determined by using the moderate shear equations in high shear regions. It would appear 

 from bathymetry measurements in high shear regions, that equation 21 can adequately 

 simulate these conditions. The shear stresses experienced at the PDS range from low 

 during ambient conditions to moderately high during storm events. 



It should be noted that the values of the site specific parameters used in these 

 methods can vary significantly. Experimentally determined values of Ao range over several 

 orders of magnitude from 1x10"^ to 5x10"^ {glen? /sec) and m ranges from 1 to 5 (Lavelle et 

 al. 1984). The experimental range of exponent m values coupled with the equation for x 

 demonstrate that the relationship between velocity and erosion is highly nonlinear (t is 

 approximately a function of V^ and g is a function of t" resulting in e approximately a 

 function of V^). Therefore, rare storm events will often produce most of die cohesive 

 sediment erosion for a given year. This is well known to occur in many rivers, lakes and 

 near shore environments. Some studies on San Francisco Bay sediments suggest that m 

 ranges from 1-2 for these sediments, assuming long consolidation periods (Parthenaides 

 1965). Higher values of m are reserved for freshwater lake and river sediments. The values 

 of Ao and Tcr for cohesive and mixed sand/cohesive sediments are also known to vary 

 significantly with depth below the sediment/water interface (McNeil et al., 1996). This 

 variation is due to the effects of agiug and consolidation of the sediment layers. These 

 effects must be accounted for when modeling the sediment bed. Choosing a single value 

 for Ao or Xcr for all vertical layers of the sediment bed will result in either excessive erosion 

 during large storms (if the values are based on the surficial sediment characteristics) or 

 insufficient erosion during moderate events (if the values are based on deeply buried 

 sediment characteristics). Choosing appropriate values for these parameters is essential to 

 accurate sediment erosion prediction. Methods used for parameter selection for the PDS 

 will be described in the next section. 



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



