Certified Storm Water Operator Practice Exam

When slopes are steeper than what ratio, may structures be more effective than vegetation to control erosion?

1. 4:1

2. 3:1

3. 1:1

4. 2:1

The correct answer is: 2:1

In the context of erosion control on sloped areas, the effectiveness of vegetation versus structural measures often depends on the steepness of the slope. When slopes are steeper than a 2:1 ratio, which means for every two units of horizontal distance, there is one unit of vertical distance, structures such as retaining walls, riprap, or erosion control blankets generally become more effective at preventing erosion compared to vegetation alone. At steeper gradients, the root systems of vegetation may not be adequate to hold the soil in place, and the risk of soil movement increases. While vegetation can help stabilize the soil and provide a protective cover, it may not be sufficient on very steep slopes where the gravitational pull on the soil is stronger. Structures are more effective in these scenarios because they can physically prevent soil from sliding or flowing downhill and can also manage runoff more effectively, reducing the impact of water on the slope. Choosing a 2:1 slope ratio indicates an understanding of the critical threshold where traditional erosion control methods like vegetation may start to fail, hence highlighting the need for more robust structural solutions.