Reports
Placement, consolidation, and erosion studies at open-water placement Site 92, 2000-2001
2003, Panageotou, W.
File Reports, Coastal and Estuarine Geology, File Report 2002-06
Abstract
A clamshell-bucket dredge was used to excavate clayey silt sediment from the Chesapeake & Delaware Canal approach channel between December 18, 2000 and March 31, 2001. The volume of sediment dredged was reported as 693,577 m3 [907,111 yd3] by the contractor, Weeks Marine, Inc. and 1,356,930 m3 [1,774,963 yd3] by the U.S. Army Corps of Engineers, Philadelphia District. The sediment was placed via bottom-release scows into the southern half of Site 92.
The placed sediments formed a large primary mound with abundant smaller secondary mounds. The primary mound encompassed the full width of the placement site and extended a short distance beyond the southwestern and eastern boundaries of Site 92. The total area covered by placed sediment was 1,259,000 m2 [1,506,000 yd2], or 126 ha [311 ac]. The combined areas covered by the spread of placed sediment outside of the site boundary was 1% of the total area covered by placed sediment. Water depths within the majority of the placement area ranged from −4.3 to −5.0 m [−14 to −16.4 ft] at the completion of operations. Some secondary mounds protruded above the surrounding bottom to depths of −3.7 to −4.0 m [−12 to −13 ft]. The area less than the authorized −4.27 m [−14 ft] depth was approximately 203,700 m2 [243,600 yd2], or 20 ha [50 ac]. This area was 16% of the total area covered by placed sediment.
The volume of placed sediment identified at completion by MGS was 1,143,500 m3 [1,495,500 yd3]. The loss of sediment during dredging and placement was estimated at 16% of CENAP's reported dredged volume. Over the six-month post-placement study period, the placed sediments underwent area, elevation, and volumetric changes. Redistribution of placed sediment into the deeper water beyond the southwest boundary of Site 92 occurred from tidal currents that run through the placement site. The total area covered by placed sediment at one and three months increased to 1,558,300 m2 [1,863,700 yd2] or 156 ha [385 ac] and 1,621,300 m2 [1,939,100 yd2] or 162 ha [401 ac], respectively, from the original coverage of 1,259,000 m2 [1,506,000 yd2]. The total area covered by placed sediment at six months decreased to 1,503,400 m2 [1,798,100 yd2], or 150 ha [371 ac]. Since completion of placement, the total area covered by placed sediments underwent a net increase of 244,400 m2 [292,300 yd2], or 24 ha [60 ac].
The total volume of placed sediment at six months after placement was 940,000 m3 [1,229,500 yd3] or 18% less than at completion. Eighty-two percent of the original sediment volume remained in-place. It was estimated that approximately one-fifth of the mean volumetric reduction was from consolidation and four-fifths from erosion. The remaining estimated capacity at Site 92 was 3.5 mcm [4.6 mcy].
