TRACER STUDY TO INVESTIGATE ISSUES RELATED TO DREDGING OF FLUID MUD


The U.S. Army Corps of Engineers (Corps), New Orleans 
District, is responsible for the maintenance dredging of the 
Atchafalaya Bar Channel. The rapid accumulation of high 
water content, fine grained sediments in the channel has 
been a persistent problem at the site and the Corps has 
undertaken numerous studies to gain a better 
understanding of the source and movement of these 
sediments. The Corps identified the following three specific 
questions to be addressed by a tracer study: 

a) Is sediment placed by the dredge at the west and east 
side dredged material disposal sites transported back 
into the channel over short time periods as suspended 
solids? 

b) Is the dredger resuspending large quantities of fluid mud 
that are redistributed throughout the mixing zone over 
several tidal cycles? 

c) How effective is the dustpan dredge at removing 
sediments from the channel? 

Tracer Being Injected into the Dredge Discharge 
SCOPE AND APPROACH 

EHI and its subcontractor, Environmental Tracer Systems 
(ETS), developed a two-week tracer study to address these 
questions. The study included the use of three silt tracers 
of different colors placed at specified locations and the 

ATCHAFALAYA BAR CHANNEL, LA 

monitoring of the movement of these tracers using 
sediment traps, water samples, and sediment samples. 
The first tracer was 
placed directly into the 
dredge discharge over 
an 8-hour period, the 
second tracer was 
placed in sediments in 
the channel just prior to 
dredging, and the third 
tracer was placed to the 
east of the channel to 
act as a control. 

Sediment Trap 
RESULTS 

The tracer results clearly showed the following: 

• A portion of the dredged sediments are rapidly
transported back into the channel as suspended 
sediment. 
• Deposited material is likely to be eroded/entrained & 
transported back into the channel even in low energy 
tidal conditions. 
• The dredging system used did not resuspend 
significant volumes of sediment in the channel during 
the dredging operation. 
• The dredging system was effective at removing 
sediments from a given portion of the channel in close 
proximity to the dredge head. 
Tracer definitively showed that the channel is an extremely 
effective sediment trap and that even though long-term 
sediment transport may be to the west, if sediment 
encounters the channel as it moves back and forth with the 
tidal currents, it will likely be captured by the channel and 
remain there. 

Project sponsors reported that the tracer study proved very 
valuable in gaining better comprehension of sediment 
transport at the site.