By Joe Royer, Kurt Chirbas and Ziad Mazboudi
The City of Laguna Niguel in Orange County, Calif., has its origins in the Rancho Niguel Mexican land grant, acquired in 1959 by the Laguna Niguel Corp. to develop one of California’s first master-planned communities, which has become predominantly a bedroom community. The city encompasses a total of 14.9 square miles (38.6 km2)—14.8 square miles (38.3 km2) of it is land and 0.05 square mile (0.1 km2) of it (0.35%) is water. The city land consists of mostly hilly terrain within the San Joaquin Hills. The average elevation is 400 feet (121.9 m); elevations range from near sea level to 936 feet (285.3 m) at the summit of Niguel Hill in the southwest corner of the city.
The city continues to review and implement its stormwater management and urban runoff management programs to meet all regulations designed to protect the quality of water in its creeks that eventually discharge into the Pacific Ocean. The current layers of regulatory compliance, which continue to evolve, affecting the City of Laguna Niguel for the stormwater requirements include:
- Federal (The Federal Clean Water Act)
- California state (Porter-Cologne Water Quality Control Act)
- Regional (San Diego Regional Water Quality Control Board)
- The City of Laguna Niguel (Stormwater Runoff Control Ordinance—Section 6-3-400)
While administering these management programs can be time-consuming and costly for the city, the city sees them as an asset for the community and its preservation of the sensitive environment that surrounds it. The city’s public works department strives to enhance its program while maintaining or reducing its annual operating costs. One way it accomplishes this is by evaluating current and new products and technologies to help support its requirements to monitor and improve the stormwater and urban runoff water quality during the dry and wet seasons. Its typical evaluation for reviewing a product includes:
- Overall cost (installation and maintenance)
- Functional longevity
- Installation requirements
- Minimal disturbances to surrounding area
- Minimal long-term maintenance
- Functionality as system approach with other products
- Environmentally friendly
- Multiple solution uses (such as weed abatement, soil stabilization and hydraulic conditions)
Background of specific runoff and erosion issues along a hiking trail
The city has miles of hiking trails—which offer breathtaking views of the surrounding mountains, valleys and the ocean—along the hilly terrain that are only accessible with small equipment (Figures 2a and 2b).
The hillside is maintained for fire protection, weed abatement and erosion control, which requires strategic vegetation management including removal or maintenance to reduce the potential of fuel for a fire. These hillsides and trails can be point sources for erosion, soil stabilization and water quality issues during the winter months. The average cumulative rainfall is 15 inches (38.1 cm), and in the 2016–2017 season it was 20 inches (50.8 cm). The heavy rains arrive in January, which typically account for 30%–50% of the total, and the rainfall can be intense within a short time period. The soils of the San Juan formation are mostly clay.
To reduce erosion control, soil stabilization and runoff water quality issues while minimizing long-term maintenance costs, the city decided to install a permanent drainage ditch to collect runoff water from the hillside and trail and direct it to storm inlet drains. This ditch would also improve the safety of the trail by reducing erosion rills and gullies. Existing storm inlets are shown in Figures 3a and 3b.
Determining the most effective armoring protection for the drainage ditch
The city reviewed several solutions for armoring the drainage ditch, specifically considering whether each option would meet the project criteria: overall installation and maintenance cost, erosion protection, water quality impact, minimum impact and disturbance to hiking trails or hillsides, increased water flow capacity, minimum disturbance to hikers and residents during construction, installation time, minimum long-term maintenance, aesthetics, and safety. The solutions considered included:
- Turf reinforcement mat
- Tied or articulate concrete block mat
- Shotcrete/poured concrete
- Fabric-formed concrete
- Geosynthetic concrete composite mat
Below is a summary of the review findings for each potential solution for the project.
Turf reinforcement mat (TRM)
TRMs are geosynthetic products that create a holding capacity for the vegetation root system to increase its permissible shear force during movement or flow of water and to minimize erosion and water quality issues. TRMs require that permanent dense vegetation be established at all times. With the concerns of the constant vegetation maintenance, increased fuel for a fire, saturating the subsurface soils of the hillside (slope stability issues), and the risk of TRM entangling with wildlife, maintenance equipment and hikers, this option was immediately eliminated.
Riprap is typically used as an energy dissipater to slow water down. The size of riprap determined for this application was 6 to 12 inches (15.2 to 30.5 cm). Using riprap would affect the hillside and hiking trail because the drainage ditch would require over-excavating to produce the volume capacity to install the riprap and the anticipated flow rate of runoff water. The other main issues with riprap were the long-term maintenance for weed control and allowing water from the drainage ditch to saturate the subsurface that could potentially cause instability for the slope and endanger hikers and their pets if they accidentally walked onto the riprap. Therefore, riprap was eliminated as an option.
