at the wall's position, are all dependent on a knov/ledge of water 

 depths at and before the wall. Therefore in this section we will dis- 

 cuss the najor phenomena which may lead to a change of water depth j a 

 seiche, a gravitational tide, a storm tide, and bottom scour. 



A seiche consists of a periodic oscillation of water over any 

 water area, determined by the inherent natural period of oscillation of 

 the body of water. Seiches have been known to attain the heights of 6 

 feet in Lake Geneva, (l) and 15 feet in Lake Erie, (2) The causal 

 phenomena may be wind, pressure difference over the water surface or 

 even a gravitational tide. 



Gravitational tides, caused by the attraction of sun and moon and 

 by the earth's rotation, are the most familiar changes in vmter level, 

 since all coasts and some lake shores axperienoa them daily. The range 

 of tide depends on local hydrographio features, and nay vary from about 

 2 feet at points in the Gulf of Mexico to some 35 feet at Dutch Harbor, 

 Alaska. 



In many locations, the most important isater level fluctuation to 

 be dealt with is the so-called storm tide. When a severe storm strikes 

 a ocastal area, high winds accompanying the storm cause "pile-up" of 

 water along the shore. Due to this, in narrowing inlets and bays, the 

 water level may rise 15 to 30 feet. Though the range of the daily- 

 gravitational tide may be as large as that of a storm tide, the latter 

 is luapredie table, and if in phase with the former, may well cause a 

 water depth increase very much larger than that due to the local 

 gravitational tide. For example, Galveston, Texas, where the normal 

 height of high tide is less than 2 feet (MLW) experienced hurricanes which 

 in 1900 caused a high of 15 feet (MLW) and in 1915 3 high of 12.5 feet 

 (MLW) (3). A hurricane in 1938 caused high water elevations as great as 

 I4..7 feet (MLW) at certain points along the Massachusetts ooast(3a), A 

 storm on 5 October 1^>6^ raised the water level at Calcutta 24 feet(l). 



It should be noted that the danger of a stcrm tide lies not only in 

 its range, which nay or may not be excessive, but also in that, by its 

 nature being caused by strong winds, the rise in water level is always 

 accompanied hy severe wave action. 



A seawall is often located in an area of erosion for the purpose 

 of preventing further loss of land landward of the wall's position. 

 However the areas tendency toward erosion may continue to manifest it- 

 self by scouring the beach before the wall. Therefore, though water 

 level fluctuation at a particular locale may be minor, depths before 

 a wall may still increase. 



Ill, Criterion for Total Effectiveness of Seawalls 



Rough measures are available for the determination of the effective- 

 ness under wave attack of seawalls whose crests- are even with or below 



