TTI hydraulics and erosion control laboratory research field performance of erosion-control blankets

LANDSCAPE AND URBAN PLANNING

ELSEVIER

Landscapeand Urban Planning 32 ( 1995) 161-167

TTI hydraulics and erosion control laboratory research field performance of erosion-control blankets Sally H. Godfrey *, Marilyn K. Curry Texas Transportation

Institute, Texas A&M University, College Station, TX 77843, USA

Accepted13January 1995

Abstract Storm water management issues facing the Texas Department of Transportation in the late 1980s led to the development of a coordinated research program with the Texas Transportation Institute. Researchers developed methodologies for evaluating the field performance of various erosion control technologies of the most widely used products within the Department’s construction and maintenance operations. From these methodologies, the Hydraulics and Erosion Control Laboratory was designed and constructed. Currently, participants include private industry (manufacturers of erosion control products), transportation researchers (TTI), and the public sector (TxDOT) The results reported in this paper reflect 2 years of erosion-control blanket research. The study objectives were to determine the effectiveness of erosion-control blankets on the growth of warm-season perennial grasses and their ability to prevent sediment loss in a sloped condition. The laboratory simulates the highway environment with the sloped plots (6 m in width) located on an earthen embankment that is 300 m in length and 6.75 m in height (94 ft. by 22 ft., vertically). A randomized experimental design was replicated on two soil types (sand and clay) for each slope condition (3: 1 or 2: 1) with a control. In general, the results indicate better combined results relating to sediment retention and vegetation establishment performance for erosion-control blankets on sandy soils (noncohesive) regardless of slope condition (3: 1 or 2: 1) or material type. A minimum of 50% more sediment was retained on the sandy treatment plots and a 45% more vegetation coverage was achieved compared with the control plots. When analyzed by material type related to performance, excelsior, synthetic blends, and straw/coconut blends performed the best. For clay soils (cohesive), regardless of slope condition (3: 1 or 2: 1), the combined results indicate a minimum of 75% more sediment was retained and a minimum of 5% more vegetation establishment was achieved compared with the control plots. When analyzed by material type related to performance, excelsior, straw, and straw/coconut blends performed the best.

.

Kqvwords: Erosion-control establishment

blankets; Erosion control; Stormwater

management;

1. Introduction The erosion control industry and the Federal Highway Administration (FHWA) recognize a wide variety * Corresponding

author: Tel. (409) 845-0133.

0169-2046/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SXMO169-2046(95)00198-O

Erosion control; Product evaluations;

Right-of-way

vegetation

of generic materials that may be used as erosion control protection. Erosion-control blankets (soil retention blankets) that met the Texas Department of Transportation’s (TxDOT) standard specifications included two primary products’ material or physical compositions. Technically, products that did not meet the material

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S.H. Godfrey, M.K. Curry/Lm&cape

and Urban Planning 32 (1995) 161-167

specification such as grid size, tensile strength, fiber type and length, etc., were excluded from the specification process. In response to this practice, TxDOT looked for other alternatives that would provide a fair system of selecting erosion control products based upon their performance. Therefore, a cooperative research study was initiated in 1989 between TxDOT and the Texas Transportation Institute (ITI). The research team developed the methodology for field performance testing of erosion-control products once they determined TxDOT’s needs and current state-of-practice in the erosion industry. A variety of laboratory tests to describe standard strength properties such as tensile strength, shear strength, heat resistance, etc. (FHWA Geotextile Engineering Manual, March 1984, Chapter 2) were generally used to describe performance characteristics of erosion-control products. These tests did not adequately describe or test field performance. Laboratory tests and field observations suggest there is great variation in strength, durability, soil-blanket interaction and vegetation response between the material classifications and between manufactured brands of similar materials. Soil-fabric interestablishment, and installation action, vegetation methods are critical factors to consider in figuring out field performance characteristics. This paper describes the current test methodology and presents field per-

Fig. 1. Aerial view of the hydraulics

formance results of erosion-control rated into the specification process.

