Effectiveness of coir-based rolled erosion control systems in reducing sediment transport from hillslopes

doi:10.1016/j.apgeog.2007.07.011

Applied Geography

Volume 27, Issues 3–4, October 2007, Pages 150-164

https://doi.org/10.1016/j.apgeog.2007.07.011 Get rights and content

Abstract

Accelerated soil erosion is ubiquitous on human-modified hillslopes. A variety of erosion control products have been developed to reduce on-site soil resource degradation, and off-site transport of sediment and sediment-associated contaminants to receiving water bodies. However, limited quantitative data are available to assess erosion reduction effectiveness, and to establish the salient properties of the erosion control products. A replicated field-based rainfall simulation study was conducted to compare the runoff and erosion effectiveness of three coir (coconut) fiber rolled erosion control systems (RECSs) with a bare (control) treatment. Detailed temporal measurements of runoff and sediment transport were made during two phases of each experiment: (1) a 110-min application of rainfall via a rainfall simulator at 35 mm h⁻¹ after runoff initiation and (2) a 30-min period, at 3 times the flow rate of phase 1, applied via an overland flow generator. All coir treatments enhanced infiltration, delayed time to runoff generation, reduced intensity of rill incision, and reduced sediment output compared to bare treatments. More importantly, statistically significant differences were observed between coir RECSs of different architecture. For the two open weave coir systems tested, the most effective design had a higher mass per area, and less open space between the regularly aligned grid of fibers. The random fiber coir architecture was the most effective, having significantly lower runoff sediment concentrations, lower sediment yields, and a lower frequency of rill initiation. The differences in system architecture are examined in light of fundamental controls on runoff and erosion processes.

Introduction

Human modification of the landscape commonly results in accelerated erosion and concomitant environmental degradation. On-site and off-site impacts associated with erosion are estimated to cost approximately $44×10⁹ per year in USA alone (Shepley, Smith, & Jackson, 2002). Today, environmental and economic costs associated with accelerated erosion are not considered acceptable. In many developed countries, a vibrant erosion control industry (ECI) has formed to mitigate erosion on slopes modified by construction activities, road, and highway building (Sutherland, 1998b).

Erosion control specialists, construction site engineers and landscape architects have a number of ‘tools’ at their disposal to keep soil on site. These erosion and sediment control practitioners are required to identify the most appropriate and cost-effective best management practices (BMPs) for their erosion control plan. For immediate surface protection, the most commonly used non-structural BMPs on construction site slopes include straw bale barriers, silt fences, loose organic mulches, rolled erosion control systems (RECSs), hydraulically applied hydro-mulches, and dust suppressants (Raskin, DePaoli, & Singer, 2005; Sutherland, 1998b; USEPA, 1995).

Research has shown that RECSs (also known as ‘geotextiles’ in UK) are one of the most appropriate BMPs for hillslope protection (Hann & Morgan, 2006; Nelsen, 2003; Sutherland, 1998b). A wide variety of RECSs are manufactured to capitalize on a multi-million dollar market. Rolled systems can be grouped into those composed of natural fibers with life spans ranging from 0.5 to 6 years (temporary), or synthetic fibers that are considered permanent fixtures. Natural fiber RECSs include jute, coir (coconut), excelsior (wood strands), and straw. Application of RECSs usually occurs on bare slopes after broadcasting a rapidly germinating seed mixture for long-term erosion protection. Natural fiber systems are increasingly favored, as they are biodegradable, less costly to produce and to apply, environmentally friendly, equally effective in reducing erosion, and generally provide a favorable microclimate for biomass production (Sutherland (1998b), Sutherland (1998c); Sutherland, Menard, & Perry 1998; Sutherland, Menard, Perry, & Penn, 1998). Coir, for example, has been increasingly applied to human-modified hillslopes. Several recent studies have found coir RECSs effective in reducing erosion from degraded hillslopes (Lekha, 2004), highway embankments (Benik, Wilson, Biesboer, Hansen, & Stenlund, 2003), railway embankments (Gyasi-Agyei, 2004), and from slopes similar to construction sites (Krenitsky, Carroll, Hill, & Krouse, 1998). Most published studies, however, have failed to examine the detailed temporal response of the systems under stress, and have overlooked links between system properties (e.g., fiber geometry) and basic physical erosion processes (e.g., splash, wash, and rill erosion). As Thompson (2001) states, “long-term progress in selecting erosion control measures can best be made by obtaining a better understanding of the interactions of the control measure and fundamental erosion principles”.

