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Module 5: Bucket (Clamshell) dredging Copy

June 3, 2021

[vc_row][vc_column][vc_single_image image=”896″ img_size=”full”][vc_column_text]Now let’s look at Bucket, also known as Clamshell dredging.

Turbidity generated by a bucket dredge operation comes from four major sources:

  • Sediment suspension occurring upon bucket impact and withdrawal from the bottom
  • Loss of material from the top and sides of a bucket as it is pulled up through the water column
  • Spillage of turbid water out of the bucket when it breaks the water surface
  • And inadvertent spillage of material during barge loading or intentional overflow operations intended to increase barge effective load.

Obviously, there are a number of variables which affect the quantity of material suspended by the dredge, such as sediment type, bucket size and type, is the bucket open or enclosed, the  volume of sediment dredged, hoisting speed, and hydrodynamic conditions at the dredging site.

Comparisons of open and enclosed bucket types indicate that surface-water suspended sediment concentrations may be reduced by 30-70% by using an enclosed bucket However,  near-bottom concentrations, have been shown to increase by as much as 50-70% due to the effect of the enclosed bucket as it descends through the water. A shock wave of water precedes the bucket and serves to suspend loosened material prior to impact.

In fact, bucket dredge induced suspension has been described as a near-field phenomenon. Sediment suspended by a dredge is likened to a small-scale storm that begins very suddenly, increases the concentrations and modifies the quality of suspended sediment fields compared with undisturbed conditions, and then produces a turbidity plume that rapidly decays following the reduction of energy required to suspend and maintain sediments in suspension.

In 1999 an investigation into sediment resuspension and loading characteristics of an open-faced clamshell bucket, an enclosed clamshell bucket, and a cable arm clamshell bucket took place. This study was done with each bucket performing under similar operating and environmental conditions. Monitoring was conducted to characterize near and far-field sediment resuspension characteristics of each bucket.

Turbidity observations were the primary near-field data collected during the study. However, a limited number of discrete water samples were taken comparable with turbidity readings.

Thirty-three samples were collected and analyzed for total suspended solids to validate the turbidity data during the bucket operations. Turbidity can be used as a surrogate for Total Suspended Solids, but factors other than sediment concentration affect turbidity.

So, what are these factors? They include particle size, shape, and organic content. Although the data correlating turbidity and TSS values in this study were scattered, they show a definite relationship.

Over 226,000 turbidity observations were collected during three partial days studying the three buckets. The primary advantage of using turbidity is the rapid number of measurements that can be obtained at very little additional cost per sample measurement. Additionally, the observations can be done in real time to gather direct knowledge about the dredging operation itself.

Although both the cable arm and enclosed buckets leaked substantially through the seals and grated vents in the upper part of the buckets, neither resulted in as much turbidity or TSS as did the conventional bucket.

The most significant difference was in the middle water column, where turbidity values were substantially less than at the bottom and near the surface.[/vc_column_text][/vc_column][/vc_row]