Recommended Erosion Frequency Procedure

To perform the EST, historical storms impacting a site are broken down into

the parameters that impact the engineering aspect of interest: storm track, maxi-

mum winds, radius to maximum, pressure deficit, etc. These variables are

termed input vectors. The storm response of interest, in this case vertical erosion

of the capped mound, is also calculated for each historical storm using an

appropriate model (in this case LTFATE is used). The response of interest is

referred to as response vector. During EST simulations, N-repetitions (say 100

or more) of T-year responses (say 100 to 200 years) of the response vector of

interest (vertical erosion for capping projects) are produced providing mean

value frequency relationships with accompanying confidence limits such that

probability of occurrence can be defined with error band estimates. In other

words, the mean vertical erosion for a range of return intervals with confidence

limits (based on the number of standard deviations) are produced by the EST

procedure.

There have been a number of applications of the bootstrap method and EST

to coastal problems. Prater et al. (1985) described error estimation in coastal

stage frequency curves for Long Island. Mark and Scheffner (1993) discuss use

of the EST to compute frequency of occurrence of storm surge elevations in

Delaware Bay. Farrar et al. (1994) describe the use of EST to estimate the fre-

quency of horizontal beach erosion as part of an economic analysis for design of

beach fills at Panama City, FL. Most recently, the EST technique was used to

predict frequency of vertical erosion estimates for capped mounds at a range of

depths at the Mud Dump disposal site located east of Sandy Hook, NJ (Clausner

et al. 1996). The work was part of a larger effort for the New York District to

determine remaining capacity of the Mud Dump site for both suitable sediments

and those requiring capping.

Application of the EST to a capping project involves a series of sequential

steps to calculate the cap erosion thickness, which are described in the remainder

of this appendix.

Recommended Erosion Frequency Procedure

To define the required cap erosion layer thickness as a function of depth at a

specific site, first the erosion frequency must be determined. It consists of a site-

specific quantitative analysis approach that requires the completion of several

sequential tasks. These tasks are (a) selection of appropriate storm events,

(b) development of storm surge elevation and current hydrographs for each

event, (c) development of four tidal phase elevation and current hydrographs,

(d) development of a wave height and period time series corresponding to each

storm event, (e) generation of input files representing the combination of tasks

2-4 to the Long-Term Fate of Dredged Material (LTFATE) model used to pre-

dict erosion, (f) execution of the LTFATE model to determine maximum vertical

erosion at the site as a result of each of the storm events, (g) development of

input files for the Empirical Simulation Technique (EST) program to generate

multiple repetitions of storm-event activity and the corresponding vertical

erosion, and finally, (h) using the EST program to generate vertical erosion

Appendix G Procedures for Conducting Frequency-of-Erosion Studies