Mixing Zone Diffuser Port Velocities
Important considerations for outfall design are the operational and maximum criteria used for diffuser port discharge velocities in the mixing zone analysis. The EPA, in Technical Support Document for Water Quality-based Toxics Control (TSD, March 1991), and other sources have suggested that an high port velocity of about 3 meters-per-second, i.e., about 10 feet-per-second (fps), can be used as a guide (see TSD Section 4.3.3, Page 71). To administer NPDES permits and evaluate MZ studies, many States rely on this passive EPA water quality guidance which does not include physical effects to fish and wildlife. When maximum diffuser port velocities are not realistically taken into account, i.e., actual port velocities in the field are disregarded, the result has been considerable misapplication of mixing zone design.
Generally, the greater the diffuser port velocity, the greater the dilution. So it follows that when operational and maximum port velocities are undefined the tendency for some diffuser designers and MZ modeling analysts is to artificially inflate port velocities, and dilutions, to unrealistic levels.
Excessively high port velocities that would cause channel bottom erosion, with mixing of discharge contaminants in bottom sediments, are frequently used in mixing zone analyses whether or not actually caused by diffuser operation in the field. In some instances, velocities used in mixing zone analysis have been extreme enough, hence exaggerating dilution, to cause cavitation. This physical condition would damage diffuser nozzles if actually occurring in the field.
An additional concern is for upstream migrating salmon, which may be adversely attracted to high velocities emanating from the diffuser discharge ports while searching out entrances to stream tributaries. This article distributes NOAA Fisheries' comments regarding maximum diffuser port velocities, which indicate velocities should be less than 8 fps to protect salmon and steelhead in the Columbia River.
Here we provide two important NOAA fisheries comments on Oregon DEQ permits that relate directly to upper limits on diffuser port velocities. The first is NOAA Fisheries' May 2003 comment on the Wauna Mill NPDES draft permit renewal on the Columbia River (May 2003, note 1); and the second is NOAA NMFS' comment on the Portland NPDES permit modification (December 1997, note 1 on page 2).