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|Title:||Evaluation of clamshell dredging and barge overflow, Military Ocean Terminal : Sunny Point, North Carolina|
|Authors:||Hydraulics Laboratory (U.S.)|
United States. Army. Corps of Engineers. Wilmington District.
Dredging Operations Technical Support Program (U.S.)
Palermo, Michael R.
Teeter, Allen M.
|Publisher:||Environmental Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: The 1987 maintenance dredging for the Military Ocean Terminal, Sunny Point (MOTSU), North Carolina, project was performed by mechanical clamshell dredge, with material placed in barges and transported to an open-water ocean disposal site. This work was the first major use of mechanical clamshell equipment in North Carolina. Resource agencies were concerned with operational procedures for clamshell dredges from the standpoint of potential resuspension of sediment during the dredging process and overflow of barges to increase load. A field study was therefore conducted to give site-specific information on the clamshell operation at MOTSU. The loading characteristics of the barges for both overflow and nonoverflow conditions and potential gain in load due to overflow were determined for three barge loads. The load gained during the period of overflow varied from 1.4 to 3.2 percent, with corresponding times of overflow from 9 to 28 min, The load gained by filling the disposal barge in one test from a level 1 ft (0.3 m) below the coaming to the point of overflow was approximately 6.9 percent. This, added to the load gained during overflow, corresponded to a total increase in load of 10.1 percent for this test. The suspended solids concentration of the overflow increased with time of overflow. The average concentration at the start of overflow was 88 g/l as compared with 248 g/l at the end of overflow. Measurements of grain size distribution of material retained in the barges and of the overflow indicated that a sorting process occurs in the barges during overflow. This could be due either to settling of sand particles or to an initially finer distribution of the more fluid fractions of the material which comprise the overflow. Plumes from the clamshell bucket spillage were observed to be patchy in nature, were advected downcurrent, and mixed with the ambient water downstream. The average suspended solids concentration of samples in the plumes generated by dredging was 47 mg/l above the background, while that for plumes generated by dredging with overflow was 65 mg/l above background. The suspended solids concentrations in the plumes were reduced to near-background levels at short distances from the dredging activity. An analytical model indicated the material in the plumes settles rapidly without being transported, and only a small fraction of the suspended material would go into far-field suspension. A literature review was conducted to evaluate the potential biological effects of the dredging and overflow. Although technical information currently available was found to be insufficient to accurately predict degrees of risk, qualitative predictions are possible. Eggs, larvae, juveniles, and adult forms of estuarine-dependent fish and shellfish species appear to be very tolerant to elevated suspended solids concentrations. When viewed against data on naturally occurring minimum, average, and maximum suspended sediment concentrations (and their temporal and spatial scales) at this site, the suspended sediment levels observed during dredging and overflow most probably did not produce any significant adverse environmental effect.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Technical Report|