By James E. Sprague and C. Joel Sprague
Sediment retention devices (SRDs) include silt fences, wattles, filter logs, compost socks and various types of stormwater inlet protectors and are a widely used best management practice (BMP) to provide sediment filtration from stormwater runoff while allowing water passage. However, SRD performance varies greatly in flow rate and filtration efficiency. Unfortunately, SRDs are frequently selected without an objective quantitative means of knowing if the device can be expected to be sufficiently effective in providing the desired balance between flow rate and filtration efficiency. There is a limited, but growing, body of performance data related to the flow rate and filtration capabilities of SRD products based on recognized standard ASTM test procedures. SRD flow rate and filtration performance can be accurately evaluated using the bench-scale standard test method, ASTM D5141, Standard Test Method for Determining Filtering Efficiency and Flow Rate of the Filtration Component of a Sediment Retention Device Using Site-Specific Soil, or in large-scale “as installed” conditions using the standard test method, ASTM D7351, Standard Test Method for Determination of Sediment Retention Device Effectiveness in Sheet Flow Applications (Figure 1). These test methods quantify both the sediment removal efficiency and the associated flow rate of an SRD, so that the potential for either excessive sediment loss or the backup of runoff can be assessed.
This article details both the bench-scale (ASTM D5141) and the large-scale (ASTM D7351) test procedures used for linear, toe-of-slope devices and presents robust data on the comparative performance of a variety of linear SRDs, providing insight into the appropriate usage of these products in design. Consequently, this article will demonstrate the ability of these test methods to differentiate product performance and, in so doing, enable specifiers to engineer the appropriate SRD system to provide the desired balance between flow and sediment retention during construction operations.
Standardized testing of SRDs
While sediment ponds have been widely studied and have generally accepted quantitative design procedures, this is not the case for most other BMPs, including SRDs. SRDs offer the potential to prevent water pollution without the large area requirement and safety concerns of a sediment pond. Unfortunately, SRDs are frequently selected without a quantitative means of knowing if the device can be expected to perform sufficiently. Without standardized test procedures, there is no generally recognized way for specifiers/designers to verify marketing claims or onetime field trials, or for innovators to reliably test new products. Standardized testing procedures assist the users of SRDs in establishing improved construction specifications. Owners and contractors can save money by installing the correct SRD for the expected site conditions. Additionally, product manufacturers have a clear, recognized methodology for establishing product capabilities.
Testing SRD performance in sheet flow applications
SRD material components can be accurately evaluated in a laboratory for hydraulic properties using the bench-scale standard test method, ASTM D5141. Yet, the effectiveness of many SRD systems is installation dependent. Therefore, a large-scale test that can incorporate the full-scale “as installed” condition is the ideal evaluation procedure. These needs are addressed by the large-scale standard test method ASTM D7351. These test methods are both able to quantify sediment removal and associated flow rate through an SRD, so that the potential for either excessive sediment loss or the backup of runoff can be assessed.
Summary of test method ASTM D5141
Test method ASTM D5141 quantifies the ability of an SRD to retain eroded sediments carried by flowing water under bench-scale conditions. In the test method, sediment-laden water of a known concentration and sediment type is allowed to flow up to and through the filtration component of an SRD. The filtration component of an SRD is often the geotextile or fabric portion of the SRD. However, some SRDs, such as compost socks, utilize both a geotextile mesh and a compost filtration component encased within the geotextile component. The filtration component of an SRD should not be confused with the structural component(s) of an SRD. Examples of structural SRD components include T-posts in a silt fence system, metal grate inserts of an inlet filter system and wooden stakes used in a compost sock installation.
At a minimum, the effluent water and associated sediment passing through the SRD is measured and compared to the influent, or delivered, water and associated sediment to quantify the effectiveness of the SRD in retaining sediments while allowing water seepage. This test method may also assist in identifying physical attributes of SRDs, such as apparent opening size, permeability or tensile strength that contribute to an SRD’s ability to capture sediment while allowing water seepage, and it is useful for comparison of products. As discussed above, since the effectiveness of SRDs can also be installation dependent, this test method may not be completely indicative of product performance.
ASTM D5141 apparatus and procedures
The test apparatus used in ASTM D5141 can accommodate a vertical SRD, such as a silt fence or wattle, or a horizontal SRD, such as an inlet filter. The full test apparatus is shown in Figure 2.
After positioning the SRD and ensuring that it is sealed around the edges, sediment-laden runoff is created by combining water and soil in the upstream mixing tank and agitating for one minute prior to initiation of the test. In the test method, it is prescribed that 13.3 gallons (50 L) of water and 0.33 pound (0.15 kg) of the prescribed soil should be mixed and introduced to the upstream face of the SRD. The soil can be either site-specific or a default silty clay. The silty clay soil prescribed in the standard test method shall conform to the target gradation shown in Figure 3 and shall have a plasticity index (PI) ≤ 15. During the test, the sediment-laden flow passing through the installed SRD is collected, and the total time required for the entire influent volume to pass through the SRD is recorded. After the completion of the test, vacuum-assisted filtration of the collected effluent seepage is used to obtain the mass of the sediment that was able to pass through the SRD. Vacuum-filtered sediments are dried and weighed, and the weight of collected sediment is compared to the initial influent amount of sediment put into suspension to determine the filtering efficiency. In addition, the total time required for the entire influent volume to pass through the SRD is used to calculate a flow rate, represented in gallons per minute per square foot (m3/min/m2).
