16 



Salient formation. Generally, a salient is the preferred response for a 

 detached breakwater system because longshore transport can continue to move 

 through the project area to downdrift beaches. Salient formation also allows 

 the creation of a low wave energy environment for recreational swimming 

 shoreward of the structure. Salients are likely to predominate if the 

 breakwaters are sufficiently far from shore, short with respect to incident 

 wave length, and/or relatively transmissible (EM 1110-2-1617). Wave action 

 and longshore currents tend to keep the shoreline from connecting to the 

 structure. Pope and Dean (1986) distinguish between well-developed salients, 

 which are characterized by a balanced sediment budget and stable shoreline, 

 and subdued salients, which are less sinuous and uniform through time, and 

 may experience periods of increased loss or gain of sediment. 



Tombolo formation. If a breakwater is located close to shore, long with 

 respect to the incident wavelength, and/or sufficiently impermeable to incident 

 waves (low wave transmission), sand will likely accumulate in the structure's 

 lee, forming a tombolo. Although some longshore transport can occur 

 offshore of the breakwater, a tombolo-detached breakwater system can 

 function similar to a T-groin by blocking transport of material shoreward of 

 the structure and promoting offshore sediment losses via rip currents through 

 the gaps. This interruption of the littoral system may starve downdrift 

 beaches of their sediment supply, causing erosion. If wave energy in the lee 

 of the structure is variable, periodic tombolos may occur (Pope and Dean 

 1986). During high wave energy, tombolos may be severed from the 

 structure, resulting in salients. During low wave energy, sediment again 

 accretes and a tombolo returns. The effect of periodic tombolos is the 

 temporary storage and release of sediment to the downdrift region. If the 

 longshore transport regime in the project area is variable in direction or if 

 adjacent shoreline erosion is not a concern, tombolo formation may be 

 appropriate. Tombolos have the advantages of providing a wide recreational 

 area and facilitated maintenance and monitoring of the structure, although they 

 also allow for public access out to the structure which may be undesirable and 

 potentially dangerous. 



Artificial headlands. In contrast to detached breakwaters, where tombolo 

 formation is often discouraged, an artificial headland system is designed 

 specifically to form a tombolo. Artificial headland design seeks to emulate 

 natural headlands by creating stable beaches landward of the gaps between 

 structures. Also termed log-spiral, crenulate-shaped, or pocket beaches, most 

 headland beaches assume a shape related to the predominant wave approach 

 with a curved section of logarithmic spiral form (Chew, Wong, and Chin 

 1974; Silvester, Tsuchiya, and Shibano 1980). Shoreline configurations 

 associated with headland breakwaters are discussed in Silvester (1976) and 

 Silvester and Hsu (1993). Figure 12 shows the headland breakwater and 

 beach fill system at Maumee Bay State Park, Oregon, Ohio, designed by the 

 USAED, Buffalo (Bender 1992). 



Wetland stabilization and creation. Breakwaters can be used as retention 

 or protective structures when restoring, enhancing, or creating wetland areas. 



Chapter 2 Functional Design Guidance 



