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.
Proposed
Solution
Denny
Hastings FLP, a developer of residential properties in Middle Tennessee,
initiated the development of a low-cost, reusable sediment tube in 2005 with the
intent of utilizing the product on its construction sites as an alternative to
silt fence. Consequently, Hastings formed a new company, Friendly Environment,
with the intent of manufacturing the new sediment tube out of an external woven
monofilament geotextile (with UV-inhibiting carbon black) filled with a
combination of washed, shredded used tires and AASHTO-certified
hardwood
chips. This material combination demonstrated effective field performance from
initial onsite trials conducted informally by the company. The manufacturer
proposes that the new sediment tube product, called the Erosion Eel, be used to
replace traditional silt fence. Like silt fence, the Erosion Eel can be applied
to intercept sheet-flow runoff by installing the tubes perpendicular to 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. Friendly
Environment contacted CEC Inc. in March 2006 to work on a set of formalized
protocols to evaluate the comparative performance between the Eel and silt
fence.
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 quality for each BMP is also important to track for each storm event.
Effluent TSS shown in the graph in Figure 3 is based on the arithmetic mean of
each storm event. The 95% confidence intervals for both the arithmetic mean and
geometric mean are given based on the effluent from all storms combined. The
geometric mean was used in an attempt to dampen out the data extremes (both low
and high). The data sets do not follow a normal distribution, and there is
uncertainty as to the distribution of the true population for the effluent data.
As a result, in an effort to develop confidence intervals with as minimal spread
as possible, the Bootstrapping Hybrid statistical technique was used to develop
the confidence intervals (Efron and Tibshirani 1993). The graph and confidence
intervals demonstrate better effluent quality from the Eel than for the silt
fence.
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
Summary
Field tests to date
have provided results that demonstrate superior suspended-solids removal
efficiencies for the Erosion Eel relative to silt fence. In addition, the field
data demonstrates that the Eel has produced better effluent quality (with
respect to TSS) than silt fence.
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.
Kevin
Wolfe, Ph.D., P.E., D. WRE, CPESC,
CPSWQ, P is a Research/Principal Engineer with Civil & Environmental
Consultants Inc. in Franklin, TN.