Standard Silt Fencing Versus Reusable Sediment Tubes

Comparative field performance data and statistical analyses

A new sediment control and stormwater diversion device developed by Friendly Environment of Shelbyville, TN, is the subject of a long-term field performance study performed by Civil & Environmental Consultants Inc. (CEC) of Nashville, TN. The manufacturer proposes that the new sediment tube product, the Erosion Eel, can be used to replace traditional silt fence, rock check dams, wattles, and temporary diversion berms. The product is a three-dimensional, tubular sediment retention device with internal filter media composed of recycled, shredded rubber tires and AASHTO-certified wood chips encased by a woven geotextile fabric. Like silt fence, the Erosion Eel can be applied to intercept sheet-flow runoff perpendicular to the direction of flow along sloped surfaces. Unlike silt fence, the product can also be placed within concentrated flow paths to act as a check dam. It can also be used to direct flow as a temporary diversion berm. To demonstrate the effectiveness of the new best management practice (BMP), field performance testing is being performed by CEC, comparing the performance of both standard silt fence (several types) and the Erosion Eel. The field test site has been under investigation by CEC for water-quality and hydraulic performance since March 2007. The field site has been designed to provide comparative evaluations between the Erosion Eel and standard silt fence where both influent and effluent total suspended solids (TSS) are measured for each storm event. In addition, particle size distributions (PSDs) collected from the July 6, 2007, storm event were analyzed at the influent and effluent of each BMP. The case study discusses the test site setup, data obtained, and statistical analyses for the field implementation. Field test results to date have provided data that demonstrate superior soil removal efficiencies for the Erosion Eel relative to silt fence.

Problem
Issues related to the performance problems with silt fence have been well documented and understood by the erosion and sediment control industry for some time. Due to the labor-intensive requirements for the installation of silt fence, research has demonstrated that a high number of installations are inadequate, resulting in poor performance and deleterious effluent water quality from construction sites. Inadequate trenching of fencing allows runoff to move underneath the fence, bypassing the intended treatment mechanism of settlement. Sprague and Carpenter (2003) examined 56 construction sites in 12 states that utilized silt fence for sediment control, and found the following:

• Seven of every 10 trenched silt fence installations have no backfill or compaction.
• Four of every 10 trenched silt fence installations will experience undermining.
• Only one of every four trenched silt fence installations effectively pond water.

Because ponding of water for sedimentation purposes is the singular attenuating mechanism for silt fence and only an estimated 25% of silt fence installations are effectively doing this, there are clearly defined problems with the effectiveness of this BMP.

Details
In order to evaluate the efficacy of the Erosion Eel relative to suspended-solids attenuation, a field test site was established in March 2007. Performed by CEC, the field testing was designed to compare the performance of both standard silt fence and the Erosion Eel. The field site was designed to provide comparative evaluations between the Erosion Eel and standard silt fence where both influent and effluent TSS are measured for each storm event. In addition, PSDs collected from the July 6, 2007, storm event were also analyzed at the influent and effluent of each BMP for comparative review. The overall intent is to see how each BMP performs in the field under the same watershed conditions, soils, and rainfall intensities relative to TSS removal efficiencies and overall TSS effluent quality.

The test site is located in Franklin, TN, 30 miles southwest of Nashville. The site consists of 0.24 hectares (0.6 acres) of denuded construction area, part of the Duke Realty Corporation commercial development on Duke Drive. The weighted-average slope for the construction watershed is 1.5%. Onsite exposed soils were sampled and analyzed as a silty clay soil with particle sizes of 90% finer than 36 microns, 50% finer than 8 microns, and 10% finer than 1.3 microns. The runoff from the site was split between two BMP locations: 1) 6.1-meter width (20 feet) of construction-grade slit-film woven silt fence; and 2) 6.1-meter width (20 feet) of Erosion Eels. The fence was installed and trenched per industry standards. The Eels were placed on top of jute mesh with the jute tucked under the front and the rear edges of the bags per the manufacturer’s requirements. Multiple discrete sample locations consisting of in-ground sample bottles were located at the influent and effluent of both the silt fence and Eel locations. Five discrete influent and five discrete effluent sample locations were established at each BMP. The sample horizontal (x,y) positions were spaced in an equivalent manner across the front and rear of each BMP to ensure that comparable results were achieved. The effluent sample locations were placed close to the effluent side of the silt fence and Eels in order to capture the representative effluent quality for each BMP. In addition, in order to minimize suspension of loose soil on the effluent side, the downslope areas behind both the silt fence and Eel were covered with clean river gravel. The site is equipped with a weather station that measures total rainfall, duration, and intensities. No polymer chemicals were used in this study.

