Erosion on construction sites generally starts with rainfall. The extraordinary amount of bare soil created by construction activity renders the ground vulnerable to washing away, and even though we’ve all seen a big mudflow, such catastrophic events start on a nearly microscopic level. Raindrops can plummet to the earth at up to 20 mph, and when they hit the ground, they disrupt the soil aggregate, sending particles up to nearly 5 feet away from where they were disturbed.
This impact and rearrangement of the displaced particles breaks up the crust and creates a surface over which water begins running faster; this increased velocity increases the erosive potential of the runoff and causes the flow to pick up more material. From this tiny point, flows may gather together, forming the more familiar sheeting, rills, and gullies that can cause even more trouble.