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A Bioengineering System for Coastal Shoreline Stabilization

by George Farek and John Lloyd-Reilley

For many years, the Shoreline Erosion Committee of the Texas State Association of Soil and Water Conservation Districts has implemented shoreline erosion control projects with smooth cordgrass (Spartina alterniflora). However, many of these projects, where shoreline bluffs were encountered, failed to completely solve the shoreline erosion problem. Either the planting would not become established or the bluff continued to erode. With the development of geosynthetic turf reinforcement mats, there is the potential to implement low-cost shoreline projects that address these highly erodible bluff sites.

Geosynthetic turf reinforcement mats (TRMs) provide a low-cost alternative to hard armor systems on eroding critical areas. The mats, along with the root reinforcement of seeded or planted vegetation, resist damage from wave energy and high velocity surface flows (9 ft/sec). On high-energy wave sites, articulated concrete blocks (ACBs) are an alternative to concrete or riprap. Both ACBs and TRMs provide the opportunity to install native, salt tolerant plant species. These plants are not only aesthetically appealing, but their roots and stems are a critical component of an effective long-term erosion control system.

In partnership with the San Patricio Soil and Water Conservation District, we implemented a shoreline project in October, 1997, under a grant from the Texas Coastal Management Program. We evaluated turf reinforcement mats and articulated concrete blocks while testing several plants, such as: marshhay cordgrass (Spartina patens); gulf cordgrass (Spartina spartinae); and marsh elder (Iva frutescens), for adaptation and added environmental and engineering enhancement.

The project is located near the city of Portland, five miles east of Corpus Christi, Texas, along the Nueces Bay. The shoreline bluff ranged in vertical height from 0 to 8 feet. The soil is a Monteola clay, which is a moderately alkaline, calcareous clay soil with low shear strength and a high shrink-swell capacity. The tidal area sloped approximately 5% and had an open fetch of roughly 3 miles. The water salinity in July of 1997 was 25 parts per thousand.

On July 1, 1997, we installed Tensar® fence with three-inch diameter posts. The fence is black plastic construction fencing that has oval slits, with openings approximately 3/4” by 4 1/2”. The posts were placed every ten feet at approximately the mean tide level. The fence acted as a wave barrier or wave energy dissipater, and was secured to the posts with 1” x 2” lath and nailed at the top and bottom.

“Vermilon” smooth cordgrass, 18-24” tall, with 1-2 stems and 6” bare roots was planted. Four rows were planted 2 feet apart, from 2” below to 12” above mean tide, and ten feet inland from the Tensar wave barrier.

From August 25-28, 1997, we installed PROTEC 420® ACB and North American Green C-350® TRM. The blocks were held together by the existing tongue and groove, or moritse and tendon construction. The blocks are approximately 12 inches in diameter and 4.75 inches in height. There are two open cells per block that are roughly 3” - 4” in diameter and represent 28% of the surface area.

We shaped the slope with an excavator at a 2.5:1 grade. We dug 1’ below ground level for the toe and installed three rows of blocks at 4:1 grade and then backfilled with locally found clay soil. We also dug 3’ into the bank and installed three rows at a 4:1 grade and then backfilled. All blocks were underlain with a nonwoven filter fabric. The blocks extended 48 feet in length and 3’ in vertical height. The TRM was placed on the bank and extended for 152 feet in length. The toe and the top of the bank was trenched to 1-1/2 ft deep. The TRM was secured with either 8” staples or 6” (60d) nails with tin caps and backfilled. The TRM was secured every 18” and overlapped 6“ to join mats.

On October 27, 1997, we planted an alternating sequence of a grass and a shrub. The grasses were gulf cordgrass (Spartina spartinae) and marshhay cordgrass (Spartina patens). The shrubs were marsh elder (Iva frutescens), armed saltbush (Atriplex acanthocarpa) and wax myrtle (Myrica pusilla). The grass and shrub sequence was chosen to provide a network of fibrous and tap roots to secure the embankment. The plants were also chosen for abundant top growth to cushion the bank against wave energy. All plants were chosen because they grow no taller than six feet, to preserve shoreline views.

The TRM was easy to install and has remained stable since planting in late October 1997. The cost of the material was $0.36 per square foot, making this erosion control material very attractive.

The concrete blocks have stayed stable under all wave conditions. However, the corner where the blocks made the transition to TRM became unstable. Therefore, we made repairs and used a Terracell® cellular confinement system, and it has stayed stable at this transition from the concrete block to the TRM. The cost of concrete blocks is $4.87 per square foot, making this material desirable only where other material is inadequate.

The four-inch Terracell cellular confinement system was a flexible material, making it easy to install. It has provided better stability than the TRM at low tide elevations and is less expensive than cellular blocks. Furthermore, when these clay soils dry out and crack during the summer and then receive heavy rainfall in the fall, they are prone to severe erosion. The cellular confinement system acts to break up the slope into 8-inch steps, or mini-dams, which capture the rainfall and prevents any gullying. Based on our experience, it appears to be adapted to 1 foot above mean tide and higher.

In February 1998, we surveyed the transplants and found 1360 out of 1400 had survived. Most of the dead plants were at the shoreline of the concrete blocks, smothered by shoalgrass (Halodule wrighti). By April of 1998, the shoalgrass was 1-2 feet thick along the shoreline, which smothered the shoreline plants, especially at the deep corner of the blocks.

On July 9, 1998, and again on February 5, 1999, we surveyed the plants for survival and growth. The grasses performed exceedingly well, with all having survival rates over 90% on both evaluation dates. Both gulf cordgrass and marshhay cordgrass have grown well at this site and appear to be adapted to 1-1/2 feet above mean tide and higher. Marshhay cordgrass not only survived well, but it extended runners from its rhizomes on 44% of the plants.

The shrubs did not perform as well as the grasses. Wax myrtle’s survival rate was only 11%. This collection of wax myrtle came from a sandy site and apparently was not adapted to the clay soils of this site. Where the bluff was a little sandy, it performed better, with a 43% survival rate.

Marsh elder had a 72% overall survival rate. However, on those sites that were 2’ above mean tide, it had a 93% survival rate. The majority of its mortality occurred where we planted it at the shoreline. Shoalgrass, which grew into a 2-foot layer, smothered many of the shrubs.

We recommend that smooth cordgrass be planted on sites where little shoalgrass is encountered and tidal slopes are less than 5%. Once the cordgrass is well established, bluffs less than 8’ in elevation can be shaped and planted with well adapted plant material. With the added toe protection, the bluff treatment has improved chances of success.

Where a smooth cordgrass stand is established, a combination of TRM and cellular confinement system with selected plant material should provide a good shoreline stabilization. If smooth cordgrass cannot be established, then a bluff treatment that includes ACBs for toe protection will be needed.

If the total length of a bluff site cannot be treated, we would discourage any attempts at bluff shaping. However, on high value commercial or residential property where adjacent landowners are protecting their shoreline, we believe this system has promising value. We also think this system may have particular value for soil stabilization and wildlife habitat enhancement on manmade spoil islands along the Texas Gulf Coast. L&W

For more information, contact the Kika de la Garza Plant Materials Center, 3409 N FM 1355, Kingsville, TX 78363-2704, (361)595-1313, John.Reilley@tx.usda.gov.

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Friday, December 1, 2000 - http://www.landandwater.com/features/vol44no5/vol44no5_1.html