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