Project Case Study: Linear park channels stormwater

April 2008 » Feature Articles
Reinforced vegetation transforms an eroded channel into a neighborhood park.
Brian Baker
Reinforced vegetation transforms an eroded channel into a neighborhood park.

Project
Mesquite Vistas’ Abbott Wash channel, Mesquite, Nev.

Engineer
G.C. Wallace

Product application
Using a high-performance turf reinforcement mat to line a development’s stormwater channel saves money and improves water quality.

Nestled in the rugged Virgin River Valley, Mesquite, Nev., is a rapidly growing community that sits on the edge of the Nevada/Arizona border. Once considered a gateway to the bright lights and big-city attractions of Las Vegas, Mesquite is now a popular tourist attraction for entertainment seekers preferring a small-town atmosphere over the hustle and bustle of Las Vegas. During the last decade, Mesquite’s population has multiplied ten-fold, making it the fastest-growing city in Nevada.

An expanding population requires new housing, and among the recent crop of developments is Mesquite Vistas, a 1,100-acre golf community. Luxury homes are surrounded by Arnold Palmer’s challenging 18-hole Oasis Golf Course and the exquisite Vistas Course. This extensive construction, along with the sandy soils found in the area, required a comprehensive blend of both defensive sediment control and offensive erosion prevention measures to help preserve the area’s natural beauty.

Erosion was prevalent in the Abbott Wash channel prior to construction of the Mesquite Vistas residential development.

 

At the time of construction, virtually all of the existing stormwater drainage channels within the Mesquite city limits were unlined, so Mesquite Vistas did not have any local design examples to follow when it came to protecting the development’s Abbott Wash channel. The project was originally designed with a 6-inch reinforced concrete lining. However, developer Keith Irons ultimately chose a vegetated solution that would allow him to turn this section of the property into a linear park and fulfill a city ordinance that required minimal disturbance. Engineer Don Haselhoff with G.C. Wallace was consulted to convert the project to a reinforced vegetated channel, which not only offers substantial cost savings compared with concrete and other hard armor systems, but also helps improve water quality.

According to Brad Cooley of Propex Geosynthetics, turf reinforcement to take advantage of the natural ability of plants to protect soil from erosion is enhanced by using long-term, non-degradable geosynthetic materials. A flexible, 3-D matrix retains seeds and soil, stimulates seed germination, accelerates seedling development, and, most importantly, synergistically meshes with developing plant roots and shoots. Numerous laboratory and field studies have confirmed that biotechnically reinforced systems can provide at least twice the erosion protection of unreinforced vegetation.

The Abbott Wash channel design was based on a 100-year storm event, which would be predicted to generate flow discharges of nearly 3,156 cubic feet per second (ft3/sec) with an average channel gradient of 0.6 percent. Using EC-Design erosion control design software available from Propex Geosynthetics, average velocities of approximately 13 feet per second (ft/sec) were calculated in an unvegetated condition for a one-hour flow duration, which represents a worst-case scenario. Assessing the maximum velocity of 13.8 ft/sec in the channel bottom versus the maximum allowable one-hour flow velocity for Propex’s Pyramat woven high-performance turf reinforcement mat (HPTRM) of 18.5 ft/sec resulted in a factor of safety of 1.34. Permissible velocity safety factors of 1.46 were calculated for both of the 3H:1V side slopes under these same conditions.

The same software also performed a shear stress analysis for Abbott Wash under the same one-hour flow conditions. The maximum shear stress generated in the channel bottom was 1.9 pounds per square foot (lb/ft2). Comparison with the maximum allowable shear stress for the unvegetated woven HPTRM of 7.25 lb/ft2 gives a safety factor of 3.81. This demonstrated that the design was well within the capabilities of the Pyramat HPTRM in an unvegetated condition. Pyramat is many times stronger than traditional turf reinforcement mats, and its UV resistance makes it a good choice in arid and semi-arid environments or for other severe applications.

In addition to being UV resistant, Propex says the woven HPTRM provides slopes and channels with long-term strength, dimensional stability, durability, and functional longevity not available in other stitch-bonded or fused monofilament permanent turf reinforcement mats (TRMs) or degradable erosion control blankets. When fully vegetated, Pyramat will resist flow velocities as great as 25 ft/sec at shear stresses as great as 15 lb/ft2.

In wetter climates, soil and vegetation may cover HPTRMs in a matter of weeks. However, in arid or semi-arid regions, sites with steep slopes or soils of low fertility, or beach areas that need erosion and scour protection, Pyramat is available in a tan color, which reflects the appearance of the surrounding landscape since it may be visible for several years or even permanently.

A high-performance turf reinforcement mat provides long-term strength, dimensional stability, durability, and functional longevity for a stormwater channel in a Nevada residential community.

