document the 

 existence of this type of cohesive shore at locations on the east shoreline of 

 Lake Michigan north of St. Joseph. This condition is a result of the presence 

 of a more erosion-resistant surface in the nearshore. The protected nearshore 

 shelf may consist of some form of bedrock or glacial till that is armored by a 

 boulder and cobble lag deposit. Shoreline (or bluff) recession on this type of 

 cohesive shore is particularly sensitive to changes in lake level. While 

 downdrift nourishment requirements for this type of cohesive shore may be 

 less in volume (i.e., less than what might be determined based on potential 

 transport rates), the timing and grain size characteristic requirements should be 

 careftilly considered. 



Chapter 6 Beach Nourishment Design Guidelines 



91 



