Last fall’s wildfires in southern California were some of the most devastating in state history. More than a dozen people were killed, thousands of homes and commercial properties were destroyed, and hundreds of thousands of acres were burned. The potential danger didn’t dissipate, however, when the flames died down in early November 2003. Left behind in the fire’s path of destruction was a thick blanket of ash and silt-what was once the native plants, grasses, and trees that dominated the landscap-covering flatlands, valleys, and hillsides. On a typical day there is nothing dangerous about traveling the state’s highways in and out of the mountain regions. But for Caltrans, California’s Department of Transportation, this layer of loose sediment resting on the hillsides was an accident, or several accidents, waiting to happen. The state highways were in harm’s way at the foot of steep embankments, raising several safety concerns.
The sediment was susceptible to being swept away by rain, creating mudslides, and clogging storm drains that could result in hazardous driving conditions and damage to the road. There was also concern about Santa Ana winds picking up and blowing the sediment onto the roadway, hindering visibility and driving conditions. These concerns brought to light the need to implement an erosion control plan to keep the silt and ash in its place, and to reestablish vegetation growth. Caltrans commissioned Dietz Hydroseeding Company to provide a quick and effective erosion control solution on the steep embankments and hillsides located along state Highways 118 and 23 between Simi Valley and Moorpark in Ventura County, and Chatsworth in western Los Angeles County. Caltrans saw erosion control as both a short-term and long-term fix. It sought to immediately control erosion, and at the same time enhance seed germination and introduce-or in this case restore-plant life.
For this particular project, a bonded fiber matrix (BFM), also known as a hydraulic blanket, offered advantages over other options, including rolled erosion control products. Rolled blankets are ideal for low-flow ditches and channels and can also be applied to accessible slopes with smooth, graded surfaces. The hillsides ravaged by the wildfires, however, were neither accessible nor smooth. A BFM works much like a blanket and interlocks on the surface after it is applied to form a blanketlike cover. It mixes with and anchors to the soil and provides more ground coverage than blankets, which is important when applying mulch to rocky and uneven terrain. BFMs are also biodegradable and nontoxic. Dietz chose Conwed Fibers 2500 from Profile Products LLC. In addition to offering the same advantages as other BFMs, it consists of thermally refined wood and a multidimensional tackifier to produce greater water-holding capacity for more complete germination and faster vegetation establishment. It was also readily available. “Getting the product in a timely manner was a non-issue,” Dietz notes. “Through the distributor, 130 tons of the material were delivered in three days. This was possible because of the Conwed Fibers 2500’s shelf life, but also because we talked to Profile about the urgency of receiving and applying the product. They made sure that we got it when we needed it.“California Highway 118 is an artery for commuters. Caltrans wanted the erosion control down as quickly as possible to beat any rain event and to lessen the impact on traffic flow.”The project began November 18. Dietz Hydroseeding used two hydroseeding machines and a crew of six to travel along the Caltrans right of way that covered a 15-mile stretch of highway, spraying 100 to 150 feet onto the hillsides above and below the road. This created an erosion control buffer zone that would help prevent sediment runoff onto the roadway, into drainage ditches, and on to adjacent properties, Dietz says.Dietz coated the landscape with two applications. The first consisted of only the BFM, while the second included the BFM and a light mix of a variety of seeds. The type and amount of seed used in the mulch was a topic of discussion among Caltrans personnel and several experts and agencies. The ash and silt contain and cover seed from the native plants and grasses that burned in the fire. The aftermath of previous fires showed that regrowth does occur without the need to reseed the landscape. “We came to a compromise,” he says. “We decided we had to include small amounts of indigenous native seed in the mulch to ensure plant establishment.”When seed is incorporated into the mulch, it usually is in the first application to provide greater opportunity for soil contact. In this case, however, the seed was applied with the second application. “The particular seed we were using is more light-sensitive than ground-temperature-sensitive,” Dietz says. “So with the seed being on the top layer of mulch, it received more exposure to the light.”The mulch was applied at 2,000 lb/ac (1,000 pounds per application), which Dietz admits was not the standard specified rate for BFM. Application rates for similar projects call for 3,000 to 4,000 lb/ac. Dietz risked using a lighter specified rate of mulch, applying a lighter layer of hydraulic blanket to encourage post-fire native species.The results? Dietz accomplished his goal of completing the job in three weeks. His crew finished December 9, having covered 15 miles while working six days a week and 12-hour days. The hydraulic blanket stayed intact and did its job during southern California’s rainy season and endured five or six rain events, Dietz says, including one that saw more than 4 inches fall in a 24-hour period in early March. “There was only clear-water runoff, no washouts, and no loss of soil,” he adds. “The BFM basically glued itself along with the fine silty material to the ground and formed a crust that held the soil in place but allowed plants to grow through it.”As a bonus, Dietz became a bit of a celebrity, as his work along the high-traffic highways attracted media attention. The Los Angeles Times featured Dietz in an article on his erosion control efforts, and a local television crew filmed Dietz and his crew as they transformed the hillside’s landscape from bare and black to a vibrant green, the die color used in the mulch. “The visual impact was striking,” he says.Dietz credits California Polytechnic State University at San Luis Obispo, Caltrans, California state officials, US Fish and Wildlife, and the distributor and manufacturer for their support in helping make the project a success. He relied on his experiences using BFMs, expert opinion, test results, and reassurance from Profile that the BFM would perform well and could be made available in mass quantities.“This was a very successful project when you consider the tremendous job the BFM did, the dollar-per-acre cost, and the impact of how many acres you can protect in a short time,” he says.
