The combined surge -tide events were analyzed at the selected locations in 

 the computational grid for which a frequency of occurrence was desired. Since 

 the total surge elevation for each storm event at a specific area was known, a 

 stage -frequency relationship could be computed for each storm from the 

 ensemble of indexed storm events. The goal of the dune erosion study was to 

 randomly select a storm of a given total surge and frequency of occurrence and 

 subject it to a dune of a specified configuration and composition. 



DUNE RECESSION-FREQUENCY OF OCCURRENCE RELATIONSHIPS 



The two requirements for generating dune recession-frequency of occurrence 

 relationships are: (1) a dune erosion model which computes erosion as a 

 function of a specified storm surge hydrograph, and (2) a data base of 

 frequency- indexed storm surge hydrographs . The mechanics of generating 

 recession-frequency curves are described in the following example: 



1. Offshore Profile. A location for which a recession-frequency diagram 

 is to be computed must be selected. Offshore profiles should be 

 available in order to compute an equilibrium profile coefficient 

 according to Equation 1. If ba thyme trie data are not available, the 

 coefficient can be estimated from the grain size relationship shown in 

 Figure 2. Example profile 286, shown in Figure 5, is located just 

 north of Manasquan Inlet, New Jersey. The equilibrium profile shape 

 coefficient A should be chosen such that the computed profile best 

 fits the actual profile. The fact that the computed profile does not 

 explicitly represent bar formations is not a serious limitation of the 

 model since the offshore computation is only intended to provide a 

 total volume of either erosion or deposition. Since the volume is 

 used to compute time -varying erosion of the dune and berm face, only 

 the total magnitude of offshore erosion or deposition is of impor- 

 tance. A value of A = 0.236 ft 1/3 was computed for the example 

 profile. 



2. Schematic Dune and Berm Configuration. Each dune and berm configura- 

 tion must be schematized according to the definitions shown in 

 Figure 3. In the example, pertinent data are: h(b) = 8.5 ft, h(d) = 

 20.0 ft, M(b) = 0.131, M(d) = 0.110, and W(b) = 65.0 ft. 



3. Stage -Frequency Diagram. A stage-frequency relationship is required 

 for the selected area. These diagrams can usually be obtained from 

 existing literature. If a relationship is not available, an inter- 

 polation between gage sites for which the stage-frequency diagrams are 

 available may be acceptable. If the study area is not conducive to 

 interpolation, a relationship should be constructed based on 

 historical data. For the present application, stage -frequency 

 diagrams computed for the Long Island study were used. Diagrams for 

 both hurricanes and northeasters, corresponding to three tide gage 

 locations, are shown in Figures 6 and 7. 



139 



