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Comparative analysis of the flow and filtration capabilities of sediment retention devices—Part 2

April 1st, 2021 / By: / Feature

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. They 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.  

In Part 1 of this two-part article, test methodologies and rationales were 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, have proven to be an objective tool for the evaluation 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 (Figure 1).

FIGURE 1 ASTM D7351 test apparatus

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 and ASTM D7351 SRD testing results

ASTM D5141 and D7351 testing are most commonly used by various state departments of transportation (DOTs) to confirm that SRDs conform to state specifications for sediment removal and sustained seepage. DOT projects are typically linear, requiring the extensive use of toe-of-slope SRDs, such as silt fence, wattles and compost socks. As a result, DOTs must often be mindful of excessive ponding behind SRDs caused by sediment buildup on SRDs that might cause a safety hazard on adjacent roadways. As noted earlier, D5141 effectively quantifies the sediment removal effectiveness and the associated sustained seepage of the filtration component of an SRD (Figure 2), while D7351 quantifies the balance of sediment removal and sustained seepage for the installed SRD system.

FIGURE 2 ASTM D5141 test apparatus

ASTM D5141 test data

Figure 3 presents results of ASTM D5141 testing of a wide range of SRD types commonly used in linear, toe-of-slope applications. SRDs tested include both woven and nonwoven geotextile-based silt fence, as well as various proprietary composite and woven fences and socks/wattles.

FIGURE 3 ASTM D5141 testing results

Discussion of D5141 data

Figure 3 also shows a clear delineation between product types and their relative performance in both filtration efficiency and flow rate. This data is comprised of six nominal categories of products tested using ASTM D5141. The nominal categories, as identified by the authors, and the numbers of tested products are as follows:

  • 32 woven slit film geotextile silt fence
  • 6 woven monofilament geotextile silt fence
  • 5 nonwoven geotextile silt fence
  • 8 nonwoven composite geotextile silt fence
  • 5 three-dimensional filter socks and fibrous wattles
  • 6 proprietary woven geotextile silt fence

Woven slit film geotextile silt fence

Since it is the category of SRDs tested most frequently using test method ASTM D5141, there is a wealth of valuable comparative data about SRD filtration components composed of woven slit film geotextile silt fence. It is apparent from the data that this category of filtration component provides excellent filtration efficiency across all tested products. All the 32 tested products provided greater than 80% filtration efficiency, and the majority of the tested products provided greater than 90% filtration efficiency. However, this category of product is also the category with the lowest flow rate, in general. These results are intuitive, as the low flow rates in this product category inherently require greater sediment retention or filtering efficiency as a function of sediments settling out of suspension, binding against the upstream face of the fabric, or simply being unable to pass through the tightly woven matrix of the slit tape fabric.

Woven slit film geotextile silt fence is the most widely used silt fence fabric, based on the number of products tested to satisfy DOT Qualified Products List requirements, and is composed of a weaving of flat two-dimensional tape-like plastic fibers that create a textile with very small openings for sediment and water to escape. It should be noted by end users and designers that while slit film fabrics provide exceptional filtering efficiency, the requisite low flow rate of the product category leads these fabric types to produce excessive ponding. Excessive ponding may be an acceptable risk when slit film fabric is used to protect sensitive water bodies, but may be an unnecessary risk along roadways when excessive ponding could lead to roadway flooding and potential risk to motorists. It must be noted that slit film fabrics, when deployed as two-dimensional silt fences, must be adequately supported by robust posts with frequent post spacing to reduce the risk of fence failure due to the load associated with high-volume ponding.

As with all SRD products, market price must be considered. Slit film fabric is generally understood to be the most economical of all silt fence fabrics and is often sourced from overseas. When considering whether slit film fabric is the appropriate SRD filtration component for a construction project, the designer must balance the economic implications, potential flooding/ponding risk and exceptional filtration capabilities of this product type.

Woven monofilament geotextile silt fence

Six products from the woven monofilament geotextile silt fence category were tested in this research. The data suggests a product category that provides moderate filtration efficiency with all tested products (excluding one outlier), providing between 65% and 85% filtration efficiency and moderately high, yet relatively uniform, flow rate within the product category. In contrast to slit film fabric, monofilament fabric is composed of rounded plastic fibers, similar to fishing line, woven together to form a fabric matrix with larger open areas and greater flow capabilities. The differences in manufacturing practices that produce fabrics with more fibers per square inch can lead to greater filtration efficiency while allowing similar volumes of water to pass.

