“The 5 Second Rule” regarding the cleanliness of fallen food, the Great Wall of China being visible from space, and shaving thickens hair – are all myths. Food is contaminated by the amount of bacteria on the floor, not how long the food stays there.The Great Wall of China is not visible to the naked eye from space (check it out for yourself on Google Earth); and put away those razors. Your hair is not going to grow in thicker. These are known as urban legends.But this is not the only place legends abound; there are plenty of myths when it comes to sediment control techniques. Let’s shed some light on common myths about sediment control.
Myth – Fiber rolls filter stormwater
No, they do not, and we can prove it. You know the story of the Three Little Pigs. What did one of the pigs use to build his house? Straw. Why, because it filters water? No. Why do many residents in developing countries still use straw on their roofs? Because it, more or less, keeps the water out. Compacted straw does not filter, but rather repels, water. The job of fiber rolls is to slow the flow by creating a miniature dam. When water velocity slows, particles settle out.
Myth – Fiber rolls rolled-out on the surface are effective
If fact or fiction was determined by popular vote, this statement would be true based on the overwhelming number of construction sites where it occurs. It is very common to find fiber rolls surrounding a site that is neither keyed-in (trenched 2-3”) nor staked down. Based on field testing, improperly installed fiber roll worsens the turbidity that is caused by turbulence of water jetting under the fiber roll.
Myth – Doubling up fiber roll and silt fence provides better protection
Sorry, false again. It mostly just wastes your money. Remember, the goal of these devices is to slow the flow, not filter. When properly installed, fiber roll or silt fence sufficiently slows the flow by itself. No significant velocity reduction is gained by doubling up the two devices.
Myth – Fiber roll and compost socks do the same thing
This is a half-truth. It is true that they both slow the flow causing sedimentation to occur. However, compost socks do something that fiber rolls do not do. They filter the water and remove sediment particles and other pollutants. These pollutants become entrained in the filter media. Compost socks are much heavier than fiber roll and become heavier as they are saturated with water and fill up with captured particles. Therefore, compost socks are “self-weighting” and will conform well to the surface without the need to key them in and stake them down. This allows compost socks to be used effectively on paved surfaces. When it is not secured, fiber roll actually will float on water.
Myth – Perimeter controls only need to be installed once and will last the duration of the project
This is the belief held by most project estimators. They typically will only include one initial installation of perimeter controls in their budgets and schedules even for multi-year projects. All BMPs need maintenance. Fiber roll, compost socks, and silt fence will all take a beating from the sun, wind, and construction activities. They will eventually wear out and need to be replaced.In addition, as site conditions change, the perimeter control strategy will also need to change. Fiber roll might have been a perfectly good sediment control measure during the grading phase, but compost socks may be more appropriate during the vertical phase.
John Teravskis is a Senior Compliance Specialist with WGR Southwest, Inc. in Lodi, California. He is a Certified Professional in Erosion and Sediment Control, a Qualified SWPPP Developer and a Qualified SWPPP Practitioner. John is also a Trainer of Record for California’s QSP/QSD program. John is the editor of WGR’s Monthly Dirt which is a free newsletter for those having to comply with California’s Construction General Permit.