Run 7 (Figure 3b) was of wave refraction in a V-shaped ohannel with side-slopes 

 of Iil2. It can be seen clearly that the waves "wheel" around by approximately 

 90° and break on the beaohes as they enter the channel. The breaking waves are 

 indicated by relatively wide and strong white lines in the photography. Almost 

 all of the wave energy is destroyed in breaking waves, and so the wave height 

 is decreasing very rapidly as they advance along the channel, from this rel- 

 atively simple experiment it can be seen that it is advantageous to use a channel 

 with flat sloping sandy beaohes to connect a harbor or basin to a body of stormy 

 waters in order to prevent the waves from penetrating into the harbor* 



"Wave Diffraction 



TOaen a wave train is interrupted by a breakwater or similar barrier a sheltered 

 region is formed. The phenomenon by which water waves are propagated into this 

 sheltered area is called diffraction. A knowledge of the diffraction phenomenon 

 has important application in the design and location of breakwaters in connection 

 with harbor development. It also has a bearing on the distribution of wave 

 energy along beaches located in the lee of headlands and offshore islands. The 

 phenomenon is analogous to the diffraction of light, sound and electromagnetic 

 waves, and theories for breakwater diffraction have been adapted from those 

 theories. 



Basically there are two types of diffraction problems, oonneoted with breakwaters i 



(i) Diffraction around the end of a semi-infinite impermeable 

 barrier, 

 (ii) The passage of waves through a breakwater gap. 



No exact solution has been found for the case of a barrier of finite length, but 

 it is believed that satisfactory results can be obtained by using the solution 

 of semi-infinite barrier for each of the ends. It has been found that this 

 assumption is allowable only in the case where the length of the barrier is long 

 compared with the wave length (see also the section under "Islands"). 



The theory of water wave diffraction will not be presented here as this is 

 readily available in the literature. The purpose of this paper is to demon- 

 strate visually, by photographs and moving pictures, the wave-characteristics 

 when diffraction ocours at various types of obstacles. 



In Huns 8 and 9 (Figure 3b) a semi-infinite breakwater was introduced. Hun 8 

 demonstrates the diffraction pattern from deep into shallow water around a 

 breakwater tip. An abrupt change of depth was introduced along the geometrical 

 shadow of the breakwater and the depth of the water in the shadow was about 3/5 

 of that outside. It can be seen that the wave-crests in the lee of breakwater 

 are almost straight, up to a line drawn about 20° from the tip of the breakwater 

 to the geometrical shadow, from here on the wave orest assumes an almost 

 circular form, with the center at the tip of breakwater. 



Hun 9 (Figure 3b) demonstrates diffraction around a semi-infinite barrier, from 

 shallow-water into deep water. The depth of water in the shadow seotion behind 

 the breakwater is about 5/3 of that outside. As in the photographs, the height 

 of diffracted waves in deep water is very small as oompared with the incident 

 waves. This is indicated by the wide, low oontrast, crest-lines. The oiroular 



