Reports
Placement, consolidation, and erosion studies at Open-Water Placement Site 92, 2009-2010
2013, Halka, J.P., Ortt, R.A., Jr., and Van Ryswick, S.
File Reports, Coastal and Estuarine Geology, File Report 2013-01
Abstract
Dredged sediment placement at open-water placement Site 92 occurred between October 25, 2009 and March 21, 2010. Based on scow loads, the contractor estimated that the clamshell-bucket dredge excavated approximately 1.72 million cubic yards (mcy) of clayey-silt sediment from the Chesapeake & Delaware (C&D) Canal approach channel. The cut volume reported by the U.S. Army Corps of Engineers – Philadelphia Distract was reported as 1.39 mcy (1,394,228 cy). The sediment was placed via bottom-release scows into a designated drop zone covering much of Site 92. Water depths across much of the designated drop area ranged from -16 feet to -18 feet (ft.) mean lower low water (MLLW).
The volume of placed sediment identified by Maryland Geological Survey (MGS) at the completion of placement was 1.25 ± 0.16 mcy. This represented a volume deficit of 0.08 mcy, or 6.02% less than the amount identified dredged by the Corps of Engineers channel surveys and using a bulking factor of 0.96 based on sediment water content data. The total area covered by placed sediment was 2.43 million square yards (msy), or approximately 502 acres. The majority of the deposit was 1.0 ft. to 2.0 ft. thick with a maximum thickness between 3 ft. and 4 ft.
The placed sediments underwent elevation and volumetric changes during the following eleven months. The maximum thickness of the placed sediments decreased from nearly 4 ft. at completion to slightly over 3 ft. at eleven months. There was a 52.0 % volume reduction at eleven months following placement. It was estimated that approximately one-fifth of the volume change resulted from consolidation and four-fifths was due to erosion.
In total, Years 1 through 12 placements used 4.75 mcy of the site volume, or 68% of the original site volume of 7.0 mcy. Although open water placement in the Upper Chesapeake Bay has ceased to take place, approximately 1/3 of the original volume estimated to be available at Site 92 remains, assuming an idealized placement configuration with a final authorized depth of -14 ft. MLLW.
