tombolo formation. Wave transmission is discussed in more detail in 

 Chapter 4, Structural Design Guidance. 



Wavelength. Generally, the amount of wave energy diffracted into a 

 structure's lee increases with increasing wavelength. Assuming 

 monochromatic waves and a flat bottom, wave length will not change the 

 pattern made by the wave crests, but will affect the wave height at each 

 location. An analysis using the diffraction diagrams provided in the Shore 

 Protection Manual (1984) can simplistically compute the amount of energy 

 that reaches the lee of the breakwater. An example problem using the 

 diffraction analysis is presented in Dally and Pope (1986). 



Wave angle. Equilibrium beach planform and degree of salient 

 development can be significantly affected by incident wave angle relative to 

 both the shoreline and structure. Design must not only consider predominant 

 wave direction, but also the average annual wave angle distribution. Salients 

 and tombolos tend to align with the predominant wave direction. Generally, 

 the feature's apex is near the center of the breakwater and is filled more on 

 the updrift than the downdrift side. If predominant waves are extremely 

 oblique to the shoreline, the beach planform and feature's apex can be shifted 

 downdrift and can change with seasonal variations in wave direction. Oblique 

 waves can also drive a regional longshore current, which may dominate local 

 effects of the breakwater and limit salient development. Increasing the 

 structure's length can subdue the effect of the oblique waves. 



Wave conditions seaward of breakwater. Waves reflected from the 

 seaward side of the structure can sometimes interact with incident waves and 

 cause a partial standing wave pattern seaward of the breakwater (EM 1 1 10-2- 

 1617). This increased wave action can cause scour on the seaward side of the 

 structure, potentially creating foundation problems. A structure's reflectivity 

 is largely determined by crest elevation, permeability, and type of construction 

 material. Rubble-mound structures are the least reflective detached 

 breakwater construction type. 



Effects of breakwater on nearshore currents 



Construction of a breakwater system can affect nearshore currents in two 

 ways: reduction of longshore current in the vicinity of the structure, and 

 creation of a net seaward flow of water through gaps in a segmented system 

 (EM 1110-2-1617). On an open-coast beach, a longshore current is generated 

 by waves approaching the shoreline at an angle. The placement of a structure 

 introduces an interruption to this natural system. The longshore current will 

 generally respond by slowing or stopping when it moves into the project area, 

 thus reducing the current's sediment carrying capacity and depositing sand in 

 the structure's lee. The structure's length and distance from shore are two 

 design parameters that must be considered when evaluating the breakwater's 

 effect on longshore currents and sediment transport. For example, a relatively 



Chapter 2 Functional Design Guidance 



29 



