The City of San Juan Capistrano in southern California was confronted with such a situation with a channel built in the 1960s. The severely eroding channel was undercutting the concrete sideslopes, causing sediment to be transported to the San Juan Creek basin that drains directly into the Pacific Ocean. The city’s objectives for rehabilitating the channel were to reduce erosion during storm events, make the channel more attractive, minimize long-term maintenance, reduce chemical particulate matter from residential runoff water, and keep construction and maintenance costs low. City engineers called P&D Consultants, an environmental planning and engineering firm in Orange, CA, to study the problem, provide recommendations, and submit a design to repair the channel. P&D reviewed many options, including conventional riprap, Reno mattresses, concrete, and turf reinforcement mats (TRMs).P&D began its assessment by calculating the flow of water within the localized tributary flowing directly into the channel. The existing channel collects runoff from more than 300 ac. The area is within a valley with hillsides higher than 200 ft. Storm drains from the existing development discharge directly into the channel. The initial flow of water entering the channel is from a 10- x 4-ft. concrete box culvert. The 1,700-ft.-long trapezoidal channel measured 4 ft. deep with a 10-ft. bottom and sideslopes of 1.5:1. The estimated flow rate of the channel was 500 ft.3/sec. This type of flow generates a shear force on the channel bottom of approximately 4.5 lb./ft.2 for an extreme storm event.After reviewing all of the options, P&D and San Juan Capistrano representatives determined that a root reinforcement matrix would be the best solution to meet the project requirements. A meeting was held with city officials, Valley Coast Landscape, and Colbond Geosynthetics to determine the most effective construction and installation methods for the channel. Low-growth vegetation that would require little or no maintenance and remove pollutants from the water prior to discharge into the basin was also selected at this time. Participants sought a root reinforcement matrix that would not restrict vegetation growth and would still meet the required flow conditions of the channel.The matrix chosen was Colbond Geosynthetics’s Enkamat R2M, a three-dimensional root reinforcement matrix that is sufficiently open to eliminate vegetation restriction. It is made from nylon, which does not float as polypropylene does. Enkamat keeps the vegetation and soil from eroding during extreme hydraulic flow conditions. City officials and P&D also liked the fact that Enkamat would not unravel during or after installation, so aquatic life within the basin would not be endangered from loose synthetic fibers floating downstream.Valley Coast Landscape suggested a mulch be applied on top of the Enkamat to ensure rapid germination, minimize seed displacement, and aid the establishment of mature vegetation. The overall cost of using the Enkamat and mulch layer provided significant savings compared to other erosion control alternatives.“Choosing Enkamat for the rehab of the channel saved considerably over traditional methods of reconstruction and provided an environmentally friendly solution to the existing erosion problem,” says Richard Wilson of P&D Consultants “Riprap would have cost an additional 15% to 20% and concrete about 40%. With these savings we were able to armor more of the channel – not to mention nearby homeowners were extremely satisfied with our solution. Adding native plants to the channel was much more pleasing to the eye than concrete or riprap.”Once construction began, the project was completed in two weeks. Valley Coast Landscape was satisfied with how conformable the Enkamat R2M was during installation and that the product didn’t lose its structural integrity when cut in the field during installation. TRMs the landscaper has used in the past have tended to unravel upon arrival at the job site and during installation, making them more difficult for crews to work with. Since the installation, several 25-year storm events have occurred in this area. Neither the channel nor the vegetation has been damaged, even during peak rainfall and hydraulic activity. City officials in San Juan Capistrano are very pleased with the success of the project and that it meets federal and state requirements of the National Pollutant Discharge Elimination System Phase II.
