THE CHALLENGE
The City of San Juan Capistrano, located in Southern California, was facing such a situation with a channel built in the 1960’s that was severely eroding and undercutting the concrete side slopes – 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:
- Minimize erosion during precipitation events
- Increase aesthetics along the channel
- Minimize long term maintenance issue
- Reduce chemical particulate matter from the residential runoff water
- Minimize construction and maintenance costs.
P&D Consultants, an environmental planning and engineering firm, was called to study the problem, provide recommendations, and submit a design to repair the channel. P&D reviewed many options including conventional rip rap, Reno mattresses, concrete, and TRMs (Turf Reinforcement Mats.)
P&D began their assessment by calculating the flow of water within the localized tributary flowing directly into the channel. The existing channel collects runoff from over 300 acres. The area is within a valley with hillsides over 200 ft. high. 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’ concrete box culvert. The 1700 ft. long trapezoidal channel measured 4 ft. deep with a 10 ft. bottom and side slopes of 1.5:1. The estimated flow rate of the channel was 500 cfs. This type of flow generates a shear force on the channel bottom of approximately 4.5 psf for an extreme storm event. After reviewing all of the options, it was determined that a TRM would be the best solution to meet the project requirements. 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. A discussion took place about which TRM would not restrict vegetation growth and still meet the required flow conditions of the channel.
THE ENKA SOLUTION
Low & Bonar’s engineers reviewed and recommended using EnkaMat 7020, a 3-dimensional, root reinforcement matrix that is open to eliminate vegetation restriction and is made from nylon, which will not float like polypropylene.
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 never unravel or lose its structural integrity both during and after installation – aquatic life would never 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 considerable savings compared to other erosion control alternatives.
INSTALLATION BENEFITS & RESULTS
Once construction began, the project was completed in two weeks. Valley Coast Landscape was very satisfied with how conformable the EnkaMat was during installation and that the product didn’t unravel or lose its structural integrity (reduction in performance) when cut in the field during installation.
Since the installation, several 25 year storm events have occurred in this area. Both the channel and vegetation have not 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 all of the federal and state requirements of the Phase II Rules of the NPDES (EPA’s National Pollutant Discharge Elimination
System) were met.
Eco Channels provide many benefits:
- Minimizes sediment loss and filters pollutants
- Increases aquatic life and stimulates oxygen production and enhances cooling effect
- Creates and beautifies recreational surroundings
‘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. Rip rap would have cost an additional 15 – 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 rip rap.’
– Richard Wilson of P&D Consultants.