1 1. Buoy update - progress 2. Plume dilution - basics OMSAP 6/29/09 Mike Mickelson, MWRA

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11/20 Data from NOAA NWS NDBC buoy (The Boston BF buoy) and the GoMOOS A buoy operated by U Maine and also designated as NOAA 44029

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13/20 2.Plume dilution - basics: Effluent plume dynamics in relation to outfall monitoring sampling design Don't sample too close or too far Within the "initial mixing zone" the plume is too patchy to sample. –Initial mixing occurs within about 10 minutes, and about 60 meters distance from the diffuser. –At that time the plume has diluted to about 100:1 The plume then becomes more uniform, and dilutes slowly over a few hours. The plume disperses and breaks up, and is undetectable after a few days, or about 20 km away from the outfall.

14/20 ocean currents Dilution in the near field due to self-induced turbulence. Dilution in the far field due to horizontal buoyant spreading and ambient turbulence. Mixing zones: Regulatory Hydrodynamic Zone of initial dilution Terminology: Mixing Zones Near fieldFar field Transition: Buoyant spreading Internal jump Diffuser Effluent plume plume rise height from Phil Roberts

15/20 stronger ocean currents. perpendicular to diffuser axis Near field dilution increases with several factors Lower effluent flow per unit length of diffuser higher rise of the plume

16/ m 0 20 m Laser imagery of dye in a scale model plume (distance legend is scaled and approximate) "0.010 C/Co" means plume concentration C is 1% of effluent concentration Co, i.e. 100:1 dilution from Phil Roberts

17/20 Processes and Scales for Ocean Outfalls Near fieldFar fieldTransitionLong-term flushing Spatial scale (m) Time scale (min) km 1 day1 week1 month 10 km100 km 1 hour Advection by ocean currents Ambient diffusion Buoyant spreading Large-scale flushing; upwelling; sedimentation Jet and plume turbulence; internal jumps from Phil Roberts

18/20 24-Hour trajectories from Brad Butman and Rocky Geyer

19/20 Plume Tracking Survey Dye results Plume moved to the southeast over 2 day survey Effluent Dilution of ~1000 in far field

20/20 Conclusions Based on MWRA data, EPA was able to certified that the outfall diffuser effectively dilutes effluent just as it was designed to do. Measurements closer than about 100m from the diffuser yield noisy data. Outfall effects are unlikely beyond 20 km from the diffuser. The plume can travel in any direction due to low-frequency currents driven by wind and river runoff Residual currents on the scale of weeks also affects circulation The plume is gone before algal abundance can increase measurably

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