"The Effect of Surface Roughness on Soil Erosion"
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By: Mike Harding
It should come as no surprise to those working in the erosion control field that a roughened soil surface is much less erodible than a smooth, rolled or compacted surface. This is because a roughened surface slows down the velocity of runoff water and increases infiltration, reducing the volume of sediment-laden runoff as well. Agriculturalists have known of this phenomenon for some time, as indeed the Soil Conservation Service – now the Natural Resource Conservation Service – since the Dust Bowl days of the 1930’s, has been encouraging farmers to use various tillage practices such as chisel plowing to create roughened conditions in harvested fields to prevent sheet and wind erosion.
It is a paradox that in construction, the same machinery that is used to disturb the soil can also be put to use to create surface conditions that reduce the soil erosion:
• Bulldozers can be used to track-walk up and down slopes (Figure 1)
• Sheep’s-foot rollers, normally used for compacting soils, can be employed to “dimple” the soil surface (Figure 2)
• Tines on the backside of bulldozers can be used on the contour to create surface roughness (Figure 3)
• Imprinting equipment, invented to address re-vegetation issues in desert climates can create a unique, moisture holding and vegetation supportive depression in the soil (Figure 4)
• Slope boarding is not only beneficial in creating contour erosion-reducing soils structure, but also assists in keeping seed and mulch on a steep slope (Figure 5)
But just how much erosion control do these roughening techniques impart? That was a question that was asked in a study conducted for Caltrans at the San Diego State University’s Soil Erosion Research Laboratory in 2000.
The San Diego State University Soil Erosion Research Laboratory
The San Diego State University Soil Erosion Research Laboratory (SDSU/SERL) integrates beneficial features from some of the foremost soil erosion research facilities in the United States at Texas A & M University, Utah State University and Purdue University.. Funding for the facility was provided by Caltrans, (California State Department of Transportation) as part of a 1998-2000 Erosion Control Pilot Study, in which design, construction and operation of the SERL was supervised by URS Corporation and SDSU faculty from the Department of Civil and Environmental Engineering. The laboratory is in general conformance with the outlined methods and scope of ASTM D6459, Standard Test Method for Determination of Erosion Control Blanket (ECB) Performance in Protecting Hillslopes from Rainfall Erosion.
A clayey-sand soil typical of Southern California conditions was placed and compacted on the test bed while it was in the horizontal position. Four candidate surface roughening practices were applied on the soil surface to simulate:
1. Track-walking by a bulldozer
2. Sheep’s-foot roller
A smooth, compacted surface was utilized as a control against which to compare the various treatments. The test bed was then raised to a specified gradient (2H:1V). Three (3) replications of six (6) different storm events were applied to each slope preparation, yielding a total of eighteen (18) rainfall events per surface treatment. All runoff from the test surfaces was collected by flume at the bottom of the slope and analyzed for volume of runoff and weight of sediment. Table A presents the “Results of Rainfall Simulation Testing for Soil Roughness”.
As can be seen from the data in Table A, roughness in of itself appears to have a beneficial effect in reducing soil erosion. Compared to the bare soil control (smooth, compacted surface) roughening practices appeared to reduce erosion in (ascending order) by:
• 12% for ripping on the contour
• 52% for track-walking up and down a slope
• 54% for sheep’s-foot rolling
• 76% for imprinting
Of side interest is the effect that the roughening practices appear to have on increasing or decreasing runoff volume. For example, ripping decreased the amount of runoff by 19%; imprinting by 4% while sheep’s-foot rolling and track-walking appeared to increase runoff by 12 and 2% respectively. This should come as no surprise in that both sheep’s-foot and bulldozer tracks tend to compact the soil.
The results of the study illustrate that soil roughening should be considered as a vital step in soil preparation for re-vegetation and erosion control. Roughening is at best a temporary practice when used as a preliminary, complementary step prior to seeding and the application of hydraulic applications, blown straw mulch or the application of wood chips, compost or rolled erosion control products.
Figure 1. Grouser tracks on a bulldozer can be used as a surface roughening technique when the tracks go up and down a slope. On the other hand, if the tracks go across a slope, erosion is accelerated.
Figure 2. A sheep’s foot roller can be used to “dimple” a slope as long as the action is not taken to an extreme, smooth and compacted surface.
Figure 3. Tines on the back side of a dozer, or similar agricultural tillage equipment, when applied on the contour and perpendicular to water flow, can increase infiltration and reduce erosion.
Figure 4. The resulting roughness created from imprinting creates an opportunity for water to and sediment to be collected and fosters the germination and growth of seed.
Figure 5. Slope boarding is generally applied to cut slopes and can provide and underlying structural support to hydraulic seed and mulching applications.
RESULTS OF RAINFALL SIMULATION TESTING FOR SOIL ROUGHNESS
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