Sector E consists of a 4-km-long stretch of uninterrupted shoreline protec- 

 tion. There is a transition zone between the feeder beach and the deeper pro- 

 files offshore of the revetment protecting the railway further to the south. The 

 profile for Line R12 shows that significant lake bed lowering in front of the 

 revetment at this area has not yet occurred (see Figure 30). However, further 

 offshore, 4 to 5 m of lake bed lowering occurred from 1945 to 1995. Both the 

 numerical modeling results and observations from aerial photos show that the 

 beach located immediately south of the Waterworks revetment is subject to 

 large fluctuations. The revetment itself probably acts as a groin structure 

 impounding at least some sediment to protect the underlying glacial till in the 

 nearshore zone most of the time at Line R12. 



Sectors F and G consist of the section of coast extending from Line R22 to 

 south of Shoreham. Here, the shore is only partly protected or entirely unpro- 

 tected. This section features long-term recession rates of 1 to 2 m/year 

 between 1945/6 and 1964/5. 



1964/65 to 1991 



The 1964/65 to 1991 period is characterized by much lower rates of deposi- 

 tion or erosion in the nearshore zone (see Figure 29) when compared to the 

 earlier 1945 to 1964/5 period (see Figure 30). The possibility that the 1991 

 bathymetry featured an error in vertical control or datum conversion was inves- 

 tigated and dismissed as a possible explanation for the discrepancy between the 

 rates of change between the two periods. An extensive review of all original 

 data and datum conversions applied to the hydrographic surveys did not iden- 

 tify any errors. The observation of low erosion rates in Sectors A and G, 

 which were previously identified as representative of the background erosion 

 condition, coupled with a low deposition rate in the Sector B fillet and erosion 

 in the Sector C fillet located south of the harbor, suggests that the driving 

 force of erosion and deposition (i.e., wave-driven sediment transport) may have 

 been less effective than during the previous period (see Figure 36). Unfortu- 

 nately, the available wave climate information only extends back to 1956, and 

 it is not possible to substantiate this hypothesis. 



A more certain explanation for reduced lake bed lowering rates in Sectors 

 D, E, and F is the influence of the Section 1 1 1 beach nourishment program, 

 which was initiated in 1976 (with some nourishment placed as early as 1970). 

 In these sectors, there was a tenfold decrease in the lake bed lowering rates. 

 In Sector G, representative of background conditions, the lake bed erosion rate 

 was lower by a factor of only 2.5. The trend for this period suggested that the 

 Section 1 1 1 Program was successful in mitigating the lake bed lowering rates 

 for Sectors D to F. While it may be argued that a beneficial effect was also 

 experienced in Sector G, it is more likely that the reduced erosion rate in this 

 sector can be explained by generally lower driving forces during this period as 

 mentioned in the previous paragraph. 



Chapter 5 Interpretation of Results - A Descriptive Model of Coastal Morphodynamics 



83 



