Record Rainfalls Test a Streambank Stabilization on a Georgia Golf Course

By Greg Northcutt

During the fall of 2003 the soil bioengineering practices on the streambank continue to minimize the amount of sediment in the stream. Fish species are now being observed that haven?EUR??,,????'???t been seen this far upstream in almost 20 years.

Stabilizing nearly two miles of streambanks and slopes wasn?EUR??,,????'???t the only challenge Garden Management Consultants, Inc. (GMC), 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. Mother Nature?EUR??,,????'???s heavy spring rains proved a fitting test of the value of soil bioengineering practices as a better, long-term solution to solving erosion problems than conventional engineering materials, like rock or concrete.

GMC met these erosion and sediment control challenges by combining soil-reinforcing coir and synthetic materials with more than 100,000 plugs, shrubs and other plant materials. This was the largest stream restoration project ever undertaken by GMC, 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. They specialize in building and rehabilitating water features such as streams, falls and ponds for parks and golf courses.

During construction, the streambank was overlaid with 3DTRM-PP a permanent, 100 percent synthetic product, made of a green polyolefin fiber layer sewn between two high-strength polypropylene geo-grids and reinforced with a polypropylene net. Also used was BioND-TRM 100, woven from machine twisted bristle coir twine, reinforced with two UV-stabilized polypropylene strands. 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 erosive force of stream flows to provide initial structural stability for the streambanks, and are a growth medium.

The seven-month long project, part of a larger project to improve drainage and other course features, was completed in October 2003. The work centered around three streams?EUR??,,????'???Peavine Creek, its south fork and Lullwater Creek, which all flow through the Druid Hills Golf Club?EUR??,,????'???s course in suburban Atlanta. Built in 1912, this 6,800-yard private course, lined with stately oaks, is one of the oldest in Georgia. The streams come into play on just about every one of the course?EUR??,,????'???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.

?EUR??,,????'??During the project, we were hit with the area?EUR??,,????'???s heaviest rainfalls in more than 100 years,?EUR??,,????'?? Bart says. ?EUR??,,????'??We stopped counting the times our work was under water less than half way through the project. By that time the number had reached 30. We got a real quick test of which erosion control practices worked and which didn?EUR??,,????'???t.?EUR??,,????'?? Over the years, erosion and sedimentation had changed the nature of the stream. ?EUR??,,????'??Following major storm events, low areas on the course would flood and remain out of play for days,?EUR??,,????'?? says Randy Delaney, the club?EUR??,,????'???s general manager. ?EUR??,,????'??That affected our revenues and it was unsettling to our members to have a hole closed for several days after a heavy rain."

Here, the streambank is shown with 3DTRM-CC which provided permanent soil reinforcement. It is similar to the first TRM, except for the inner fiber layer. Instead of a synthetic material, it?EUR??,,????'???s made of natural coir, a 100 percent organic brown mattress fiber from processed coconut husks that features a high wet-tensile strength and is naturally resistant to rot, molds and moisture.

A Soft Alternative to Hard Walls

One purpose of the restoration project was to improve the appearance of the stream, which had been re-routed over the years. He reports the entire stream had been lined with riprap at one point or another and several areas, some as long as about 200 yards, were covered with shotcrete. In other areas, rock or poured concrete walls, up to about six feet high and 200 feet long had been constructed to support adjacent tees or storm drains. These structures had proved less than ideal for controlling erosion. ?EUR??,,????'??Every wall had been rebuilt at some time,?EUR??,,????'?? Bart says. ?EUR??,,????'??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.?EUR??,,????'??

Garden Management Consultants proposed a more natural, permanent fix that involved using turf reinforcement mats (TRM) made of coir and/or synthetic materials to build reinforced soil structures, and coir rolls combined with turf reinforcement mats to stabilize streambanks until vegetative canopies and root systems could establish to hold the soil in place permanently.

?EUR??,,????'??A lot of people who aren?EUR??,,????'???t familiar with soil bioengineering have a hard time believing that these materials offer a less-expensive, more environmentally-friendly alternative to concrete and riprap in controlling erosion,?EUR??,,????'?? Bart says. He and his crews used TRMs to build fabric-encapsulated soil lifts, creating the earth structures. In all, they built eight of these structures. Most of them replaced existing rock or poured concrete walls that protected stream banks or were used elsewhere to support tees or a cart path.

Depending on the application, they used one of three different types of TRMs, manufactured to build the structures. They began construction by first 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 fabric encapsulated lift on top of the first, setting it back about two to six inches, and repeated this process until reaching the desired height of the structure. Vegetation, either sod or plants, were then installed on the face and top of the structure to complete the reinforced earth wall.

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.

