15_ 



period February-May 1996. These results are in agreement with the SAIC turbidity data. 

 Some resuspension was calculated for lesser wave events, but the resulting concentrations 

 were negligible. The parameter values for the lower layers shown in Table 1 are 

 reasonable estimates for fairly well consolidated cohesive sediments below the surficial 

 layer. To determine values more accurately for the PDS site would require extensive 

 testing of the proposed cap sediments to determine resuspension potential. Without such 

 data, the above mentioned values represent a reasonable first estimate for the change in 

 sediment characteristics within the upper one to two feet of sediment and fall within the 

 expected experimental range (Lavelle, 1984). 



As previously mentioned, the values of Ao and Xcr for the fost two layers presented 

 in Table 1 were chosen based on a comparison of the SAIC data derived suspended solids 

 values for April 6-18 to LTFATE model output for the same period. Various values of the 

 parameters within the experimental range were tested and those that seemed to compare 

 reasonably well to the field data were selected. The results of the simulations for the April 

 6-18 period indicated that using either current inputs, SAIC current meter data or ADCIRC 

 simulation results, produced similar magnitudes of erosion. The total erosion for the two 

 simulations differed less than 8%. This is because the dominant force in the sediment 

 resuspension is the wave generated stresses. The current velocities from the two sources 

 were similar in magnimde but varied significantly in direction. Therefore the direction in 

 which the sediment plmne traveled and thus the TSS concentrations at a specific location 

 varied between the two simulations. Both simulations resulted in peak vertically averaged 

 suspended solids concentrations for the first of three events in the calibration period 

 (maximum wave height of 3.45 m) of approximately 4 mg/1 at the site of the SAIC 

 measurement tripod. This is significantly less than the approximately 65 mg/1 (maximum) 

 measured by the near bottom turbidity instrument, but, as previously stated, a direct 

 comparison is not possible because the field data are near bottom. Vertically averaged 

 concentrations (model output) will be significantly lower than near bottom concentrations 

 and an order of magnimde reduction was considered an acceptable match for these 

 simulations. Therefore 4 mg/1 concentrations vertically averaged over the entire water 

 column may indicate close to 65 mg/1 near bottom concentration. Similarly, LTFATE 

 model simulations predict maximum TSS concentrations of approximately 1 mg/1 at the 

 tripod during the second event on April 10, compared to 8 mg/1 maximum from the field 

 data. Maximum vertically averaged concentrations for any location during the simulation 

 were approximately 25 mg/1 near the center of the LTFATE PDS configuration. 



LTFATE simulations indicate an order of magnitude less maximum TSS 

 concentrations at the tripod during the third storm (maximum wave height of 5.38 m) in 

 the April 6-18 calibration period when compared to the first event. The SAIC data indicate 

 maximum concentrations of approximately 45 mg/1, compared to only 0.3 mg/1 for 



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



