Author
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Bennett, Sean |
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Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 3/19/1999 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 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. This soil was subjected to rainfall that produced a surface seal, followed by overland flow that caused erosion to occur at the step. The same flow discharge was used in each experiment, but bed slope varied from 1 to 10%. The rate of headcut migration was constant within each experiment, but higher bed slopes generally resulted in lower migration rates. At bed slopes 2% and smaller, sediment yield and scour hole geometry remained constant as the headcut migrated upstream. For bed slopes 3% and greater, as the headcut migrated upstream, the depth of scour increased. The slope of the sediment deposit downstream of the migrating headcut was 2.2% for all experiments, suggesting that flow discharge and not initial bed slope controlled transport capacity and downstream bed adjustment. This information will form the framework for predicting sediment load and enhancing the assessment of soil loss in upland areas impacted by headcut formation. Technical Abstract: Experiments were conducted to examine soil erosion by headcut development and upstream migration in rills typical of upland areas. Soil material, simulated rain, overland flow discharge, and initial headcut height were held constant in each experiment, but initial bed slope varied from 1 to 10%. Sandy loam soil was incrementally packed into a laboratory channel with a pre-formed headcut. Simulated rain produced a well-developed surface seal that minimized surface soil detachment. Following the rainfall, overland flow was released onto the bed, soil erosion occurred at the headcut overfall, and a scour hole developed. The rate of headcut migration was constant within each experiment, but higher bed slopes generally resulted in lower migration rates. At bed slopes 2% and smaller, the overfall nappe at the headcut brinkpoint remained submerged, and sediment yield and scour hole geometry remained constant as the headcut migrated upstream. For bed slopes 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. The slope of the sediment deposit downstream of the migrating headcut was 2.2% for all experiments, suggesting that flow discharge and not initial bed slope controlled transport capacity and downstream bed adjustment. |
