REFLECTION AND TRANSMISSION CHARACTERISTICS 

 OF POROUS RUBBLE-MOUND BREAKWATERS 



by 

 Ole Seahev Madsen and Stanley M. White 



I . INTRODUCTION 



Porous structures consisting of quarry stones of various sizes 

 often offer an excellent solution to the problem of protecting a harbor 

 against the action of incident waves. When used for this purpose it is 

 important that the coastal engineer is able to assess the effectiveness 

 of a given breakwater design by predicting the amount of wave energy 

 that will transmit through the breakwater. 



Both the transmission and the reflection characteristics of a 

 porous structure are important. Thus, the severity of the wave motion 

 resulting from the partially standing wave system on the seaward side 

 of a breakwater will determine the accessibility of the harbor during 

 storm conditions. This wave motion outside the harbor will determine 

 the sediment transport patterns near the structure, and will also affect 

 the wave motion within the breakwater enclosure by governing the wave 

 motion at the harbor entrance. Therefore, the ability to predict the 

 reflection and transmission characteristics of a porous structure is 

 of utmost importance to an overall sound engineering design. 



The interaction of incident waves with a porous sturcture is a 

 rather complex problem, one which probably will defy an accurate 

 analytical solution for the foreseeable future. With incident waves of 

 various frequencies and the possible occurrence of wave breaking on 

 the seaward slope of the structure, the problem is amenable to an 

 analytical solution only by the adoption of a set of simplifying 

 assumptions. With the addition of the energy dissipation associated 

 with frictional effects on the seaward slope as well as with the flow 

 within the porous structure, it appears the only possible solution is 

 to perform scale-model tests. 



Performing a scale-model test for the interaction of waves with a 

 porous structure is, however, not a simple matter, and it presents a 

 separate set of problems even when tests are limited to normally inci- 

 dent v^7aves . It has recently been possible to perform tests for inci- 

 dent waves composed of several frequencies by using programmable wave 

 generators. However, some difficulties are associated with this type 

 of testing procedure. Thus, it is possible to perform tests corre- 

 sponding to a given incident wave spectrum only by trial and error. 

 Even limiting testing conditions to periodic incident waves leaves some 



