Reports
Placement, consolidation, and erosion studies at open-water placement Site 92, 1998-1999
2001, Panageotou, W.
File Reports, Coastal and Estuarine Geology, File Report 2001-01
Executive Summary
A clamshell bucket dredge was used to excavate clayey silt sediment from the Chesapeake & Delaware Canal approach channel between December 23, 1998 and March 31, 1999. The volume of sediment dredged was reported as 833,695 m3 [1,090,367 yd3] by the contractor, Weeks Marine Inc., and 580,740 m3 [759,534 yd3] by the U.S. Army Corps of Engineers, Philadelphia District (CENAP). The reported volumes typically differ because of the different methodologies used to determine the quantities dredged from the channel. The sediment was placed via bottom-release scows into the northernmost section of Site 92, in the tug channel, known as the West Sailing Course, that traverses the placement site. Placement was designed to create a sediment berm, not to exceed an elevation of 4.27 m [14 ft] below mean low water (MLW), in the northernmost section of the site. This would form an enclosed basin within the site that would minimize the potential for sediment migration out of the site during any subsequent placement operations.
Studies are routinely conducted by Maryland Geological Survey (MGS) on the dredged sediments to monitor their placement locations, elevation changes, physical characteristics, volumes occupied, and the changes in these attributes over time. The studies showed that placement resulted in a berm that filled the tug channel in the northern section of Site 92. The contract specification of 4.27 m [14 feet] below MLW was confirmed over the site. The 30H:1V [0.0333] sediment slope that was used in planning site management and for determining the setback from the boundary was not achieved during this placement operation. The steepest slopes identified along the northeast side of the placement area fell between 35H:1V [0.0286] and 50H:1V [0.0200].
The initial area of the completed berm was approximately 719,000 m2 [860,000 yd2]. The volume of placed sediment identified by MGS at Site 92 after the completion of placement was 792,000 m3 ±73,000 m3 [1,035,500 yd3 ±95,000 yd3]. This represents an excess of 211,500 m3 [276,000 yd3] or 36% more than the CENAP reported dredged volume and a deficit of 41,500 m3 [55,000 yd3] or 5% less than the contractor’s reported dredged volume. Based on the total volume of sediment identified and discussions with CENAP personnel, the volume that CENAP reported dredged is pay yardage removed by the contractor and an underestimate of the gross quantity removed from the channel.
Although all sediments were placed within the site boundaries, a small amount of the placed sediment extended beyond the northeast site boundary at the completion of placement operations. This sediment extended a maximum distance of 100 m [330 ft] to the east of the site boundary and had an estimated volume of 3,500 m3 ±2,300 m3 [4,600 yd3 ±3,000 yd3]. This represents less than one-half of one percent (0.4%) of the placed sediment identified at the site and of the volume reportedly placed by the contractor. Sediment that was placed at the top of the berm during the latter weeks of the placement period likely moved downslope on the berm’s east steep embankment and came to rest at the base of the slope in the deeper portion of the trough to the northeast. In addition, tidal currents may have spread some of the less consolidated sediments beyond the drop zones.
Over the eleven month post-placement study period, as expected the berm underwent elevation and volume changes. Redistribution of sediment within three months after completion of placement resulted in the area of the placed sediments increasing by two-thirds to approximately 1,200,000 m2 [1,432,000 yd2] but did not result in a measurable change in the total volume. The redistribution included slumping of sediment to a short distance beyond the site boundary, within a month after completion of placement. Sediment appeared to have moved over the peripheral areas of the berm and deposited as a thin layer in the tug channel to the northeast and in the basin within the site to the southwest. Between three and six months after placement, a reduction in the elevation of the berm and thinning of the sediments in the peripheral areas resulted in an 11% volume reduction. Between six and nine months after placement, the sediments that had previously spread into the peripheral areas were largely eroded, contributing to an overall 20% volume reduction. Between nine and eleven months after placement, there was an additional 2% volume reduction. The net area covered by the berm sediments was reduced to approximately 533,000 m2 [637,000 yd2] or three-quarters of the original footprint. The maximum elevation of the placed berm decreased by 0.6 m [2 ft] after completion to 3.1 m [10.2 ft] at eleven months.
At the end of the eleven month post-placement period, 67% of the original sediment volume was identified at Site 92 with a net decrease of 263,500 m3 [344,500 yd3], or 33% less than the volume identified on the completion survey. Bulk property data indicated that one-third of the volume change, approximately 12% of the originally placed volume, was due to dewatering and consolidation. The remaining two-thirds of the volume change, representing 21% of the original volume, was attributed to erosion of sediment from the surface of the deposit. In past studies of clamshell dredged and scow-placed sediments, it has been found that one-third to two-thirds of the total volumetric reduction could be attributed to either consolidation or erosion. The sediments placed in this operation exhibited amounts of consolidation and erosion similar to those placed in previous years in the northern Chesapeake Bay.
It is recommended that future placement near site boundaries should avoid developing a sediment pile (lift) of similar thickness and slopes (less than 50H:1V [0.0200]) as those in this year’s placement operation in deep areas such as this. Shallower slopes should be anticipated and a greater setback from the site boundary identified for scow drops to minimize the potential spread of sediment outside of the site boundary. However, it is unlikely that any future operations in Site 92 will result in these conditions occurring given the bathymetry at the site. Close coordination between CENAP, the dredging contractor, Maryland Environmental Service (MES), and MGS and development of a suitable site management plan will minimize the potential for spread of sediment outside of the site boundary and slumping events in future placement operations.
