Project Case Study: Long-term erosion control

May 2005 » Feature Articles
Stabilizing nearly two miles of streambanks and slopes wasn't the only challenge Garden Management Consultants, Inc., faced when tackling a stream restoration project on a Georgia golf course. The landscape design and construction firm also had to contend with record rainfalls and heavy stream flows.

Soil bioengineering practices are tested by storms on a stream restoration project.

BY GREG NORTHCUTT

Stabilizing nearly two miles of streambanks and slopes wasn't the only challenge Garden Management Consultants, Inc., faced when tackling a stream restoration project on a Georgia golf course. The landscape design and construction firm also had to contend with record rainfalls and heavy stream flows.

It was a fitting test of the company's confidence in the value of soil bioengineering practices as a better, long-term solution to solving erosion problems than conventional engineering materials such as rock or concrete. In this case, the company met the challenges by combining soil-reinforcing coir and synthetic materials with more than 100,000 plugs, shrubs, and other plant materials.

It was the largest stream restoration project ever undertaken by the company, which is based in Gainesville, Ga. The firm is owned by Claudia Hodges, a landscape planner and designer, and her husband, Bart, who directs the construction work.

The seven-month-long project was completed in October 2003. The work centered around three streams that flow through the Druid Hills Golf Club's course in Atlanta. Built in 1912, this 6,800-yard, private course, lined with stately oaks, is one of the oldest courses in Georgia. The streams come into play on just about every one of the course's 18 holes. However, the streams, which drain a watershed consisting mostly of impervious roof, sidewalk, and street surfaces, converge on the course at the lowest point of the watershed, and, over the years, erosion and sedimentation have changed the nature of the drainage network. “Following major storm events, low areas on the course would flood and remain out of play for days,” said Randy Delaney, the club's general manager.

During the project, the area was hit with the heaviest rainfalls in more than 100 years, which inundated the work site more than 30 times. “Almost any rain event can bring the stream up and out of its banks within 15 to 20 minutes,” said Bart Hodges. “We got a real quick test of which erosion control practices worked and which didn't.” One purpose of the restoration project was to improve the appearance of the stream, which had been re-routed over the years. The entire stream had been lined with riprap at one point or another; several areas, some as long as 200 yards, were covered with shotcrete. In other areas, rock or poured concrete walls had been constructed to support adjacent tees or storm drains. These structures had proved less than ideal for controlling erosion.

“Every wall had been rebuilt at some time,” Hodges said. “Also, we found many areas where riprap had failed to prevent streambank washouts and more riprap had been added. People were surprised at just how much riprap had been installed over time in an attempt to control erosion.” Garden Management Consultants proposed a more natural, permanent fix-using turf reinforcement mats (TRMs) made of coir and synthetic materials to build reinforced soil structures, and coir rolls combined with TRMs to stabilize streambanks until vegetative canopies and root systems could establish to hold the soil in place permanently.

Depending on the application, Garden Management Consultants used one of three types of TRMs, manufactured by RoLanka International, Inc., Stockbridge, Ga., to build fabric-encapsulated soil lifts. In all, they built eight of these earth structures.

Most replaced existing rock or poured concrete walls. They began construction by anchoring part of the unrolled TRM onto the wall foundation, using rebar or wood stakes. Then they added and compacted soil on top of this fabric to a depth of 12 inches. Next, they wrapped the remaining fabric over the front of this soil lift to cover the layer and anchored this top fabric to the soil layer. They formed a second fabricencapsulated lift on top of the first, setting it back about 2 to 6 inches, and repeated this process until reaching the desired height. Vegetation, either sod or plants, then were installed on the face and top of the structure.

One of the TRMs used was RoLanka's 3DTRM-PP, a permanent, 100-percent synthetic product, which is made from a green polyolefin fiber layer sewn between two, high-strength polypropylene geogrids and reinforced with a polypropylene net.

The second product was 3DTRM-CC, which provided permanent soil reinforcement.

