Flexible Channel Lining Systems:
The Benefits of Geosynthetically Reinforced Vegetation Over Rock Riprap

Authors:
Tim Lancaster, North American Green, Inc., Don Lutyens, American Excelsior Company, and Deron Austin, Synthetic Industries

In the last several years many areas of the U.S. have been drastically affected by torrential rainfalls and flooding. Although most of these "natural" disasters are indeed caused by nature, man’s impact on the environment and land use has magnified the destructive forces and the resulting damages. A major contributor to the flooding problems, which seem to occur all too often, is the "hardening" of our natural landscapes. The replacement of forests and fields with impervious sidewalks, streets, parking lots and roofs has resulted in increased run-off volumes which exceed the capacity of our existing drainage systems.

Ironically, even man’s own attempts to manage storm water and control erosion have added to the problem. The use of "hard armor" materials such as rock riprap, concrete and asphalt to line and stabilize drainage channels inhibits water infiltration, reduces filtration of sediment and other potential pollutants, and increases run-off volumes. The result - more polluted water feeding into our over-burdened streams and reservoirs at a faster pace. In addition to adverse environmental effects, hard armor erosion control materials pose other concerns, prompting today’s engineers and designers to employ more natural, vegetative solutions. To extend the erosion resistance capabilities of vegetative "soft armor" channel linings, modern geosynthetic turf reinforcement mattings (TRMs) are widely used for immediate erosion protection and long-term vegetative reinforcement. This article will explore the use of geosynthetically reinforced vegetation for permanent channel stabilization and its many benefits over one of the most commonly used types of "hard armor" protection, rock riprap.

Engineering Performance

"Hard armor" materials such as rock riprap were once thought to be the only suitable lining systems for channels exposed to high water flows. Vegetative channel linings were simply out of the question where expected flow velocities exceeded 8 feet per second (ft/s) (Chow, 1959) or shear stresses topped the 3.7 pounds per square foot (lbs/sf) mark (Chen and Cotton, 1988). Since their first use in 1979, TRMs have proven their ability to substantially increase the erosion resistance of natural vegetation. Two independent channel lining studies performed by CIRIA (Hewlett, etal, 1987) and the Texas Department of Transportation (Northcutt, 1997), suggest that some types of TRMs can more than double the erosion resistance of vegetation, enabling its use in channels where velocities exceed 15 ft/s and when shear stresses reach 8 lbs/sf. Compared to the Federal Highway Administration’s recommendations for rock riprap, this level of protection is equivalent to that of 24 inch (d50) stone (Chen and Cotton, 1988). This revelation is to the delight of many designers who appreciate the lower cost, lower maintenance, safety and environmental benefits of "soft armor" vegetative linings.

Aesthetics

In the past, aesthetics was not considered by engineers an important factor in erosion control materials selection. Today, as the urban sprawl lays a maze of paved roads, sidewalks and parking lots, "greenways" have become scarce. This loss of green space, in conjunction with heightened public environmental awareness and the ever increasing demands of owners, has engineers re-thinking the aesthetic aspects of materials used to stabilize drainage channels. On urban projects, these areas may offer the only opportunity to provide a natural setting within an otherwise grey, manmade landscape. Though rock is a completely natural material, it’s associated with and causes the same "hardening" effect as manmade pavements such as concrete or asphalt. Hence, the use of vegetation to soften and naturalize the appearance of drainage channels is becoming the preferred method.

Wildlife Habitat

Vegetation not only improves aesthetics, vegetative channel linings function as living components in the surrounding ecosystem. Grasses and flowers can provide food and habitat to draw and support preferred wildlife species. Though rock riprap may provide improved fish habitat when used in perennial streambeds, when used in intermittent drainage channels, it often harbors undesirable species such as mosquitoes, snakes and rodents.

Clean Water

Vegetative channel linings are also efficient at trapping and filtering sediment and pollutants in storm water. In recent research conducted on three different types of vegetation with varying densities and heights, in-stream vegetative stands increased sedimentation from 200 to 700 % versus unvegetated stream beds, with up to 81 % retention of the trapped sediments following clean water flush events (Abt et.al., 1995). Plants, along with microorganisms within the soil, can metabolize and convert many pollutants often carried by sediments into non-harmful compounds. Though rock riprap linings will also capture some sediment, rock has no way of "neutralizing" these pollutants. The sediment and its attached pollutants trapped during one storm event will often be washed out of the rocks and carried downstream during the next.

Noise Abatement

Furthermore, vegetation offers an absorptive buffer for noise. This can be quite beneficial in channels along highways through heavily populated areas where noise pollution is a problem. Rock linings have little capacity for sound absorption.

Maintenance

At first thought, one might expect riprap to require less maintenance than vegetation. Just after it's installed, rock has a nice, neat appearance. But what happens a year or two down the road? Water- or wind-carried seeds (often from weeds) deposit in the void spaces between rocks, resulting in an unattractive combination of rocks and weeds. Since rock cannot be safely mowed over with riding equipment, maintenance must be performed with weed-eaters or pesticide sprayers to eliminate weed growth. Furthermore, the voids in a rock lining not only harbor weeds, they also collect debris and litter. Pulling litter out of rock linings can be a very laborious and even dangerous task for maintenance crews.

Other potential costs and/or hazards associated rock riprap linings, include: increased maintenance time due to use of extreme care when mowing around the rock; excessive wear and tear on machinery from accidentally hitting rocks with mower blades; lost work days from employees due to injury from flying debris; damage to personal property from flying rocks and debris; and possible law suits from injury and vehicle damage.

