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Designing a defense against erosion

May 2011 » Products » ENGINEERED SOLUTIONS
Projects highlight strategic use of hard, soft, and organic technologies.
Bob Drake

Hard armor, soft armor, and vegetation are all important defensive weapons in the battle against erosion and its consequent impacts on landscapes, infrastructure, and stormwater quality. And as in war, victories depend on effective and strategic use of a varied arsenal as situations “on the ground” — literally — dictate. The following applications of a range of erosion control technologies in transportation and drainage projects highlight strategies that civil engineers and erosion control professionals can consider when designing such a defense.

Airport Drive, Charlotte, N.C.
Sanders Utility Construction of Charlotte, N.C., recently installed 19,000 feet of steel water supply pipe along Airport Drive, adjacent to the Charlotte-Douglas International Airport. Airport Drive had a severe dip in it where it crossed a watercourse and the project designer, Camp, Dresser and McKee, saw an opportunity to eliminate the severe dip as part of the utility installation. Fill was imported and the road was elevated, thereby eliminating the severe dip. However, steep side slopes, with slope gradients exceeding 2:1, resulted from the fill placement. The project designer specified a triple-net coconut turf reinforcement mat (TRM) to protect the side slopes from erosion and to facilitate establishment of vegetation. East Coast Erosion Blanket ECC-3, a triple-net TRM with a coconut matrix, was installed after the side slopes were graded and seeded. The ECC-3 protected the slopes from raindrop splash erosion and provided a permanent mat that allowed the vegetation to grow into and through it. The TRM now provides a strong base that is incorporated with the plants to protect the slope.

On a roadway elevation project, East Coast Erosion Blanket ECC-3, a triple-net turf reinforcement mat with a coconut matrix, was installed after the roadside slopes were graded and seeded.
East Coast Erosion Blankets LLC

East Coast Erosion Blanket’s ECC-3 triple-net coconut TRMs are designed to provide permanent slope protection. The TRMs are manufactured with a top and bottom, medium-weight 8# PMSF, UV-stabilized polypropylene net and a middle heavyweight 24# PMSF, UV-stabilized net. All three nets are UV stabilized to resist the effects of natural ultraviolet radiation. The coconut fiber matrix is 100 percent organic and will slowly degrade as it is exposed to the forces of nature and as the vegetation penetrates through the netting.

The 100-square-yard TRM rolls were easy to install. The product was anchored at the top of the slope and allowed to unroll down the slope. The TRM was stapled to the slope with 6-inch, 11-gauge steel staples. Vegetation quickly became established.
Information provided by East Coast Erosion Blankets LLC

State Highway 385 overpass, Memphis, Tenn.
The Tennessee Department of Transportation (TDOT), like many state transportation departments, has traditionally used rock rip rap as its primary erosion control solution for bridge drain applications. But in the case of the extreme water flows from one of its overpasses, located at U.S. Highway 72 and State Highway 385 just southeast of Memphis, rip rap wasn’t the appropriate remedy.

The Tennessee Department of Transportation used approximately 1,000 square feet of ScourStop transition mats (above – photo during installation) with a combination of sod and turf reinforcement mats (below – photo after vegetation) to replace rip rap at a bridge drain near Memphis.
Landmark Earth Solutions Inc.

This bridge had a drainage channel with a 3:1 slope. Combined with the stress of concentrated water flows, gravity easily dislodged and displaced the rock, resulting in erosion at the overpass and a continued headache for the TDOT maintenance team.

The site also became a safety hazard for motorists because of sediment running onto the roadway. TDOT incurred continuing costs from the frequent need to remove sediment and repair the rip rap after each major storm event.

ScourStop transition mats were presented to TDOT as a solution to deal with the steep slopes and extreme water velocities. Approximately 1,000 square feet of ScourStop mats were used with a combination of sod and TRMs to replace the rip rap at the bridge drain. The ScourStop solution addressed the erosion issues and stabilized the drainage system. TDOT no longer has to return to repair or replace the dislodged rip rap and the sediment is no longer being carried onto the roadway.

