85 



constructed in 1925-1928. The project has performed well, with annual 

 losses on the order of 76,500 m^ , and has provided upland protection during 

 several major hurricanes. Several islands provide some shelter to the 

 project from the Gulf of Mexico and may be partially responsible for its 

 longevity. It was renourished with 1.5 million m-^ of fill in 1972-1973, 

 following the effects of hurricane Camilla (1969), which caused storm tides 

 locally in excess of 6 m. Relative sea level is estimated to have risen 

 only 8 cm during the life of the project (Hicks et al . , 1983), forestalling 

 conclusions of the fill's stability in response to sea level rise. 



6.4 COST OF COASTAL WORKS 



Although the effect of a rise in relative sea level on the cost of a 

 coastal structure or beach nourishment project can only be accurately 

 determined on a case-by-case basis, several crude indicators are available. 

 For rubble mound structures, the cost increases with the required 

 individual weight of the armor stone. Using the well known Hudson formula, 

 found in the Shore Protection Manual (Army Corps of Engineers, 1984), the 

 weight (W) increases with the cube of wave height. From section 5, 

 expression 5.10 for the generated wave height and 5.18 for the height after 

 bottom friction can be used to determine the relative increase in stone 

 weight. For the example presented (sea level rise of 1 m, wind speed of 

 30 ra/s, fetch length of 50 km and shelf depth of 10 m) , the ratio of 

 weights is 



W (after s.l.r.) (0.96)^ ^1.60 ^^-^^ 



W (before s.l.r.) 3 



or a 60% increase in stone size. We see that sea level rise may have a 

 significant Impact on the design and cost of rubble mound structures. 

 Also, the future cost of buying additional land required to raise the crest 

 of a levee or dike often is much greater than the additional material 

 required, and should be considered in long-range planning and design. 



For beach nourishment projects, the increase in the rate of losses can 

 be examined by assuming the transport rate (Qg) to be proportional to wave 