Tied or articulate concrete block mat
Tied or articulate concrete block mats are used similarly to riprap as an energy dissipater. They are manufactured concrete blocks formed to a certain shape that are secured together by their capability of interlocking or mechanically tying together via a grid or cable system. The thickness ranges from 2 to 12 inches (5.1 to 30.5 cm). It was determined that the block mat has essentially the same concerns as riprap mentioned above and so was eliminated as an option.
Shotcrete or poured concrete was reviewed, but access to the site is limited and concrete trucks could not enter the trail. The trail has a single access point with limited width (a tight 10 feet [3.0 m]), steep slopes and no place to turn around. So, any concrete or shotcrete work was deemed unfeasible.
Revetment mats are special woven fabrics that are sewn together to create pockets or envelopes to be filled with a pumpable fluid concrete mix. It met the criteria, eliminating water peculation and vegetation issues and was easy to install. However, the difficulty for the fabric-formed concrete was it required over-excavation and access for the pump truck at the site, so it was eliminated from the options.
Geosynthetic concrete composite mat (GCCM)
A geosynthetic concrete composite mat (GCCM) is a flexible concrete-impregnated geotextile on a roll. When deployed, it hardens on hydration to form a durable, fiber-reinforced concrete layer. The material is 0.2 to 0.5 inch (0.5 to 1.3 cm) thick and weighs from 1.9 to 5.3 psf. The material met all the criteria and exceeded expectations, including no special contractor (in-house labor could install it), flame resistance (fire protection), no weather constraints during installation, easy-to-follow ground surface contours, and minimum construction crew and equipment, which would allow the hiking trail to be open during the installation. This product was selected for the project.
The selected GCCM could be used for multiple functions and applications besides armoring a drainage ditch, including slope protection, weed control, fire control, geosynthetic liner armoring, relining inverts of culverts, shoring line protection and more. The city could inventory the material and then use it when a need arose, especially when extreme rainstorms occur, to minimize erosion and water quality issues at unexpected locations throughout the city limits. The GCCM material can be delivered to the site in large bulk rolls weighing nearly 3,500 pounds (1,587.6 kg). The rolls can also be supplied in batch or person-portable sizes, which are less than 150 pounds (68.0 kg). The lighter rolls can be transported by smaller equipment and installed without large equipment footprints. A summary of comparative options is shown in Table 1.
Installation procedures for concrete cloth GCCM at the project site
The contractor initially graded the trail to reverse the drainage toward the proposed trapezoidal gutter design (back toward the toe of the slope). To minimize excavation and construction time, the contractor manufactured a steel template to attach to the mini-backhoe and cut a trapezoidal gutter that provided a flat 8 inches (20.3 cm) at the bottom and a 1H:1V side slope with a flat 6 inches (15.2 cm) on top. The flat bottom was selected to allow for easy maintenance using a flat shovel to remove any silt, leaves or debris compared to a typical V-gutter shape. Once the gutter was cut, the contractor crews deployed the GCCM and laid it within the gutter. The crews used and deployed bulk rolls along the gutter, which minimized the number of overlaps of the GCCM fabric. At overlaps, crews used construction caulk or adhesive and screws. Once the GCCM was installed, the crew used a water tank attached to a pickup truck with a pump to hydrate it. The completed gutter is shown in Figure 4.
After the gutters were completed, the crews used the GCCM to construct a presediment basin. The completed basin can be seen in Figure 5.
The GCCM was also used for the construction of midslope interceptor weirs to slow down sheet and concentrate flow from the steep hillside slopes to minimize erosion issues. Gravel geotextile bags were stacked three courses high, and the GCCM was draped over to protect the geotextile bags from UV and abrasion to create a long-term structure that could be removed with minimal effort. The weirs are shown in Figure 6. The city used to replace gravel bags and/or fiber rolls annually and hoped that the use of a GCCM would eliminate the need for this annual chore.
The GCCM provided the city with a product that was easy to handle, that was easy to install and that performs equivalently to regular concrete construction. In addition to the speed of construction, the project was completed with a highly reduced cost compared to the initial budget estimate, and the city council was very pleased with how the project was accomplished. The city completed another project with GCCMs following the completion of the trail project. GCCMs are now a tool in the staff’s toolbox and supported by management for use as needed.
Joe Royer, Ph.D., is development manager with Clockspring-NRI in Spartanburg, S.C
Kurt Chibas, P.E., CPESC, is western regional manager for concrete solutions for Clockspring-NRI in Sacramento, Calif.
Ziad Mazboudi, P.E., D.WRE, is senior civil engineer for the City of Laguna Niguel Public Works in California.
Photographs courtesy of the authors.