HYDRAULIC MODEL STUDY FOR BOSTON OUTFALL .2. ENVIRONMENTAL PERFORMANCE

Hydraulic model tests were done on the Boston Harbor out fall diffuser with the number of risers and their spacing maintained constant at their final design values. It was found that eight ports per riser resulted in higher dilutions than 12, because with 12 ports the plumes merged and collapsed to a single rising column. Extended duration tests showed that the waste field was stable, that its thickness increased very slowly with time, and that dilution decreased very slowly with time. The flux-average dilution was estimated from the movement of dye streaks to be only 1.1-1.2 times higher than the minimum dilution, a much smaller ratio than usually assumed. The mathematical model ULINE was found to be generally conservative in predicting dilution. Better predictions were obtained with a new mathematical model RSB, which well simulated the observed waste-field characteristics for the final design. The effect of varying the number of risers was not so well predicted by RSB, however, due to the very large effective port spacing for this diffuser. Thus, hydraulic model tests are still needed for atypical diffuser designs, such as encountered here.