Background/Objectives. Combined sewer overflow (CSO) sediments had accumulated within the Hendrix Street Canal, creating mounds that became exposed during diurnal tidal events resulting in the generation of nuisance odors. To mitigate this situation, the New York City Department of Environmental Protection (NYCDEP) removed the sediment mounds to a depth of 2.5 feet below mean low water. The primary goals of the project were to identify dredging and dewatering methodologies to allow for the removal of CSO sediments that comply with regulatory guidance documents, minimize sediment resuspension, maximize solids capture and provide the selected contractor with a suitable staging area to perform the work. Approach/Activities. Based upon a bathymetric survey of the Canal, the CSO sediments that lie above the datum 2.5 feet below mean low water had accumulated within the uppermost 1,500 feet of the Canal. Chemical characterization of the CSO sediments to be removed indicated that the sediments are “Class C material; High contamination, Acutely Toxic to Aquatic Life”, in accordance with New York State Department of Environmental Conservation Guidance Series documents. Additionally, sediment cores taken within the Canal showed that contaminant concentrations increased with depth. Therefore an engineered sand and carbon isolation cap was required to protect the water quality within the Canal. Both hydraulic and mechanical dredging via a clamshell bucket was evaluated. In order to evaluate dewatering technologies, sediment samples were collected and bench scale tests conducted for plate and frame and belt filter presses as well as geobag technologies. Resultant filtrate from these dewatering technologies was also collected and analyzed to determine if they could be returned to the water column within the area of active dredging. Results/Lessons Learned. Hydraulic dredging with mechanical dewatering was selected to remove the CSO sediments from the head end of the Canal. Major construction challenges overcome during dredging and capping activities included modification to both the elutriate water clarification system and the process used for placement of the sand containment cap. The clarification system originally provided resulted in excessive suspended solids being returned to the Canal from the dewatering process. To mitigate this issue, a barge was added downstream of the dewatering system, to function as an intermediate clarifier in order to remove the fine grained organic material present in the dredged slurry. The contractor’s procedure for placing the sand cap was also modified after initial pilot testing indicated that the in-situ sediments remaining in the Canal after
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