Dredging
Adapted from the Final GEIR on Eutrophication and Aquatic Plant Management in Massachusetts
(MA-EOEA. 2004.) http://www.mass.gov/dcr/waterSupply/lakepond/geir.htm
Dredging can be an effective technique to deepen a shallow lake or for the control of excessive
algae and invasive macrophytes. To control macrophytes, dredging must typically either reach
mineral, nutrient-poor sediment or deepen the lake to at least ten feet to achieve light limitation.
Dredging can be accomplished by a variety of methods grouped into the following five
categories:
•
Dry Excavation involves draining the lake to the extent possible, dewatering the
sediments by gravity and/or pumping and then removing sediments with conventional excavation equipment such as backhoes, bulldozers, or draglines.
•
Wet Excavation: For this technique, the lake is either not drained or is only partially
drawn down to minimize downstream flows. Excavation of sediments is conducted
with bucket dredges mounted on cranes or amphibious excavators.
Hydraulic dredge
Hydraulic Dredging involves the use of floating equipment, combining the use of a cutter head to loosen
sediments and suction to pump sediments out of the
lake as a wet slurry. The slurry, which is 80-90% water,
must be de-watered outside of the lake, allowing the
sediments to dry out for later disposal and the water
to drain back to the lake.
•
Pneumatic Dredging uses air pressure to pump solids out of the lake at a higher solids
content (50-70%) than hydraulic dredging. As a result, this approach requires
much less area for sediment containment and dewatering. However, few of these
dredges are operating in North America and there are limited freshwater project
examples.
•
Reverse Layering is grouped with dredging because it involves the movement of
sediment, but differs because sediment is not actually removed from the lake. Sandy
substrates beneath layers of muck are pumped upward, burying the nutrient–rich
muck and creating a new top layer of presumably low-nutrient sand.
Dredging projects are subject to an extensive permitting process requiring local, state and
federal permits and approvals. In Massachusetts, the most involved and expensive permitting process is for the submittals required under the Massachusetts Environmental Policy Act
(MEPA), which can typically range in cost anywhere from $50,00 to $250,000. Other required
submittals/approvals include a Notice of Intent under the Wetlands Protection Act (local Conservation Commission, MA-DEP), Section 401 Water Quality Certification for Dredging Activities
(MA-DEP), Section 404 permit under the Clean Waters Act (Army Corps of Engineers), and a
Drawdown Notification to the MA Division of Fisheries and Wildlife.
Dredging pg.1
Massachusetts Nonpoint Source Pollution Management
Because the per acre cost of dredging varies depending on the volume of material removed,
costs are usually expressed per cubic yard (cy) of material removed. Generally, the larger the
project, the smaller the cost per cubic yard. A typical range of costs for dredging projects in
recent years is $7 to $20/cy, with $10/cy suggested as a rough estimator for considering the
general magnitude of a project under initial consideration. It is important, however, to develop a more careful estimate during further project planning.
For a detailed discussion, literature review and case studies of dredging, please refer to the
Final GEIR on Eutrophication and Plant Management in Massachusetts .
Dredging pg.2
Massachusetts Nonpoint Source Pollution Management