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From BioCycle Journal of Composting &Organics Recycling November 2001, Page 43 |
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FILTRATION WITH MULCH
USING WOODY MATERIALS FOR EROSION CONTROL
Connecticut researchers conduct a series of field trials and laboratory analyses to determine how particle size and other characteristics affect mulch performance.
Kenneth Demars, Richard Long and Jonathan Ives
WHEREVER a soil slope is exposed to rainfall or running water, methods are needed to prevent erosion. Some sites only need protection during construction – buildings, roadways, development; other sites have a continuing challenge with barren slopes. Wood residuals have been effective at preventing fine silt and clay particles from damaging waterways.
Residuals can consist of a mix of bark, wood shavings, wood chips, wood scraps and mineral grit that are by-products of the lumber, paper and construction industries. In a series of tests at the University of Connecticut, material characteristics were correlated with field performance to get a clearer idea of how effectively erosion could be controlled. The overall objective was to quantify the properties and behavior of mulches made from wood residuals as a medium to control erosion, when applied to slopes at a thickness of three-quarters inch up to three inches.
Use of natural materials to reduce erosion has been well established, especially with composted feedstocks. Research trials going back to 1962 show how soil erosion has been controlled with composted materials in the vineyards of Europe, and more recently in general applications as well. State Departments of Transportation have increased their use of compost at highway construction sites to prevent runoff.
PHYSICAL AND CHEMICAL PROPERTIES
When wood particles are used as a mulch at a thickness of three-quarters inch or greater, the amount of soil eroded can be reduced to one-fiftieth or less than the erosion from an untreated surface. For wood residues to be used effectively on large-scale projects, procurement specifications – based on physical properties of materials – are needed to ensure the proper effect is achieved (see sidebar).
Three wood residual products were used for the trials – a paper mill wood waste, a pine bark mulch, and ground stump/wood waste mulch. All samples were subjected to laboratory testing for physical properties; some additional chemical tests for toxicity assessment were completed before these materials were placed at the field test site. Table 1 shows the results of the physical property tests. The organic matter content was highest for the pine bark mulch, followed by the ground stump mulch. The paper mill wood waste had the lowest organic matter content. The friction angle measures the stability (amount of mass movement or sliding) of these products under the gravimetric forces that were imposed during the field tests. Based on the laboratory tests, all three products were expected to be stable on the slope angle of the test site (1 vertical to 2 horizontal or 26.6 degrees versus the lab sites which were all greater than 43 degrees).
The model procurement specifications for source separated compost adopted by the Coalition of Northeast Governors (CONEG) in 1996 specified chemical properties for usable composts and mulches. Chemical properties of the three wood residual products used in this study and the base soil were measured, including stability, pH and conductivity (Table 2). Stability (an indicator of the degree of composting of the raw feedstock) was measured using the Solvita test with a numerical system from one for raw non-composted feedstock to eight for fully composted/biodegraded and cured materials. This test verified that the ground stump mulch was fresh raw material and the other two materials are further along in the composting process as evident from their color and appearance. The CONEG specifications recommend that erosion control materials should be very stable to stable, which was not the case for the fresh ground stump material.
TRIALS AND RESULTS
Large-scale erosion control testing was done at a field site with a 1:2 slope (the steepest soil slope normally used by the Connecticut Department of Transportation [Conn-DOT]). The base soil is an easily eroded silty sand with some gravel. Fourteen test cells (5-feet by 30-feet) were constructed on this slope in May 1999 (Table 3). Two cells were left untreated as reference cells and three other cells were untreated but contained erosion control structures including a wood waste filter berm (made from the paper mill’s mulch product), geosynthetic silt fence (silt fence), and hay bale silt barrier near the bottom of the cell. The other nine cells were prepared with different wood waste mulch treatments; each were placed at thicknesses of 0.75, 1.5 and 3.0 inches. None of the cells were seeded or stabilized other than by the wood waste surface treatments. Weed control was performed every month or so and some weeds were pulled or cut to keep roots from developing as an erosion retardant. As a result of this weed control, the late fall storm events were expected to test the erosion resistance of these surface treatments before growth could be established.
The erosion control performance of each cell treatment was evaluated for 11 storm events of varying rainfall magnitude and intensity. Calibrated tipper boxes were used to measure the amount of runoff from each cell and collection buckets were used to sample runoff and determine the mass of sediment eroded from each cell. Evaluating the performance of the various surface treatments when subjected to the expected range of rainfall magnitude and intensity was difficult to achieve because of the random nature of storms. It was accomplished, however, by visiting the field site after each storm for the period of June through mid-October 1999. Rainfall during the storm events varied from 6 mm to a maximum of 110 mm (4.35 inches) for Hurricane Floyd on September 16, 1999.
A day or two after each storm event, each runoff collection bucket was sampled for analysis of suspended solids in the runoff water. These suspended solids measurements indicate average erosion from each cell treatment and storm event, and are combined with the total runoff measured by the tipper boxes to determine the mass of sediment particles eroded from a slope. (The sediment mass eroded had to be corrected for the mass of coarse particles that tended to collect on the apron at the bottom of a slope and not enter the collection system.)
RESULTS AND CONCLUSIONS
The runoff volumes varied among the cells with the amount of total rainfall in the storm. In one case, when a storm event led to a heavy rainfall of 27 mm (1.06 inches) over a long weekend, the suspended solids concentration in the runoff was very high for the two untreated test cells (>10 g/l of solids). These erosion levels are more than an order of magnitude greater than the erosion for any of the treated test cells (<1 g/l).
Cells 8, 9 and 10 are used to compare the performance of the paper mill wood waste filter berm in cell 10 with the effectiveness of a silt fence in cell 8 and hay bales in cell 9. These cell surface areas are untreated like the control cells. Their retention of the flow allows some of the runoff water to percolate into the soil. During all storm events, the wood waste filter berm was more effective in retaining erosion runoff than either the hay bales or the silt fence. Figure 1 shows the ratio of the amount of soil reaching the measuring systems below the retention structures to that measured at the control cells. The results are presented as a percentage. All erosion control structures are effective in significantly reducing the amount of eroded soil that gets past the structure. The silt fence is more effective than a hay bale berm, but the wood waste filter berm reduces the amount passed by nearly an order of magnitude.
The primary conclusions of the study are as follows:
• Wood waste materials are effective in minimizing erosion when applied to the soil surface as an erosion control mulch with a thickness of 0.75 inches or more. An untreated soil surface produced over 50 times more sediment than a treated surface.
• Wood materials are particularly effective in reducing runoff during storms under 0.5 inches by absorbing rainwater and by promoting percolation.
• The thickness of 0.75 inches allows vegetation to root and grow through wood waste materials.
• The wood waste erosion control filter berm was more effective than either hay bales or silt fence at controlling erosion. While all erosion control structures were effective compared to no treatment, hay bales and silt fence released about an order of magnitude more sediment than the filter berm made from paper mill mulch.
Kenneth Demars is Associate Professor and Richard Long is Emeritus Professor at the University of Connecticut in Storrs. Jonathan Ives is a project engineer with URS Greiner in Rocky Hill, Connecticut. This work was sponsored by the New England Transportation Consortium at the University of Connecticut in Storrs.
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