Based on stability tests carried out in a hydraulic model, Kamel 

 (1967) developed suggested breakwater sections for Hilo, Hawaii (Figs, 

 56 and 57). Allowable overtopping heights are given in Table 5. 



Table 5. Allowable overtopping heights 

 (after Kamel, 1967) . 





Allowable height 





of overtopping (m) 



Slope of 



9. 1 -metric ton 



18.2-metric ton 



harborside of barrier 



Armor stones 



Armor stones 



1:2 



0.3 



1.0 



1:2.5 



0.8 



1.2 



1:3 



1.3 



1.5 



1:3.5 



1.6 





1:4 



2.0 





1:4.5 



2.3 





1:5 



2.4 





1:6 



2.7 





1:7 



2.9 





Iwasaki and Horikawa (1960) show typical cross sections of seawalls 

 at locations on the northeast coast of Honshu (see Fig. 58). In some 

 instances, such as in fishing ports or harbor areas, it is undesirable 

 to have high seawalls directly on the waterfront. The seawall at Yamada 

 (Fig. 59) is in two stages. A low seawall along the waterfront allows 

 access to the water; a higher seawall, set back from the shoreline, 

 protects the town from higher waves . Figures 58 and 59 show that the 

 seaward toe of a wall is protected by rubble to deter scouring. Also, 

 the area behind the top of the shoreline wall, such as at Yamada, is 

 paved to prevent saturation and erosion of the backfill material. 



The protection provided by a breakwater depends on its location and 

 the width of the navigation channel through the breakwater. Iwasaki, 

 Miura, and Terada (1961) ran model tests for breakwaters in Kesennuma 

 Bay. They discovered that a breakwater at the mouth of the bay would 

 substantially reduce wave heights in the bay for all wave periods tested. 

 As expected, the greatest reduction in wave height occurred when the area 

 of the breakwater opening was the least. When the ratio of the breakwater 

 opening area to the cross -sectional area of the bay was equal to about 

 0.1, the wave height was reduced to about 0.25 times the height which 

 would occur without the breakwater. Surprisingly, when the breakwater 

 was moved to the mouth of Kesennuma harbor in the model, at the inner 

 end of the bay, the breakwater had almost no effect in reducing wave 

 heights. The location of the breakwater would be expected to affect the 

 resonant periods of the bay and the harbor. Therefore, care should be 

 exercised in placing a breakwater in any bay or harbor. 



162 