blankets incorpo-

2. Research program 2. I. Laboratory facility description Field tests were and are performed at the Hydraulics and Erosion Control Laboratory located on the Texas A&M University Riverside Campus, 6.5 km (4 miles) west of Bryan, Texas. The laboratory (originally a military airport facility) is positioned on a ridge east of the Brazos River. The site is influenced by heat energy stored in, or reflected from the surrounding pavement. These environmental conditions closely simulate those experienced in typical highway roadside environments that were desirable for the researcher’s data collection. Fig. 1 shows an aerial view of the facility. Study plots for the 2: 1 (west orientation) and 3: 1 (east orientation) slope conditions measure 9 1.14 m2 and 128.34 m* (1000 ft.* and 1400 ft.*), respectively. They were located on a 6.75 m (22 ft.) vertical height, earth-fill embankment with sediment collection boxes along the base. The embankment was constructed from two types of soil located within the 5 ha (12.5 acre) site according to TxDOT standard construction meth-

and erosion control laboratory.

S.H. Godfrey, M.K. Curry /Landscape and Urban Planning 32 (1995) 161-167

ods. One-half of the embankment was built and capped with a sandy loam soil (SL) (K= 0.38) and the remaining portion was built and capped with a clay soil (C) (K= 0.20) (post-construction soil samples were analyzed by SASI Inc. with reference made to the National Soils Handbook, July 1983, fig. 603- 1, “Soil Texture Triangle”; K values were determined on postconstruction soil samples following the SCS soil erodibility nomograph, Predicting Rainfall Erosion Losses-A Guide to Conservation Planning). The physical properties of these two soils represent the erosive properties frequently encountered on highway roadsides in Texas. Compaction was controlled by the density control method. Rainfall simulators were used to generate the primary data for sediment retention performance. The simulators were 6.2 m (20 ft.) wide with irrigation arms spaced 1.5 m (5 ft.) apart which covered the entire plot. Each unit calibrated to provide the appropriate amount of precipitation, 25-300 mm ( l-l 1.8 in.) range, produces representative natural rainfall droplets. The water supply system for the rain simulators was provided by an on-site reservoir. Additional facility features include an on-site suite of recording weather instruments, an additional water reservoir for storage, and soil moisture instruments. 2.2. Methodology The experiment was established under a completely randomized design consisting of 12 treatments of two replicates for each soil (sand, clay) type by slope. Treatments consisted of erosion-control blankets (soil retention blankets) overlaying seeded embankments on clay and sandy loam soil on a 2: 1 and/or 3: 1 slope. Experimental control consisted of four plots receiving the same vegetative treatment for each soil type with no erosion-control blanket in place. Treatment plots were evaluated for sediment retention and vegetative density with respect to soil type and slope. The rainfall intensity determination was based upon rainfall intensities of 30.2 mm hh’ (1.19 in. hh’), 145.5mmh-1(5.73in.h-‘)and183.6mmh-1(7.23 in. h-l). These amounts were calculated intensities from storms of a 10 min duration and l-year, 2-year and 5-year return frequency (99%, 50%, and 20% probability of occurrence in a given year), respectively. The l-year storm intensity was derived from historical

163

data of Brazos County rainfall events obtained from the State Climatologist’s Office, Texas A&M University. The method used to derive the 2-year and 5-year values was the modified “Steel Formula” recommended in the Texas State Department of Highways and Public Transportation (now TxDOT), Bridge Division (D-5)) Hydraulics Manual, Third Edition, 1985. b j= (t,+dY where b, d and e are constants. The values of the constants b, d, and e were derived from the National Oceanic and Atmospheric Administration (NOAA) National Weather Service (NWS) Technical Paper No. 40, “Rainfall Frequency Atlas of the United States” and (t,) was equal to the storm duration. Recommended constants used for each county were from table 6 of the TxDOT Hydraulics Manual. The intensity values used were derived by computing the median values of i for the 36 counties located in the Houston, Austin, and Dallas triangle. The research team needed data that would accurately depict the vegetative density or apparent vegetative cover for the first growing season. After experimenting with several data collecting methods, the team chose to use a computer-based process to analyze the samples. The process was chosen since it was a reproducible and cost-efficient method to collect and analyze the samples. VeCAP or Vegetation Coverage Analysis Program was developed to calculate the percentage of pixels in a sample image by color. Sample images were recorded in the field, converted to single digital images using a Targa 16 board and TIPS software, and imported into the VeCAP program. The images were analyzed and a percentage of vegetation was determined. The sediment retention and vegetation density data were statistically analyzed by the variance test of the Statistical Analysis System Institute Inc. (Car-y, NC), and significant means were separated by Duncan’s muitiple range test (P < 0.05). Material performance was documented but no data were included in the statistical analysis. 2.3. Procedures Study plots were cleared of all vegetation, graded to a uniform condition, and hand raked to a fine grade.

