MODEL STUDY OF OFFSHORE WAVE TRIPPER 
by 
Frederick F. Monroe 
Research Division, Beach Erosion Board 
The Galveston District of the U. S. Army Corps of Engineers recently 
requested that the Beach Erosion Board's staff perform a series of small- 
scale model tests on a proposed offshore structure. This structure would 
act as a wave tripper, causing offshore breaking of larger waves, thus 
preventing waves of height greater than a particular value from reaching 
the shoreline structures. The system of a shoreline structure combined 
with an offshore structure was to be considered for providing protection 
during hurricanes to the shoreline of Texas City, Texas. 
The study was carried out at the Beach Erosion Board laboratory in 
a wave tank approximately 74 feet long, 1.5 feet wide, and 2 feet deep. 
Waves were generated by a vertical-bladed, pusher-type wave generator, 
operated by an arm attached eccentrically to a drive wheel. A bottom 
profile, generalized from a hydrographic survey sheet provided by the 
Galveston District, was modeled at a 1:50 scale in the wave tank. A 
comparison between the bottom profile, as taken from the hydrographic 
survey sheet, and the generalized profile used in the model, is shown in 
Figure 1. A prototype storm surge level of 15 feet above mean sea level 
was used for all tests, as was a prototype wave period of 6 seconds. These 
conditions remained constant throughout the duration of the test series. 
Two types of offshore structures were included in the test program. 
One type simulated a breakwater of rubble-mound construction, and the 
other a vertical-face structure. Two structure elevations were tested; 
one of 6 feet and the other 2 feet above mean sea level. In conjunction 
with the offshore structure variations, two types of shoreline conditions 
were also tested. One consisted of a smooth concrete slope to an eleva- 
tion of 15 feet above mean sea level, with a rubble absorber landward of 
that point, permitting wave reflection to only a negligible extent. The 
other shore condition tested consisted of a rubble shore structure that 
extended from mean sea level to an elevation of 23 feet above mean sea 
level, and which permitted considerable reflection of wave energy. 
Wave heights were measured at a number of locations in the wave tank 
without an offshore structure present, and both with and without a struc- 
ture present on shore, in order to determine the characteristics of the 
incident waves in undisturbed form. Then, wave height measurements were 
made with each type of offshore structure in place, each with two eleva- 
tions, and with different shoreline conditions. The locations of wave 
gages relative to the bottom profile may be seen in Figure 1. Figure 2 
Shows the wave gage locations with respect to the various test conditions. 
Wave heights were measured by means of parallel wire resistance 
probes suspended in the tank at various locations. These were calibrated 
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