Scour Stop transition mats — 4 feet by 4 feet by 1/2-inch-thick polymer — are installed easily on steep slopes without requiring heavy equipment, and are a no-maintenance, vegetated, NPDES-compliant alternative to hard armor solutions. Hard armor has long been the standard remedy for abating the effects of extreme hydraulic conditions. But with the combination of stress from concentrated water flows and gravity, loose-placed rip rap frequently moves, requiring repeated maintenance and replacement to ensure the integrity of the engineered drainage system.

In steep drainage designs, ScourStop manages these stresses more effectively through its combination of deep-seated anchoring and lower profile. In addition, the ScourStop system allows the incorporation of vegetation into the steep drainage channel, which in turn decreases the velocity of the flows, cleans the runoff water, and enables infiltration into the soil.
Information provided by Landmark Earth Solutions Inc.

Interstate 385, South Carolina
The Interstate 385 renovation project in South Carolina began in January 2010. Led by the South Carolina Department of Transportation, the $60 million rehabilitation and widening of a 15-mile stretch of roadway included a massive erosion control assignment.

More than 300 acres of steep slopes needed vegetation established quickly to keep soil from eroding in an area of the state dissected by two dozen streams. Further complicating matters, the work had to take place during the winter and spring seasons that saw heavy rains and cool temperatures throughout construction — not exactly an ideal scenario for such a large erosion control application.

A worker hydraulically applies Profile Products’ Flexterra High Performance-Flexible Growth Medium to more than 300 acres of steep roadside slopes on a South Carolina interstate renovation project.Profile Products LLC

The state of South Carolina man-dated that the project be fast-tracked because the revamped stretch of interstate was so well traveled — about 21,900 vehicles each day. The condensed construction schedule saved the state more than $34 million.

However, that required the project, which would typically take two years to complete, to follow a tight eight-month timeframe. The interstate was closed to one direction of traffic to hasten construction — the first time an entire direction of traffic had been closed on an interstate in the state’s history. Consequently, the erosion control products selected had to work on first application to prevent the project from incurring fines for liquidated damages. The contractor, McCarthy Improvement Co., would be fined $50,000 for each day the roadway remained closed to traffic past its mid-August deadline.

The project’s numerous erosion control challenges were solved with Profile Products LLC’s Flexterra High Performance-Flexible Growth Medium (HP-FGM).

Flexterra HP-FGM stood out to officials because it is applied hydraulically and designed to bond immediately to soil, even under wet conditions. In independent testing, Flexterra HP-FGM was shown to be 99-percent effective. Vegetative establishment was documented to grow eight times faster than bare soil and twice as fast as rolled erosion control blankets. It’s also faster and less expensive to use than blankets because no fine grading is required to smooth the slopes before application. Patented HP-FGM is non-toxic, 100-percent biodegradable, and manufactured from 100-percent recycled wood fibers and naturally derived biopolymers.

A two-step application technique was required. Seed, fertilizer, and agronomic amendments with a small amount of HP-FGM were applied first. After the first pass, McCarthy applied the remaining HP-FGM in opposing directions to cover and protect the previously applied layer of seed and amendments to achieve the specified application rate.
Information provided by Profile Products LLC

West City Ditch, Kissimmee, Fla.
The city of Kissimmee, Fla., experienced extensive flooding on July 11, 2009, from a flash flood where unofficial recordings of 7 inches of rain in two hours were reported. The nearest city rain gauge reported 4.35 inches in two hours. The flash flood was caused by two colliding fronts that converged over a 1,200-acre area of the city. The subsequent runoff severely eroded a major conveyance ditch, named the West City Ditch, which drains half of the city’s stormwater into Lake Tohopekaliga. The ditch is designed to convey 1,500 cubic feet per second for a 100-year storm event; however, the flash flood was equivalent to a 500-year storm event and the resulting overtopping flow eroded the ditch banks by more than 2 feet along certain reaches of the canal and flowed over three collector roads crossing the ditch.

