Predicting erosion magnitude and rate

Predicting erosion magnitude and rate

Predicting erosion thicknesses, which consists of computing a resuspen-

sion rate (the volume or mass of material put into movement by the cur-

rents per unit of time and area), net transportation rate (how fast is the

sediment mass or volume moved horizontally), net transportation gradient

(is more sediment moving out of a given area than moving in), and the

duration of the erosion, is a difficult task that requires a sophisticated

numerical model to obtain reasonable results at an open-water site.

Erosion of fine-grained cohesive sediments is even more complicated

than for cohesionless particles because of interparticle forces (i.e., cohe-

sion), the fact that cohesive forces can vary with depth (i.e., become more

erosion resistant), cohesive forces are time dependant (density and cohe-

sion increase with time), and other factors (e.g., salinity). In contrast,

cohesionless sediments are considerably simpler because the erosion resis-

tance does not change with depth, time, or sediment chemistry. Thus,

modeling erosion of cohesive sediments is much more difficult than for

cohesionless sediments.

A model was developed as a part of the USACE Dredging Research

Program (DRP) to evaluate the long-term fate of a mound, i.e., mound

stability over periods ranging from months to years (Scheffner 1991a,b).

This model is called the Long-Term FATE of dredge material (LTFATE)

model (Scheffner et al. 1995). In LTFATE, hydrodynamic conditions at a

site are considered using simulated databases of wave and current time se-

ries or actual wave and current data as driving forces. These boundary

conditions are used to drive coupled hydrodynamic, sediment transport,

and bathymetry change models that predict erosion of dredged material

mounds (of specific dimensions, grain size, and water depth) over time.

LTFATE uses empirically derived methods to estimate either noncohesive

(Ackers and White 1973) or cohesive (Lavelle, Mofjeld, and Baker 1984)

sediment resuspension, transport, and deposition. Results from this model

indicate whether a given site is predominantly dispersive or nondispersive

and predict potential erosion and migration of a mound for the given cur-

rent and wave conditions, mound geometry, and sediment characteristics.

Typical results from the model are shown in Figure 27. Appendix F de-

scribes the model in more detail by providing background, major assump-

tions and limitations, input requirements, and sample output.

The LTFATE model has recently been applied in hindcasting the stabil-

ity of a capped mound located in the Mud Dump site, a designated ocean-

disposal site in the New York Bight, during a severe storm that occurred

in December 1992 (Richardson et al. 1993). In this application, wind and

wave data from a directional wave buoy operated by the National Data

Buoy Center of the National Weather Service, data on current and tidal

fluctuation from a verified Bightwide numerical hydrodynamic model, and

data on historical storm and surge effects in the area were used to develop

bottom currents for a range of storm-induced conditions at the proposed

capped mound location. The model was used to predict the magnitude of

resulting cap material erosion. Long-term stability of the mound was also

evaluated using empirical criteria from nearshore berms to determine the

potential for significant movement of the overall capped feature using

criteria from other monitored sites. This study provides a model for

Chapter 8 Long-Term Cap Stability