Hydraulic Dredging of a Stormwater Pond - Geotube
LOCATION: Toronto, Ontario
DATE OF INSTALLATION: 2013
SCOPE OF WORK: Dredging of sediment in a large pond using Geotube hydraulic dredging.
PROJECT OWNER: City of Vaughan , CLEANOUT PARTY: Layfield Canada Ltd.
Geotube®Dredging Background Information and Challenges
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In Ontario, municipalities typically require landowners to remove sediment accumulated in stormwater management ponds before they will assume them. The dredging of Pond 4 was undertaken by the Block 11 Landowners Group in 2013 in order to render the pond suitable for assumption by the City of Vaughan.
Based on the completion of a bathymetric survey, it was determined that a total of 1230 m3 of sediment had accumulated in the pond. Over and above the primary objective of removing this accumulated sediment, the method selected for the dredging of Pond 4 was also required to meet the following secondary objectives.
Avoid the clearing of vegetation on pond embankments and surrounding areas so as to keep restoration and replanting costs to a minimum.
Return the pond to its original design capacity for the purpose of assumption by the City of Vaughan
Minimize disturbance to local residents and individuals who use the walking trails around the pond
Minimize the ecological disturbance of dredging activities to wildlife that inhabit the pond area, particularly waterfowl
Prevent the release of sediment to the receiving stream
Complete project on schedule and budget
Geotube® Dredging Solution
Based on the Geotube® dredging method selected, large excavators were not required in the pond and surrounding area. As a result, there was no significant vegetation removal and the need for erosion and sediment control measures was minimal. In order to mitigate the release of sediment from the pond during dredging activities, work was scheduled in the summer during a period over which dry weather was forecasted. Because more than 60% of accumulated sediment was located in the forebay part of the pond, the risk of sediment release to receiving waters was considered to be low. The primary activities associated with preparing the site for dredging were:
The preparation of a lay down area for the sediment dewatering bags;
The placement of the hydraulic dredge and steel cable system in the pond; and
The installation of the pumps, pipes and hoses conveying material between the dredge, polymer supply trailer, and sediment dewatering bags.
The area designated for the placement of the Geotubes® (measuring 45 by 100 feet) was a vacant gravel parking lot immediately south of the pond. the sediment slurry was suctioned by the dredge, pumped to the trailer for inline polymer injection and mixing, and then pumped to a Geotube® for dewatering. The Geotubes® used to contain the sediment were left to drain for a period of two months. This drying period was longer than what would normally be required, reflecting delays in scheduling of hauling rather than the actual time needed for sediment to be dry enough. Typically, a few weeks of drying time would be sufficient, but this can vary significantly according to the type of material being dewatered. For this project, the solids concentration was 56% at two weeks and roughly 60% at two months post dredging, demonstrating that the additional drying time resulted in a relatively marginal benefit.
Geotube® Dredging Results
The dredging of Pond 4 was carried out successfully and as planned with the removal of 1300 m3 of sediment.
Based on a bathymetric survey of the pond completed in 2011, an estimated total of 1230 m3 of in-situ sediment had accumulated in the pond – 770 in the forebay and 460 in the wet cell. The total removed sediment was 800 m3 from the forebay and 500 m3 from the permanent pool for a total of 1300 m3, which indicates that the method used to estimate the volume of accumulated sediment was accurate. The total volume of sediment-laden pond water removed by the hydraulic dredge was 4,336 m3.
The preparation of the laydown area was carried out within three days at the end of May 2013, and dredging was subsequently carried out over the course of ten days, from June 3rd to 13th. The project was carried out as planned and on budget, with no significant delays reported by the project team.
Layfield is a leading vertically integrated company that specializes in providing environmental containment systems and solutions. Our products and services are commonly used by the Oil & Gas, Mining, Energy, Municipal Water, Agriculture, Civil Construction, and Waste Management sectors. Layfield has been in business since 1978 and is an ISO 9000 registered company.
For more information contact Layfield –1.800.840.2884