Effects of breakwater on onshore-offshore transport 



Breakwater construction can reduce offshore transport by presenting a 

 physical barrier to offshore transport and by reducing wave heights and wave 

 steepness, which tends to promote onshore transport of material in the 

 breakwater's lee. However, for segmented systems, especially low-crested, 

 impermeable structures, a net seaward return flow of water can occur through 

 the gaps, promoting offshore loss of sediment. Reduction of seaward flow 

 through the gaps was discussed in the previous section. 



Influence of other coastal parameters 



Water levels. Water level variations influence the magnitude of wave 

 energy in the lee of the breakwater, which in turn influences shoreline 

 configuration and consequently must be considered in functional design. 

 Dally and Pope (1986) suggest that water level fluctuations of over 1.5 m will 

 tend to hinder permanent tombolo formation, especially if significant wave 

 overtopping of the structure occurs, and may prevent the salient from attaining 

 a smooth equilibrium shape. The Winthrop Beach, Massachusetts, project 

 experiences a relatively large tidal range (2.7 m) and has two distinct 

 planforms during high and low tide conditions (Figure 23). Projects 

 constructed on the Great Lakes or Chesapeake Bay will experience less 

 dramatic water level fluctuations; however, variations in water level may 

 cause significant seasonal or longer period changes in the equilibrium beach 

 planform. 



Sediment characteristics. Sediment particle size and distribution affect 

 longshore transport and profile shape, and therefore have some influence over 

 the resulting beach planform. Because a coarse-grained beach equilibrium 

 profile will be steeper, a structure should be placed in relatively deeper water 

 (Dally and Pope 1986). 



Data Requirements for Design 



Data requirements for both functional and structural design depend on the 

 methods and evaluation tools used in the specific project design. This section 

 discusses data requirements necessary for an understanding of site 

 characteristics and coastal processes relative to functional design of detached 

 breakwaters. 



Water levels 



Both the functional and structural design of detached breakwaters require 

 data on the range of water levels that can be expected to occur at a project 

 site. Prevailing water levels will determine where waves may affect the beach 



Chapter 2 Functional Design Guidance 



31 



