LABORATORY STUDY ON MACRO - FEATURES OF WAVE 

 BREAKING OVER BARS AND ARTIFICIAL REEFS 



PART I: INTRODUCTION 



Background 



1. Wave breaking is the most dynamic phenomenon in the nearshore zone. 

 A wave approaching a beach enters shallow water, which causes it to become 

 steeper and shorter. The wave eventually becomes unstable, and the crest 

 either spills down or plunges over its front face, releasing its kinetic and 

 potential energy. The crest of a spilling breaker rolls down the front face 

 of the wave and creates intense turbulence at the surface. A plunging breaker 

 crashes into the water surface shoreward of it, displacing a large volume of 

 water that also plunges shoreward. The vortex of entrained air created by the 

 overturning crest penetrates below the water surface. Plunging and, to a 

 lesser extent, spilling waves create turbulence and currents that mobilize and 

 transport beach sediment. Longshore currents, driven by waves breaking at an 

 oblique angle to shore, transport sediment along the coast. Cross -shore cur- 

 rents generated by breaking waves move sediment across shore to form bars. 



The point of wave breaking and plunging varies according to the height and 

 length of the incident waves, as well as the beach slope and water level 

 (tidal stage). Thus, bars are constantly changing shape and location, moving 

 seaward in storms and shoreward under calmer conditions. Coastal structures 

 such as breakwaters and jetties are constantly subjected to breaking wave 

 forces. Forces exerted by breaking waves can crack concrete dolosse and 

 displace armor units weighing several tons. 



2. Placement of clean dredged material offshore in linear berm config- 

 urations to both protect and nourish the beach has become an area of recent 

 interest (McLellan 1990) . Depending on depth and orientation, the presence of 

 offshore berms may alter the incident wave characteristics and influence the 

 breaking wave properties. Although offshore berms have desirable qualities in 

 concept, research in this field has been fairly limited. 



3 . Breaking waves also provide a recreational environment for surfers . 

 Recently, a surfing reef (shoal) was designed to be placed at Patagonia, Cali- 

 fornia, and the project is in the final stages (Pratte 1989). Walker (1974a) 