164

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Each treatment plot received a seeding mixture of native and introduced grasses as recommended by the 1992 TxDOT Standard specifications, Item 164, Seeding for Erosion Control, rural mixture. The seed (and fertilizer) mixture was hydraulically installed with a hydroseeder immediately before product installation. Grass mixtures varied based upon soil type. For clay or tight soils the following grasses were used: Green Sprangletop, Bermudagrass, Little Bluestem, Indiangrass (Lometa), K-R Bluestem, and Switchgrass (Alamo). For sand or sandy soils the following grasses were used: Green Sprangletop, Bermudagrass, and Bahiagrass (Pensacola). Fertilizer was applied at the rate of 102 kg ha-’ (225 lb. acre-‘). Installation of blankets began in May of 1991 and was repeated for the 1992 cycle. All material installations were according to manufacturers’ published recommendations. To maintain rainfall uniformity throughout a multiple year testing program, all results were based on artificially generated rainfall. The onsite weather monitoring system recorded natural rainfall and any unusual weather events that might skew test results. Each study plot was subjected to a series of simulated rainfall events. The first rainfall events were of l-year return frequency, 30.23 mm hh’ (1.19 in. h-l). The next rainfall events were of a-year frequency, 145.54 mm h- ’ (5,73 in. h - ’ ) . Final rainfall events were of 5-year frequency, 183.64 mm hh’ (7.23 in. h-l). All simulated events were for 10 min durations. The following criteria were used for the rainfall simulation process: (1) rainfall simulation did not occur within 24 h of a natural rainfall or during any precipitation; (2) rainfall simulation was not done under wind conditions where most of the precipitation would be blown onto the adjacent plots; (3) adjacent plots were covered with plastic while the test plot received simulated rainfall. After each simulated rainfall event the sediment and water were suctioned with a wet-dry vacuum into buckets and weighed. Soil samples of uniform size were collected from each bucket, capped, labeled and stored. Each soil sample went through a drying process to find out the moisture-to-sediment ratio. Each soil sample was weighed, recorded and then emptied onto a microwave cooking dish. The soil was dried in a microwave oven for several minutes and weighed. This process continued until three consecutive weights

and Urban Planning 32 (1995) 161-167

became constant. Dry weights were recorded and averaged with the other replication samples to determine an average wet/dry ratio. This ratio was divided into the total weight of sediment to calculate total dry weight of the collected sediment from each plot. The sample dry weights were then divided by the number of 9.3 m* ( 100 ft.*) for each plot to determine the total sediment loss per 9.3 m2 ( 100 ft.‘) area. Vegetation establishment observations began 4 weeks after installation and continued at approximately 6 week intervals until the end of the growing season ( 15 November). The following process was done for each round of vegetation data collection: ( 1) each plot was subdivided on a graph into a grid of 0.5 m* (5.38 ft.*) sections; (2) a random sampling pattern was established using a table of random numbers; (3) observations from 20 random sections were recorded for the 3:l slope plots and 16 random sections were recorded for the 2:l slope plots (observations were recorded using a Hi-8mm video camera) ; (4) the video analog images were converted to digital images using a Targa 16 board and TIPS software; (5) single sample images were imported and analyzed with the VeCAP program to calculate the percentage of vegetation coverage. 2.4. Description of materials Erosion-control blankets were categorized into three varying degrees of definition as shown in Table 1. All of the materials were classified by generic material type, primary material classification, and trade or brand names in the first three columns. The last column documents steepness of slope conditions as requested by the manufacturer.

