Table 2 



"Exposure Ratios" for Various Prototype Multiple Breakwater 



Projects 1 (Modified from EM 1110-2-1617) 



Project 



Exposure Ratio 



Shoreline Response 



Winthrop Beach, MA 



0.25 



Permanent tombolos (low 

 tide); well-developed salients 

 (high tide) 



Lakeview Park, Lorain, OH 



0.36 



Subdued salients 



Castlewood Park, Colonial Beach, VA 



0.31 to 0.38 



Permanent tombolos 



Central Beach, Colonial Beach, VA 



0.39 to 0.45 



Periodic tombolos 



East Harbor, State Park, OH 



0.56 



Limited 



Presque Isle, Erie, PA 

 (experimental prototype) 

 (hydraulic model) 



0.56 to 0.66 

 0.60 



Permanent tombolos 



The "exposure ratio" is defined as the ratio of gap width to the sum of the breakwater 

 length and gap width. It is the fraction of shoreline directly exposed to waves and is equal 

 to the fraction of incident wave energy reaching the shoreline through the gaps. A 

 "sheltering ratio" that is the fraction of incident wave energy intercepted by the 

 breakwaters and kept from the shoreline can also be defined. It is equal to 1 minus the 

 "exposure ratio." 



breakwater parallel to incoming wave crests will protect a greater length of 

 shoreline and reduce toe scour at the breakwater ends. 



Location with respect to breaker zone. If the breakwater is placed 

 substantially landward of the breaker zone, tombolo development may occur. 

 However, a significant amount of longshore transport may continue to pass 

 seaward of the breakwater, thus alleviating the effects of a tombolo on 

 downdrift shorelines. A disadvantage of a breakwater within the breaker zone 

 may be substantial scour at the structure's toe. Generally, detached 

 breakwaters designed for shore protection along an open coast are placed in a 

 range of water depths between 1 and 8 m (Dally and Pope 1986). 



Structural mitigation methods for impacts on adjacent shorelines. End 



effects from a breakwater project can be reduced by creating a gradual transi- 

 tion or interface between the protected shoreline and adjacent shorelines 

 (Hardaway, Gunn, and Reynolds 1993). Hardaway, Gunn, and Reynolds 

 (1993) document various methods for structurally transitioning the ends of 

 breakwater systems in the Chesapeake Bay. Structural methods used at the 12 

 sites investigated include shorter and lower breakwaters, hooked or inclined 

 groins, small T-head groins, and spur-breakwaters. Based on project experi- 

 ence in the Chesapeake Bay, Hardaway, Gunn, and Reynolds (1993) recom- 

 mend hooked or skewed groins where adjacent effects are predicted to be min- 

 imal; T-head groins where the dominant direction of wave approach is shore- 

 normal; and short groins, spur-breakwaters and low breakwaters placed close 

 to shore when the dominant wave direction is oblique. The use and design of 



Chapter 2 Functional Design Guidance 



25 



