GenCade Val:LSTF

A model validation of shoreline evolution in a laboratory setting was performed for GenCade following a similar case presented in Hanson et al.(2006). The laboratory experiment was conducted in the USACE-ERDCCHL Large-Scale Sediment Transport Facility (LSTF) (Hamilton et al.2001) and the methodology and results are described in Gravens and Wang (2007). The validation experiment analyzes the capability of the GenCade model to calculate shoreline and salient evolution and tombolo formation in the lee of a detached breakwater in a controlled laboratory environment. It should be noted that Hanson et al. (2006) performed the GENESIS validation with a research version of GENESIS, which employed a different sediment transport formulation from what is employed in the release version of GENESIS-T (i.e., the version included with the CEDAS interface package) (Hanson and Kraus 1989) and in the current release of GenCade. Therefore, the results presented herein may be used to compare agreement between the measured results of the laboratory experiment and the calculated results from a different sediment transport formulation.A model validation of shoreline evolution in a laboratory setting was performed for GenCade following a similar case presented in Hanson et al.(2006). The laboratory experiment was conducted in the USACE-ERDCCHL Large-Scale Sediment Transport Facility (LSTF) (Hamilton et al.2001) and the methodology and results are described in Gravens and Wang (2007). The validation experiment analyzes the capability of the GenCade model to calculate shoreline and salient evolution and tombolo formation in the lee of a detached breakwater in a controlled laboratory environment. It should be noted that Hanson et al. (2006) performed the GENESIS validation with a research version of GENESIS, which employed a different sediment transport formulation from what is employed in the release version of GENESIS-T (i.e., the version included with the CEDAS interface package) (Hanson and Kraus 1989) and in the current release of GenCade. Therefore, the results presented herein may be used to compare agreement between the measured results of the laboratory experiment and the calculated results from a different sediment transport formulation.

GenCade model setup and parameters

Model domain

The GenCade model domain was developed to represent the LSTF lab experiment test case 1. The model grid was 60 m long with a 4 m long detached breakwater positioned in the center of the domain 4 m offshore of the initial shoreline position with both diffracting tips at a depth of 0.17 m. Model grid cell resolution was set to a constant 0.1 m (600 cells total). The model experiment was simulated for 24 hours as was the case in the laboratory case. GenCade lateral boundaries were selected as pinned beach boundaries. The median grain size (d50) was set at 0.15 mm as represented in the LSTF experiments. The berm height was set to 0.35 m and the depth of closure was set to 0.5 m. Figure 1 shows the LSTF domain. Additional model parameters are described in the Model Parameters section.

Figure 1: GenCade LSTF domain, with detached breakwater in orange

Model forcing: waves

Wave forcing applied to the GenCade simulations was held constant throughout the simulation such that breaking wave heights were H=0.26 m, T = 1.5 sec, and Dir=6.5-deg as measured in the LSTF experiment. Wave inputs were supplied at the 0.9 m depth contour.

Model parameters

GenCade calibration parameters were adjusted to calibrate the model to measured transport rates and salient shape. First calibration was conducted with the K1 parameter to obtain average longshore transport rates in agreement with the measured average longshore transport rates. The measured average longshore transport rates were reported at 2,194 m3/yr (Hanson et al. 2006) and GenCade simulations resulted in average longshore transport rate of 1,991 m3/yr, and within 10 percent of measured rates with K1 set to 0.18. Next, K2 and ISMOOTH parameters were adjusted to improve calculated salient shape. Table 1 lists the best fit values obtained.

Table 1: Model Parameters

Parameter	Value
K1	0.18
K2	0.06
ISMOOTH, # of cells in smoothing window	1

GenCade model results and discussion

Figure 2 presents the GenCade results compared to results obtained from the LSTF experiment (Gravens and Wang 2007) and the GENESIS-T validation (Hanson et al. 2006) for shorelines after 24 hours. This figure shows that GenCade calculated tombolo formation after 24 hours. The GenCade calculated shoreline and shape of the tombolo and salient show excellent agreement with the measured shoreline at the center of the tombolo. However, the transitions immediately updrift and immediately downdrift of the breakwater are not as well represented and appear to transition more abruptly in the GenCade calculations than is observed in the measurements and the GENESIS (Hanson et al. 2006) results. This may indicate an area for future improvement of sediment transport formulation or diffraction routines.

Figure 2: Calculated vs. measured shoreline position in the LSTF.

References: Gravens, M. B., and P. Wang. 2007. Data report: laboratory testing of longshore sand transport by waves and currents; morphology change behind headland structures. Technical Report, ERDC-CHL-TR-07-8, US Army Engineer Research and Development Center, Vicksburg, MS.

Hamilton, D.G., B.A. Ebersole, E.R. Smith, and P. Wang. 2001. Development of a large-scale laboratory facility for sediment transport research. Technical Report ERDC/CHL-TR-01-22, US Army Engineer Research and Development Center, Vicksburg, MS.

Hanson, H. and N.C. Kraus. 1986. Seawall Boundary Condition in Numerical Models of Shoreline Evolution, Technical Report CERC-86-3, US Army Engineer Waterways Experiment Station, Coastal Engineering Research Center, Vicksburg, MS.

Hanson, H., M. Larson, N.C. Kraus, and M.B. Gravens. 2006. Shoreline response to detached breakwaters and tidal current: Comparison of numerical and physical models. Proceedings of 30th International Coastal Engineering Conference, World Scientific, 3,630-3,642.