J4 



demonstrated that there is a critical percent of cohesive sediment in mud/sand bed mixtures 

 above which the muds form a network of strong chemical bonds and the bed must be 

 treated as a cohesive sediment bed. Below this critical percent, the mixture can be treated 

 as non-cohesive (Toorman, et al., 1995). Due to the nature of cohesive sediments, it does 

 not take a large percent mixed with sands before the mixtore starts behaving in a cohesive 

 manner. Whitehouse, et al. (1995) reports the amount at 3-15% fmes by weight, depending 

 on the types of mud and sand in the mixture. Therefore, choosing correct parameters for 

 the cohesive sediment equations can accurately represent a bed comprising of a significant 

 portion of sand mixed with the silt and clay. 



After selecting the type of sediment to be modeled, the appropriate parameters for 

 the sediment erosion rate equations needed to be selected. Unfortunately the grain size 

 distribution data (collected at the PDS and Portland Harbor) alone can not be used to 

 further refme the erosion potential parameters in equation 21. Therefore other data were 

 used to define erosion potential parameters. The suspended solids concentrations measured 

 by SAIC were near bottom values. It is not possible to compare directly near bottom 

 concentrations (field data) and vertically averaged concentrations from the model. The 

 former tend to be much larger because the water column is not, in general, well mixed at 

 these depths. To convert the field data to total water coltmm load (i.e., vertically averaged 

 concentrations) would require an understanding of the distribution of suspended sediments 

 above this near bottom layer. However, the near bottom data do provide a qualitative 

 understanding of conditions for which erosion occurs and a first estimate of the magnitude 

 of erosion to compare to model output and to assist in estimating values for parameters Ao, 

 Xcr, and m in equation 21. 



For application of LTFATE to the PDS, the value of exponent m was set at two, a 

 reasonable value for ocean sediments (Partheniades, 1965). The value of coefficient /lo 

 generally changes dramatically with depth, often over orders of magnimde (Ziegler and 

 Lick, 1986). The values are larger for the surficial sediments and decrease with depth 

 reflecting the greater resistance to erosion of the more dense, deeply buried sediments as 

 well as the effects of sediment armoring on increased erosion resistance. Table 1 shows 

 values of ^0 used for all PDS simulations. Similarly, the values of the critical shear stress, 

 Xcr, increase with depth to reflect the increased resistance to erosion. The surficial layer 

 sediments are often recently deposited and are kept in a less dense, loose state by such 

 factors as bioturbation and the agitation of currents and waves at the sediment/water 

 interface. These sediments have a critical shear stress less than 1 dyne/cm^ and are easily 

 resuspended. Higher values of Xa between 2 and 10 dyne/cm^ for sediment buried more 

 than a few inches are typical for well consolidated ocean sediments. Table 1 shows values 

 of Tcr used for all PDS simulations. The value of Xcr chosen for the first two layers of 

 sediment resulted in noticeable resuspension under 2-3 m waves for the events during the 



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