3. Results and discussion The material performance of each product is shown in Tables 2 and 3. Results from the sediment loss study indicated that control plots yielded significantly greater sediment than all other treatments within each of the four soil and slope conditions. There were no significant differences among products with respect to sediment retention performance on cohesive (clay or tight) soils regardless of slope condition. Means were spread for sandy loam soils suggesting that the erosion-control

S.H. Godfrey, M.K. Curry/L.andscape and Urban Planning 32 (1995) 161-167 Table 1 Description

of erosion-control

Generic classification

blankets in the 1991 and 1992 cycles Material classification

Brand name of material evaluated

Cycle

Slope

Excelsior

Amer. Excelsior Curlex” Xcel Regular@ Xcel Superior@ ANTI-WASH” /GEOJUTEa (Regular) GEOCOIR@‘/DEKOWE@ 700 North American Green” S75 North American Gree@’ S 150 verdyola ERO-MAT@’ North American Green@ SC150 Airtrol” Plaster

91+92

2:1+3:1 2:1 3:l 2:l

Jute

Straw

Straw/coconut Gypsum

Polypropylene

Synthetic

blends

performance

assessment

of erosion-control

91 91 91 92 91 91 91 91 92

POLYJUTEm 407GT Polyfelta TS220 GREENSTREAK” PEC-MATm

PVC

Table 2 Comparative

165

91 91

2:l 2:l 2:1+3:1

blankets on sediment loss for the 1991 and 1992 cycles

Clay soil Sediment loss mean (kg/sm)

per grouping

Sandy loam soil Sediment loss mean (kg sm) per grouping

21 slope treatment Curlex@ (92) Curlex@ (9 1) North American Greena SC150 PoIyfelt@ TS22 DEKOWBa 700 North American Greena S150 POLYJUTETM 407GT Airtrolm Plaster GRBENSTREAKm PEC-MATTM ANTI-WASH@ GEOJIJTEa Xcel Superiora Control

NA -0.3912 - 0.4346 - 0.4437 -0.4491 - 0.4608 - 0.4857 - 0.4962 -0.5100 - 0.5565 - 0.6555 -3.0056

(a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (b)

-44.50 -60.81 - 42.49 -51.27 NA -48.81 - 38.30 - 77.32 -63.56 -61.83 -31.99 - 134.36

31 slope treatment Curlexa (92) Curlex@ (91) verdyol” ERO-MATa GRBENSTREAKe PEC-MATTM Airtrol@ North American Green” S75 Xcel Regular@ Control

- 0.2367 -0.3017 -0.3129 -0.4107 -0.5011 - 0.5598 - 0.6559 - 2.9939

(a) (a) (a) (a) (a) (a) (a) (b)

- 8.454 (a) - 9.043 (a) - 18.634 (ab) - 33.667 (c) -25.429 (bc) - 16.624 (ab) - 9.672 (ab) -63.526 (d)

Mean separation by Duncan’s different. NA, not available.

2:l 2:l 3:l 2:l 3:l 2:l 2:1+3:1

(ab) (bc) (ab) (abc) (ab) (ab) (c) (bc) (bc) (a) (d)

multiple range test (P< 0.05); values within each column followed by the same letter are not significantly

S.H. Godfrey, M.K. Curry/Landscape

166

Table 3 Comparative

assessment of the effects of erosion-control

21 slope treatment Xcel Superior‘” Curlex” (9 1) POLYJUTErM 407GT North American Green”’ S150 ANTI-WASII~/CEOJUTEQ’ North American Green” SC150 GREENSTREAK’ PEC-MATTM Control Airtrol”’ GEOCOIR‘R’ DEKOWE” 700 Polyfelt”’ TS22 Curlex”’ (92) 31 slope treatment Curlex@ (92) North American Greenm S75 GREENSTREAK” PEC-MATTM Xcel Regular” verdyol” Eromat” Airtrol’F Control Curlex’” (9 1) Mean

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blankets on vegetation density production

for the 199 1 and 1992 cycles

Clay soil Vegetative density mean (%) per grouping

Sandy loam soil Vegetative density mean

98.814 97.834 96.151 92.014 90.058 89.979 87.580 86.400 86.094 73.717 35.909 NA

(a) (a) (a) (a) (a) (a) (a) (ab) (ab) (b) (c)

85.805 52.674 74.302 84.746 51.372 76.409 38.863 40.123 41.882 38.716 46.051 47.335

(a) (b) (a) (a) (b) (a) (b) (b) (b) (b) (b) (b)

98.125 96.187 90.524 90.166 87.808 86.444 67.286 63.230

a a a a a a b b

33.232 77.904 63.385 72.263 73.202 68.749 47.553 60.937

d a b ab ab ab c bc

(%) per grouping

separation by Duncan’s multiple range test (P
different.