Because of the severe damage caused by the flash flood and the desire to prevent this type of damage in the future, the city applied for an Emergency Watershed Restoration Grant with the U.S. Department of Agriculture – Natural Resource Conservation Service (NRCS). The city was awarded the grant to make the necessary repairs to restore the ditch banks and install erosion protection measures. Initially, the parties desired a cost-effective and green solution instead of using traditional methods such as rip rap. Having several positive experiences with engineered solutions and services supplied by CONTECH Construction Products Inc. and Propex for stream bank stabilization, the NRCS recommended the city use a combination of ArmorMax (soft armor) permanent erosion solutions and ArmorFlex (hard armor) to restore the eroded streambanks and to provide erosion protection measures for the future.

The ArmorMax anchored reinforced vegetation system, manufactured by Propex and supplied by CONTECH for this project, is composed of a high-performance TRM (HPTRM) and earth percussion anchors. ArmorMax is an engineered solution and provides additional factors of safety against erosion and scour. It also can be engineered to failures when needed. The ArmorMax system initially was used on the West City Ditch to prevent surface erosion, but upon further investigation, it was determined that the ditch embankments were unstable. Therefore, a structural ArmorMax system was needed to prevent erosion caused by surficial slope failures. Propex provided the necessary engineering/slope stability analysis needed to complete the design, and 6-foot, 9-foot, and 12-foot anchors were used to stabilize the slopes, while the HPTRM component of the system provided for permanent surface erosion protection.

ArmorFlex concrete articulated block mats, manufactured by CONTECH, make a flexible matrix of concrete blocks with uniform size, shape, and weight. ArmorFlex blocks are designed to have specific hydraulic capacities and are laced longitudinally with cables to ease handling and installation. The blocks come in an open-cell or closed-cell configuration, depending on the aesthetic and vegetation needs. The blocks were designed to withstand the hydraulic forces that would be anticipated in the next severe storm event and multiple block sizes and thicknesses were used on the project, ranging from 4 inches to 6 inches and using both open- and closed-cell configurations.
Information provided by Propex Inc.

Mary Creek, Archbold Biological Reserve, Fla.
Mary Creek, which drains approximately 1,000 acres of preserve and wetland ground in Archbold Biological Reserve, Fla., was experiencing erosion caused by high flows from storm conditions. The naturally meandering creek had straightened due to erosion and the force of the stormwater. For purposes of wetland restoration, it was desired to realign Mary Creek to its former meandering state. When the first round of dirt work to realign the creek failed after a major storm event, it was decided that the creek would need to be armored to prevent major erosion.

The initial thought was to hard armor the creek with articulated concrete block or rock Reno mattresses. However, with the wildlife consideration of the site, soft armor designs ultimately were desired. To provide permanent soil protection, Tensar’s North American Green’s P500 Turf/Earth Reinforcement Mat (TERM) was selected. After grading and channel realignment were completed, the P500 TERM was installed. Two sections of Mary Creek were protected — one upstream and one downstream of a newly constructed bridge. The upstream section was realigned to the natural meandering form of the creek with its many tight curves, while the downstream section was a straight shot that extended beyond the new bridge.

The P500 TERM was selected because of its long-term erosion control and vegetation reinforcement capabilities facilitated by excellent UV stability and high tensile strength. As with any TRM, the P500 TERM is designed to protect the soil temporarily and permanently aid in the reinforcement of the vegetation roots and stems as the site develops and matures. A series of fasteners including earth anchors and 24-inch pins were used to provide permanent deep strata anchoring and to ensure intimate mat-to-soil contact. Sod was installed on top of the P500 TERM in lieu of pre-seeding. Minimal irrigation was needed onsite, and the sod was installed on the channel side slopes, leaving the continually wet bottom of the channel to vegetate naturally. As the sod established, the strength of the complete system continued to increase. With minimal installation costs, and material costs lower than hard-armored systems, the P500 TERM resulted in a high-strength, low-cost, and natural alternative.
Information provided by Tensar International Corp.

Propex ArmorMax anchored reinforced vegetation system — soft armor (background) — and CONTECH ArmorFlex concrete articulated block mats (foreground) were combined to protect West City Ditch in Kissimmee, Fla., from erosion.
Propex Inc.
The complete Mary Creek drainage channel was protected with a system including Tensar’s North American Green P500 Turf/Earth Reinforcement Mat and earth anchors.
Tensar International Corp.

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