103 



using a eustatic sea level rise of 1.2 nun/year and a retreat/rise 

 multiplier of 100. Data provided by the Jacksonville District of the U.S. 

 Army Corps of Engineers for the period 1980 to 1985 indicate that 

 approximately 50% of the east Florida coast material dredged was still 

 being disposed at sea during this period. This amount (38,000 m-^) , again 

 using the Bruun Rule, is sufficient to more than offset their retreat due 

 to the eustatic sea level rise rate employed in the preceding example. 



The role of inlets in Florida has been well documented in two cases. 

 The entrance to St. Andrews Bay was cut in 1934 on a previously stable 

 beach. Over the next 50 years, the beach receded at a maximum rate in 

 excess of 2 m/yr where accretion of 1 m/yr had occurred prior to cutting 

 the inlet. Fig. 7.8. The second example illustrates both the adverse 

 effect of cutting the entrance to Port Canaveral in 1951 and the beneficial 

 effects of a beach restoration project carried out in 1974. Again as shown 

 in Fig. 7.9, a beach that had been stable previously underwent dramatic 

 erosion immediately downdrift (south) of the inlet. 



Weggel (1986) has examined the economics of beach nourishment under 

 the scenario of a rising sea level. Methods were presented for computing 

 the present worth costs of perpetual renourishment for sea level rising at 

 a uniform rate and projects of limited life (e.g., 50 and 100 years) for 

 increasing sea level rise rates as predicted by Hoffman (1984) . The 

 tradeoff between renourishment (repeated costs) and stabilizing structures 

 (initial cost only) was examined and, based on the reduced required 

 frequency of renourishment due to the structures, the justified cost of 

 structures is presented. It is concluded that perpetual beach nourishment 

 is not economically justified under the sea level rise rates predicted by 

 Hoffman. 



7.3 PHYSICAL PRINCIPLES 



There are two general types of considerations that can be applied to 

 beach profile response. Kinematic considerations relate to sand budget 

 components regardless of the causes of the transports and associated 

 forces. Dynamic considerations relate to the forcing mechanisms. Each of 

 these will be discussed briefly in the paragraphs below. 



