Reports
Placement, consolidation and erosion of the sediment berm created at the G-West open-water placement site, January-September, 1994
1995, Halka, J.P., Panageotou, W., Ortt, R.A., Jr., and Shadid, J.
File Reports, Coastal and Estuarine Geology, File Report 1995-03
Project Description
The dredging of shipping channels in the northern Chesapeake Bay is routinely required to maintain navigational access to the Port of Baltimore. Portions of the sediment dredged from these operations have historically been placed overboard, on the Bay bottom, in open-water sites adjacent to the shipping channels. A designated site, identified as G-West, was approved for the acceptance of sediments dredged from the approach channel to the Chesapeake and Delaware Canal during the winter of 1993-1994. The sediments were dredged by the clamshell method and placed in split-hull scows for transport to the site. Placement of the sediments was performed in such a manner as to create an underwater berm across the southern section of the G-West site (Maryland Environmental Service, nd). The purpose of the berm was to enclose a underwater basin suitable for the placement of hydraulically dredged and discharged sediments in subsequent years' operations. Thus the stability and competence of the berm was a concern of both port planners and environmental managers.
Construction of the berm began on January 14, 1994 but was discontinued the next day due to extremely cold weather and the formation of extensive ice in the area. Dredging and placement resumed on February 15 and continued until March 26, 1994. Pre- and post- dredging bathymetric surveys of the channels were conducted by the Philadelphia District, Army Corps of Engineers. The time span over which the surveys occurred precluded a completely accurate determination of the dredged sediment volume. The total volume of sediment agreed to by the Corps and the dredging contractor totaled 404,972 m3 (529,652 yd3) (Maryland Environmental Service, nd.).
Fine grained sediments deposited in shallow water from split-hull scows have been examined during previous operations conducted in the northern Chesapeake Bay (Halka, 1993). The material has been shown to fall to the bottom below the point of release and to commonly develop stable side slopes of approximately 50H:1V. In this placement operation slopes averaged 34H:1V on the northern side of the berm and 50H:1V on the southern side (Maryland Environmental Service, nd.). Subsequent to placement the deposited sediments are subjected to volume changes due to two processes. Resuspension and erosion operate to remove particles from the site, and consolidation of the deposited sediments and underlying foundation serve to change the volume and height of the deposit. To insure the integrity of the berm for retaining future hydraulically deposited sediments in the G-West area and evaluating the environmental effects of the operation both of these processes needed to be documented and understood.
The objectives of the studies conducted by the Maryland Geological Survey were to determine the relative contributions of erosion and consolidation to the changing configuration of the berm over time subsequent to placement. Sediments from the shipping channel and the G-West berm site were collected prior to dredging and placement operations for determination of sedimentological and bulk properties. Following the completion of sediment placement, bathymetric surveys of the site were performed periodically to measure the changes in berm height, configuration, and volume. In addition, sediment samples were collected from the berm and foundation materials to evaluate the changing bulk properties of the sediments over time. These data were analyzed to determine the volumetric changes which could be attributed to consolidation. Resuspension and erosion could be estimated by subtracting the volume changes which could be attributed to consolidation both of the berm sediments and the underlying foundation sediments from the total volume change.
