Reports
Placement, consolidation, and erosion studies at open-water placement Site 92, 2003-2004 (Year 6)
2005, Panageotou, W.
File Reports, Coastal and Estuarine Geology, File Report 2005-03
Abstract
A clamshell-bucket dredge excavated 0.823 mcm [1.075 mcy] of clayey-silt sediment from the Chesapeake & Delaware Canal approach channel between October 9, 2003, and December 18, 2003. The sediment was placed via bottom-release scows into a rectangular shaped drop zone near the center of Site 92. Placement occurred in water depths that ranged from -4.3 to -6.7 m MLLW [-14 to -22 ft].
The volume of placed sediment identified at completion of placement by Maryland Geological Survey was 0.563 mcm [0.736 mcy]. This represented a mean volumetric deficit of 0.260 mcm [0.339 mcy], or a mean percentage deficit of 31.5%. The total area covered by placed sediment was 647,000 m2 [773,800 yd2], or 64.7 ha [160 ac]. The majority of the deposit was from 0.5 to 1.5 m [1.6 to 5 ft] thick with a maximum thickness of 2.0 m [6.6 ft]. The resultant bottom in the southern one-half of the placement site was relatively level with average water depths of -4.75 to -5.0 m [-15.6 to -16.4 ft].
The placed sediments underwent area, elevation, volumetric, and sediment mass changes. After completion of placement, there was an additional 14% volumetric reduction at two month, 29% at six months, and 38% at nine months. It was estimated that approximately one-third of the volumetric change occurred from consolidation and two-thirds from erosion. Since completion, the area of placement underwent an overall increase in water depths of 0.25 m to 0.6 m [0.8 to 2.0 ft] with a few localized areas increasing in water depths by as much as 0.75 m [2.5 ft]. Over the combined placement and post-placement periods, there was a 51.6% loss of sediment mass (-232,100 tonnes) identified at Site 92.
Placement Year 6 utilized 0.49 mcy of site volume capacity, leaving a remaining 4.45 mcy. In total, Placement Year 1 through Year 6 used 2.55 mcy, or approximately one-third of the site volume, leaving two-thirds for future placement. Placement Years 1, 2, and 3 used the majority of the site volume capacity, 2.42 mcy, or 35%. Placement Years 4, 5, and 6 used 0.13 mcy of site volume capacity, or 2%.
