1.0 INTRODUCTION 



The USAGE New England District (NAE) is currently analyzing resuspension 

 potential of sediments at the Portland Disposal Site (PDS). This site is used for placement 

 of both contaminated and clean sediments removed as part of maintenance dredging of 

 Portland Harbor, Maine. To effectively manage the site, NAE must determine if the placed 

 sediments are stable, i.e., do they remain essentially in place under severe weather 

 conditions. An understanding of the resuspension potential of these sediments as well as 

 any proposed cap is necessary before decisions can be made concerning future sediment 

 placement or cap design. The Long Ter m FATE (LTFATE) model (Scheffher, 1996) has 

 been modified and applied to the PDS to assist in predicting the stability of the placed 

 sediments. This study is one component of the capping investigations being conducted by 

 NAE. Sediments at the PDS lie in water depths from 40 m to 70+ m. The site is 

 characterized by rough bottom terrain, and is exposed to unlimited fetch from the east to 

 the south. Because of this exposure, the PDS is directly impacted by both tropical and 

 extra-tropical wave events causing bottom sediments to experience significant wave 

 generated shear stresses. It will be shown that under normal conditions the site is stable, 

 but despite these depths, storm waves from low frequency (long return period) events have 

 potential to cause moderate erosion that needs to be factored into cap design and 

 monitoring. 



LTFATE is a disposal site-analysis program that uses coupled two-dimensional 

 hydrodynamic, sediment transport, and bathymetry change sub-models to compute site 

 stability over time as a function of local waves, currents, bathymetry, and sediment 

 characteristics. 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. The model estimates the site 

 stability for time periods ranging from days (for storm events) to years (for ambient 

 conditions). If the site is demonstrated to be dispersive, model output will provide an 

 estimate of the temporal and spatial fate of the majority of eroded coarse-grain cohesionless 

 material. The ultimate fate of the majority of fine-grain cohesive material is not estimated 

 by LTFATE because these sediments can disperse over a large area beyond die boundaries 

 of the localized LTFATE model. LTFATE will, however, estimate the local deposition of 

 these cohesive sediments but this will usually be only a small fraction of. the total amount 

 resuspended from the site. The prediction of fine-grain sediment transport is further 

 complicated, as will be described later, by the significant variation in erosion rates with 

 depth below the sediment/water interface when compared to coarse-grain sands which 

 erode at relatively consistent rates with variation in depth. 



A Predictive Model for Sediment Transport at the Portland Disposal Site, Maine 



