where t is the total bottom shear stress due to currents and waves, pw is the density of 

 water, g is the acceleration of gravity, Vwc is the perceived bottom velocity due to currents 

 and waves, and Cz is the Chezy roughness coefficient. This method of calculating the shear 

 stress compares favorably to more complex combined current/wave approaches like 

 Christoffersen and Jonsson (1985), generally agreeing within 20%. However, this method, 

 like the others, is influenced by bottom roughness parameters. These parameters were not 

 measured for the sediments of interest and the results may change significantly depending 

 on their values. Bottom roughnesses for typical ocean sediments were used in the absence 

 of acmal data from the PDS. 



The factors influencing the resistance of a cohesive sediment bed to erosion may be 

 best described by Ariathurai and Krone (1976) as: "(1) the types of clay minerals that 

 constitute the bed; (2) structure of the bed (which in mm depends on the environment in 

 which the aggregates that formed the bed were deposited), time, temperature, and the rate 

 of gel formation; (3) the chemical composition of the pore and eroding fluids; (4) stress 

 history, i.e., the maximum overburden pressure the bed had experienced and the time at 

 various stress levels; and (5) organic matter and its state of oxidation." It is obvious from 

 this description that the resistance of the bed to erosion will be different not only from site 

 to site, but also potentially with depth at a given location. Therefore, erosion potential is 

 usually considered a site specific function of shear stress (and sometimes depth). Methods 

 have been developed to determine erosion based on stresses, but these equations require 

 parameters whose values are site specific. A commonly used metiiod of relating erosion to 

 shear stress has been incorporated into LTFATE. This method relates erosion as a function 

 of shear stress to some exponential power. The equation for the erosion rate, e , in g/crn^ 

 /sec is: 



e = Ar 



(21) 



where Ao and m are site specific parameters which vary with depth (and are usually 

 determined by laboratory or field experiments on the sediments of interest), x is the shear 

 stress due to currents and waves, Tcr is the site specific critical shear stress below which no 

 erosion occurs (assumed to vary with depth), and Xr is a reference shear stress (assumed to 

 be 1 dyne/cm^. Most research on cohesive sediment erosion has been performed in 

 laboratory settings at moderate shear stresses less than 20 dynes /cm^ (Lavelle et al. 1984). 

 The above described method was developed for moderate stresses. Data for high shear 

 stresses are sparse and the experimental methods are still under development (McNeil et al. 

 1996). Despite limitations in the understanding of high shear bed erosion, a lot can be 



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



