water levels. The presence of the coarse sand (2.0 mm) on the beach protected 

 this area from significant erosion, even during high water levels. However, the 

 zone of transition from coarse beach sediment to fine sediment offshore was 

 vulnerable to considerable erosion. 



At Line R 14 the shoreline is protected by the revetment. The results of the 

 profile response runs for an average water level are shown in Figure 18. For 

 all water levels, there was minor erosion of the sand in the troughs between 

 the nearshore bars. The trends in profile response for R14 with multiple sedi- 

 ment sizes were similar to the results presented in Parson, Morang, and Nairn 

 (1996) for single grain sizes across the entire profile. For all three water 

 levels, the model predicted accumulation at the toe of the revetment and minor 

 adjustment in the position of the large bar. 



COSMOS-3D Modeling 



Methodology 



COSMOS-3D is referred to as a quasi-3-D model because the coastal processes 

 are more fully integrated across the profile than in the alongshore direction. 

 The 3-D model is based on the deterministic COSMOS-2D model for the pre- 

 diction of coastal processes across a nearshore profile (see Nairn (1993)). The 

 profile model is extended to represent a 3-D situation through the linkage of 

 1 1 of the individual profile lines along the St. Joseph study area. In this 

 model, the profiles are treated independently in a hydrodynamic sense, but are 

 linked morphodynamically by consideration of the differential rates of along- 

 shore transport between adjacent profiles. Although the 3-D model grid is rec- 

 tilinear, the sediment is transported alongshore in a direction coincident with 

 an input "marker depth" contour. For the runs performed as part of this inves- 

 tigation, two different marker depths have been utilized: (a) the most land- 

 ward 2.5-m depth contour (parallel to the first large bar, which is the primary 

 pathway for fine sediment transport) (b) the 0.5-m depth contour, which gives 

 the alignment of the upper beach and is the major pathway for the transport of 

 the 2-mm grain size. The model is only quasi-3-D and is restricted in its 

 application to cases where the 3-D circulation is negligible or of secondary 

 importance to the morphology change. In this respect, the St. Joseph study 

 area, which features a relatively straight coast with parallel near-shore con- 

 tours, is an ideal site for the application of C0SM0S-3D. 



The grid for the 3-D modeling consisted of 1 1 of the profile lines, with 

 RlOa excluded due to its close proximity to RIO and Rll (Figure 19). In the 

 northern beachfill area, there was 200- to 300-m spacing between profiles, 

 while south of R12, the spacing was 800 m or more. The lines varied in 

 length (perpendicular to the shore) between 1 ,000 and 1 ,700 m, with about 

 200 calculation points describing each line. The input depth ranged from 8 to 

 13 m. 



Chapter 4 Analyses of Coastal Processes and Geomorphology 



37 



