Reports
Placement, consolidation, and erosion studies at open-water placement Site 92, 2001-2002 (Year 4)
2004, Panageotou, W.
File Reports, Coastal and Estuarine Geology, File Report 2003-06
Executive Summary
A clamshell-bucket dredge was used to excavate clayey silt sediment from the Chesapeake & Delaware Canal approach channel between November 15, 2001 and December 7, 2001. The contractor, Great Lakes Dredge and Dock Company, reported that a sediment volume of 140,500 m3 [183,500 yd3] was removed from the channel. The U.S. Army Corps of Engineers, Philadelphia District, reported a total sediment volume of 230,006 m3 [300,819 yd3] removed from the channel. The sediment was placed via bottom-release scows into the southern half of Site 92. The volume of placed sediment identified at completion of placement by MGS was 146,500 m3 [191,600 yd3]. The total area covered by placed sediment was 412,700 m2 [493,600yd2], or 41.3 ha [102 ac]. The expected loss of sediment during dredging and placement operations was estimated at 21 to 25% of the average of the reported dredged volumes.
The placed sediments formed a hummocky surface marked with abundant small mounds. The majority of these mounds were densely concentrated in a 450 m [1,500 ft] wide band parallel to the southeast boundary of the drop zone, forming semi-continuous positive-relief features of 0.5 to 1.0 m [1.6 to 3.3 ft]. Water depths within the majority of the placement area ranged from -4.3 to -5.0 m [-14.0 to -16.4 ft]. Some mounds protruded above the surrounding bottom to depths of -3.8 to -4.0 m [-12.5 to -13 ft]. The area less than -4.3 m [-14 ft] deep, including several pre-existing mounds from the previous year’s placement, covered approximately 104,600 m2 [125,100 yd2] or 10.5 ha [26 ac]. This represented 25% of the total area covered by placed sediment.
Over the post-placement study period, the placed sediments underwent area, elevation, and volumetric changes. The total area covered by placed sediment after six months, was 277,200 m2 [331,500 yd2], or 27.7 ha [68.5 ac], a 33% reduction in area since the completion survey. The area less than -4.3 m [-14 ft] in depth, including the pre-existing mounds, decreased by 77% since completion to approximately 24,400 m2 [29,200 yd2], or 2.4 ha [6 ac]. The total volume of placed sediment at six months after placement was 83,500 m3 [109,500 yd3]. This represented 57% of the original sediment volume identified at completion. Water content data indicated one-third of the volumetric reduction, or 13% of the placed volume, was from consolidation and two-thirds, or 30% of the placed volume, from erosion. In past studies of clamshell-dredged and scow-placed sediments, it has been found in most cases that one-third to two-thirds of the total volumetric reduction could be attributed to either consolidation or erosion. The two-thirds loss from erosion falls within the expected range, but at the high end of the loss range observed in previous studies.
Over the combined placement and post-placement periods, there was a 48% loss of the sediment mass identified at Site 92 (50,200 of 103,700 tonnes). This loss was due to transport of suspended sediment in turbidity plumes during dredging and placement operations and erosion of the deposited sediments after placement. Although the total percentage loss was at the high end of the range, the mass loss was relatively small compared to previous years due to the relatively small volume of sediment dredged and placed during this year’s operation.
Year 4 placement used 0.11 mcy of capacity in Site 92 by the end of the study period. To date, a total capacity of 2.53 mcy has been utilized. The remaining estimated capacity at Site 92 is 4.47 mcy, based on an original estimated capacity of 7.0 mcy.
