4. Simulation of the Longshore Sand Transport Study at Channel Islands Harbor . 

 Cal ifornia . 



The CIH Longshore Sand Transport Study (Bruno, et al . , 1981) was the 

 only field study found suitable for verification purposes. Wave data 

 collected included the LEO data and a two pressure-sensor gage array. 

 Although the pressure gages were not in operation throughout the study, it 

 was expected that the data they produced would be superior to that of the LEO 

 data. However, these data were not available in a reduced form, so the LEO 

 data were used. An adjustment of 11° was made to the breaker angle to orient 

 the angle with respect to the base line, rather than to the local shoreline 

 orientation angle. Observations had been taken twice daily at three 

 locations; the middle location was used (observer No. 5714). Waves which 

 approached the shoreline at angles too large to have originated in a depth of 

 10 meters, according to Snell ' s law, were set equal to 90° at that depth 

 (crest of wave perpendicular to the baseline). The 10-meter depth was chosen 

 because it is the approximate depth at the tip of the offshore breakwater 

 (for this reason, it was also chosen as the depth of the step beyond the y(I, 

 JMAX + 2)lll contour). It was assumed that each of the two daily 

 observations occurred for 12 hours and using a time-step of 6 hours, this 

 meant two time-steps per wave. In cases where parts of the wave data (Hb, 

 ab, or T) were missed by the observer or were equal to zero, the data were 

 ignored (no computations were made), but the time was included. Because the 

 time rate of change is important for this simulation, the variation of Cqff 

 outside the break point was used. 



The period chosen to model was 20 April through 1 December 1976. The 

 initial survey was taken after dredging of the sediment trap and for this 

 reason was known to be out of equilibrium. The bathymetric surveys were con- 

 ducted using several methods, the most advanced being a Lighter Amphibious 

 Resupply Cargo vessel (LARC) proceeding along shore-perpendicular lines 

 (approximately in the vicinity of each survey station) taking fathometer 

 readings e^ery 10 seconds, with positioning systems trilaterating the 

 vessel's position concurrently. These data were recorded on tape. The 

 beach-face data were taken using standard surveying methods. Because the 

 data fluctuated randomly about the stations, depending on the speed of 

 the craft, the (x, y) coordinate positions had to be altered to fixed changes 

 in X and y. This was accomplished using an interpolation routine. The x 

 values were made to coincide with the stations used in the surveys, and the y 

 values were determined at 100-foot intervals beginning from the base line. 

 Stations 100+00 and 118+00 were located at the north jetty and termination of 

 the detached breakwater, respectively (these correspond to I values of 16.5 

 and 34.5 in the model). See Figure 19. 



Monotonic profiles of the form h = A(y - ydel)2/3 were fit to the data 

 along each station line, "ydel" represents the zero location of the fitted 

 shoreline, the value of which was unknown. Because dredging had been done in 

 the lee of the breakwater, there was no reason to expect the A value to 

 correspond to the value upcoast where the influence of the structure and the 

 dredging was negligible. For this reason, the profiles of Stations 122+00 

 through 134+00 were evaluated separately to determine an A value for the 

 equilibrium profile to be used in the numerical model. For the detailed 

 method used (LaGrange Multipliers and Newton-Raphson Method for nonlinear 



46 



