Mirant Canal LLC (Mirant) uses the eastern entrance to the Canal for its vessel traffic carrying fuel for the Canal Power Generating Station. Although the Cape Cod Canal is a federally maintained channel, the U.S. Army Corps of Engineers (USACE) is authorized to dredge the Canal to a depth of only 34 feet; Mirant requires a depth of 42 feet for its use of deeper draft fuel tankers. Mirant assumes responsibility for maintaining this greater depth.

Historically, Mirant dredged the Canal on roughly a decadal frequency (1980, 1991, 2001) to maintain adequate channel depths. Following dredging in 2001, the channel began to shoal more rapidly, and additional maintenance dredging was performed in April 2004. Within six weeks of this dredging, the channel started to shoal again to a depth of 37 feet, according to surveys performed by the USACE. This increase in dredging frequency represented a significant sediment management dilemma for Mirant.

The shoaling at the east end of the Canal was assessed through collection of field data and a review of historical data. Woods Hole Group performed 2 separate one-day field survey operations to collect data to provide insight related to the shoaling processes and the sediment sources. The surveys included 3-D current measurements, bathymetric data, sub-bottom data, and side-scan sonar.

The findings of the study included increased detail about the geometry of the entrance channel, shoal fields, recent sedimentation, and build-up of sediment in the study area. Woods Hole Group designed and delivered a detailed two-dimensional conceptual model of the dominant shoals, sand sources, and sand transport pathways. The conceptual model quantified three significant shoals that restrict Mirant’s deeper draft fuel tankers from entering the Cape Cod Canal with full loads.

The results of the assessment study provided a better understanding of the shoal forming processes on the canal, identification of sediment sources, and estimates of dredging frequency projected into the future.

In summary, the findings and recommendations of the study provided Mirant with details about the source and cause of the shoaling processes and a means to evaluate improvements to the dredging including insight into a more cost-effective solution to the shoaling problem.