18 



for the January 1979 event would be approximately 20% greater if no re-deposition of 

 locally eroded sediments occurred. 



The values of Ao presented for the lower layers of sediment in Table 1 are 

 reasonable for well consolidated open ocean sediments, but there is a possibility that the 

 PDS sediments, especially those recently placed as part of maintenance dredging, are more 

 easily eroded than, for example, San Francisco Bay sediments from which the value of Ao 

 for the lowest layer was derived (Parthenaides, 1965). To reflect this possibility, the 

 storms were simulated again assuming the value ofAo for each layer except the top two 

 (which were calibrated with the TSS data) was five times the value presented in Table 1 

 (four times the value for layer 3) and the bottom layer was combined with the seventh 

 layer above it. The values for Ao for these simulations are presented in Table 3. As would 

 be expected, the erosion amounts were significantly higher. The predicted erosion 

 amounts, however, were not five times as great due to the layers reflecting increased 

 resistance to erosion. The results of these simulations are presented in Table 4. The 

 maximum erosion for any storm using the Table 3 parameters is 0.22 m during the 1978 

 34 day event (Figure 7). The other storms of record experienced approximately 0.09-0.1 m 

 maximum erosion. The total volume of erosion predicted for the 1978 event is 2.6x10^ m^ 

 and half or less of this amount for the other storms. Agaia, these erosion depths and 

 volumes would be reduced if off-site sediment were advected to the PDS. 



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