 

Calculated flow conditions for Abbott Wash are within the performance limits for conventional stitch-bonded polypropylene and fused monofilament TRMs. However, because of the challenging site conditions and anticipated installation and maintenance demands, the designer was more comfortable specifying a woven HPTRM because of the additional safety factors above those of a conventional TRM. This is an important consideration in areas of accelerated development where increasing runoff and discharge rates are likely because of land-clearing activities upstream.

Despite the advantages of the woven HPTRM, a 12-foot-wide, low-flow concrete channel bottom was incorporated so that Mesquite Vistas’ residents could use it as a walking and bike path. Additionally, the concrete low-flow channel bottom facilitates removal of accumulated sediment deposited when major rainfall events erode unlined natural channels upstream.

The engineer, Don Haselhoff with G.C. Wallace, and contractor, Randy Johnson of Alford, Inc., both selected Propex’s Pyramat HPTRM and rejected other TRM brands because of hydraulic performance, installation, durability, maintenance, and UV-resistance issues. They identified the tan colored version of the product as the ideal solution for this application to visually blend with the sandy soil. They judged it to be the strongest HPTRM available, and because of Mesquite’s arid climate—average annual rainfall is only 4.34 inches—it was beneficial that this HPTRM contained no degradable components and provided superior UV resistance throughout all the woven, interconnected yarns.

Installation
Approximately 25,000 square yards (yd2) of tan Pyramat were installed and filled with 1/2 inch of soil. To prepare the channel for installation of the HPTRM, the channel was graded to a trapezoidal shape, and soils were compacted to 90 percent proctor using a smooth drum roller. The concrete low-flow channel was constructed first. Anchor trenches were then constructed around the outside perimeter of the HPTRM installation, and check slots were dug every 27 feet across the channel with a trencher. Steel fabric stake pins, 18 inches long and 3/16 inch in diameter with 3/4-inch washers, were used to anchor the woven mat in the sandy soils. The contractor used a pneumatic air hammer to install the pins, which greatly accelerated the project’s construction.

After the HPTRM was anchored in the soil, it was then affixed to the concrete low-flow channel cutoff walls using anchor bolts on 2-foot centers. A trench was constructed adjacent to the inside perimeter of the cutoff wall and the mat was folded over the concrete channel and bolted to the cutoff walls. The trench was then backfilled and compacted using the drum roller to 90 percent proctor, and the HPTRM was then folded back over the exposed soil and anchored per the project specifications. Conventional stitch-bonded or fused monofilament TRMs could not have been bolted in this manner, since their dimensional stability would be compromised by penetration of the bolts. The site was left fallow for a couple of months, until various site amenities and a permanent irrigation system were constructed. The channel side slopes were then sprigged with Bermuda grass and lined with Palms.

Pyramat was installed on this project at a cost of $14.50 per square yard (yd2). This was compared with a 6-inch slab of concrete, which would have cost approximately $65/yd2 to install in this region of the United States. Not only was the developer able to receive credit for turning the channel into a park, but also the use of the HPTRM instead of concrete saved nearly $1.2 million in product and labor costs. According to Quinn Brown of Attila Environmental Products, the success of the Mesquite Vistas project as a stormwater best management practice for using a vegetative reinforced channel has spurred similar projects throughout Southern Nevada, including a project in Boulder City that was specified with almost the exact same layout.
"The design and installation went so well that the Abbott Wash channel has become the model for other residential channel construction in the area," said Brown. "We’ve had nothing but compliments on it since we finished—the residents have been extremely happy with the vegetated look."

Using the Pyramat woven HPTRM as the flexible channel lining material on the Mesquite Vistas project proved to be an effective, economical, and technically sound permanent erosion control solution. The woven HPTRM was installed at approximately 22 percent of the cost of concrete and resulted in a total savings of nearly $1.2 million on this project. In addition to an economically beneficial solution, the mat offered the developers a solution that provided multiple environmental benefits in a very demanding arid region of the United States, including natural appearance, water cleansing, infiltration and exfiltration features, and promotion of sediment capture. And because of the challenging site conditions—aridity, high amounts of UV radiation, recreational and maintenance activities, demanding installation requirements, and desired safety factors—use of the woven HPTRM was a more effective permanent erosion control solution than conventional turf reinforcement mats.

This high-flow event severely damaged the golf course and several houses, but the high-performance turf reinforcement mat was still anchored firmly in place and left completely intact.

 

The HPTRM was so effective as a permanent erosion control solution that it withstood a 100-year storm event in 2005, which the designers had planned for but hoped to avoid. According to Brown, the storm event severely damaged four holes of the golf course and destroyed many homes in the community.

"I went back to take a look at the channel to see how the HPTRM had withstood the high-flow event," said Quinn. "While there was some vegetation stripped away from the channel in limited areas, the mat was still anchored firmly in place and completely intact. Crews had to remove silt from the channel floor that had been deposited from upstream, but there was no maintenance required on the HPTRM."

Brian Baker is an account executive for The Johnson Group. He can be contacted at bbaker@johngroup.com.


Upcoming Events

See All Upcoming Events