POINTS OF CONTACT - doere120020

ERDC TN-DOER-E12

July 2000

southern sites as the Providence River widened into the Upper Bay. Highest concentrations at the

Fox Point Reach dredging site ranged from 54 mg/L to 150 mg/L, varying approximately linearly

with the release rate. Concentrations dropped to below half that amount within 150 m (500 ft)

upstream or downstream of the site and below one-third that amount within 300 m (1,000 ft) of the

site. The relatively low concentrations in near surface waters peaked every half tidal cycle in

response to TSS buildup during slack water conditions. The absolute highest peak concentrations

occurred at a depth of 10-12 m (33-39 ft) during every tide, with the location of the instantaneous

highest concentrations varying over the vertical throughout the tidal cycle. The ebb and flood plumes

followed the axis of the dredged channel since it was aligned with the main flow direction.

The maximum concentrations at the Bullock Point Reach dredging site ranged from 20 mg/L to

53 mg/L and at the Rumstick Neck Reach site from 22 mg/L to 60 mg/L. Even though the release

rates were the same, the concentration drop-off at 150 m (500 ft) and 300 m (1,000 ft) was less than

at the Fox Point Reach site, indicating more effective dilution. As in the Fox Point Reach, the plume

axis at the Bullock Point Reach was aligned with the channel axis. However, the plume axis was

25 clockwise of the channel axis at the Rumstick Neck Reach site since the current direction was

not aligned with this channel.

Using a 2 percent release rate, a series of multiple release cases was also simulated. There is little

interaction of the adjacent plumes generated by simultaneously dredging at multiple sites since the

typical concentrations where the plumes overlap has dropped to approximately 7 mg/L. In general,

the plume extents are shorter than the separation distance of the release sites. Thus, little

increase in concentrations above ambient would be expected if multiple dredges were operating

simultaneously.

Because decisions regarding the ultimate need for environmental windows rest with representatives

of agencies involved in the dredging project coordination process, no attempt is made herein to

interpret model results to that end. However, it is believed that application of SSFATE clearly

provides a useful source of relevant information upon which to base such decisions. In addition to

providing characterizations of suspended sediment plume dynamics that can be coupled with

available information on responses of fish and shellfish to predicted concentration gradients,

SSFATE can be used to explore numerous alternative dredging project scenarios. Ideally, such

explorations may identify operational measures that pose minimal risk to biological resources, and

thereby maintain maximum flexibility in dredging project schedules.

POINTS OF CONTACT: For additional information contact Dr. Billy H. Johnson (601-634-3425,

johnsob1@wes.army.mil), Mr. Allen Teeter (601-634-2820, teeter@hl.wes.army.mil), and

Dr. Douglas G. Clarke (601-634-3770, clarked@wes.army.mil), or the manager of the Dredging

Operations and Environmental Research Program, Dr. Robert M. Engler (601-634-3624,

englerr@wes.army.mil). This technical note should be cited as follows:

Swanson, J. C., Isaji, T., Ward, M., Johnson, B. H., Teeter, A., and Clarke, D. G. (2000).

"Demonstration of the SSFATE numerical modeling system," DOER Technical Notes

Collection (ERDC TN-DOER-E12), U.S. Army Engineer Research and Development

Center, Vicksburg, MS. www.wes.army.mil/el/dots/doer