It is similar to the first TRM, except that the inner fiber layer is made of natural coir-a 100-percent organic, brown mattress fiber made from processed coconut husks. It features a high, wet tensile strength and natural resistance to rot, molds, and moisture.

This composite TRM offers more flexibility and greater water-holding capacity than a synthetic mat, supports seed germination and seedling growth, and is easy to walk on. Both products are recommended for flow velocities as fast as 30 feet per second and shear stresses as great as 8 pounds per square foot.

The third TRM used on the project was BioND-TRM 100. It is woven from machine-twisted bristle coir twine and reinforced with two, UV-stabilized polypropylene strands. The coir fiber biodegrades in four to six years and the synthetic mat structure continues to provide permanent support for the soil and plant roots. It is designed for fabric-encapsulated soil layers in wetland areas and features a woven structure with moveable intersecting points to prevent entrapment of wildlife.

“We used a lot of this product,” Hodges said. “Unlike the other two TRMS, we could cut openings into the fabric for planting root balls and secure the fabric back around the base of the plant after planting.

Not only are these walls more aestheticallypleasing, they cost about 40 percent less than a concrete or rock wall.” Protecting streambanks Hodges and his crew used RoLanka's BioD-Roll coir rolls to stabilize the toe of streambanks. Made from coir fiber densely packed into tubular coir outer netting, the product is available in various sizes. In this case, 12-inch-diameter, 10-foot-long rolls were installed along more than 7,000 linear feet of streambanks. With a design life of about five years, these logs resist the water's erosive force to provide initial structural stability for the streambanks. Also, they are a growth medium. Over time, sediment deposited around the rolls will support riparian vegetation.

In some cases, the TRMS were anchored to the coir rolls and used to stabilize the banks above the logs before installing plants. Toward the end of the project, the TRMS protected bare areas above the coir logs during the winter until sod could be installed the following spring. “Once the coir logs filled with water, they became fairly heavy, providing a good base for tying the TRMs,” Hodges said. “They also prevented the mat from being undercut by the stream.” All exposed areas next to the stream were covered with a geotextile to protect them temporarily when the crew wasn't working on them. Elsewhere, after planting, disturbed ground was stabilized with RoLanka StrawMats, which are stitched straw blankets designed for temporary use on moderate slopes, and BioD-Mats, which are more durable, higher-performance solutions woven from bristle coir twine designed for steep slopes.

Sediment control during the project included the use of such conventional practices as silt fence and sediment ponds, as well as more innovative approaches. In some cases, polyacrylamide flocculent blocks were placed in drainage channels to remove sediment. Crews also built check dams made of large river rock to slow stream flows and to collect sediment for removal. After the project was completed, these dams were broken down and the rocks were left in place, enhancing the natural look of the stream.

Prior to working in the streams, the crew installed RoLanka BioD-Pillow, a 2-inchthick mattress of coir encased in a woven bristle coir blanket, across shallow areas downstream of the work site to form a sediment barrier. After the earthwork was completed, the sediment was removed and the coir mattresses were placed on the streambank to degrade naturally.

“Sediment levels in the stream were monitored after every rain event,” Hodges said. “The water in the stream leaving the property was consistently cleaner than the streams as they entered the golf course. The soil bioengineering practices continue to minimize the amount of sediment in the stream. Fish species are now being observed that haven't been seen this far upstream in almost 20 years.” Passing the tests The stream restoration work has met performance expectations. A citizens group has followed the project with interest and is pleased with the effectiveness of the erosion and sediment control techniques. Since the project was completed, it has been tested by about six major storms, which have dumped as much as several inches of rain in a few hours. “We had to make some minor revisions in a few areas,” said Delaney, “but the erosion control features have done what they are supposed to do.”

Greg Northcutt is president of Northcutt Communications, Inc., an editorial services firm in Port Orchard, Wash., which has been covering environmental issues and the construction industry since 1988. He can be contacted at 360-895-1887, or via e-mail at gregnorthcutt@att.net.


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