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CMC and Hanes Erosion Control Overview. What is Erosion?. Erosion.
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Erosion • Raindrop impact is the major cause of soil particle detachment which can result in the particles moving down slope in flowing water (as sheet erosion) during a rainfall event. Flowing water can also detach soil particles if the velocity is high enough, usually where water starts to concentrate (rill and gully erosion)
How do we prevent Erosion? • If the velocity of flowing water is reduced sufficiently, particles will settle out. The velocity at which settling begins is dependent on particle size and density, as is the time required for the particles to settle out. Large, dense particles, such as grains of sand, settle first. Fine clays settle out slowly and only in relatively still water. Mechanical filtration is also used.
Why do we have to prevent Erosion? • Runoff from construction sites is the major source of sediment in urban areas under development. There are two main reasons why construction activities increase pollutant loads in runoff. First, the volume and rate of runoff typically increase, providing a larger capacity to transport pollutants to rivers and lakes. Second, the vegetation is removed, leaving bare soil that is much more vulnerable to erosion, resulting in more sediment moving into receiving waters. Typical sediment loading rates from construction sites vary from 100 to 200 tons per acre per year and can range up to 1,100 tons per acre per year.
But WHY do we have to prevent Erosion? • Let’s face it- there is not a product in this category that customers buy because they want to. They buy them because they have to. • Erosion control measures are dictated by DOT regulations, municipalities, and the NPDES Guidelines. • Annnnd they’re all different. KNOW. YOUR. SPECS!
Spec Differences • Austin specs are different from San Antonio which are different from Corpus which are different from TXDOT which are different from USDA specs which are different from Corps of Engineers. • Best you can do is ask your customers- and there is ALWAYS a spec for all of these products. That being said, here are a few differences…
Austin vs. San Antonio • Silt fence: 6oz nonwoven on 12.5ga wire w/ 4’ 1.25lb/lf posts • Curlex required as erosion blanket, even when on TXDOT projects where straw would otherwise be acceptable • Silt Fence: 70gm woven fabric on 14ga wire w/ 4’ 0.85lb/lf posts • Straw blankets accepted for ECBs
KNOW YOUR SPECS! • Have the Standard Specifications for Construction and Maintenance of Highways, Streets, and Bridges. • Have your City and surrounding City’s specs printed and in binders- on your desk AND in your truck. Yes. They’re 600 pages each. Have hard copies. • It takes research and dedication. And it sets you apart.
Silt Fence • Install before land is disturbed • Install on down slope side of site parallel to contour of the land • Stake posts on downhill side • Extended ends up slope enough to allow water to pond behind fence • Turn ends of fence uphill • Bury 8 inches of fabric in trench • Leave no gaps. Overlap sections of silt fence, or twist ends of silt fence together. • Inspect and repair once a week and after every rain. Remove sediment if deposits reach half the fence height • Maintain until a lawn is established • Remove upon project completion and disturbed soil is vegetated
Silt Fence • COMMON MISTAKES AND PROBLEMS • Not adhering to daily inspection/maintenance schedule • Fabric not tucked into trench • Posts on uphill side of fence • Silt fence used in areas of concentrated flow • Not repairing silt fence after it has been damaged or knocked down
Erosion Control Logs • Common sizes: 9”, 12”, and 20” diameter • Matrix: Straw, Aspen, Coconut Fiber, or Mulch • Require stakes, generally 24”-36” wood, when installed • Generally required to overlap ends, so allow for slight waste when calculating total logs needed
Erosion Logs: the Why • Serve same purpose as silt fence, basically. • Install before land is disturbed to prevent jobsite runoff. • Placed perpendicular to flow rate, aside from stream bank establishment applications. • Lower impact, lower installation cost, and biodegradable so often do not require being removed. • One warning- just like straw erosion blankets, straw logs can be destroyed by pesky wildlife. Mention to customers!
