370 waves (see plots in Figure E19 in Appendix E). At 750 waves, the profile with the vertical seawall 

 showed increased scouring of the surf zone and decreased height of the offshore bar feature. At the 

 equilibrium condition (after 1,650 waves), quite distinct differences were observed between tests T03 and 

 TIO, although the time required to reach the equilibrium profile appeared to be very similar in both tests. 



214. The 1,650- wave comparison in Figure 30 shows that the seawall promoted increased erosion of 

 material from the surf zone and caused a decrease in the offshore bar height. The extra material eroded 

 was used to satisfy the sediment demand in the region immediately seaward of the bar feature. The 

 removal of sediment from the bar crest was probably due to a combination of increased breaking wave 

 height as the incident wave interacted with the wave reflected off the vertical wall and offshore return flow 

 patterns different from those generated in the absence of a vertical seawall. However, wave statistics 

 presented in Tables B3 and BIO (Appendix B) indicated that measured waves and reflection coefficients at 

 the nearshore gages were quite similar for both tests, suggesting that any increjised reflection caused by the 

 vertical wall was attenuated as the reflected wave returned through the surf zone. 



215. Cross-tank variations in the profile occurred during test TIO as well as test T03. Figure 31 

 compares the final profiles at the glass sidewall (top), the center line (middle), and the concrete wall 

 (bottom). The additional eroded area resulting from the seawall was calculated for all three profile 

 comparisons; then an average was subsequently calculated to determine the eroded volume for comparison 

 with the withheld volume. The withheld volume per unit width of the flume was about 1.54 ft^/ft; and 

 the calculated average volume of additional erosion was 1.58 ft^/ft, resulting in the eroded volume being 

 only 3 percent greater. Eroded volumes per unit width for the individual profile comparisons were 



1.92 ft^/ft at the glass wall, 2.13 ft^/ft on the center line, and 0.68 ft^/ft at the concrete wall, resulting 

 in the average of 1.58 ft^/ft. 



216. The near equivalence between the additional eroded volume in front of the seawall to the volume 

 retained behind the seawall conforms to the field observations of earlier investigators, and it also follows 

 the conclusions given by Dean (1986); however, the present result is substantially different from the average 

 of laboratory results presented by Barnett (1987). The difference between the two may well lie in the fact 

 that this is a single test result, whereas Barnett examined 11 different cases. Further tests are needed to 

 examine a wider variety of vertical seawall conditions. 



Irregular Wave Comparisons 



217. The irregular wave comparison consisted of running the same irregular waves used in test T09, 

 but with the seawall installed on the sloping revetment as described above. Thus, the only difference 

 influencing profile development between cases T09 and Til was the presence of the seawall. Figure 32 

 shows comparison plots between vertical seawall test Til (solid line) and non-seawalled test T09 (dashed 



86 