This product category may be exceptionally valuable to designers in areas where the risk of excessive ponding and roadway flooding would lead to unnecessary motorist risk, but where downstream impairment is of relatively low risk. Often within the erosion and sediment control industry, SRD products nominally termed “high flow” will be composed of a filtration component made with monofilament geotextile.

Nonwoven geotextile silt fence

Five products from the nonwoven geotextile silt fence category were tested in this research. The data suggests that the product category provides excellent filtration efficiency, with all tested products providing greater than 95% filtration efficiency and moderately high, yet uniform, flow rate. Nonwoven fabrics are different from woven fabrics in that they have randomized open spaces. The vast array of nuance in nonwoven fabric manufacturing can, however, lead to products that are produced for purposes other than filtration, and caution should be used by a designer when specifying nonwoven fabric for deployment as a silt fence. Nonwoven fabrics are utilized for reinforcement, separation and filtration, and it is unlikely that every nonwoven fabric produced has the material characteristics necessary to meet the filtration and flow properties found in this research.

Due to cost, nonwoven fabric may not be the most appropriate solution for all silt fence applications and may be most appropriately utilized in areas where runoff concentrates prior to being released off-site, thus requiring both high filtration efficiency and moderately high flow rate.

Nonwoven composite geotextile silt fence

Eight products from the nonwoven composite geotextile product category were tested via ASTM D5141 in this research. Aside from the two noticeable outliers in Figure 5, this product category provides excellent filtration efficiency, with all products providing greater than 90% filtration efficiency and high flow rate. In contrast to the nonwoven geotextile silt fence category, products in the nonwoven composite geotextile product category are composed of a multilayer nonwoven geotextile component impregnated, or filled, with natural fibers. The combination of multilayer nonwoven fabric and fiber impregnation leads to a high-strength, relatively thick filtration component that provides a structurally three-dimensional profile, allowing high water flow and soil capture. This product category, based on knowledge gained during the research, is relatively unique and not widely represented within the industry.

Similar to nonwoven fabrics, the nonwoven composite fabrics may not be the most appropriate solution for all silt fence applications due to cost, but may be most appropriately utilized in areas where runoff concentrates prior to being released off-site and, thus, both high filtration efficiency and high flow rate are required. There may be applications for the product category in capturing nonsediment pollutants. However, further investigation is required.

Three-dimensional filter socks and fibrous wattles

Five products from the three-dimensional filter sock and fibrous wattle category were tested via ASTM D5141 in this research. The data shows that this product category provides moderately high filtration efficiency, with all tested products providing greater than 80% filtration efficiency and excellent flow rate. Three-dimensional filter socks and fibrous wattles are composed of either a geotextile fabric or geotextile mesh filled with compost, rock, straw, or other agricultural or synthetic media.

Compared to all other tested products, this product category is unique in that it is a three-dimensional SRD and provides a much lower vertical profile for filtration in the field. This product category may be both an economical and functional alternative to two-dimensional silt fence materials. However, caution must be used by designers in specifying products from the three-dimensional filter socks and fibrous wattles category, as the lower vertical profile will lead to greater maintenance/clean-out requirements compared to two-dimensional silt fence products. Lack of maintenance/clean-out will lead to soil deposition on the upstream face of the SRD. This soil buildup will eliminate any vertical profile for filtration and cause constant overtopping and lack of product efficacy.

Proprietary woven geotextile silt fence

Six products from the proprietary woven geotextile silt fence category were tested via ASTM D5141 in this research. The data shows that this product category provides poor filtration efficiency, with products tested providing between 30% and 70% filtration efficiency and high flow rate. It should be noted that most of the products from this category were open-weave geotextiles, which may be appropriate for use as a prefilter prior to runoff reaching a point of concentration before being released off-site or as a filtration component in areas where trash migration is an issue.

ASTM D7351 test data

Figure 4 presents results of D7351 testing of a wide range of SRD types commonly used in linear, toe-of-slope applications. SRDs tested include both woven and nonwoven geotextile-based silt fence, along with various proprietary composite and woven fences and socks/wattles.

FIGURE 4 ASTM D7351 performance testing results

Discussion of D7351 data

ASTM D7351 was used for the evaluation of 12 unique two-dimensional silt fence SRD products and 10 unique three-dimensional sock/wattle SRD products in this research. All the products were subjected to the theoretical 30-minute peak discharge of a design storm event, as prescribed in the standard calculated using the MUSLE. The resulting influent sediment concentration is prescribed at approximately 6%, and the soil type used in this research was loam with a PI ≤ 8.