A Rose by Any Other Name… • Erosion Control Blankets • Rolled Erosion Control Blankets • Vegetation Mats • Turf Reinforcement Mats • Erosion Control Rolls • “That green stuff TXDOT uses…”
ECBs and TRMs • All of these products are the same concept: A rolled blanket or mat that prevents erosion before vegetation is established by growing through the material itself. • On permanent mats there is a nondegradeable component that reinforces the root zone to strengthen the vegetation’s resistance to shear stress of water moving across it.
ECB vs. TRM? What is the difference? • ECB stands for Erosion Control Blanket and generally refers to any Temporary or Biodegradable blanket. I.E. Straw mats or curlex mats • TRM stands for Turf Reinforcement Mats and are generally permanent and withstand higher flow rates
Why vegetation at all? • Vegetative cover is normally the most effective and practical control of erosion and sediment loadings but takes time for establishment. Sodding can reduce the time required for grass establishment and can be used to control erosion in some channels in lieu of riprap or concrete. The roots of vegetation, such as grass, binds soil particles together to resist erosion. Vegetation helps absorb the impact of raindrops to prevent detachment of soil particles. Vegetation can flatten and, like shingles on a roof, allow the runoff to flow above the soil. Stiff, standing vegetation can slow runoff (which may allow larger particles to settle out) and act as a filter to remove soil particles from flowing water.
Geotextiles Filter Fabrics are always geotextiles, but geotextiles are not always filter fabrics! There are two main types of Geotextiles: Woven and Non-Woven
Woven Geotextiles • Woven geotextiles are composed of 2 different kinds of “thread”: Slit Tape and Monofilament • Slit Tape is less expensive and substantially more common in the market, Monofilament is much stronger
Nonwoven Fabrics • Nonwoven fabrics look like felt: uniform but without a visible weave pattern. • Have a very high flow rate and are good for separation and the stronger ones for stabilization • Nonwovens are described by their weight in ounces per square yard. • Used behind retaining walls, in French drain systems, and under rock for separation
Woven and Non-Woven Fabrics Can be used for Road Reinforcement
Road Reinforcement: Which to Use? • Woven Fabrics tend to be stronger with less stretch, BUT can be punctured easily • Non-Woven Fabrics have more “give”, so are more resistant to punctures, but don’t provide the permanence of woven fabrics • Nonwoven Fabrics have a higher flow rate • Sooooo….
Road Reinforcement: Which to Use? • Woven Fabrics are good for soft and saturated soils: i.e. sandy and muddy conditions. Generally used along the coast more than in central Texas • Nonwoven fabrics are good for conditions with rock: i.e. stabilized construction entrances, temporary roads, etc. • Woven Monofilament will always have very specialized applications: i.e under articulated block, in jetties, etc. Least common geotextile. • This is ALWAYS an engineered product unless contractor is slapping something together for private work
Most Common Geotextiles • Terratex SD- TXDOT Type 1 approved Filter Fabric. Used under small rock (4”), French Drains, and behind retaining walls. 4.2oz Nonwoven Fabric • Terratex N08- TXDOT Type 2 approved Filter Fabric. Used under gabions and large riprap, also in Construction Entrances. 8oz. Nonwoven Fabric • Terratex GS- (stands for Ground Stabilization) Most common stabilization fabric for sandy and soft soils. Equivalent to Mirafi 500x
Geogrid • Geogrid- used for aggregate containment and reinforcement under roads • Used either to strengthen the base material OR to reduce amount of aggregate needed for desired strength. • Rectangular aperture Geogrids are the TXDOT approved geogrids, triangular aperture are NOT! • Type 1 Terragrid RX1100- used 90% of the time • Type 2 Terragrid RX1200- used predominantly in ports and freight yards. Stronger and less expensive than TX5!
Other Products • Gabions • Turbidity Barriers • Drainage Mats • Pond Liners • Hydraulically Applied Products • Rock Berm Wire • Seed • Safety Fence • Etc.
