Soil Erosion Processes

	Richard A. McLaughlin, Ph.D.
	Soil Science Department
	North Carolina State University

Water Quality Problems: EPA Ranking

	Sediment: widespread problems in surface water.  Ruins habitat, clogs waterways, fills lakes and reservoirs.
	Nutrients: cause algal blooms and ultimately oxygen depletion.  Yuck.
	Pathogens: local problems especially on coast.  Many sources and no way to tell them apart!
	Organic Stuff:  degrades and robs oxygen.
	Heavy Metals:  biological impacts
	Pesticides: mostly a private well problem.

Non-Point Pollutant Sources: North Carolina Rivers

But…Construction Sites Have Highest Erosion Rates

Erosion: Two Phases

	Detachment:  individual particles are loosened from the soil mass.
		Rainsplash > running water > wind
	Transport:  water or wind carries the detached particles downslope or downwind.
		Flow in rills is the most important.

Factors in Soil Losses

	Rainfall: intensity, duration, and energy.
	Soil erodibility: texture, structure, organic matter content.
	Topography: slope length, steepness.
	Surface Condition: vegetation, mulch, bare, etc.
	Erosion Control Practices:  contours, terraces, silt fences, basins, etc.

Factors in Soil Losses

	Rainfall: intensity, duration, and energy.
	Soil erodibility: texture, structure, organic matter content.
	Topography: slope length, steepness.
	Surface Condition: vegetation, mulch, bare, etc.
	Erosion Control Practices:  contours, terraces, silt fences, basins, etc.

Rainfall Factor

	Intensity: the volume of water per unit of time e.g. inches/hour.
		Records:  1.23” in 1 minute (MD, 1956), 19” in two hours (WV, 1889), 45” in 3 days (FL, 1950).
	Duration: how long the storm lasts.
		Norfolk has an average of 603 hours of precipitation/year, or 6.9% of the time. Of that, 12 hours exceed 0.5 in per hour.
	Energy: droplet size and velocity.
		Heavy rain (0.6”/hr) has 30 times more energy than light rain (0.04”/hr).

24-Hour Rainfall Amounts for a 25-Year Recurrence

Rainfall Erosivity: Duration + Intensity