LONG TERM STABILITY OF CAPPED DREDGED MATERIAL DISPOSAL MOUNDS: STAMFORD NEW HAVEN NORTH AND CAP SITE 2 IN LONG ISLAND SOUND

摘要

The first two open water capped disposal mounds to be engineered (rather than formed by dredging sequence) were created in Long Island Sound in 1979 by the Disposal Area Monitoring System (DAMOS). In 1983, two more mounds were created to further test the approach. In all of these mounds, dredged sediments considered unsuitable for unconfined open water disposal (UDM) were point-dumped. The mounds were surveyed to define the disposal footprint on the seafloor and then covered by controlled disposal of capping dredged material (CDM, material suitable for unconfined open water disposal). Physical, biological and chemical monitoring of the mounds established that UDM layers were isolated from the marine environment; coring studies in 1990 provided additional evidence of stability and isolation.rnIn 2004, two of these mounds were revisited, cored and subsamples of sediment layers analyzed to compare with previous results. The Stamford-New Haven North mound (STNH-N) provided a 25-year record and the Cap Site 2 mound (CS-2) provided a 19-year record of placement and capping of dredged material.rnThe results of the coring study provided clear and consistent data showing that the CDM over UDM sequence remained intact with a well-defined interface between the intervals at both mounds. At STHN-N, the thickness of the CDM interval compared well with the distribution of CDM mapped following the original formation of the mound, taking into account the expected long-term consolidation of the hydraulically dredged CDM. At CS-2, the thickness of the CDM was more variable, reflecting the mechanical dredging that was used in the project, but there was no apparent reduction of CDM thickness over time. At both sites, a surficial layer was noted above the CDM, indicating net deposition since formation of the mounds. The maintenance of the CDM thickness over time and the overlying net deposition provide evidence that the UDM interval remains physically isolated from the overlying waters and unaffected by potential erosive events or other surface disturbances.rnSediments in cores were classified visually into horizons (surficial, UDM, CDM, older dredged material, native sediments). Bulk sediment data and detailed sedimentological analysis supported these classifications: physical contacts between layers were distinct and undisturbed. Comparison of 1990 and 2004 sediment data indicated similar horizons were observed in both surveys. The 2004 analytical results did not suggest any physical mixing of sediments from the UDM into the CDM, supporting a conclusion that UDM has been physically stable and isolated within the sediment column of these disposal mounds in Long Island Sound for up to 25 years.