Figure 3b. Rounding of the corners will practically eliminate t secondary 

 capillary wave system. This is demonstrated in Run 12 (Figure 3h) wherein 

 diffraotion occurs around the leeward end of a vertical-walled ^..age. 



Wave Reflection 



Wave motion in harbor basins often is increased due to waves fceing reflected 

 from vertical or nearly vertical walls. In small-scr.le models involving wave 

 action the side walls and wave generator of the mode 1 3 or tanks may result in 

 reflections which can produce erroneous or misleading results. Total reflection 

 will occur at smooth vertical, rigid,, impermeable barriers, while for almost 

 total dissipation a flat sloped permeable wall is necessary. There is a transi- 

 tion region between total reflection and total dissipation. Besides the slope 

 of the wall and the characteristics of its construction (rigidity, permeability, 

 roughness of the surface etc.) the following factors effeot the dissipation of 

 wave energy: (i) the water depth, (ii) the wave length, (iii) the wave height, 

 (iv) the angle of wave approaoh. 



Runs 11 and 12 (Fig. 3b) were made to demonstrate the reflection of gravity 

 water waves at a rigid, vertioal -walled barrier. In Run 11 a barrier was 

 introduce at 45° to the direction of wave approaoh, while in Run 12 a quarter 

 of a oylinder was used with the tip of the wedge faoing the approaohing waves 

 and the sides of the wedge inclined by 45° to this direotion. The depth of 

 the water was uniform for both of the runs. Prom the few experiments performed 

 it appears that (i) the angle of incidence was equal to the angle of refleotionj 

 (ii) the wave lengths and velocities remain -unchanged - only the direction of 

 travel being ohanged; (iii) in case of a smooth, rigid impermeable vertioal 

 wall, the individual wave heights were not greatly affeoted by the reflection. 



Conclusions (i) and (ii) are verified by the fact that the incident waves and 

 reflected waves formed almost perfect squares when the angle of incidence was 

 45°, as can be readily seen in the photographs of both Run 11 and Run 12 (Figure 

 3b). The third conclusion came from the fact that the intensity of white crest- 

 lines of reflected waves close to the obstaole was almost the same as the 

 intensity of the inoident waves (see Run ll). 



Runs 15 to 25 (Figures 3f and 3g) also show reflection patterns, where the 

 pattern of reflected waves appear to be circular. Total reflection may result 

 in doubled wave heights due to the superposition of incident and reflected 

 waves. This is demonstrated in Run 11 (Figure 3b) by the high intensity of the 

 intersections of inoident and reflected wave trains. An engineer, designing 

 a harbor should take this fact under consideration. Breakwaters to be built 

 close to navigation channels, should be constructed so as to absorb all or most 

 of the wave energy without considerable reflection. There are many harbors where 

 this faot was not taken into considerations the result being that the entranoeg 

 to the harbors are dangerous to navigation, even for incident waves of medium 

 height* 



Islands 



When a train of waves is interrupted by an island, there is generally a zone of 

 "wave shadow" in the lee of the island. Since the regular pattern of a long 

 swell is disturbed even at great distanoes beyond islands, the early navigators 

 were able to use this phenomenon as a guide to new islands. The factors which 



