relative position of secondary waves to the primary wave causes a variable 

 asymmetry in the velocity field under waves and resulting variation in the 

 shape of the profile and rate of profile change. The critical distance 

 in cases of secondary wave occurrence is the overtake distance, the dis- 

 tance from the generator to the point where the secondary crest of the 

 first wave has been overtaken by the primary crest of the second wave. 

 The overtake length for this wave condition in the 2.33-foot (0.71 meter) 

 water depth was about 26 feet (7.92 meters) and the difference in initial 

 test lengths was 39,3 feet (12.0 meters). Therefore, secondary waves 

 could account for the difference in rate of profile change. 



The second way in which the initial test length can be important is 

 in the difference (between the two experiments) of the phase difference 

 between the re-reflected waves and the generated waves. The difference in 

 initial test lengths was 39.3 feet or 2.7 wavelengths. The average inci- 

 dent wave height for the first 20 hours in experiment 70X-06 was 0.35 foot 

 (10.7 centimeters) and in experiment 70X-10 was. 0.37 foot (11.3 centimeters). 

 However, the lower incident wave height is associated with the greater 

 initial erosion rate. Thus, it is not apparent how the re-reflection 

 affected the beach. 



c. Water Temperature . The water temperature varied from 30° to 7° 

 Celsius for the experiments which began in May and August and continued 

 into early December. The dynamic viscosity varied from 1.7 x 10"^ to 

 3.0 X 10"5 pounds-second per square foot (0.798 x lO'^ to 1.430 x 10"2 

 grams-second per square centimeter) (Daily and Harleman, 1966) . Quantifi- 

 cation of temperature effects is not possible here because of the unquanti- 

 fied effects of the different tank dimensions and the lack of temperature 

 data for the first 40 hours of experiment 70X-10. However, two points 

 can be made. At 22 hours in experiment 70X-06, the water temperature 

 dropped from 28° to 18° Celsius and the rate of shoreline recession in- 

 creased from 0.06 to 0.14 foot per hour (Fig. 37). This supports the 

 hypothesis that colder, more viscous water increases the sediment trans- 

 port capacity of waves (Fairchild, 1959). The gradual increase in viscosity 

 may have prevented the profile from reaching equilibrium by continuing to 

 increase the sediment-carrying capacity of the water. 



V. CONCLUSIONS AND RECOMMENDATIONS 



1 . Conclusions . 



(a) In two experiments with a water depth of 2.33 feet, a wave period 

 of 1.90 seconds, and a generator stroke of 0.39 foot, the nominal incident 

 wave height was 0.36 foot. Reflection measurements in the control tanks 

 with a fixed-bed profile varied from 0.03 to 0.07, indicating that the 

 wave generators were operating uniformly and that the measurement error 



in determining the reflection coefficient, K^, was ±0.02 (Table 6). 



(b) % varied from 0.08 to 0.20 in experiment 70X-06 and from 0.04 

 to 0.19 in experiment 70X-10. The variation in K^ correlates with pro- 

 file changes. K^ more than doubled in the first few minutes of wave 



83 



