60 



detached breakwaters. The Lakeview Park project consists of three rubble- 

 mound detached breakwater segments protecting a placed beach fill, contained 

 by groins on both ends. 



Again, the first step in the numerical modeling application was to evaluate 

 all relevant studies and data sets available for the project area. Ten 

 calculation cells were set up behind each detached breakwater, for an 

 alongshore grid spacing of 7.6 m. A one-year wave time series was used in 

 the calibration. First, the parameter K } was initially varied until calculated 

 overall net transport rates were close to estimated values. Then K 2 and the 

 boundary condition at the west groin were varied to obtain the approximate 

 magnitudes of sand inflow from the west. Then the transmission coefficients 

 were varied to achieve correct salient sizes, with a best fit obtained for K T = 

 0.50, 0.22, and 0.30 from west to east. Finally, the location of the eastern 

 breakwater was translated two grid cells to the east to obtain better agreement 

 of the eastern-most salient position. The result provided good agreement 

 between calculated and measured shoreline positions (Figure 35), with a mean 

 absolute difference of 1.2 m. Calculated and measured volumetric changes 

 also compared well (3,360 cu m predicted versus 3,290 cu m measured). 



For the model verification time period, the boundary at the west groin was 

 observed from aerial photographs to have retreated approximately 18 m. 

 Therefore, this boundary condition was altered in the model setup, and model 

 verification proceeded using a 13-month wave time series. Reasonable 

 agreement was obtained, although sensitivity testing indicated that increasing 

 the wave heights by 10 percent would result in a more accurate prediction, as 

 shown in Figure 36. The mean absolute difference between measured and 

 calculated shoreline positions was approximately 1.2 m, and the calculated and 

 measured volumetric change compared well (-238 cu m calculated, -256 cu m 

 measured). 



Prior to evaluating alternative structure configurations, model sensitivity to 

 key parameters should be examined. The authors investigated sensitivity of 

 the calibrated model to variations in Kj, K 2 , D 50 , and K T . Of particular note 

 is that, when grain size was halved, the predicted shoreline position indicated 

 an increase in sand volume. This is due to the more gently sloping 

 equilibrium beach profile used in GENESIS, which moves the breaker line 

 further offshore. However, GENESIS does not account for cross-shore 

 movement of material, which would be greater for the smaller diameter fill. 

 Using an average value for K T for each segment produced acceptable results, 

 although differences in transmission coefficients could be possible due to 

 differences in breakwater construction, wave transformation across an 

 irregular bottom, and differential settling. 



The authors used the verified model to evaluate alternative designs for 

 maintaining the beach fill in place. Simulations with detached breakwaters 

 only, groins only, and groins extended further seaward were evaluated. 

 Hanson and Kraus conclude that the combination of detached breakwaters and 

 short groins, as constructed, is superior to simpler designs for preserving 



Chapter 3 Tools for Prediction of Morphologic Response 



