Overview of LTFATE

Overview of LTFATE

LTFATE is a site-analysis program that uses coupled hydrodynamic, sedi-

ment transport, and bathymetry change models to compute site stability over

time as a function of local waves, currents, bathymetry, and sediment size.

LTFATE was developed to simulate the long-term fate and stability of dredged

material placed in open water with an initial intended use for classifying existing

or proposed disposal sites as dispersive or nondispersive. If the site is demon-

strated to be dispersive, model output will provide an estimate of the temporal

and spatial fate of the eroded material. This determination is often difficult to

quantify because the movement of sediment is a function of not only the local

bathymetry and sediment characteristics, but also the time-varying wave and

current conditions. LTFATE overcomes these difficulties by using an informa-

tion database to provide design wave and current time series boundary condi-

tions that realistically represent conditions at the candidate disposal site.

The wave simulation methodology and the elevation and current databases

referenced in this report were developed through the Dredging Research Pro-

gram (DRP) at the U.S. Army Engineer Waterways Experiment Station (WES).

The procedures for generating stochastic wave height, period, and direction time

series are reported in Borgman and Scheffner (1991). The database of tidal

elevations and currents for the east coast, Gulf of Mexico, and Caribbean Sea are

described in Westerink, Luettich, and Scheffner (1993), and the database of

tropical storm surge and current hydrographs is reported in Scheffner et al.

(1994). These data are used to generate wave and current boundary condition

data for use as input to LTFATE for evaluating mound stability. If these data-

bases are not available for the geographic area of interest to the user, then

replacement input files will have to be supplied by the user and copied into the

appropriately designated files.

Noncohesive mound movement

The LTFATE model uses four coupled subroutines to predict dredged mate-

rial movement of various types of noncohesive material during different stages

of mound evolution. These subroutines simulate hydrodynamics, sediment

transport, mound cascading, and bathymetry change. LTFATE uses the equa-

tions reported by Ackers and White (1973) as the basis for the noncohesive

sediment transport model. The equations are applicable to uniformly graded

noncohesive sediment with a grain diameter in the range of 0.04 to 4.0 mm

(White 1972). Because many disposal sites are located in relatively shallow

water, a modification of the Ackers-White equations was incorporated to reflect

an increase in the transport rate when ambient currents are accompanied by

surface waves. The modification is based on an application of the concepts

developed by Bijker (1971) and enhanced by Swart (1976). This preliminary

model was verified to prototype data by Scheffner (1991) and was shown to be a

viable approach to providing quantitative predictions of disposal-site stability.

Appendix F LTFATE Model