Reports
Placement, consolidation, and erosion studies at open-water placement Site 92, 1999-2000
2002, Panageotou, W.
File Reports, Coastal and Estuarine Geology, File Report 2002-02
Abstract
A clamshell bucket dredge was used to excavate clayey silt sediment from the Chesapeake & Delaware Canal approach channel between February 13 and March 31, 2000. The volume of sediment dredged was reported as 386,665 m3 [505,709 yd3] by the contractor, Great Lakes Dredge and Dock, Inc. and 350,740 m3 [458,724 yd3] by the U.S. Army Corps of Engineers, Philadelphia District. The sediment was placed via bottom-release scows into the southernmost section of Site 92. By mid-March 2000, the 50% completion survey indicated that dragging operations would be necessary to achieve the authorized depths of -4.27 m [-14 ft] MLW. Dragging operations occurred concurrently with the final stages of placement in order to comply with the dredging window specified in the Water Quality Certification and complete the contract by March 31, 2000.
Turbidity plume monitoring found that relatively rapid settling rates occurred during slack tide resulting in a dense turbidity plume primarily concentrated in the immediate vicinity of dragging. The strong directional currents found near Site 92 tend to disperse sediment suspended in the upper and mid water column over large areas and consequently decrease the suspended sediment concentrations. During ebb tide, with peak current velocities of about 1 knot, the turbidity plume was tracked approximately 1 km [0.6 miles] down-current. Elevated levels of suspended sediment (>500 NTU) were generally localized and detected only within Site 92. Within 500 m [1,600 ft] of the dragging site, higher levels of turbidity (>100 NTU) were rapidly dispersed to levels below 100 NTU throughout the entire water column, except near the sediment-water interface. Resuspension from strong bottom currents results in higher turbidity levels in the lower water column.
Most of the triangular shaped drop zone located at the southern end of Site 92 was covered with a layer of placed sediment that was 0.1 to 1.0 m [0.3 to 3.3 ft] thick. The initial area of placed sediments was 920,000 m2 [1,100,000 yd2] or 92 hectares [228 acres]. Placed sediment was not detected beyond the site boundaries. Although the bottom was dragged in an attempt to achieve the specified -4.27 m [-14 ft] MLW final elevation, a mound in the northeastern corner of the drop zone protruded above the bottom to a depth of -3.7 m [-12 ft]. This mound covered an area of approximately 20,000 m2 [24,000 yd2] or 2 hectares [5 acres] and occupied 2% of the total area covered by deposited sediments. Over the seven-month post-placement period, the placed sediments underwent a reduction in the elevation of the placed sediments by up to 0.5 m [1.6 ft]. The area encompassing the 0.5 m [1.6 ft] contour or greater was approximately one-half of that on the completion survey. The area less than -4.27 m [-14 ft] was reduced to 4,000 m2 [5,000 yd2] or 0.4 hectares [1 acre].
At the end of the seven-month post-placement study period, 83% of the original sediment volume was identified. Of the 17% volumetric reduction that occurred, 10% was due to dewatering and consolidation and 7% was due to erosion. Thus, approximately three-fifths of the mean volumetric reduction was from consolidation and two-fifths from erosion. This is consistent with past studies of clamshell dredged and scow placed sediments, where it has been found that one-third to two-thirds of the total volumetric reduction could be attributed to either consolidation or erosion. The remaining estimated capacity at Site 92 seven months after the completion of placement was 4.4 mcm [5.8 mcy].
