Natural erosion occurs over time, but erosion can also be greatly accelerated by human activities. This includes construction sites of roads and buildings, mining, and even recurrent walking in a particular trail or path. Undisturbed soil has a natural groundcover that protects it, but repeated activity in that area of soil can cause that protection to deteriorate. Thus, the absence of that natural surface cover increases the risk of soil erosion. Erosion prevention methods are unique to each site based on the site’s topography, climate, vegetation, and soil type. Because of the harmful environmental impacts that erosion can have, including the destruction of natural habitats, the removal of vegetation, the transport of eroded soil into bodies of water, it is crucial to understand which erosion prevention method would work best in a certain location based on its geographical characteristics.
In my project, I assessed the erosion prevention effect of four different ground covers under different topography and rainfall intensity: Creeping Jenny (Lysimachia nummularia) groundcover plants, gravel, straw, and mulch. I set up four boxes, each box separated into five categories with small Creeping Jenny plants, gravel, straw, mulch, and control with the soil fully exposed. See Figure 1 for an illustration of the set-up. To measure the efficacy of these four erosion prevention methods under different topographic conditions – variations in slope steepness- I elevated two of my four boxes to seven degrees and the other two boxes to twelve degrees. A seven-degree slope steepness represents a gentle to moderate slope, while a twelve-degree slope represents a steeper slope. To simulate rain, I used a typical watering bucket with a shower head to water 2 gallons of water in each of the four boxes. Of each set of two boxes with the same degree, I set one box to be watered at a height of one foot and the other to be watered at a height of two feet.
Figure 1. The Set-up of the Four Boxes
With this set-up, I measured the effect of rain intensity on the four erosion prevention methods, assuming that greater rain height amounts to greater rain intensity. See Figure 2 below for an illustration of the four different combinations of slope degrees of box and rain heights.
Figure 2. Four Different Combinations of Box Slopes and Rain Heights
To measure the degree of soil erosion, I stuck a thin rod into the two separate sides of both the top and bottom parts of each section and measured the soil height with a ruler. I then took the average between the two measurements for the top and bottom to achieve greater accuracy. Finally, I calculate the differences in depth at the top and the bottom. The difference in depth will be the measure of the degree of soil erosion. See Figure 3 for an illustration of this measuring process.
Figure 3. Illustration of Measurement of Soil Erosion
After about nine weeks of watering, I put together my data to analyze the overall trend. Figure 4 below illustrates the degree of erosion of each type of ground cover in each box. Figure 5 depicts the effect of the slope degree on soil erosion by keeping the rain intensity constant, and Figure 6 depicts the effect of rain intensity on soil erosion by keeping the slope degree constant.
Figure 4. Average Degree of Soil Erosion (Inches) of Four Erosion Control Methods
Figure 5. Effect of Slope Degree on the Amount of Soil Erosion
Figure 6. Effect of Rain Intensity on the Amount of Soil Erosion
Figure 7. Best Erosion Prevention Method for Each Condition
Creeping Jenny Groundcover
The Creeping Jenny grew to the largest size in box one. Because plant roots function to hold the soil, I inferred that as a plant grows larger and its roots increase in length, the plant should be able to prevent erosion more effectively. In box 1, the soil in the section with the three Creeping Jenny plants eroded an average of -0.225 inches. Additionally, in box 2, the section with the Creeping Jenny plants eroded a greater average of -0.325 inches (Figure 5). The plants in box 2 also grew fairly well, almost expanding outside of the box edges. However, in boxes 3 and 4, which were set up on a 12o slope, the Creeping Jenny plants were approximately the same size as when they were first planted (Figure 1). Although the average amount of erosion in the Creeping Jenny sections in boxes 3 and 4 was not drastically different from that in boxes 1 and 2, the slope difference did hinder the ability for the plant to grow. Therefore, in landscapes with a steeper slope and more intense climate conditions, utilizing native plants as an erosion prevention method may not be as effective as methods like mulch and straw, especially if the native plants are used permanently. Instead, using Creeping Jenny plants would be adequate in areas with a level to moderate slope to simultaneously promote plant growth and prevent erosion.