Summary of test method ASTM D7351
ASTM D7351 is a standard full-scale performance test commonly used to characterize installed SRD system performance, including sediment and flow retention, and structural behavior under hydraulic loading. Sediment-laden water is allowed to flow up to and through an installed SRD. At a minimum, the amount of sediment-laden flow and associated sediment passing through the SRD is measured. The measurement of flow (seepage) and sediment that passes through the SRD subtracted from the amount in the upstream flow, and then divided by the amount in the upstream flow, is used to quantify the water-retention percentage and filtration-effectiveness percentage of the SRD under full-scale conditions.
This test method may also assist in identifying physical attributes of the SRD system that contribute to its sediment control performance, and it is useful for comparison of different SRDs and their unique installation requirements. In contrast to ASTM D5141, D7351 evaluates the “as installed” SRD system performance. This is an important difference in the test methods. As an example, a silt fence system with a low-flow, high-sediment-retention filtration component would require robust post size and frequent post spacing, as the filtration component will retain a high volume and, consequently, high weight of water that may break posts of weaker construction or less frequent spacing. In another example, a compost filter sock installed with a 3-inch (76-mm) installation trench may provide better soil retention than a compost filter sock with no installation trench. Since the effectiveness of SRDs is installation dependent, this test method is indicative of actual field performance when installed with the appropriate techniques.
D7351 apparatus and procedures
The test method is commonly executed using the following equipment:
- A mixing tank with an internal paddle mixer device mounted on scales
- A sufficient source of water and associated pumping equipment to fill the mixing tank in a timely manner
- A collection tank mounted on scales of sufficient volume to accommodate all runoff passing the SRD
The mixing and collection tanks are separated by areas, or zones, as shown in Figure 4. A nonpermeable slope surface immediately below the mixer discharge spreads the initial discharge and provides a retention zone above the installation zone. The installation zone is comprised of prepared soil subgrade to allow full-scale installation of the SRD to be tested. The area below the installation zone is nonpermeable to facilitate efficient transmission of runoff passing the SRD to the collection tank.
A representative sample of the SRD to be tested is installed in the installation zone in accordance with the manufacturer’s recommendations. A sediment-laden runoff is then created by combining water and soil in the mixing tank and maintaining agitation during the test. A typical “construction phase” sediment concentration for testing is 6%. The rationale for the “construction phase” sediment concentration used in the test method is described in the standard as follows: “An important variable in any testing procedure is the establishment of test ‘conditions.’ For a sediment control performance test this means selecting an appropriate design storm event and associated runoff along with an expected amount of sediment to be transported by the runoff. For this testing, a standard 10-y, 6-h storm event (mid-Atlantic region of US) was selected. This return frequency is commonly used for sizing sediment control ponds and, thus, was deemed appropriate for the testing of other SRDs. Using this criterion, a 4-inch (100-mm) rainfall was selected. It was also assumed that approximately 25% of the storm would occur during the peak 30 minutes, and that 50% of the rainfall would infiltrate the ground (Goldman et al. 1986). A theoretical contributory area of 100 feet (30 m) slope length by 20 feet (6 m) wide was selected to limit runoff to sheet flow conditions (Richardson 1990). Runoff and associated sediment were calculated using the Modified Universal Soil Loss Equation (MUSLE).”
As described above, MUSLE was used in the standard test method to produce test quantities of runoff of 5,000 pounds (2,250 kg) and sediment load of 300 pounds (136 kg) for a 20-foot (6.1-m) SRD installation, or a sediment concentration of approximately 6%. A constant valved discharge is released evenly across the slope for 30 minutes to replicate the peak 30 minutes of the prescribed storm event.
Periodic grab samples are taken as influent discharge flows through, over or under the installed SRD and into the collection tank. Depth and weight of the collected seepage inside the collection tank is measured and recorded at the same intervals. Observations such as height of ponding, undermining, overtopping and the associated times are recorded. The grab samples are evaluated in a lab to determine percent solids content of the filtrate and to determine the percent filtering efficiency of the installed SRD.
In Part 1 of this two-part article, test methodologies and rationales have been presented for the evaluation and quantification of SRD performance in both bench-scale and large-scale “as installed” scenarios. These standardized test methods, ASTM D5141 and ASTM D7351, provide a means for objective evaluations of sediment filtration and water retention performance of SRD systems. These objective evaluations can be used by designers, specifiers and regulators to compare product-to-product performance and make decisions on the appropriate SRD system for a given project.
In Part 2 of this article, the results of historical testing efforts will be presented, providing a robust and compelling data set, and an in-depth analysis of the results will be discussed. Products will be defined by product category, and categories will be compared for general performance. These objective, data-driven comparisons will help designers, specifiers and regulators make informed decisions on the appropriate SRD system to use on any given jobsite.
ASTM D5141. Standard Test Method for Determining Filtering Efficiency and Flow Rate of the Filtration Component of a Sediment Retention Device Using Site-Specific Soil. American Society for Testing and Materials, West Conshohocken, Pa.
ASTM D7351. Standard Test Method for Determination of Sediment Retention Device Effectiveness in Sheet Flow Applications. American Society for Testing and Materials, West Conshohocken, Pa.
James E. Sprague is lab director for TRI Environmental in Greenville, S.C.
C. Joel Sprague is senior engineer and technical director for TRI Environmental in Greenville, S.C
All figures courtesy of the authors.