LTFATE Cohesive Sediment Transport Model

Technical Note DOER-N1

April 1998

LTFATE Cohesive Sediment

Transport Model

PURPOSE: The Long-term Fate of Dredged Material (LTFATE) model is a combined local

hydrodynamics and sediment transport model used to determine the long- and short-term stability

of dredged material mounds. This technical note (TN) introduces a new cohesive sediment transport

submodel for LTFATE, which includes a combined current-wave shear stress calculation and a

layered sediment bed model. LTFATE can be accessed via the Internet at the following address:

http://www.wes.army.mil/el/elmodels.

BACKGROUND: At some open-water placement sites, transport of fine-grained sediments

outside the site can be a concern. First, fine-grained sediments often are cohesive and attract

hydrophobic contaminants such as polychlorinated biphenyls (PCBs). The transport from dredged

material mounds comprised of these sediments is significant because of the potential effects of the

associated contaminants. Second, even uncontaminated fine-grained sediments can cause negative

biological or aesthetic impacts if sufficient quantities are transported into sandy areas.

Unlike sands, which tend to settle back to the sediment bed rather rapidly, cohesive sediments

remain in the water column for significantly longer time periods and therefore can be transported

farther. This can result in significant bed contamination a considerable distance from the original

mound location. It is therefore important to determine the stability of a mound, i.e., those conditions

under which sediments in a mound remain in place. LTFATE (Scheffner et al. 1995; Scheffner

1996) was developed at the U.S. Army Engineer Waterways Experiment Station (WES) originally

for simulating sand transport from dredged material placement sites. Sand often is used to cap

contaminated sediments. However, before capping occurs, cohesive mounds are exposed to the

water column and could experience sediment transport. In addition, due to the additional cost

sometimes associated with obtaining sand for capping, there is interest in using suitable fine-grained

cohesive sediments as cap material for open ocean sites. Therefore, LTFATE was expanded to

include fine-grained, cohesive silt and clay transport. However, the original LTFATE cohesive

sediment sub-model was basic and could not simulate the effects of large storms accurately. Thus,

the improved cohesive sediment transport sub-model described in this TN has been developed.

Unlike sands, the interparticle forces of fine-grained sediments (due to their small mass) are

significant when estimating transport processes. Sand erosion can be relatively simply related to

the grain size. This is not true for fine-grained cohesive sediments. Several factors, including grain

size distribution, mineralogy, bulk density, and organic content have been demonstrated to signifi-

cantly affect the erosion rate. In fact, sediments that at first glance may seem similar may have

orders of magnitude difference in their erosion rates (Lavelle 1984). Because of the various

processes influencing erosion, the rates tend to decrease with depth below the sediment/water

interface even for sediments of consistent grain size and mineralogy.