The data for the gravel sections in boxes one and two depict the gravel as one of the most effective erosion-prevention methods, with the lowest overall average of -0.35 across all four boxes. However, in boxes three and four, the gravel sections have a much greater average slope change. In fact, by the end of the nine-week period, I noticed that the gravel in boxes one and two was fairly spread out throughout the section, and barely any of it had shifted due to its moderate slope. On the other hand, the gravel in box three was crowded together in many sections in the soil, leaving some parts of the soil exposed. In box four, the gravel only covered the bottom three-quarters of the section, leaving the upper quarter of soil completely exposed with about 3 or 4 small rocks remaining (Figure 4).
According to Figure 4, comparing the data from boxes elevated to the same slope degree, gravel proves to be an effective erosion prevention method in locations with moderate slopes, which allow the gravel to stay in place. Specifically, in the boxes with a slope of seven degrees, the gravel section experienced a slope change of only -0.2 and -0.15 inches in boxes one and two, respectively. In the two boxes with a strong slope of twelve degrees, the gravel section experienced soil erosion of -0.4 and -0.65 in boxes three and four, respectively (Figure 4). Lastly, when comparing boxes with different rain heights, I noticed that the rain intensity only slightly affected the movement of the gravel downhill. The difference between the average degree of erosion of the gravel was 0.05 inches in boxes one and three and -0.25 inches in boxes two and four, which had a greater rain intensity (Figure 6). Although rain intensity did have some impact on the efficacy of the gravel, slope steepness was a much larger factor in its ability to hold the soil. Thus, in locations with intense climate conditions like heavy rain and wind and a strong slope, gravel would not be an effective erosion prevention method. Instead, according to Figure 7, gravel is best used in level to moderate slopes, and it is capable of holding the soil for long periods of time.
The mulch in box one, after a nine-week period, showed barely any shifting in the section. Considering this observation and how the mulch in box one had a larger slope change than all the other erosion prevention methods in box one and even the mulch section in box two, which had a greater rain intensity, this piece of data seemed to be an outlier. Specifically, in box one, the mulch section experienced a degree of erosion of -0.375 inches, while in box two, the mulch section experienced a lesser slope change of -0.325 inches. Throughout all four boxes, the mulch did not show significant changes in movement, and the data for slope change in the mulch sections, according to Figures 5 and 6, do not portray that either slope degree or rain intensity has much impact on its efficacy.
Overall, the average data for straw proved that it was the least effective erosion method, but its average degree of erosion in boxes one and three was significantly lower: only -0.175in and -0.35in, respectively. However, in boxes two and four, aside from the control, the straw sections experienced the greatest degree of erosion of -0.425in and -0.65in, respectively. According to my observations, by the end of the nine-week period, the straw in boxes two and four was very well-packed into the soil, barely observable from a distance. It no longer retained its dry texture, and instead became very condensed and moist. In fact, in box four, the straw contracted so that it only covered the center of the section, leaving the edges exposed. Straw would make a good temporary erosion control method because too much rain causes it to soften and lose its effectiveness. Comparing the data from boxes with different slope heights, it does not seem that steepness of a slope impacts the straw’s efficacy as much as rain intensity does.
Figure 8: Use of Erosion Prevention Methods
Before deciding on which erosion prevention method would be best for a certain site, it is imperative to understand the qualities of the erosion site. These qualities include the environment of the surrounding area, soil type of the site, the climate of the site location, and the topography of the location. For example, planting native plants as ground cover in construction sites would be ineffective. Whereas, they would be good for natural sites where the land has a level to moderate slope and is not usually disturbed (Figure 8). This includes backyards, gardens, and alongside walking trails, and other locations where native plants are able to grow. Gravel would work best in areas with pedestrian-induced erosion and low slopes, such as next to level roads and trails (Figure 8). Gravel can also be seen in varying sizes near storm drains to prevent water erosion; larger rocks hold much better in steeper slopes than do small gravel. Straw would make a great temporary ground cover due to its ability to trap sediments on a soil slope. However, in long periods of time with heavy rain, the straw will degrade and leave some parts of the soil exposed. Therefore, straw is often temporarily used as an erosion prevention method in construction sites or along roads (Figure 8). Mulch, unlike the other three prevention methods, is capable of withstanding steeper slopes. They would work well in both hilly landscapes and along more level walking trails (Figure 8). Mulch is also often used with native plants in order to hold the soil for the plants to grow. This can be effective if native plants are being planted on steeper slopes.