On the other hand, vegetative channel linings can typically be maintained simply by mowing to the preferred height using conventional riding equipment. In a well manicured setting, this may require frequent mowings and occasional fertilizer applications. However, in many situations where native grasses or low maintenance species are used, mowing and fertilization may only be necessary once a year. In regards to litter removal, hand collection can be done safely, and the use of a power rake where litter is heavy can simplify the task.

Safety

The potential hazards of rock riprap go beyond those associated with maintenance activities. If the channel is in a residential or public area, riprap poses a risk for children and pedestrians who may trip and fall when playing on or walking across the jagged surface. Along roadsides, riprap linings are an obstacle to vehicles that stray from the pavement, causing reduced control of the vehicle and possibly more severe damage to the car and its passengers.

Vandalism

When riprap is installed, the only thing holding it in place is the weight of the rock itself. Though its weight may be sufficient to resist movement under high velocity water flow, it can be helpless against the most damaging force known to man - the little boy next door. A report from the field tells the story. The use of detention basins around subdivisions is a typical approach to water and sediment management. In one state, the most commonly used lining for the out-fall channels from the basins is rock riprap. Inspections in the spring by Department of Natural Resources officials after installation of many of these channels the previous autumn revealed a puzzling loss of rock from the channel surfaces. Knowing that the rock didn't wash away, state inspectors conducted an investigation. What they found out was, the rocks "walked" away over winter, carried by neighborhood children up to the detention basin and thrown onto the frozen surface of the detention basin to try to break the ice.

Materials Installation

The benefits of vegetative linings are realized even before they are in place. Installation of rock riprap typically requires excavation of a bed at least twice as deep as the mean diameter stone, lining the bed with a geotextile, and placement of two or more layers of stone in the bed. This entire process, aside from geotextile installation, requires heavy equipment. From dump trucks hauling loads of rock to the site to backhoes excavating the beds and placing the rock, each step is a time consuming, expensive task. [With just a little more effort it takes to hand place the geotextile under riprap, a TRM can be installed.] —(note: use this statement also in a box) No excavation, heavy equipment, or skilled equipment operators are required. The area is simply raked to form a seedbed and treated with seed and any necessary soil amendments. The TRMs are rolled out over the treated area and secured in place with relatively small trenches, stakes and/or staples, and in some cases top-dressed with a fine layer of soil and more seed.

Fig. 1) Proper installation of rock riprap requires an expensive, time-consuming three step process with heavy equipment. Installation becomes more labor intensive and potentially hazardous when gabion baskets, installed by hand, must be used to encapsulate the rock.

Cost Effectiveness

Putting all other benefits aside, the first consideration in erosion control materials selection must be cost-effectiveness. For channel lining applications, many engineers have adopted the design procedures presented by the Federal Highway Administration (FHWA) in the most recent HEC #15 manual (Chen and Cotton, 1988). This design methodology utilizes maximum shear stress calculations in determining the suitability of various lining materials. However, at the time the HEC #15 was developed, little was known about the vegetative reinforcement properties of geosynthetic mattings. Furthermore, this design methodology lacks the cost information needed to determine the most cost-effective lining.

Fig. 2) Both 12 inch riprap (foreground) and a turf reinforcement matting (downstream section) were installed in this channel in September. Following heavy winter rains, the rock portion shows signs of erosion. In comparison, the TRM downstream has worked with early vegetation growth to prevent scouring of the channel surface.

The following Tables 1. and 2. present up-to-date permissible shear stresses for one type of TRM and hard armor material, along with approximate installed costs for the Indianapolis, Indiana area (Lancaster and Spangler, 1996).

Table 1. Comparison of Performance Limits and Unit Cost

* Costs may vary considerably for different regions and types of TRMs

1 Based on FHWA HEC #15 Recommendations for double layered rock lining (size = d50)

2 Based on laboratory flume testing with a commercial TRM on bare soil (failure @ .5 inches soil loss)

3 Extrapolated from laboratory flume testing

4 Based on laboratory flume testing and outdoor channel testing at the Texas Transportation Institute (failure @ .3 inches soil loss)

This information can be used in general to complement HEC #15 design procedures. These figures suggest that certain turf reinforcement mattings can provide similar protection, even before vegetation growth, as 8 inch rock riprap, at less than 1/3 the cost. Once vegetation begins to establish through the matting, the degree of protection increases to that of 18 inch rock, at 1/5 of the cost. Once vegetation becomes fully established, it has similar erosion resistance as 24 inch riprap, at 1/6 the cost!

To see how this affects overall project costs, let's consider linings for two small channels where calculated shear stresses exceed the limits of unreinforced vegetation.

Table 2. Comparison of Installed Costs (12 ft x 750 ft channels (1,000 sys))

Based on these applications, using a TRM to control erosion and reinforce vegetation in place of rock would save $ 28,000 - $ 38,000. This money could be put into the owner's pocket, or used by the engineer to address other areas which may have been disregarded due to a limited budget.

Fig 3) Like electricity, water follows the path of least resistance. The roughness of rock can create problems for improperly placed linings. Here, water has eroded around a rock-lined chute. Weed growth in the rock poses further headaches for highway maintenance crews.

Conclusions

Geosynthetically reinforced vegetation can provide a lower cost, lower maintenance, less hazardous and more aesthetically and environmentally pleasing alternative to riprap in areas requiring high performance erosion protection (see Table 3.). However, the vegetation and its stand characteristics, along with the erodibility of the soil, will significantly affect the erosion resistance of this type of system. Further analysis through the United States Department of Agriculture design procedures for "Grass-Lined Open Channels" (Ag Handbook #667) (Temple, etal, 1987) can assist in evaluating different vegetative stands and soil types. Areas where vegetation is difficult to establish and maintain as a dense, healthy stand may necessitate hard armor protection.

Table 3. Summary of Lining System Features