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 COASTAL CURRENTS |
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| November 2008 Update |
Engineering News and Information from
Coastal Engineering Company |
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When Planning to Dredge, Plan Ahead
Local Cape towns and owners of private marina facilities are continually faced with the need to perform periodic dredging of sediment deposits to maintain their channel and basin areas. While environmental permitting to obtain the required permits for these dredging operations is necessary to the project's success, the bigger issue is the dredge material disposal dilemma - created by the physical condition of the site - that must first be addressed in order to permit and dredge a project site.
To this end, an evaluation/consultation assessment needs to be performed that looks at dredging methodologies, dredge material make-up, de-watering alternatives, disposal alternatives, and long-term maintenance options and associated costs. The goal of this assessment is to accurately analyze and characterize the dredge material and identify the most efficient and cost-effective way to dredge and dispose of it.
Dredging Methodologies
The different methods for dredging to be used for a specific project are typically based on the type of material to be dredged and the disposal method. Most of the work on Cape Cod is done with small hydraulic dredges and small excavators. The shallow draft hydraulic dredges and excavators can operate in as little as three feet of water and are very portable. Bucket dredging is also often used, but is primarily for projects where there is an easy means of offloading or disposing of the material. |
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Dredge Material Make-up
The material that is to be dredged is one of the most important parameters that must be understood prior to undertaking any dredge program. To this end, dredge material must first be subjected to laboratory grain size analyses to determine its sand and organic content. Typically, material in marina basins is primarily an organic sludge that is a byproduct of sedimentation caused by decomposition of organic materials from the adjacent coastal estuaries. Due to its organic make-up, the material has a very strong odor. In addition, the material has no structural qualities at all in this state, so the material will continue to flow into the dredged areas and will increase the actual amount of material the dredge has to remove.
The result is that the material will have to be de-watered and disposed of either in a landfill, as composting material, or as non-structural fill. This will involve Massachusetts Department of Environmental Protection (DEP) chemical analysis testing for any material to be disposed of in lined or unlined landfills, as opposed to other upland areas. Due to the high moisture content of the material, tests also need to be performed to determine whether the material is suitable to be disposed of in a town landfill once it has been properly de-watered.
Disposal of dredge materials may also fall under the jurisdiction of the U.S. Environmental Protection Agency (EPA) if the material is to be disposed of offshore or in a CAD cell-type disposal method (see below). This would involve not only chemical analysis, but also bio-assay testing for EPA compliance which can involve extensive lab work and substantial fee costs. |
De-watering Alternatives
 De-watering - the process of removing water from dredged "sludge" - can be accomplished in a number of different ways, all of which are a function of time, available space, and expense. Alternatives include: the use of sedimentation basins; utilizing geo-tubes; and mechanical de-watering using presses.
The key to both geo-tubes and mechanical de-watering is the use of odor control chemicals and polymers that are environmentally safe and have been tested extensively. Read more... |
Disposal Alternatives
Disposing of the dredged material is usually the real physical and financial challenge of these types of dredging projects. Due to the high organic nature of the dredge material, it is unsuitable for any type of beach re-nourishment or as an additive to a gravel operation. This leaves only four available disposal alternatives for the dredge materials:
- Offshore Disposal;
- Upland Disposal;
- Beneficial Re-Use; or
- Confined Aquatic Disposal (CAD) Cells.
Read more... |
Long Term Maintenance Options and Costs
Research showing when a channel area was last dredged and then comparing that pre-dredge survey with today's conditions will show how long the site has been filled in. Using typical sedimentation rates for the type of dredge material, calculations can be performed to determine what level of draft the channel should retain for a certain number of years. Provisions can then be made to dredge a certain volume of material every number of years to maintain the channel.
Costs for dredging projects will vary depending on the actual quantity of material to be disposed of, the dredging methodology used, and the disposal option selected - and can run from $30/c.y. to $200/c.y. However, partnering with other dredge projects can help cut costs by reducing mobilization costs and finding the open space needed to de-water the material. |
Featured Project - Town of Harwich, Round Cove Channel Maintenance Dredging 
After a three-year effort to put in place a comprehensive permit for dredging its harbor channels, the Town of Harwich received key approvals last summer from the state of Massachusetts and the U.S. Army Corps of Engineers to establish a dredging and beach nourishment program. Part of this permit approval process involved coordinating efforts with the county dredge program and re-nourishing both public and private beaches in the Round Cove area.
Click here to learn more about this dredging project on our website. Also, read how Harwich's Harbormaster Tom Leach plans to use the comprehensive permit for town and private beach association nourishment projects, or view a brief video of the dredge in action. | |
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