Land managers, department of transportation personnel, and erosion consultants are faced with a wide variety of RECSs to choose from, but little rigorous quantitative data for optimal decision making. Even for a given natural fiber there may be several design architectures, each with unique cardinal properties (i.e., physical, chemical, and hydraulic). Coir RECSs have two common architectures. The first is a randomly oriented set of loose fibers stitched with thread between two nets. This type typically has a low mass per unit area (200–300 g m⁻²), and limited open space between fibers (<10%). The second type of coir system is an open weave architecture with spun coir forming an interlocking grid with significant open space. Open weave systems have higher mass per unit area compared to the random fiber architecture, with values at the low end ranging from 350 to 500 g m⁻²; at the upper end 700–900 g m⁻². Open space between the grid of fibers ranges from 30% to 40% for high mass per area systems, to 50–80% for the lower mass per area systems (cf. Sutherland, 1998b).

The primary objectives of this study are to quantify the hydraulic and erosion response of various coir systems to different flow stress levels; and to link basic erosion processes with specific system design criteria. To vary flow stress levels, we applied rainfall with a field-based rainfall simulator, followed by overland flow with an overland flow generator on bare surface treatments and coir protected slopes.

Section snippets

Treatments, site selection, and soil preparation

Three commercially available, and widely applied, coir RECSs were selected for this study. Two architectures were examined, a random fiber coir (RFC) system (Fig. 1), and an open weave coir (OWC) system (Fig. 2, Fig. 3). We examined two open weave products, manufactured by the same company, differing in mass per unit area and degree of open space. The open weave system with the lowest mass per unit area (and greatest proportion of open space) is designated as OWCL (Fig. 2); the system with a

Water input and runoff generation

Rainfall application did not differ significantly between treatments (α=0.05). The overall mean rainfall intensity was 35±3 mm h⁻¹ (±1 standard deviation), with the bare treatment=34±4 mm h⁻¹; OWCL=35±1 mm h⁻¹; OWCH=36±2 mm h⁻¹; and RFC=36±2 mm h⁻¹.

During phase 1, the time required to initiate runoff differed significantly (α=0.05) between treatments (Kruskal–Wallis test followed by post hoc testing). The bare treatments were the first to generate runoff, with an average time of 33±4 min. Runoff

Runoff generation and overland flow leading edge velocity

The coir RECSs significantly delayed the time to runoff generation and enhanced infiltration during the early portion of the rainfall phase compared to the bare surface treatment. Enhanced infiltration below RECS-treated slopes reflects the lower probability of raindrops directly impinging on the soil surface, and causing aggregate disintegration. Therefore, this would help to maintain a stable hydraulic interface. Once runoff was initiated on all treatments there were no significant

Conclusions

Human-modified slopes are a ubiquitous feature associated with urban growth. With slope disturbance, comes accelerated soil erosion and the increased potential for deleterious downstream impacts. To effectively mitigate these impacts, erosion and sediment control specialists require quantitative data from replicated studies to identify optimal practices, such as RECSs, under different stress conditions. In the present study, a replicated field experiment with separate rainfall and overland flow

Acknowledgments

Financial support for this work was provided to the Geomorphology Laboratory by the Department of Geography at the University of Hawaii at Manoa. Dr. Mark A. Nearing is gratefully acknowledged for arranging the use of the Norton ladder-type rainfall simulator. We are indebted to Mr. Trae Menard, and the Sutherland clan for their help and entertainment during the field portion of the experiment. The comments of two anonymous reviewers were greatly appreciated, and enhanced the final quality of

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Effectiveness of coir-based rolled erosion control systems in reducing sediment transport from hillslopes

Abstract

Introduction

Section snippets

Treatments, site selection, and soil preparation

Water input and runoff generation

Runoff generation and overland flow leading edge velocity

Conclusions

Acknowledgments

Geotextiles and Geomembranes

Catena

Geotextiles and Geomembranes

Performance of erosion control products on a highway embankment

Transactions of the American Society of Agricultural Engineers

Elementary processes of soil-water interaction and thresholds in soil surface dynamics: A review

Earth Surface Processes and Landforms

Optimum use of erosion control blankets and waste ballast (rock) mulch to aid grass establishment on steep slopes

Journal of Hydrologic Engineering

Evaluating erosion control measures for biorestoration between the time of soil reinstatement and vegetation establishment

Earth Surface Processes and Landforms

Erosion development of streams and their drainage basins: Hydrophysical approach to quantitative morphology

Geological Society of America Bulletin

Runoff and sediment losses from natural and man-made erosion control materials

Crop Science

Mathematical simulation of the process of soil erosion by water

Transactions of the American Society of Agricultural Engineers