Selecting the Proper Methodology for Determining Frequency-of-Occurrence Relationships

topographic evolution of dredged material mounds over time periods ranging

from hours to centuries (Scheffner et al. 1995). A detailed description of

LTFATE is found in Appendix F.

The second major modeling advance was the development of a series of

databases containing the hydrodynamic driving force time series needed to run

LTFATE - water levels and currents. Prior to the DRP, obtaining the hydro-

dynamic data to run LTFATE was a virtually impossible task because actual

storm surge elevation and current data are unavailable except for a few recent

storms at selected locations. The water level and current data needed for

LTFATE required modeling tides and their associated currents and storm surges

due to tropical and extratropical storms over a large area. To accomplish the

modeling effort, the DRP funded the development of a state-of-the-art three-

dimensional circulation model, called the advanced circulation model, or

ADCIRC. A series of reports (Bain et al. 1994; Bain et al. 1995; Luettich,

Westerink, and Scheffner 1992; Westerink, Luettich, and Scheffner 1993; and

Westerink et al. 1994) describe the model, its development, and testing.

A primary application of ADCIRC for hydrodynamic input required by

LTFATE was to compute tides and currents for the east and Gulf coasts. The

20,000 point grid over which ADCIRC computed surface elevations and currents

is shown in Figure G1. A companion effort was to compute storm surge levels

and the associated currents for 134 major tropical storms (hurricanes) on the east

and Gulf coasts (Scheffner et al. 1994). A similar effort has also been conducted

for extratropical storms. A comparable effort has been started for the West

Coast (Luettich, Westerink, and Scheffner 1994), but the full suite of data

needed for routine application of LTFATE for erosion-frequency studies on the

West Coast and Great Lakes Coast are not yet available.

Wave data required as input to LTFATE are more readily available, both

from gauges and from the Wave Information Studies (WIS) (Hubertz et al.

1994). The WIS series of reports provides hindcast wave heights, periods, and

directions data at over one thousand coastal sites on all United States coasts for

periods of 20 years or more. WIS wave data are provided at widely spaced

(1 degree of latitude) deep-water sites and closely spaced locations (1/4 degree

of latitude) in shallow water (typically about 10 m). Wave data can be accessed

via a series of WIS reports, more recently electronically via the Coastal Engi-

neering Data Retrieval System (CEDRS) available in Corps Coastal District

offices (McAneny, in preparation), and the data are now available on the internet

(ref).

Selecting the Proper Methodology for Determining

Frequency-of-Occurrence Relationships

There are two methods that have been used by Corps' Districts in coastal

design projects for computing frequency-of-occurrence relationships: (a) limited

historical data and the selection of one or more "design storms" and/or

Appendix G Procedures for Conducting Frequency-of-Erosion Studies