shown in Table 4. On the basis of several tests, it was determined the datum 

 used in survey 2 was in error by 0.5 ft. This is not surprising since the 

 reach of channel surveyed was far away from the tide gages used to locate the 

 datum. The datum for this survey was adjusted accordingly. 



107. The field data were examined in two ways. First, surveys 1, 2, 

 and 7 were used to determine average yearly erosion/deposition rates. At each 

 of the 35 locations along the channel corresponding to computational grid cell 

 centers, depths across the width of the channel were averaged, and the 

 erosion/deposition rates were computed and extrapolated to feet/year values. 

 (Following a similar procedure, but computing the average depth for each cell 

 from 16 spatially distributed points in the cell, yielded results that were 

 close to the results obtained from averaging the cross-section depths). Next, 

 the total period was broken down into three separate periods of approximately 

 4, 2, and 6 months, based on the survey dates. At each of the 35 locations, 

 the erosion/deposition rates obtained for these periods were converted to 

 feet/year values, and the extreme values at each location were determined. 

 Figure 20 is a plot of the average and extreme values from the prototype data 



-15 

 -10 



CO 



Q_ QC -5 



Lu <: 

 a LU 



>- 



I 



E?5 °c 5 

 o 



QC 



10 

 15 



LEGEND 

 PROTOTYPE AVERAGE 

 PROTOTYPE EXTREMA 



-100 -50 



50 100 150 

 DISTANCE, 100 FT 



200 250 300 350 



Figure 20. Prototype data on erosion/deposition rates 



61 



