HYDRAULIC MODEL STUDY FOR BOSTON OUTFALL .1. RISER CONFIGURATION

The hydraulic model study to determine the diffuser design for the Bos ton Wastewater Outfall is described in two papers. The outfall is tunn eled with a diffuser consisting of multiport risers, and in this paper the tests leading to the final choice of the riser configuration are described. The study was done in a large density-stratified towing tan k. It was found that, for zero current speed with a deep thermocline, a thick, laterally homogeneous, submerged waste field was formed. For widely spaced risers, additional mixing in the plumes following collap se caused the minimum dilution to be higher than predicted by simple p lume formulae. As the number of risers is decreased below about 50, th e dilution decreases fairly rapidly, but increasing the number above a bout 50 caused the dilution to increase only slowly. The number of ris ers was chosen to be 55. This resulted in a savings of about $38,000,0 00 in construction costs compared to the preliminary design.