Formatted Title:

Evaluation of Sediment Dredging, Transport, Dewatering and Disposal Approaches for Large Sediment Sites

Background/Objectives:

Remedial action at large sediment sites that require a significant volume of sediment to be dredged must incorporate processes to dewater or stabilize a sufficient volume of sediment each year to complete remediation within a timeframe that may last several years. For the largest environmental dredging projects completed to date, this has included design and construction of an upland processing facility (UPF) for sediment desanding and dewatering prior to disposal, construction of a confined disposal facility (CDF), and/or construction of dewatering pads for stabilization or dewatering followed by disposal. Sediments are also frequently capped in place as another component of the remedy. Additional upland support sites are usually also needed at various locations along the river for processing sediment and/or storage of capping material and equipment. In addition to designing sediment dewatering and water treatment facilities, a sediment processing site may be designed to manage stormwater runoff; provide facilities for workers; provide storage/staging areas for materials, debris, and capping materials, and to manage waste transport. In-river work may include hydraulic and/or mechanical dredging, sediment transport to the UPF or CDF for dewatering or to a stabilization area, waste transportation and disposal; as well as various sampling and monitoring activities required during remediation. This presentation will discuss the key considerations and evaluations that should be completed during the planning and design phases of large-scale projects to develop the most cost-effective and efficient remedial design approaches for dredging, processing, and disposal of dewatered sediment.

Approach/Activities:

The approaches considered for remedial action at these sites will help justify the selected RA components and provide cost saving benefits and numerous environmental benefits. While preliminary evaluations are performed as part of remedial investigations and feasibility studies, selection of methods for dredging, dredged sediment transport, dewatering, and disposal are typically deferred to the detailed design phase. At the time the RA is bid, it’s also advantageous to allow flexibility in means and methods to the extent these can be best determined by the RA contractor’s team. Experience suggests that minimizing waste tonnage through hydraulic dredging and transport of the sediment to a CDF for dewatering and disposal, or to a sediment processing facility where sand and much of the water are removed, can provide significant cost avoidance for some sites. Hydraulic dredging provides additional benefits such as odor control, less sediment resuspension, ease of pumping dredged sediment past low clearance obstructions, and fewer vessels needed on the river for remediation, versus mechanical dredging with scow transport.

Results/Lessons Learned:

There are many factors to consider when selecting the optimal approaches for sediment dredging, dewatering or stabilization for a multi-year remediation project. In addition to the evaluations for dredging and dewatering technologies that are expected to result in cost savings, other potential benefits to consider include sand separation and beneficial use of the sand, schedule reliability (e.g., minimizing downtime), minimizing resuspension from vessel traffic on the river, and minimizing odors, noise, and disruptions to shoreline property owners. Sediment volume to be remediated and schedule requirements for RA completion will typically require sediment dewatering to be performed at a much higher rate than would be needed for smaller sites, but this objective can be reliably achieved while also achieving other significant benefits.

Brian Delaney

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Evaluation of Sediment Dredging, Transport, Dewatering and Disposal Approaches for Large Sediment Sites