Wave data necessary for these applications are derived from the Wave Information 

 Study (WIS) hindcast for the Atlantic Coast (Brooks and Brandon, 1995). An outline of 

 derivation of specific LTFATE inputs is included later in this text. Tidal current and tidal 

 elevation data necessary for boundary condition inputs to LTFATE simulations are 

 developed using the ADCIRC ocean circulation model. The ADCIRC model is described 

 in Luettich et al. (1992), the database of tidal elevations and currents for die east coast. 

 Gulf of Mexico, and Caribbean Sea are described in Westerink et al. (1993), and the data- 

 base of tropical storm surge and current hydrographs is reported in Scheffner et al. (1992). 

 The ADCIRC east coast grid (Figure la) was refined (resolution increased) in the region 

 near New England so it could more accurately predict currents and elevations at the PDS. 

 Figure lb shows the ADCIRC grid detail near the PDS. 



LTFATE has the capability of simulating both non-cohesive and cohesive sediment 

 transport. In addition, consolidation of cohesive sediments is accounted for to more ac- 

 curately predict physical processes which occur at the site. Many sediment transport 

 equations require near bottom velocities, but the methods incorporated in LTFATE were 

 developed and work well using vertically averaged velocity of flow reflective of conditions 

 outside the wave and current boundary layers. Unlike near-bottom velocities, vertically 

 averaged velocities are not significantly effected by bottom roughness. This is an advantage 

 in regions where bottom roughness is unknown or continually changing (however, as will 

 be described later, uncertamties in the bed roughness estimates will contribute to 

 imcertamties in erosion rates). Following are sections describing the effects of waves on 

 the sediment/water interface, non-cohesive sediment transport, cohesive sediment 

 transport, and application of LTFATE to die PDS. 



1 . 1 Effect of Waves at SedimentAVater Interface 



Most non-cohesive sediment transport equations are developed for a current only 

 environment. Areas of interest where LTFATE is applied normally include bottom stresses 

 due to both currents and waves. Therefore die effects of waves must be included in 

 estimatmg sediment transport. A modification of the transport equations proposed by 

 Bijker (1971) is incorporated into LTFATE to reflect an increase in the transport rate if the 

 ambient currents are accompanied by surface waves. The modification, in the form of an 

 effective increase in the depth-averaged current velocity used to compute sediment 

 transport, is based on equations reported by Swart (1976). This increased velocity can be 

 thought of as the current velocity that would produce a bottom stress equivalent to the 

 stress due to the combined effects of ambient currents and waves. The effective increase in 

 velocity for currents accompanied by waves Vwc , is written as a function of the current 

 velocity Vc in the absence of waves as follows: 



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



