This technique, which is explained and Illustrated in CERC Field Guidance 

 Letter 1^-h (Esteva, 1979), is used to determine C^r in the present study. 



III. COMPARISON WITH FIELD DATA 



Simultaneous observations of a variety of significant wave heights, 

 periods, and energy spectrum shapes were chosen from available field data to 

 illustrate possible weaknesses or strengths of Bretschneider and Reid's (1954) 

 theory in all types of wave climate. The wave data selected were obtained 

 from two Waverider buoy gages located in an area outside the breaker zone 

 where sediment characteristics indicate that bottom friction is the predomi- 

 nant dissipation mechanism. Using conditions at the outer gage (Waverider 

 gage 620) as input for Bretschneider and Reid's predictive equations, result- 

 ing calculated wave characteristics at the nearshore gage (Waverider gage 610) 

 are compared to observed wave height values. Results are shown in Table 1 and 

 Figure 3. Negative deviations from observed wave heights indicate the pre- 

 dicted value is lower than actually observed; i.e., the theory predicts more 

 frictional energy loss than is observed. The range of friction coefficients 

 used is 0.004 to 0.07. Most of the large underpredictions occur when no 

 change or an actual increase in wave height is observed from offshore to 

 inshore, possibly due to strong wind-wave generation. Overprediction indi- 

 cates that other dissipation processes are occurring. Table 2 summarizes the 

 results of this study. Figure 3 indicates that negative deviations are more 

 pronounced for broad or multipeaked spectra, while narrow or single-peaked 

 spectra correspond to slightly overpredicted wave heights. General trends 

 show that the theory corresponds closely to observed wave conditions with 

 maximum deviations of 60 percent but most conditions are within 15 percent. 

 Examining only the data points for the narrow, single-peaked spectra, over- 

 prediction occurs for lower wave heights; underprediction occurs for larger 

 waves which tend to be more nonlinear at the same shallow depth. 



Table 3, which presents the results of Bretschneider and Reid's theory 

 using Baylor staff gages (manufactured by Baylor Company, Houston, Texas) 

 along the pier at FRF, provides an example of the theory's inapplicability 

 where bottom contours are not straight and parallel. The irregular pier- 

 induced topography causes the theory to overpredict wave height at Baylor gage 

 665 (located 350 meters from shore), inshore of Baylor gage 625 (located 630 

 meters from shore), indicating that other processes (e.g., refraction, bottom 

 scattering) are affecting wave heights. As shown in the table, preliminary 

 runs of a more advanced, nonlinear model indicate that the additional observed 

 losses are likely due to refraction. This example shows that caution must be 

 taken in applying the Bretschneider and Reid theory near manmade structures or 

 in areas of irregular bathymetry. 



************** -[v, EXAMPLE PROBLEM *************** 



GIVEN : A wave with the following wave height and period at gage 620 at an 18- 

 meter depth: 



"s620 = ^-"^ r^et&Ts 

 T = 10 seconds 



FIND : The wave height 2,200 meters closer to shore in a depth of 10 meters. 

 Assume a dq^ of the sediment to be 0.3 millimeter. 



10 



