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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #75641
Title: DYNAMICS OF HEAD-CUTS IN UPLAND CONCENTRATED FLOWS

Author
item Bennett, Sean
item Alonso, Carlos
item PRASAD, SHYAM - UNIVERSITY OF MISSISSIPPI
item Romkens, Mathias

Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 6/1/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Erosion due to head-cut development and migration is a serious problem on farms and in streams in watersheds throughout Mississippi due to the low cohesive strength of the soil materials. Such localized erosion commonly occurs very quickly, even within single storm events, and sediment yields can increase dramatically as a head-cut forms and begins to move. Thus, a series of experiments have been initiated to investigate the nature of pre-formed head-cuts in a laboratory channel. Crushed, air-dried soil was packed incrementally into a laboratory flume. Rain was allowed to fall for a period of time onto the soil, and this produced a well-developed surface seal with some surface runoff. With the rainfall stopped, shallow clear-water flows were then released onto a pre-formed head-cut, thus allowing localized scour to occur. Within a few minutes, a steady state situation ensued: the head-cut migrated at approximately a constant velocity, the scour hole morphology remained unchanged, and the amount of soil caught at the downstream end of the flume remained constant. Notably, once the head-cut reached this steady state, its rate of migration became invariant to flow discharge, hence the presence of the surface seal plays a significant role in controlling rates of soil loss and sediment yield. These data will be compared to larger-scale experiments and field observations of head-cut morphology and rates of migration.

Technical Abstract: Erosion due to head-cut development and migration is a ubiquitous problem on farms and in streams in watersheds throughout Mississippi due to the low cohesive strength of the soil materials. Such localized scour commonly occurs very quickly, even within single storm events, and sediment yields can increase by orders of magnitude as a head-cut forms and begins to migrate. Thus, a series of experiments have been initiated to investigate the nature of pre- formed head-cuts in a laboratory channel. Crushed, air-dried Neshoba soil (6% clay, 48% silt, 47% sand) was packed incrementally into a 2 m long and 0.16 m wide flume to a height of about 0.2 m. A rainfall intensity of 30 mm/hr for a duration of 4 hours at a 5% slope produced a well- developed surface seal with approximately 15 mm/hr runoff. With the rainfall stopped, clear- water concentrated flows ranging in depth from 0.01 to 0.02 m and discharges from 25 to 82 l/min were then released onto a pre-formed head-cut 0.025 m high, thus allowing localized scour to occur. After a short period of time (1 to 3 min), a steady state situation ensued: the head-cut migrated at approximately a constant rate (1.2 to 1.7 mm/s), the scour hole morphology remained unchanged (scour hole length-to-depth of about 1.1), and sediment transport rates remained constant (from 0.003 to 0.013 kg/s). Notably, once the head-cut reached this steady state, its rate of migration became invariant to flow discharge, hence the presence and maturity of the surface seal plays a significant role in controlling rates of soil loss and sediment yield. These data will be compared to larger-scale experiments and field observations of head-cut morphology and rates of migration.