K =\^ 0.6 Hb 



where here H is the maximum expected wave height at the wall's posi'tion. 

 Relative effectiveness of lower Trails is shofin on Figure 7. 



VIII., - An Actual Case - Galveston, Texas 



Unfortunately little data are avai]a ble on the type of storm 

 attack experienced by those seawalls' which have withstood such attack. 

 Descriptions of coastal s'tructuroc turning back c]3ms.gin'^ vrave action , 

 are usually-.,, ^^raohic , containi'ne •o.hL'ase3--suGh as ."a huf^e wive'* which 

 ■■' are ^;g'^]:ittl^''practic3l value. . The report on Galveston's seawall^^/ 

 is an exception, tWough even here,' a,,, larger portion of descriptive 

 material, is wholly subjective. 



After a violent hurricane in 1900 which caused damage to most 

 of the city, a seawall was constructed to a crest height of 17 feet, 

 MLW, (the hurricane caused storm tide heights up to 15 feet). In 

 I915j another storm of comp rable intensity accompanied by a storm 

 tide of 12.5 feet struck the protected area. This tide height left 

 about 10 feet or more water depth at the toe of the wall, and a wall 

 free-board of only 4. 5 feet. Though no description of the waves is 

 given in the report, it is easy to guess that wave action accompanying 

 the storm overtopped the wall. Portions of the report read, "Con- 

 siderable quantities of vrater came over the wall, seriously d-^naging 

 the embankment back of it in places.....'*, "The volume of vrater passing 

 over the Tjall was surprisingly large. One observer reports that at 

 Sixth and Broadvray, the water appeared to be coming over in a continuous 

 sheet estimated' to average 2 feet deep." 



The distance from tide level to a point 2 feet above the seawall 

 crest is about 6.5 feet. Therefore the wave height (equation A) should 

 have been about 6.5/0.7 = 9 feet. Other parts of the report estimate 

 that vraves "of any material frequency were about 5 feet higher than 

 the ?rall|" in this case the wave height itself Yrould have been 

 9.5/0.7 = 13.5 feet. Considering that the TJind attained a maximum 

 velocity of 93 miles per hour at Galveston, waves of these heights 

 are not excessive . 



Bibliography 



(1) Johnson, D. 1^., Shore .Processe.a. and Shore_line ,I)e.velo.pra3.nt, New 



York, John 'liley and Sons, 1919 



(2) Krecker, F. H., Periodic Oscillation in Lake Erie, Ohio, State 



University, The Franz Theodore Stone Laboratory, Contribution 

 ■■•I, 1928. '. 



(3) House of Representatives, Doc. n-693, 66th Congress, 2d Session, 



"Galveston Island and Galveston Channel" 

 Doc. #173, 81st Congress, 1st Session, "Galveston Harbor and 

 Channel" 



16 



