Effects of Agitation Dredging in Savannah Harbor
Type:
Presented during:
WODCON XVIII - 2007 Orlando, Florida, USA
Authors:
Way, et al
Abstract: Historically, tracking suspended sediment plumes resulting from dredging operations or passing container ships has been difficult due to the dynamic nature of suspended sediment transport. By using a new acoustic tracking technique, Applied Technology and Management (ATM) was able to accurately study a dredge plume from source to dissipation. While this study focused on dredging in Savannah Harbor, the method described can be applied to estimate suspended sediment concentrations resulting from fluvial erosion, offshore sediment disposal or runoff from disturbed uplands.
The Savannah Harbor is a high sedimentation environment that presents challenges for maintaining navigable depths on this fast-growing port. Agitation dredging has been used to maintain berths since at least 1932. In addition to agitation dredging, hydraulic cutterhead dredging with direct pipeline discharge is also currently permitted in the Harbor. Both methods utilize the tidal and river currents to flush the suspended sediments down the river. However, resource agency concerns over these dredging practices prompted a comprehensive study to evaluate environmental effects.
The data collection required three simultaneous efforts: water chemistry sampling, in situ water quality profiling, and monitoring the dredge plume. This last effort utilized SEDIVIEW, an acoustic Doppler post-processing software, to quantify total suspended solids (TSS) concentrations from the acoustic backscatter signal. Chemistry samples and in situ profiles were collected both upgradient and downgradient of the dredged material discharge to quantify released constituent concentrations and to measure changes to water quality (especially, dissolved oxygen). The fate of the suspended material was also modeled using SSFATE.
The results of this study addressed natural resource managers' concerns about suspended sediment and other water quality effects from agitation dredging. The final report includes complete discussions of water chemistry, sediment chemistry, suspended material, and dredge plume shape. Water quality changes were nearfield and brief in duration for both agitation dredging and passing container ships.
Keywords: Ships, berth maintenance, suspended sediments, plumes.