Reports
Placement, consolidation, and erosion studies at open-water placement Site 92, 2002-2003 (Year 5)
2004, Panageotou, W.
File Reports, Coastal and Estuarine Geology, File Report 2004-05
Executive Summary
A clamshell-bucket dredge excavated clayey-silt sediment from the Chesapeake & Delaware Canal approach channel between October 17, 2002, and November 11, 2002. The contractor, Norfolk Dredging Company, reported that a sediment volume of 246,759 m3 [322,729 yd3] was removed from the channel. The U.S. Army Corps of Engineers, Philadelphia District, reported a total sediment volume of 265,476 m3 [347,209 yd3] removed from the channel. 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 -4.75 to -5.25 m [-15.5 to -17.0 ft] MLLW.
Maryland Geological Survey identified a placed sediment volume of 162,000 m3 [212,000 yd3] at completion of placement. The total area covered by placed sediment was 368,300 m2 [440,500yd2], or 36.8 ha [91 ac]. The loss during this year’s operation was estimated at 32.5% of the average of the reported dredged volumes.
The placed sediments formed a hummocky surface marked with abundant small mounds. The mounds were located in a 200 m [650 ft] wide band in the lower one-half section of the specified drop zone. The mounds ranged in relief from 0.25 to 1.0 m [0.8 to 3.3 ft]. Water depths after completion of placement ranged from -4.0 to -4.75 m [-13.0 to -15.5 ft]. The area less than -4.3 m [-14.0 ft] deep covered approximately 9,550 m2 [11,400 yd2], or 0.9 ha [2.4 ac]. This represented 2% of the total area covered by placed sediment.
Over the post-placement study period, the placed sediments underwent area, elevation, and volumetric changes. After placement, there was an additional 22% volumetric reduction at one month, 28% at three months, 38% at six months, and 49% at nine months. It was estimated that approximately one-fifth of the volumetric change occurred from consolidation and four-fifths from erosion. By nine months after placement, there was a 24% reduction in coverage area (87,200 m2 [104,300 yd2], or 8.7 ha [21.5 ac]) and a decrease in thickness of 0.5 to 0.6 m [1.6 to 2.0 ft]. At that time the bottom was relatively level with average water depths of -4.75 to -5.0 m [-15.6 to -16.4 ft]. North of the placement area, the bottom sloped into deeper waters, -5.5 to -7.2 m [-18 to -23.6 ft] in depth.
Over the combined placement and post-placement periods, there was a 65% loss of sediment mass (-94,500 tonnes) identified at Site 92. This loss was due to suspended sediment transport in turbidity plumes during dredging and placement operations and erosion of the deposited sediments after placement. The relatively high percentage loss likely resulted from a combination of variables that allowed wind and storm driven currents to more effectively transport sediment from the site. These variables included a greater surface area per unit mass of placed sediment, higher water contents of dredged and placed sediments, relatively shallow water depths prior to and after placement, and perpendicular orientation of the deposit to the prevailing currents that traverse Site 92. It is also possible that a portion of this high percentage “loss” is actually a result of the error inherent in the bathymetric comparisons. Although the total percentage loss was at the high end of the range observed in past dredging operations, the 0.09 million tonnes loss was relatively small compared to the 0.11 million tonnes average loss in previous years due to the relatively small volume of sediment dredged during this year’s operation.
Year 5 placement used 0.10 mcy of capacity by the end of the nine-month study period. An updated National Tidal Datum Epoch (1983-2001) increased water depths by 0.081 m [0.27 ft] MLLW in the northern Chesapeake Bay. This resulted in an additional capacity 0.40 mcy of at Site 92. Hurricane Isabel passed through the mid-Atlantic area on September 18-19, 2003, and caused some additional erosion of Bay bottom sediments after the nine-month study period. The amount of capacity created at Site 92 from Hurricane Isabel can only be roughly estimated. The estimate of capacity gained from Hurricane Isabel is 100,000 m3 [0.131 mcy] to 163,000 m3 [0.214 mcy]. To date, a total of 2.36 to 2.44 mcy of capacity has been used. The remaining estimated capacity at Site 92 is 4.90 to 4.98 mcy based on an original capacity of 7.0 mcy.
