Soil Categories and Erodibility

This week centered around the relationship between soil categories and erodibility. This is a big aspect of environmental engineering because building on the wrong kind of soil can be absolutely catastrophic to your project. This is also where the work with the rain garden comes into play.

The concept behind a rain garden is straightforward: it is designed with a central depression to retain rainwater runoff and give it time to seep into the soil. This infiltration helps “recharge” groundwater and protects local water quality by reducing polluted runoff. Rain gardens usually grow native plant species that thrive without fertilizers and pesticides. These native plants also have elaborate root systems that create channels into the soil causing it to more readily absorb water. So then comes the central question of this week- based on erodibility and soil stability, which soil category would be best for building a rain garden?

Most soil can be divided into six categories- silty, sandy, clay, loamy, peaty, chalky. And of course, combinations of all six exist naturally as well. The optimal soil for a rain garden would be one that is nutrient rich enough to allow plants to grow, but also drains freely enough to absorb stormwater in about 2 days. And of course, like all science, categorizing these soils comes with a little bit of math.

The soil erodibility factor (K-factor) is a quantitative description of the inherent erodibility of a particular soil; it is a measure of the susceptibility of soil particles to detachment and transport by rainfall and runoff. {The preferred method for determining K-factors is the nomograph method based on the work by Wischmeier (1971) and is mathematically represented as follows}-

screenshot 5

The K values for the most common types of soil are as follows-

screenshot 4

After understanding the differences between these types of soil, my job was to make another guidesheet for the kids (of course, without all the higher level math). They would be using their soil stability kits to determine the numerical value of the soil stability, then from there they would use this sheet (Soil Categories and Erodibility) to determine the type of soil they had tested. The next step in their process is to answer whether or not their school’s soil would be optimal for constructing a rain garden.

This is also where the next phase of the project comes into play- I will hopefully get to interview some teachers at Wheeler Elementary about the success of the unit thus far- questions like “How well do you think the students are understanding the material and experiments”, “Do you think the students are engaged in the material and experiments”, and “How do you think the students could engage more effectively with the material.” I’ll keep you updated!


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