Results
As of this date, there have been 13 storm events where samples have been taken and analyzed. Figure 1 presents the comparative data for TSS removal for each storm event from March through September 2007. Data are normalized relative to the performance of silt fence for comparative analysis. The presentation of normalized data better demonstrates comparative performance of BMPs in lieu of publishing discrete removal efficiencies that can be erroneously misused by applying the removal efficiencies to all sites and conditions. The values in Figure 1 are based on TSS removal efficiencies calculated for each storm event using arithmetic mean values of influent and effluent TSS for each BMP. The Eel retained a greater concentration of suspended solids in each separate storm event than the silt fence (as demonstrated by the normalized value for the Eel being greater than 1.0 for each category).

Figure 2 gives values that are based on TSS removal efficiencies calculated for each storm event using the median values of influent and effluent TSS for each BMP. The median is important to examine as an indicator of the middle or “average” of the data distribution, especially if the data set is not normally distributed, as is the case with these data. The Eel retained a greater concentration of suspended solids in each separate storm event than the silt fence, with the exception of one storm (the 16.51 mm (0.65-inch) storm event on April 14, 2007), as demonstrated by the normalized value for the Eel being greater than 1.0 for most categories.

Figure 1. Normalized TSS Removal Versus Rainfall Totals Per Storm Event (Based on Average Influent and Average Effluent Values) – Duke Property, Franklin, TN

Figure 2. Normalized TSS Removal Versus Rainfall Totals Per Storm Event (Based on Median Influent and Median Effluent Values) – Duke Property, Franklin, TN

The effluent TSS shown in Figure 4 is based on the median TSS of each storm event. Again, the data sets do not follow a normal distribution, and due to the uncertainty as to the distribution of the true population for the effluent data, Bootstrapping was used to develop the confidence intervals. The 95% confidence intervals for the median are based on the effluent from all storms combined. The graph and confidence intervals demonstrate better effluent quality from the Eel than for the silt fence.

Overall field performance descriptive statistics for the Erosion Eel from March to September 2007:

• TSS solids removal efficiencies for all storms combined based on the arithmetic mean value of all influent and effluent concentrations =  82.1%.
•   TSS solids removal efficiencies for all storms combined based on the median value of all influent and effluent concentrations =  84.6%.

Overall field performance descriptive statistics for the Woven Slit-Film Silt Fence from March to September 2007:

• TSS solids removal efficiencies for all storms combined based on the arithmetic mean value of all influent and effluent concentrations= 52.9%.
•   TSS solids removal efficiencies for all storms combined based on the median value of all influent and effluent concentrations= 63.3%.

Figure 3. Average Effluent Quality (TSS) per Storm Event – Duke Property, Franklin, TN

Figure 4. Median Effluent Quality (TSS) per Storm Event – Duke Property, Franklin, TN

A probability plot for all of the influent and effluent data combined for the Eel and silt fence over the 6-month study are shown in Figure 5. The linear regression lines on the plot are based on Weibull distributions. The Weibull distribution is the closest distribution fit for the data based on Goodness-of-Fit analysis for each of the influent and effluent data sets. The plot clearly demonstrates that the reduction in TSS from the Eel is greater than the reduction in the TSS for the effluent from the silt fence (relative to the difference in influent data groupings from effluent data groupings).

Figure 5. Probability Plots for Franklin, TN Field Test Site—All storms from March 2007 through September 2007

Efron, B. and R. Tibshirani. 1993. An Introduction to the Bootstrap. Chapman and Hall.

Sprague, C. Joel, and Thomas Carpenter. April 2003. “Silt Fence Performance Revisited.” GFR Magazine.