Communications Project

Document Type:Master's Thesis
Name:Lisa Marie Hoechst
Title:Evaluating the Effectiveness of the Skimmer Versus the Perforated Riser in Sedimentation Basins
Degree:Master of Science
Department:Biological Systems Engineering
Committee Chair: Dr. B. Blake Ross
Committee Members:Dr. Theo A. Dillaha, III
Dr. David F. Kibler
Dr. John V. Perumpral, Department Head
Keywords:sedimentation basin, skimmer, retention efficiency, perforated riser
Date of defense:December 4, 1997
Availability:Release the entire work immediately worldwide.


Erosion, transportation, and deposition of sediment into receiving waters can have substantial environmental and economic impacts. Sedimentation basins are a remediation technique used to limit sediment transport from earth disturbance activities. Retention efficiency is used as a measure of a sedimentation basin's effectiveness.

Several factors influence retention efficiency including the type of principal spillway used. The most common spillway is the perforated riser which dewaters the basin throughout its entire vertical profile. However, a relatively new outlet device, the skimmer, has been developed, which dewaters the basin from the water surface.

A laboratory study was conducted to compare the skimmer with the perforated riser for three different soil types and determine if there were any significant differences in the trapping efficiencies of the two outlets. The test basin dewatered over a three hour period. The parameters observed were dewatering rate, effluent sediment concentration, sediment loss rate, and retention efficiency.

The skimmer treatments consistently had higher values of sediment retention efficiencies. A statistical analysis performed on the retention efficiency data showed that retention efficiency was not influenced by any combination of outlet and soil type and that outlet was significant at the 5% level. Overall, the skimmer outperformed the perforated riser for all soil types tested.

Additionally, retention efficiencies were predicted for shorter dewatering times. The results indicated shorter dewatering times may have smaller impacts on the retention efficiency of basins where the skimmer is utilized rather than the perforated riser.

List of Attached Files


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