Comparative Effectiveness of Low Sea-walls - By the same method 

 used on page 7, we may determine the relative effectiveness of seawalls 

 of a height not capable of completely turning back the expected wave 

 attack. The primary source is the same paper by Morison(^9) dealing 

 with rectangular barriers, this time using his results for steep waves 

 over an horizontal bottom, (The range of wave action at a seawall's 

 probable location on a slope will resemble this model). The results for 

 the heights of barrier reported on follow. 



Depth of barrier crest Eatio of transmitted Ratio of transmitt- 



. below still -water level to incident wave ed "Energy" to that 



(in terms- of wave hgt.) height incident 



H 0.8 0.64, 



O.AB. 0,6 0,36 



O.A 0,16 



There is one other source which, in a broad way, confirtns two 

 important results of Morison's paper. One -type of barrier tested by 

 the Beach Srosion Board(27) -was a vertical plane (e.g. a sheet pile 

 bulkhead). If the results of this study are plotted as the ratio of 

 depth over the barrier to incident wave height versus the relative 

 height of transmission (Figure 5) a wide scatter of points is noted. 

 However an average cur-ve draim through these points lies close to a 

 curve dra-vm' thro-ugh points plotted from Morison's data. Actually 

 Morison's points show higher transmission values, and therefore the 

 use of his results should be conservative. If in addition a plot is 

 made (Figure 6) of the ratio of relative energy (actual) transmitted 

 H4.^ L+/E±^ L^ to the square of the relative transmitted height {R\, / 

 Hj2), -these points show little scatter, indicating that energy trans- 

 mission may be approximated by (Ht^/Hi^), (the assumption made pre- 

 viously) . 



Therefore, the comparative effectiveness from an energy stand- 

 point, of low barriers may be obtained from Figure 7, a plot of the 

 ratio depth of water over the barrier vs. //^ v 

 incident wave height iHr J 



VI. Seawall Shoreward of the Breaker Zone 



If a seawall is placed far up a slope, and the water level 

 fluctuation at its location is not great, damaging -waves -will break 

 before reaching the structure. Indeed this is a common situation at 

 many points on open sea coasts. Unfortunately, this problem is the 

 most difficult of the three to analyze, for little has been done to 

 find the dynamic characteristics of waves after breaking. Instead of 

 undergoing predictable laminar orbital motion, the water is in a 

 highly complex turbulent state which defies analytical breakdown. 



13 



