By Joel Sprague and Jay Sprague – Sediment retention devices include silt fence, wattles, filter logs, compost socks, and various types of stormwater inlet protectors. They are a widely used best management practice (BMP) to remove sediment from stormwater runoff as water passes through the BMP. However, SRD performance varies greatly with regard to flow rate and filtration efficiency.
The body of performance data related to the flow rate and filtration capabilities of SRD products based on recognized standard ASTM test procedures is limited but growing. Here at TRI, we regularly conduct standardized SRD tests to help private companies, project owners, and the overall field build its understanding of and confidence in the selection of sediment retention devices.
Of note, 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.” Large-scale, “as installed” conditions can also be tested using ASTM D7351, “Standard Test Method for Determination of Sediment Retention Device Effectiveness in Sheet Flow Applications.” 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 back-up of runoff can be assessed.
TRI conducts both bench and large-scale testing at its erosion and sediment control labs. Contact Jay Sprague for more information.
SEDIMENT RETENTION DEVICES – AN UPDATE ON CHARACTERIZATION
Figure 3 shows results from ASTM D5141 testing, in which a wide range of SRD types commonly used in linear, toe-of-slope applications were evaluated. The SRDs tested included both woven and nonwoven geotextile-based silt fences as well as various proprietary composite and woven fences and socks/wattles.
The chart shows a clear delineation between product types and their relative performance in both filtration efficiency and flow rate. For example, the slit-film silt fence exhibits high filtration efficiency but modest to very low flow capacity while monofilament silt fence has much improved flow capacity but reduced filtration performance. In this we see that the nonwoven composites and three-dimensional socks/wattles are able to provide higher flow rates while maintaining high filtration efficiency.
Figure 4, meanwhile, shows ASTM D7351 results for 12 unique two-dimensional silt fence-type SRD products and 10 unique three-dimensional sock/wattle-type SRD products. All of the products were subjected to the theoretical thirty minute peak discharge of a design storm event as prescribed in the standard, which uses the Modified Universal Soil Loss Equation (MUSLE). The resulting influent sediment concentration is prescribed at approximately 6%, and the soil type used was Loam with a PI ≤ 8.
The data displays a delineation in performance between the two-dimensional and three-dimensional SRDs. Two-dimensional products consistently demonstrate high sediment retention efficiency, typically because they also exhibit high water retention. Conversely, all three-dimensional products provided high sediment retention efficiency and consistently low water retention effectiveness percentage.
GOOD, BETTER & BEST
Standard testing procedures are available to assess the relative performance of SRDs under conditions typical of construction sites. While yet to be invented is the SRD that has 100% seepage and 100% sediment retention, performance testing can effectively differentiate products and show which perform pretty well, which perform better, and which are the very best at this time.
In our next lab update, we’ll look at the standards being developed for post-construction Manufactured Treatment Devices (a.k.a. stormwater control measures). This work is largely taking place in ASTM Subcommittee C27.70 on Precast Concrete Products for Stormwater Management. Three standards (C1745, C1746, and C1814) are being revised, and several more are being introduced and should become standards soon!
In the meantime, we’d love to hear from you.
Joel Sprague, P.E. is Technical Director and Jay Sprague, CPESC is Lab Director for TRI Environmental – South Carolina. Learn more about their work at www.erosiontest.com.