Effectiveness of Undersized Sediment Basins
University Park, PA

Grantee: PSU Department of Agricultural & Biological Engineering
Basin Program Funds: $13,189
Non-federal Funds: $12,233
Project Duration: 06/1996 - 09/1997
Status: complete

Problem Statement
In Pennsylvania, sediment basins are required to be large enough to completely capture a 5-year return period runoff event. Poor design, construction, and maintenance practices have led to unacceptably poor sediment retention. There is a need for research to determine appropriate methods for selecting basin size and detention time.

Background
Sedimentation basins are currently used for two reasons: to remove the eroded soil mass from the effluent leaving a site; and to store that sediment. Experiments in 1992 and 1995 at Penn State University showed that from 90% to 97% of influent sediment is captured and retained by sedimentation basins constructed according to Department of Environmental Protection guidelines. However, silt and chemically active clays make up most of the sediment discharged to the uncontrolled environment. Thus, the project sought to find a basin design that captures greater portions of these chemically active particles. Recent studies have shown that floating risers and in-basin barriers may be more effective methods of capturing and retaining sediment. In addition, the proper selection of basin detention time and basin size, relative to local hydrologic constraints, remain poorly understood.

The amount of sediment released to the environment is controlled by the basin's outlet structures. The basin's principal spillway regulates the release of water, thus controlling the vertical location from within the basin where water and suspended soil particles are selectively discharged. Presently, the perforated riser is the cheapest and most popular method of controlling basin dewatering. However, non-uniform practices of perforation sizes, spacings, and locations have made dewatering difficult to predict and sediment settling difficult to control. These poor design practices, compounded by improper or careless construction practices, short-circuiting of sediment-laden water, and poor maintenance practices have resulted in unacceptably low rates of sediment detention.

This project was a continuation of several sedimentation basin studies undertaken by the Department of Agriculture and Biological Engineering at Penn State University since 1992. This research has evaluated several concepts, including a floating dewatering device and in-basin barriers, each designed to improve water quality from earth disturbance sites by confining eroded sediment to sedimentation basins. To date, efforts have focused on quantifying sediment removal from basins that meet Pennsylvania Department of Environmental Protection's (DEP) size regulations. The specific objective of this work was to evaluate and demonstrate the effectiveness of undersized sedimentation basins, relative to watershed area, with respect to reduced effluent sediment concentrations and portion of each soil particle size retained.

Activities
The new undersized basin to be used for conducting experiments was constructed. An earth moving contractor was hired to excavate the basin and install all of the piping required to get the controlled inflow water and sediment to the basin, and collect and monitor the basin outflow using an HS-Flume and an ISCO sampler. Because it was possible to collect the new basin's effluent into the same sampling apparatus used in a 1996 basin evaluation, the cost of site construction and preparation were greatly reduced.

Four principal spillway/in-basin barrier combinations identical to those used in the 1996 basin evaluation were prepared. The apparatus included a skimmer and a perforated riser, each tested alone and then in combinations with internal basin barriers designed to subdivide the basin into three equal volumes. Each of the four combinations was tested three times with 85,000 liter, simulated runoff events containing 454 kilograms of soil. Basin outflow and sediment concentration rates were monitored during each 24 hour inflow-outflow event.

Results
Experiments carried out on the basin demonstrated that the skimmer yielded the best overall performance, retaining 94.3% of the soil injected; the perforated riser yielded the poorest performance retaining 90.4% of the sediment. The barriers proved ineffective at enhancing either spillway's sediment capture ability. When compared to the results from the larger basin the smaller one performed better than the larger one until the basin filled to the level of the emergency spillway which meant a significant amount of unsettled water left the basin. 50% to 77% of the soil lost from the small basin was through the emergency spillway.

Particle size distributions were also determined on all discharge water samples collected. All particles larger than 30 micrometers were retained in the basin for all treatments. The most poorly retained particles were 3.7-11 micrometers and 11-20 micrometers (the silt range). The skimmer retained 59% and 80% of those particles respectively, while the perforated riser retained only 28% and 67%. Retention of clay and sand particles was nearly 100%.

Contact: Dr. A.R. Jarrett, (814) 865-5661