48 



The input wave condition for Test ST10A was narrow-banded random 

 waves with H^ = 0.8 m and T p = 3.0 sec, where H^ is the (significant) 

 wave height calculated from the zeroth spectral moment, and T p is the peak 

 period of the input spectrum. The wave condition for Test ST30A was 

 H^, = 0.4 m and T p = 8.0 sec. Positive and negative rates respectively 

 denote offshore and onshore transport. 



The profile in Test ST10A experienced significant erosion around the 

 still-water shoreline, together with formation and migration seaward of a 

 prominent breakpoint bar. For the random wave accretionary Test ST_30A, 

 the transport rate and profile change were less extreme in rate and magnitude, 

 respectively, than the erosional Test ST10A. There was slight erosion of the 

 foreshore as it adjusted under the higher runup of the long-period waves of 

 Test ST30A, and the bar slowly migrated onshore, decreasing in crest eleva- 

 tion and filling in the shoreward trough. 



The net cross-shore transport rate is calculated by integration of the mass 

 conservation equation, a process that involves the difference in consecutive 

 profile-survey depths. Small errors in survey measurement, either in the hori- 

 zontal or vertical, or changes in depth due to loss or gain of sand by local 

 compaction or expansion of the bed under wave action, enter cumulatively in 

 such a calculation and are manifested by nonclosure of the transport rate in 

 deeper water. In calculation of the cross-shore transport rate by integration of 

 profile elevation differences, therefore, one of the two profiles may have to be 

 translated vertically by a small amount (Larson and Kraus 1989) to achieve 

 closure of the rate offshore, where no transport occurred, as evidenced by 

 absence of change in the profile between surveys. In examination of the 

 SUPERTANK profile data, it has been found that some transport rates do not 

 close without vertical adjustment, typically on the order of 0.5 cm. The next 

 section discusses the reliability of the profile survey measurements. 



Reliability of survey procedure. Although the accuracy of Geodimeter 

 T140 was satisfactory for the SUPERTANK project, it was necessary to esti- 

 mate the reliability of the total survey system and operating procedure. 

 Reliability was assessed in part by repetitive surveys of the same profile line 

 (Kraus, Smith, and Sollitt 1992) after a particular day of testing with 

 monochromatic waves, which left the profile with a well-defined breakpoint 

 bar with steep vertical gradients. The 10 repetitive surveys were made with 

 different personnel operating the Geodimeter, survey carriage, and rod to 

 obtain an upper bound on potential survey error. 



Repetitive surveying of a well-defined bar is expected to provide error 

 bounds associated with the overall profiling system. Figure 2-6 shows the 

 superposition of the 10 profile surveys that were made along the center line of 

 the channel. The profile surveys began at Section I (Figure 2-3). The mea- 

 surements in Section I, made with a hand-held rod, are not of direct interest 

 here because the objective is to assess measurement of profile change pro- 

 duced by wave action as determined with the survey carriage and rod. The 

 error associated with the hand-held rod is less than 0. 1 ft (0.03 m), which was 



Chapter 2 Beach Profile Surveys and Sediment Sampling 



