The magnitude of the response inside the tube to the wave 

 fluctuations outside was deemed excessive, as leading to appre- 

 ciable error in actual field measurements. Accordingly the gage 

 was modified to reduce the ease of water interchange between the 

 outside and inside of the tube by reducing the effective bottom 

 opening to 0.040 square inch. The measurements were then repeated 

 with this reduced section. Values obtained are shown in Figure 2, 

 As may be readily seen, they were reduced considerably, the ride-up 

 inside the gage never amounting to more than a quarter wave height, 

 and most being less than a tenth. The highest value observed for 

 tests in the small laboratory tank was 0.05 foot (for a 0.56-foot 

 wave of 2.63-second period). For a number of test waves, parti- 

 cularly with the shorter periods, no measurable rise was recorded. 

 In the prototype tank in only one case did the ride-up inside 

 the gage exceed 0.3 foot (when it reached 0.5 foot for a 6-foot, 

 7.9-second wave) and most values were on the order of 0.2 foot. 



The general degree of reduction in ride-up inside the tube 

 with the reduced opening is also indicated in the photographs in 

 Figure 3. Figures 3a and 3c show the gage as a wave crest passes 

 by; note than while the original gage shows a water level rise of 

 almost 0,1 foot (about one-third the wave height), the choked 

 gage shows no rise. Figures 3b and 3d show the conditions at 

 maximum water level rise inside the tube. For the original gage, 

 this was 0.15 foot, as compared to only 0,02 foot in the choked 

 gage. Note that, due to time lag in the response of the water 

 level in the tube, the maximum rise always occurred considerably 

 after the passage of the crest. 



The lesser response to wave fluctuations of the water level 

 inside the tube with the choked down opening was considered much 

 more acceptable, and it is believed that most field installations 

 by the Corps of Engineers since these tests have utilized gages 

 with the lesser size opening. The effective area of 0.04 square 

 inch approximates that of a 7/32-inch hole (0.038 square inch). 

 Actually as many of the standard river crest gages supplied by 

 the U. S. Geological Survey (through their Equipment Development 

 Laboratory) come with a bottom-cap drilled with six 1/4- inch 

 holes, a standard gage with five of these six holes closed up 

 probably would give acceptable results for hurricane surge stage 

 determination. Attention is called to the fact that a series of 

 tests in Southern California ocean waters in the 1930' s showed that 

 if the bottom of the gage is continuously submerged, algae growth 

 will close a 1/4-inch or smaller diameter hole. Holes of 1/2- inch 

 in diameter were not closed by algae, however. 



Although the tests were made to determine feasibility of using 

 the gage to give a measure of the maximum hurricauie high water 

 stages, the results with the gage having the larger bottom opening 

 would seem to indicate that the applicability of the general gage 

 to measuring peak stream flood flows might also be reexamined to 

 determine the possible effect of superficial wind waves and ship 

 waves on these measurements. 



33 



