Overland Flow n Once rainfall exceeds infiltration, water begins to flow. –Clay is hard to pick up due to cohesion. –Sand is hard to pick up due to size. n Most of the sediment in sheet flow comes from rain detachment. n THIS IS WHY COVERING THE SOIL IS SO EFFECTIVE! NCSU Soil Science R. A. McLaughlin

Rills Starting… NCSU Soil Science R. A. McLaughlin

Flow Along Waterway NCSU Soil Science R. A. McLaughlin

Formation of Rills n Water begins to collect near the top of the slope. n Rills generally deepen downslope. –Flow itself results in erosion. –Headcutting moves upslope. n Sediment comes into the rill from overland flow. NCSU Soil Science R. A. McLaughlin

Rills With Sandy Deposits NCSU Soil Science R. A. McLaughlin

Overland vs. Rills n Studies have shown that both erosion processes are important. n Relative importance depends on soil, slope, and storm intensity. n Rills can carry large materials. NCSU Soil Science R. A. McLaughlin

Gully Formation Overland or Rill Flow NCSU Soil Science R. A. McLaughlin

Gully Formation Nick or depression forms NCSU Soil Science R. A. McLaughlin

Gully Formation Headwall forms NCSU Soil Science R. A. McLaughlin

Gully Formation Scouring at headwall base Headwall retreats by undermining and collapse. NCSU Soil Science R. A. McLaughlin

Gully Formation: Piping Water emerges at slope base, then erodes upslope. Restricting layer forces percolating water along slope. NCSU Soil Science R. A. McLaughlin

Gully After One Storm NCSU Soil Science R. A. McLaughlin

Headwall retreat continues… NCSU Soil Science R. A. McLaughlin

Note Headwall Scouring…Nothing to Stop It! NCSU Soil Science R. A. McLaughlin

Gullies Form Even in Flat, Sandy Soil NCSU Soil Science R. A. McLaughlin