The data displays a clear delineation in performance between the two-dimensional and three-dimensional SRD products. Two-dimensional SRD products tested via ASTM D7351 provide consistently high sediment retention efficiency, with all tested product providing no less than 82% sediment retention efficiency. However, performance of the two-dimensional SRD category, in terms or water retention efficiency, ranges greatly, from 7% water retention effectiveness to 98% water retention effectiveness over the course of the design storm. This relationship is most likely a result of the material characteristics of the tested product, specifically, weave geometry. In the data, the products with lower water retention effectiveness are all composed of plastic monofilament fibers, while the products with higher water retention effectiveness are composed of slit film fibers. As discussed earlier, the differences in manufacturing of these filtration component fabrics leads to a marked difference in flow rate, and thus water retention effectiveness. Encouragingly, these results provide reasonable agreement with the performance of product categories tested in the ASTM D5141 testing program described previously.

Separately, the data displays relatively consistent performance from the three-dimensional SRDs tested in this program. All three-dimensional products provided high sediment retention efficiency, with performance ranging from 88% to 98% efficiency. However, as a product category, and as opposed to two-dimensional SRDs, three-dimensional SRDs have a consistently lower water retention effectiveness percentage (Figure 5). This is a result of the vertical profile, or effective height, of the three-dimensional SRD tested and the lack of a burial trench. Larger nominal diameter allows for greater ponding area upstream of the SRD and means a greater volume of product fill per unit length, which typically leads to a higher unit weight per length of the product. Also, in contrast to two-dimensional SRDs, which are typically installed with a 6-inch (152-mm) burial trench sealing the fabric to the soil surface, three-dimensional SRDs are often installed in a shallow installation trench or laid directly on the soil surface. This lack of a sealing burial trench allows water to more freely seep under the installed three-dimensional SRD, further relying on the unit weight per length of the product to seal against the soil surface and slow seepage. The three-dimensional SRDs tested in this research ranged in nominal diameter, and thus nominal height, from 8 to 24 inches (203 to 610 mm). Unsurprisingly, the products with the highest water retention effectiveness were all of larger nominal diameter. The two 24-inch (610-mm) nominal diameter products produced 29.8% and 23.3% water retention effectiveness, and the 18-inch (457-mm) nominal diameter product produced 22.0% water
retention effectiveness.

FIGURE 5 ASTM D7351 toe-of-slope filter sock testing (upstream)

There are two caveats relating to the performance of three-dimensional products that should be discussed. The first caveat to the data presented here is that the nominal diameter/height and functional height are often different. Installation of three-dimensional SRD products often includes trenching, staking, stapling or strapping and can affect the functional height of the installed SRD. Thus, a three-dimensional product with a nominal height/diameter of 24 inches (610 mm) and a deep installation trench could have similar functional height to a three-dimensional product with a nominal height/diameter of 18 inches (457 mm) and a shallow installation trench. The second caveat is that unit weight of the three-dimensional SRD should be considered. A lightweight three-dimensional SRD of a given nominal height/diameter may not provide the performance of a heavyweight three-dimensional SRD of a similar nominal height/diameter. Unsurprisingly, this phenomenon is represented in the data above. A 9-inch (229-mm) nominal height/diameter straw wattle had lower performance in both sediment retention effectiveness and water retention effectiveness (88.5% and 15.7%) than an 8-inch (203-mm) nominal height/diameter compost filter sock (89.7% and 19.7%). The straw wattle has a lower unit weight (kg/m) than the compost filter sock. This lighter weight of the straw wattle can potentially lead to floating or movement under hydrostatic load, which could be the cause for the differences in performance.

Based on the data presented above and the discussion of variables in three-dimensional SRD performance, designers must balance factors such as nominal height/diameter, functional height, unit weight and installation details when determining the appropriate three-dimensional SRD for a sediment control application.

Conclusions

This two-part article has presented robust data, via two standardized test methods, on the performance of both SRD filtration components and “as installed” SRD systems. These standard testing procedures are available to assist the users of SRDs in establishing improved construction specifications that will guide owners and contractors to install the correct SRD for the expected site conditions. Yet to be invented is the SRD that has 100% sediment retention. Typically, when sediment retention, a.k.a. filtration efficiency, is increased, loss of seepage, or retention efficiency, also increases. Both ASTM D5141 and D7351 can differentiate between SRD types used in toe-of-slope applications. There are clear performance differences, using both methods, when evaluating filtration components and installed systems. Within the erosion and sediment control industry, manufacturers, users and designers must balance the known performance characteristics when determining the appropriate SRD to use in a specific sediment control application.

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.


References

ASTM D5141. (2018). 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. (2019). Standard Test Method for Determination of Sediment Retention Device Effectiveness in Sheet Flow Applications. American Society for Testing and Materials, West Conshohocken, Pa.