Author
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Bennett, Sean |
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Submitted to: Laboratory Publication
Publication Type: Government Publication Publication Acceptance Date: 9/1/1999 Publication Date: N/A Citation: N/A Interpretive Summary: Headcut erosion plays a critical role in initiation of drainage systems, rill and gully formation, and landscape evolution. In the agricultural areas of northern Mississippi, soil erosion due to headcut formation can be devastating because of the low cohesive strength of the soil. Such erosion processes lead to significant soil losses that may impact both ecology and water quality, reduce crop production, and cause ephemeral gully formation. Experiments were conducted to examine the effect of flow discharge and bed slope on the growth and development of headcuts in a laboratory channel. Soil indigenous to Mississippi was packed into a flume with a pre-formed step or headcut. The soil was subjected to simulated rainfall that produced a surface seal, and then overland flow that caused erosion to occur at the step. Steady-state soil erosion was observed in both experiments where the headcut migrated upstream at a constant velocity, the scour hole shape remained the same, and the sediment yield remained constant. Increasing the flow rate or bed slope caused the headcuts to increase in size but their geometry remained the same. Sediment was deposited downstream of the headcut, and the slope of this deposit was controlled by the rate of overland flow. The systematic variations in headcut erosion described provide a means for enhancing the assessment and prediction of soil loss in upland areas impacted by headcut development. Technical Abstract: Two series of experiments were conducted to examine soil erosion by headcut development and migration in concentrated flows typical of upland areas. In a laboratory channel, packed sandy clay loam soil beds with pre-formed headcuts were subjected to simulated rain followed by overland flow. The rainfall produced a well-developed surface seal that minimized surface soil detachment. Using a bed slope of 1%, overland flows ranging from 20 to 80 l/min caused the formation of steady-state scour hole where the headcut migrated at a constant rate, the scour hole morphology remained unchanged, and sediment yield remained constant. A four-fold increase in flow discharge resulted in larger scour holes, yet aspect ratio was conserved. Using an overland flow of 52 l/min, the rate of headcut migration decreased as bed slope increased from 1 to 10%. At bed slopes of 2% and smaller, the overfall nappe remained submerged, and a steady-state condition was achieved. For slopes on the bed of 3% and greater, the overfall nappe became aerated, and as the headcut migrated upstream, the depth of scour increased. Higher bed slopes resulted in deeper scour holes. A sediment bed was deposited downstream of the migrating headcut, and its slope was controlled by flow discharge. |