A Trio of Mats

One of the TRMs used was RoLanka?EUR??,,????'???s 3DTRM-PP. Depending on the type of vegetation, the green color offered an attractive look to a site until the vegetation became established. The second product was 3DTRM-CC, This composite TRM offers more flexibility and higher water holding capacity than a synthetic mat, supports seed germination and seedling growth and it?EUR??,,????'???s easy to walk on. Both products exceed industry standards for 100 percent synthetic and composite TRMs in terms of tensile strength and elongation for improved performance during the critical unvegetated stage. Each is recommended for flow velocities up to 30 feet per second and shear stresses up to eight pounds per square foot.

The third TRM used on the project was BioND-TRM 100. 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, unlike most TRMs, features a woven structure with moveable intersecting points to prevent entrapment of wildlife. ?EUR??,,????'??We used a lot of this product,?EUR??,,????'?? Bart says. ?EUR??,,????'??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. ?EUR??,,????'??Not only are these walls more aesthetically-pleasing, they cost about 40 percent less than a concrete or rock wall.?EUR??,,????'??

Crews also built check dams, made of large river rock, to slow stream flows and collect sediment for removal. After the project was completed, these dams were broken down and the rocks left in place, enhancing the natural look of the stream.

Protecting Streambanks

Bart and his crew used 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 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 over the winter until sod could be installed the following spring. ?EUR??,,????'??Once the coir logs filled with water, they became fairly heavy, providing a good base for tying the TRMs,?EUR??,,????'?? Bart says. ?EUR??,,????'??They also prevented the mat from being undercut by the stream.?EUR??,,????'??

?EUR??,,????'??Although the coir logs are pretty stiff, they were flexible enough to bend around most of the contours in the stream,?EUR??,,????'?? Bart says. ?EUR??,,????'??Sometimes, instead of reshaping all of a bank, we were able to push the logs into undercuts to provide an anchor for the TRMs before filling in behind the logs. The logs acted like shock absorbers to prevent damage to the TRMs from the impact of some fairly heavy debris in the flowing stream.?EUR??,,????'??

These streams draining a watershed consisting mostly of impervious roof, sidewalk and street surfaces, converged on the golf course at the lowest point of the watershed. Almost any rain event brought the stream up and out of its banks within 15 to 20 minutes. The restoration solves a drainage problem while giving golfers a hazard to shoot over, and several bird species, including kingfishers and blue herons, have taken up residence, adding to the appeal of the course.?EUR??,,????'??

Flexible Solutions

All exposed areas next to the stream were covered with a geotextile to protect them temporarily when the crew wasn?EUR??,,????'???t working on them. Elsewhere, disturbed ground was stabilized after planting with StrawMats, which are stitched straw blankets designed for temporary use on moderate slopes and more durable, higher-performance BioD-Mat woven from bristle coir twine designed for steep slopes. The wide range of available coir and composite materials proved valuable. ?EUR??,,????'??We came up against a variety of situations throughout the project,?EUR??,,????'?? Bart says. ?EUR??,,????'??The combination of materials allowed us to use them in a variety of ways, other than textbook applications, to solve different problems.?EUR??,,????'??

Prior to working in the streams, the crew installed BioD-Pillow, a two-inch thick mattress of coir encased in a woven bristle coir blanket, across shallow areas downstream of the work site to collect sediment (sediment barrier). Once the earthwork was completed, the sediment was removed and the coir mattresses were placed on the streambank to degrade naturally. These sediment control practices proved very effective. ?EUR??,,????'??Sediment levels in the stream were monitored after every rain event during the project,?EUR??,,????'?? Bart says. ?EUR??,,????'??The water in the stream leaving the property course 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.?EUR??,,????'??

Other features of the golf course renovation projection include a wetland, which was created as part of drainage improvement. It covers several acres where the three streams meet, which once were part of a fairway. ?EUR??,,????'??This wetland captures and holds runoff, releasing it slowly to reduce flooding,?EUR??,,????'?? Delaney explains.

By the spring of 2004 the restored watershed was blooming with natural flowers. In many areas, smaller perennials, including ferns and cardinal flowers, were planted directly into the coir logs. In others, sod was installed directly on top of the logs. Over time, sediment will be deposited around the rolls and support riparian vegetation planted in and around the coir rolls (logs), which will in turn provide permanent erosion control.

Passing the Tests

The stream restoration work has met performance expectations. Since the project was completed, it has been tested by half a dozen or so major storms that have dumped as much as several inches of rain in a few hours. ?EUR??,,????'??We had to make some minor revisions in a few areas, but the erosion control features have done what they are supposed to do,?EUR??,,????'?? says Delaney. He is also a member of the board of directors of the Peavine Watershed Alliance, a citizens group, which has followed the project with interest. That group is also pleased with the effectiveness of the project?EUR??,,????'???s erosion and sediment control techniques, he reports. Bart sees increased use of soil bioengineering and other environmentally-friendly practices in future golf course renovation projects. ?EUR??,,????'??It offers a better way than previous hard armor techniques to improve and protect riparian areas,?EUR??,,????'?? he concludes.