40 



Output for COSMOS-3D consists of profile change plots for each of the 

 1 1 profile lines, which are similar to the 2-D result format. In addition, net 

 alongshore transport for the duration of the storm is plotted. The boundary 

 profiles at R8 in the north and R23 in the south are assumed to remain fixed in 

 the numerical calculation scheme. The results must be considered in relation 

 to the large profile spacing (i.e., 200 to 800 m). The predicted changes to the 

 morphology will be limited to features with lengths in the range of 200 to 800 

 m or greater. Closer alongshore spacing was not possible owing to the limited 

 number of survey lines. Unfortunately, the 1991 bathymetry survey was not of 

 sufficient detail to pick up the shape of nearshore bars, and therefore could not 

 be used to supplement the profile lines for 3-D input. The SHOALS 1995 

 bathymetry was not available at the time the 3-D modeling was completed. 



Wave and water level information can be input at each of the 1 1 profile 

 lines if information is available on variations in these parameters along the 

 shore. For this investigation, the only variation that was considered consisted 

 of wave sheltering effects for Lines R8 and R9 during northwest wave attack 

 (i.e., in the lee of the south jetty of the harbor entrance). 



General results 



Profile change predictions from the cross-shore (2-D) modeling indicated 

 that the middle and outer sections of the surf zone (which feature one or more 

 bars) were relatively stable over the duration of a single storm event. A series 

 of 3-D runs were performed to investigate the morphologic response of the 

 sand cover under the combined influence of cross-shore and alongshore sedi- 

 ment transport during storm events. These experiments were also specifically 

 directed to describing the mobilization and transport of the beach nourishment 

 and to assessing the exposure and downcutting of the underlying glacial till. 



Table 8 summarizes all of the 3-D results. The three initial runs consisted 

 of an assessment of morphologic response under pre-fiU conditions, with a 

 grain size of 0.2 mm for three different storm events. These storm events are 

 described in Parson, Morang, and Nairn (1996). The 2 November 1991 event 

 represents one of the largest storms from the southwest (in terms of wave 

 energy) over the two hindcast periods (1956 - 1987 and 1991 - 1993). The 

 14 January 1992 storm featured northwest waves and was the largest storm in 

 terms of wave energy over the two hindcast periods. In the first section of this 

 chapter, entitled "Results of the Alongshore Sediment Transport Calculations," 

 we concluded that net transport is directed to the south owing to the predomi- 

 nance of northwest storms. The 24 January 1994 event features waves which 

 swung from southwest to northwest through the duration of the storm (with an 

 average direction of west). This type of storm occurs frequently, and the mag- 

 nitude of this particular event represents a storm that would occur once per 

 year on average. The 24 January 1992 storm was used as input for all of the 

 cross-shore (2-D) evaluations described in the section titled "Results of Cross- 

 shore Modeling with Multiple Grain Sizes." 



Chapter 4 Analyses of Coastal Processes and Geomorphology 



