We all tend to describe the steepness of slopes in different ways. Some of the most common descriptors are:
1. Degree of inclination (i.e., 26.87°, 18.43°, etc.)
2. Percentage of vertical rise over horizontal run (i.e., 50%, 33.3%, etc.)
3. Ratio of horizontal run to vertical rise (i.e., 2:1, 3:1, etc.)
Although (3) is the most common form of description, many governmental agencies actually reverse the relationship and describe slopes in terms of vertical rise to horizontal run (i.e., the Oregon Department of Transportation and Caltrans denote slopes as 1:2, 1:3, etc.).
Being able to visualize the steepness of a slope regardless of the manner in which is described is an important aspect of erosion control. Slope inclination can affect the erosion potential of a site, the ease to which various best management practices (BMPs) can be applied or installed and is one of the research parameters commonly investigated to determine the effectiveness of various BMPs. As suggested in the accompanying white paper on “Slope Measurement”, slope inclination can have a marked effect on the amount of mulching material estimated for a job site.
Table A is a Slope Conversion Chart to help you visualize slope inclination, no matter how it’s described.
Finally, in addition to slope steepness the roughness, configuration and orientation of a slope is also an important consideration in erosion control and re-vegetation planning.
• Roughness of the soil surface is generally considered beneficial for erosion control, sediment retention and plant establishment (See white paper entitled: “The Effect of Surface Roughness on Soil Erosion”); • Slopes that are convex or concave can have different erosion characteristics; and, • The direction a slope faces is critically important in drier climates and at altitude. In the Western United States north-facing slope typically achieve a higher percentage of vegetation coverage than south-facing slopes that are hotter and drier (Figure 1).
Slope orientation needs to be taken into account in re-vegetation planning, particularly in fire-prone areas, as slow regeneration of indigenous plants on south-facing slopes has the potential to result in greater runoff and erosion after burn events.
Figure 1: The effect of slope aspect or orientation on vegetation establishment. In this photo, the highly vegetated slope faces north and the weakly vegetated slope faces south. Both sides of this berm were treated at the same time with a hydraulic mulch matrix. TABLE A
SLOPE INCLINATION CONVERSIONS