4 Structure Design 



The objective of this study is to examine various approaches (including 

 strucures) for extending the beach-renourishment cycle for the Monmouth Beach 

 hot spot to the same 6-year cycle as the adjacent beaches. Of the alternatives 

 exained that accomplished this objective. Alternative 3 involved structural 

 additions and modifications. This chapter reviews the existing structures along 

 the Monmouth Beach shore and describes the armor-unit design and structure 

 cross section as specified in Alternative 3, a 100-ft seaward extension at 

 Groin 44. 



Existing Structures 



Groin performance (impoundment and bypassing) is primarily a function of 

 the ratio of net and gross longshore sediment transport rates, groin length 

 (parameterized by the ratio of the depth at the toe of the structure and a 

 representative wave height), and structure pemieability and crest elevation 

 (Kraus, Hanson, and Blomgren 1994). Length controls bypassing; permeability 

 controls through-passing: and crest elevation controls overtopping of sediment. 

 Structure condition also enters consideration of groin performance because if 

 stones break, deteriorate, or shift, the groin length, permeability, and crest 

 elevation may change and thereby reduce groin effectiveness. Because of the 

 many variables involved in groin functional design, inspection of the perfor- 

 mance and structural integrity of groins at or near a project site can provide 

 reliable information for achieving a successful design. 



Groin 44 appears to be impounding sand updrift (to the south) as intended 

 (Figures 34 and 35). Groin 46 also impounded sand until the fall of 1997 when it 

 was notched (a 100-ft-long (at mlw line), 7-ft-deep section of the central portion 

 of the groin was removed) to increase its permeability. Since then, no sand has 

 been impounded. Little influence on shoreline position by the notching is 

 evident at the remaining groins (e.g.. Groin 45, Figure 36). 



The relation between structure condition (stone breakage, deterioration, or 

 shifting) and structure performance at Monmouth Beach is not known. Though 

 Groin 44 is impounding sand, its "fair" condition gives no indication of the 

 existence of any broken or deteriorated stones. Groin 46 was performing as 



38 Chapter 4 Structure Design 



