Erosion Control Methods for Steep Slopes

Highway construction projects create special needs in erosion control.

Erosion-control-Geosynthetics-coir-mats

Credit: TERRAFIX GEOSYNTHETICS
After revegetation

Editor’s note: This article first appeared in the May 2015 edition of Erosion Control.

Huge projects like the widening of US 202 in Pennsylvania span several years and several seasons. Stormwater and erosion control is an important part of the planning process, and a variety of soil erosion control devices play a role. Changing topography and limited highway easements sometimes result in steep slopes which present unique challenges for stormwater and erosion control and mitigation projects.

Background: What to Do With High-Volume Traffic

A Pennsylvania Department of Transportation (PennDOT) report states, “Some of the fastest growing areas in the Greater Philadelphia Region are located along US 202. Many of these areas have experienced rapid residential, commercial, and light industrial growth, the result of acres of undeveloped land and the corridor’s reputation as a high-tech growth area.”

To relieve congestion and meet existing and future travel demands, there was a huge need to increase the highway’s capacity. The project was divided into eight sections, including a segment known as Section 320. In addition to major road construction and the addition of an Intelligent Transportation System (ITS), Section 320 needed bridges reconstructed, retaining walls, improvements to the stormwater drainage systems, and erosion control measures.

RAM-T Corp. worked closely with Richard E. Pierson Construction Company, the lead contractor on the “Congestion Mitigation Program on U.S. 202 Sect. 320” in Chester County. RAM-T’s part of the massive highway project was basin stabilization and stormwater mitigation at East Whiteland Township. RAM-T Corp. of Bernville, PA, is a leading manufacturer and installer of erosion control products, and with owner/president Deborah Turner leads the way as Pennsylvania’s largest woman-owned business.

Christine Rovner, purchasing agent with RAM-T Corp., explains that the stormwater mitigation basins along Section 320 incorporate many erosion and sediment control best management practices (BMPs), each used for different purposes. “The basin bottoms were stabilized with seed and fiber mulch so that the inlets and culverts do not fill with the typically used straw mulch, causing clogs,” says Rovner. “The side slopes were stabilized with the designated seed and S1 matting, and the emergency spillways were installed with seed and P2 matting to ensure permanent, heavy stabilization.”

Working with the erosion and sediment control plans designed by Michael Baker Jr. Inc. and landscaping plans prepared by Menke & Menke LLC, both of Horsham, PA, the crews from RAM-T Corp. began by protecting the mitigation areas first with super silt fence and compost filter socks to control stormwater and remove sediment.

“These were PennDOT mitigation projects,” explains Don Holladay, RAM-T Corp. site superintendent. “These sites are for stormwater control in particular. And the plants will act like a rain garden.”

Using a large Finn HydroSeeder, crews hydraulically applied the bonded fiber matrix mixed with, primarily, a Formula L hydroseed mix from Seedway Seeds. Formula L seed mix incorporates hard red fescue (Festuca longifolia), creeping red fescue (Festuca rubra), and annual ryegrass (Lolium multiflorum) and is seeded at a rate of 24.0 pounds per 1,000 square yards, unless otherwise specified. It’s all in the specs, Holladay notes. For instance, at some other highway projects, crews can dry-seed the mixtures. However, for the Section 320 area, the engineers had specified that seed was to be applied with the fiber mulch mixture.

“It was mixed with rye seed, which gives instant germination so you get a good plant stand,” he says. “The mulch keeps it all warm and the rye comes up right away. Then the other seeds get a chance to germinate.”

Annual ryegrass, like that used in the hydroseed mix applied at Section 320, is considered very ecofriendly. It’s nonaggressive, provides vigorous growth, and provides soil stability with its root system. It typically germinates in five to 14 days. It offers some nematode control and will die off after a year cycle in most climates. At Oregon State University Hyslop Farms, where grass seed is grown and studied in the Willamette Valley, the grass reaches maturity, then lodges and goes to seed. The lodging, or lying down of the plant stand, is what allows other seeds to have protection to germinate.

In the hydroseed mix Formula B that was used elsewhere along the US 202 corridor, perennial ryegrass (Lolium multiflorum) usually takes longer to germinate but is a more permanent choice because it handles the cold and warm extremes better than annual ryegrass. With both ryegrasses, planting is most successful when soil temperatures are between 50 and 65°F, which usually correlates with 60–75°F. air temperature. The nature of ryegrasses allows the seeds to germinate with less soil contact, which makes them a good selection for roadways and conservation areas, but because the hydroseed mix usually include one or more fescue grasses, soil contact and good seedbed preparation is critical. For this, good single- or double-net matting helps hold soil and seed contact through germination and plant stand.

To ensure protection on the lower basin bottoms, those with 3:1 slopes, RAM-T crews installed ECS-1 single-net straw blankets, and on the side slopes along the highway with 2:1 slopes, they used ECS-2 double-net straw blankets, both manufactured by East Coast Erosion Blankets of Bernville, PA. Other matting used included ECP-2, a TRM, and jute matting for erosion control.

“Flexterra was used above a roadside retaining wall in place of the originally specified matting,” says Ashley Kreutzer, vice president with RAM-T Corp. “This substitute was made to ensure slope stabilization in an area that was too difficult to access by hand. The use of the HydroSeeder and a man-lift allowed us to safely stabilize the slope with a matting equivalent.”

Flexterra is a high-performance flexible growth medium (HP-FGM) from Profile Products that bonds immediately to soil, even on severe slopes. Because it gives increased sheet flow resistance, turbidity is reduced, and soil loss is minimized, thereby giving seeds a good start.

“The sites were then planted with riparian plantings to help with post-construction stormwater and erosion control,” says Kreutzer. “With the removal of existing earth and the addition of nonpervious roadways, more plants were needed to control the additional water runoff on the project.”

Woody plants native to Pennsylvania, Canada, and the eastern United States were planted in riparian zones; they included dogwood (Cornus sp.), arrowwood (Viburnum sp.), elderberry (Sambucus sp.) and alder (Alnus sp.), says Holladay. “Some of the areas got live stakes. When we do that, mostly it’s dogwood and the live stakes get driven in the back on the slopes. They take hold really well.”

Construction on the US 202 project is estimated to continue through 2016. When finished, it will include a 2-mile two-lane collector-distribution road, two new bridges, new sign structures, retaining walls, sound barrier walls, improved stormwater drainage systems, and additions to the ITS that allow traffic notices and estimated distances and times to be displayed to travelers.

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