CONFINED AQUATIC DISPOSAL (CAD)
Because finding new placement capacity is a challenge in the Baltimore region, in addition to diked containment facilities and innovative and beneficial uses of dredged material, there remains a third option called Confined Aquatic Disposal, or CAD, that is being investigated as a potential solution.
What is CAD?
Confined Aquatic Disposal (CAD) is an emerging dredged material management approach that safely places Harbor dredged material in an underwater space created by excavating material from the waterway bottom.
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How Does CAD Work?
The construction of a CAD cell usually begins by dredging a thin layer of silt and clay on the river bottom and placing that dredged material in a DMCF. Next, a layer of sand and gravel is removed to deepen the cell, but only in the areas that have a relatively thick, impervious clay layer known as the Arundel Formation. Finally the resulting cell (or depression) is filled with dredged material from the Baltimore Harbor. The thick clay underneath the cell creates a barrier, effectively eliminating the potential for deposited sediments to interact with aquifers that lay still deeper beneath the surface. The sand that is removed will be used in nearby construction.
CAD programs identify suitable underwater locations where sandy material can be recovered/dredged and innovatively or beneficially reused. Sediment removed from the shipping channel is confined in the resulting depression.
With limited dredged material placement capacity at our Harbor Dredged Material Containment Facilities (DMCF), exploring innovative new approaches to dredged material placement is necessary to maintain the deep water channels of the Port of Baltimore.
A Successful Pilot Project
In 2016, MPA constructed its first CAD cell in Baltimore Harbor and began an extensive 2-year study of this new approach in Maryland. CAD has been used successfully in other areas of the United States, including Boston and Newark Bay.
The 2016 CAD Pilot Project identified planning goals to be taken into consideration while exploring a second CAD project: evaluate different site conditions that may influence other locations within the harbor region; operate a larger capacity cell with multi-use or multiple cells within an established area, and determine cost effectiveness.
Preliminary site analysis for a second pilot project included an environmental assessment, hydrodynamic modeling, and geotechnical investigations. Potential sites were refined and focused for further study, ultimately resulting in a recommended location in the open water region southeast of Cox Creek DMCF. The geotechnical investigations at the proposed location identified a large volume of sand near the surface, allowing easier access and better value for removal and reuse.
Communities have raised concerns about the proposed location for the next CAD pilot so the project has been paused to ensure there is a thorough education and outreach process.