blankets’ effectiveness on sediment loss is more variable under this soil type. Xcel Superior@ yielded significantly less sediment than all other treatments. PolyjuteTM 407GT, North American Greena SC150, Curlex@ (92)) and North American Green@ S 150 performed similarly and retained significantly more sediment than Polyfelt@, Pecmat@, Geojute@, and Curlex@ (91). Results from the sediment loss test suggest that the selection of erosion-control blankets is critical with more erodible soils than cohesive soils regardless of slope condition. As expected, sediment loss was significantly greater on the erodible soil (K=0.38) than on the erosion resistant soil (K= 0.20), regardless of the slope condition. The erosion resistant soil is more cohesive than the erodible soil. This would explain the soils enhanced capability to resist the forces of rain splash. Generally, the organic products reduced the amount of sediment loss significantly greater than the synthetic products. This finding may be a result of the organic products tendency to burrow down into the soil to form a soil/

material bond that was not apparent with the synthetic products. In contrast, the synthetic products (Polyfelt@, Pecmat and Polyjute 407GT) tended to span the surface of any rill formations that developed, instead of conforming to the shape of the slope. In the vegetation study, Polyfelt@ TS220 supported significantly less vegetation than all other treatments of 2:l slope and clay soil. Xcel Superior, Polyjute 407GT, North American Green S 150, and North American Green SC150 supported significantly more vegetation than Curlex (91), Geojute, Pecmat, Dekowe@ 700, and Polyfelt@ under conditions of 2:l slope regardless of soil type. For the 3: 1 slope and clay soil, the control and Curlex (91) yielded significantly less vegetation than all other treatments. Interestingly, Curlex (92) supported the greatest vegetation density for 3:l slope and clay soil, while it yielded significantly less vegetation than all other materials placed on sandy loam soils with same slope condition. Data indicated that the erosion-control blankets tested support vegetation at a relatively similar level

S.H. Godfrey, M.K. Curry/Landscape

for clay soils. This level is generally greater than that of the control plots, although not always significantly. Further, erosion-control blankets seem to be more important in the establishment of vegetation on plots with sandy loam soils regardless of slope. Overall apparent vegetative cover on the erosion resistant (K = 0.20) soil was more abundant than on the erodible soil (K= 0.38)) regardless of the slope condition. This finding might be attributed to a higher percentage of clay, silt, and organic content found in erosion resistant soil which could have promoted better germination and growth. The following trends were also observed in the vegetation study: ( 1) products containing straw, excelsior, or PVC as the primary component were the top vegetation producers on the 3: 1 slope regardless of soil condition; (2) products composed of excelsior, straw, straw/coconut, or polypropylene were the top producers on the 2: 1 slope regardless of soil condition.

4. Conclusions Results of the study indicate that vegetative cover on erosion resistant clay soil (K = 0.20) proved significantly (P
and Urban Planning 32 (1995) 161-167

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significantly (P < 0.05) higher percentage of vegetation than the other organic and synthetic materials analyzed. Moreover, controls produced significantly less vegetation cover than the plots that were protected by erosion-control products. The erosion control data indicate that sediment loss is significantly (P < 0.05) greater on the erodible sandy loam soil (K= 0.38) than the erosion resistant clay soil (K= 0.20), regardless of the slope condition (2: 1, 3: 1) . This observation may be contributed to the cohesive properties of the erosion resistant soil which may enhance the soil’s capability to resist the forces of rain splash. Sediment loss was also shown to be significantly (P < 0.05) reduced by the use of organic products compared with synthetic materials. Furthermore, the bare soil plots (controls) yielded significantly (P < 0.05) greater quantities of sediment than the plots protected by an erosion-control blanket. As a result of this study, the Texas Department of Transportation (TxDOT) established an Approved Materials List and standard installation details for approved materials. These products are incorporated into the construction bid set and are referenced in the Department’s Standard Specifications for Construction of Highways, Streets, and Bridges, 1993. The list and detail sheets are updated on an annual basis by the Department based upon the research results. Standard specifications and details allow the designer/specifier choices and flexibility in product selection while maintaining a standard quality performance level. The documents also expedite the design process by eliminating the need for the designer to generate such documents and construction inspectors prefer the standard layout and thorough installation information.