Hazardous Substance Research Centers/South & Southwest

Surface Discharge Techniques

Conventional equipment
Cap material discharged from the surface with conventional equipment descends rapidly to the bottom as a dense jet. Surface release of material from barges where the sediment has been largely dewatered is fastest and results in the mostcompact mound. Material released from a hopper dredge descends slower and produces a looser mound. However, the slowest descent and loosest mound occurs when capping material is released at the water surface through a pipeline. This method results in the most displacement of underlying sediment and the highest turbidity in the water column. View animation

Spreading with barge movement
Bottom-dump barges are effective for gradual spreading of coarse-grained, sandy capping material over sediment beds. The cap is released in a sprinkling action while the barge moves back and forth over the capping site. Silts or clays are not effective when they gather at the opening of the barge and their accumulated weight causes them to be released all at once. Whether this accumulation occurs depends on the placement technique chosen. The use of a water spray to force the cap off the end of the barge may eliminate the problem.View animation

Pipeline with baffle plate or sand box
This technique is useful for surface discharge of cap materials when the goal is to gradually spread the cap in thin layers over an extensive area. The baffle plate or sand box is attached to the end of a pipeline. These devices spray capping material out of the pipeline, where it loses velocity and descends vertically to the bottom.