30 



long breakwater will cause a greater reduction of longshore current in the 

 project area than a short breakwater. 



If the breakwater's crest elevation is sufficiently low and overtopping 

 occurs, water level behind the breakwater is increased and flow occurs around 

 the structure. In a multiple segment system, this results in a net seaward flow 

 through the gaps, which can cause offshore sediment losses, structural scour, 

 and create a hazard to swimmers. The magnitude of return currents through 

 the gaps can be reduced by increasing crest elevation, gap width, and/or 

 structure permeability. Seelig and Walton (1980) present a method for 

 estimating flow rate through the gaps of offshore segmented breakwaters 

 caused by wave overtopping. The effects of wave height and period, 

 breakwater freeboard, breakwater length and spacing, distance offshore, water 

 depth, and shore attachment are considered relative to flow rate through the 

 gaps. Seelig and Walton (1980) recommend that the gap velocity should not 

 exceed 0.5 ft/sec (0.15 m/sec) for extreme design conditions. Velocities 

 greater than this could cause significant offshore losses of sediment and scour 

 around the structure's foundation. 



Effects of breakwater on longshore transport 



The longshore transport rate Q is the rate at which littoral material moves 

 alongshore in the surf zone from currents produced by breaking waves. 

 Detached breakwaters can significantly reduce longshore transport through a 

 project area. Reduction of wave heights and wave diffraction around the 

 breakwater's ends primarily determines the reduction in transport capacity. If 

 a salient forms, longshore transport can continue to move through the project 

 area; however, a tombolo can act as a total barrier of longshore transport 

 causing a sediment deficiency at downdrift beaches. Some longshore transport 

 may be redirected seaward of the breakwater, but may also result in an 

 offshore loss of material. Structure length, distance offshore, crest elevation, 

 and gap width may be modified to vary the resulting transport rate during 

 design of a breakwater system. Once constructed, modifications to the 

 transport rate are more difficult; however, reduction of crest elevation or 

 increasing permeability can be undertaken to allow more wave energy to 

 penetrate the structure. This was conducted at the Redington Shores, Florida, 

 detached breakwater project where tombolo formation and subsequent 

 blocking of longshore transport occurred (Chu and Martin 1992). 



The effects of a breakwater on the shoreline depend on both net and gross 

 transport rates. Shoreline response both at the structure and on adjacent 

 shorelines can occur rapidly if transport rates are large, or can take several 

 years for low transport rates. If net transport in a project area is nearly zero, 

 but gross transport is not zero, the breakwater's major effects will be limited 

 to the general vicinity of the structure; however, some effects of the structure 

 can be experienced on updrift and downdrift beaches over time. 



Chapter 2 Functional Design Guidance 



