Cost-Effective Erosion Control With Blankets and Mats – Part 3

Colorado: Mountainous and Unpredictable

According to Tom Boyce, water-quality program manager for the Colorado DOT (CO DOT) in Denver, unlike Pennsylvania, Colorado makes extensive use of erosion control blankets on newly graded and seeded highway slopes and embankments steeper than 2:1. He says Colorado is an extensive user because the state has many steep slopes, and when rain comes, it often comes suddenly and intensely. Furthermore, the highly variable climate means that the growing season for grass is short, and as a result it takes several years to get a good stand of grass growing. Finally, the state’s scenic beauty is one of its most important assets, and therefore it is important that slopes get adequate grass cover.

Colorado has a very varied terrain and climate, and in many areas of the state the growing season is short. Often it is too dry or too cold for seed to germinate. Temperature levels and moisture levels fluctuate wildly. Because of this short growing season, Boyce says, an erosion control blanket must last for two to four years before biodegrading.

CO DOT landscape architect Mike Banovich says that the following are the most common types of erosion control blankets CO DOT uses on newly graded and seeded slopes steeper than 2:1:

• Straw blanket, with plastic netting on the bottom and top–least costly of the erosion control blankets, with an installed cost of $1.50/yd.² With a typical lifetime of one to two years, this fast-biodegrading blanket does not allow enough time for good vegetative cover to become established in some parts of the state. Its best feature: an open, less dense structure compared to a coir blanket of similar weight, allowing seed in underlying soil to get more moisture and germinate more quickly.
• 70% straw/30% coir blanket, with plastic netting on the bottom and top–the installed cost is $2/yd². It is a very popular choice in Colorado because it permits faster germination than a 100% coir blanket does.
• 100% coir blanket, with plastic netting on the bottom and top–installed cost is $3/yd². Coir blankets have a more densely woven structure than straw blankets, making it more difficult for moisture to penetrate and potentially slowing seed germination. Nonetheless, Colorado uses such blankets extensively, especially in areas where there is more rainfall and where planners anticipate it will take several years to grow an adequate stand of grass. Even with the intense ultraviolet exposure in elevated Colorado, a coir blanket will last six years, this durability being this blanket’s most appealing asset.
• Excelsior blanket, with plastic netting on the bottom and top–composed of curled wood shavings (excelsior) from Aspen trees and similar in performance to the popular straw-coir blanket. Typically an excelsior blanket will last for four years. Installed cost is $2/yd². CO DOT has worked with American Excelsior to come up with a less dense, lighter-weight blanket, thereby permitting faster germination and growth of grass seed.

For areas with rocky soil, where blankets are difficult to properly staple down, CO DOT sprays on a hay mulch with a tackifier. Netting is not used over the mulch, explains Banovich, because it tends to blow away. Instead, the tackifier bonds the straw fibers to one another and to the soil.

For flat or moderately sloped (less than 2:1) areas that have just been seeded and graded, CO DOT spreads on a straw or hay mulch, then sprays on a tackifier (200 lb./ac.) that glues the straw or hay strands together. Even on a flat grade, CO DOT favors a mulch to protect the soil against erosion and to hold in soil moisture. The installed cost of applying such a sprayed-on mulch is about $0.50/yd.²

Banovich cautions that correct installation of erosion control blankets is critical, for the highest-quality blanket is worthless if improperly installed. A blanket needs to be oriented transversely to the longitudinal axis of the slope, unrolled from the top to the bottom of the slope. The blanket ends must be entrenched at both the top and the bottom of the slope. Adjacent parallel rolls must be overlapped. And each roll must be properly anchored, with staples placed at the right intervals (about every 3 ft.) in the right places. In Colorado, private contractors install erosion control blankets, but CO DOT employees inspect the site carefully for proper installation.

The other major use for temporary erosion control blankets and permanent TRMs by the CO DOT is for lining newly graded and seeded ditches and channels along the state’s roads and highways. Banovich says more than 90% of the countless miles of ditches in the state need only a thick stand of grass growing in them to protect against soil erosion.

But for the remaining 10% of the ditches, with gradients ranging from 2% to 10%, stormwater flow velocities are considerably higher, so much so that natural grass all by itself is not vigorous enough to resist the shear and erosive forces of the surging stormwater over an extended period. CO DOT installs permanent synthetic TRMs, especially along highway shoulders or in median strips, where safety considerations rule out lining the ditches with riprap or concrete.

“When we install a turf-reinforcing mat in a ditch or in a channel,” explains Banovich, “we expect it to be permanent, we expect it not to rot out or to biodegrade. That is why the mats we specify for such applications must be made of synthetic–not natural–materials. We want the mat to remain permanently in place to continue to reinforce the roots of the grass decade after decade.”

CO DOT uses two basic approaches to reinforcing its ditches, depending on how steep the ditch is. For ditches or channels with gradients between 2% and 5%, CO DOT contractors first prepare the soil and seed the ditch, then staple down the TRM directly on top of the soil. The grass will subsequently shoot up through the mat, and the mat will remain in place indefinitely, reinforcing the dense network of grass roots.

For ditches or channels with a gradient between 5% and 10%, Banovich says the approach–worked out through long experience–is different. First, the mat is stapled directly on the soil surface of the ditch and 2 in. of topsoil is placed directly on top. The topsoil is seeded and covered with a biodegradable erosion control blanket to protect it and keep the seed from washing away. As the temporary mat degrades, the germinating seed sends grass upward and a dense tangle of roots downward through the permanent synthetic turf-reinforcing mat.