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Approximately 6000 cubic yards of sand/silty maintenance dredge material was successfully dredged from the Intake Canal at Vermont Yankee Nuclear Power Plant, Vernon, VT on the Connecticut River. The dredging was performed by Inner Space Services, Inc., Dredging Contractors, Braintree, MA.

The critical flow cross section, in front of the intake, was dredged to a depth of 20' and pumped by an Mud Cat 370 Mud Cat™ cutterhead dredge, through 12" HDPE pipeline, for a distance of approximately 2200 linear feet, with a vertical lift of 42' to a field in the upland area of the plant. The maintenance material was pumped into 12 - 300' by 30' circumference geotextile tubes, fabricated by TC Mirafi, a Nicolon Corporation.

The "Geotubes" were placed side by side, in an area that had been enclosed by a haybale and filter fabric barrier to direct the flow of discharge water through a natural swale to a storm outfall and eventually back to the Connecticut River.

Geotubes being filled
at Yankee Nuclear Power Plant

The project was permitted by both the Army Corps of Engineers and the State of Vermont, Water Quality Division. Water quality standards were monitored hourly for a NTU and color units not to exceed 10 and 25 units, respectively, above background.

Mud Cat Series 370 Mud Cat™ Dredge

BACKGROUND: Vermont Yankee Nuclear Power Plant in Vernon, Vermont is located on the Connecticut River. This plant, which started commercial operation on February 28, 1973 is a GE Mark 1, BWR 1593 MWT reactor.

The plant's cooling system makes direct use of the Connecticut River in the fall and winter months when the additional heat of the discharge water would not adversely impact the environment. The cooling water is drawn into the intake and circulated through the plant's cooling system. Over the years, natural river sedimentation accumulated in an area in front of the intake, causing concern that the critical flow cross section was being severely impacted.

A Hydrographic survey conducted by Normendeau Associates, an environmental consulting firm under the direction of Tom Osterhoudt, Project Manager for Vermont Yankee, confirmed that the area was in need of immediate dredging to maintain the critical cross section throughout the winter. Permits for the removal of 10,700 cubic yards of maintenance material were applied for and received from the New England Division of Army Corps of Engineers and the State of Vermont, Water Quality Division.

Concern for the minimum cross sectional flow prompted Vermont Yankee Management to explore the possibility of dredging the minimum area that could be accomplished during the fall prior to the river ice setting in and with the plant in full operation. After interviewing several dredging contractors, Vermont Yankee chose Inner Space Services, Inc., Dredging Contractors from Braintree, MA. Knowing that time was of the essence and the winter ice was fast approaching, Inner Space Services, Inc. suggested the use of TC Mirafi/Nicolon geotextile tubes as a means for dredge spoils containment. These 300' long by 30' circumference "Geotubes" would be used instead of the originally designed, traditional settling basin.

Mud Cat Series 370 "Dragon" Dredge
Pumping Material to Geotubes

Due to the limited area for the containment basis (which ultimately would not have contained all the materials or would have had to be cleaned out during the project) and time and costs of building it, the Geotubes were submitted to and granted approval by the permitting agencies. This not only added to the expediency of the project, but also provided an environmentally clean and cost effective way to handle the dredge materials.

THE PROCESS: The area to be dredged was surveyed and laid out by Normandeau Associates. Responding to the immediate mobilization request, Inner Space Services, Inc. brought in 2200 linear feet of 12" HDPE pipe, which was fusion-welded and placed from the dredge area up a 42 foot vertical rise to the field in which the containment area was designated. A "berm" constructed of haybales, filter fabric, and heavy mil polyethylene plastic was constructed around the perimeter of the containment area to ensure that the effluent water that "bled" through the Geotube fabric while entrapping the spoils material, would be directed to the natural swale and ultimately to the outfall and back into the river. Environmental permits contained stringent water quality requirements.

The Geotubes not only provided excellent filtration of the effluent water, but also allowed the contractor to continue working as the tubes filled, moving to the next consecutive tube, where as a traditional stilling basic would have had to settle and decant prior to the dredging operations continuing. The Vermont Water Quality permit required hourly NTU and color monitoring against background river samples, with a "shut down" requirement if those standards were exceeded.

Only once during the course of the dredging did the readings exceed the standard, due to a pocket area of silty muck dredge spoils. Even then, the project was back on track in a matter of hours after the effluent waters had settled and decanted. With freezing temperatures a daily norm, time was critical and the use of the Geotubes proved to greatly speed the project along. As each tube filled, the material was allowed to settle and dewater, eventually, to be trucked off and recycled in the ensuing warmer water.

Inner Space Services, Inc., working hand in hand with Vermont Yankee to ensure highest standard of safety, security and continuing plant operation during the dredging, brought in an Mud Cat Series 370, 10" cutterhead hydraulic dredge, whose size afforded portability and efficient mobilization in this remote area without sacrificing production capability and performance. The dredge ultimately dug the area to an elevation of 10' below the river level (with some areas of 12' to 14' of dredged material), pumping the material uphill, with a 42' vertical lift, to the Geotubes, ultimately removing some 5000 to 6000 cu.yd. of material. This increased the plant's critical cross flow section by a factor of five.

Source - Land & Water Magazine
& Inner Space Services Inc.

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