height he above MLW is 



U) he = ht + 0..7 % 



Comparative Effectiveness of Low Seawalls - Though it is possible 

 with the relationship just determined to design a seawall to be completely 

 effective in turning back the highest tide and wave expected at its 

 p>osition, it is quite likely^ due probably to economic considerations, 

 that such a wall would not be feasible to construct. The question then 

 arises of a wall's relative effectiveness when its crest height is below 

 that level which would completely ttirn back a certain height of wave. 



Theoretically the problem has been solved for surface waves of 

 aiall aaplitude (23 ) by considering the energy distribution of a wave 

 in the vertical, and assuming that that portion of the energy which 

 impinges on the submerged wall is not transmitted. (This criterion is 

 an extension of the one adopted previously for total effectiveness of 

 a wall). The results of this particular analysis cannot be extended 

 to the case at hand for, by considering waves of small amplitude, the 

 expression for the ratio of transmitted wave height to incident wa ve 

 height to incident wave height becomes (in shallow water) EtM±- j/z-J^ 

 where h and d are respectively the wall height and water depth before 

 the wall* This indicates that when a wall is at the height of still 

 water , nothing may be transmitted over it* Practically this if far 

 from true* 



Similarly we must reject^for our case, another attack on the 

 problem made on the basis of shallow water wave theory (24) (25) 

 (theory of tides). In this derivation, the expression for the trans- 

 mission coefficient, (Ht/^i) becomes 2 when h = d, but conservation 

 of energy demands that this transmitted wave be propagated with zero 

 velocity. 



Since neither of the two theoretical treatments may be applied, 

 we must take recourse to any observed or experimental work done on the 

 problem. One study (26) made in an attempt to correlate wave parameters 

 (especially length) to depth of water over a reef predicts (as would 

 be expected from the theoretical treatments) a decrease in ratio of 

 wave length over the reef to that before the reef, but unfortunately 

 gives no information as to relative wave heights. Other studies (27,28) 

 deal with underwater barriers of various cross-sections, all of which 

 however are located seaward of the breaker zone* 



The only study of which application may be made in the present 

 case is one by J. Morison (29,30) on the damping effect of submerged 

 rectangular barriers, some of which were located in the breaker zone. 

 Even here, the application must be limited, for the problem at hand is 

 essentially that of a nearly horizontal reef in shallow water, while 

 Morison dealt with a rectangular barrier of finite width. However, 

 broad relationships may be derived which deal with the amount of energy 



