|Tour Stop #10: Dynamics of Head-cuts in Upland Concentrated Flows|
Erosion due to head-cut development and migration is a ubiquitous problem on farms and in streams in the DEC watersheds of Mississippi due to the low cohesive strength of the soil materials. Such localized scour commonly occurs on very short time-scales, say within a single runoff event, and sediment yields can increase by orders of magnitude as a head-cut forms and begins to migrate. These increased sediment loads can have significant impact on water quality and ecology, both in rivers and streams and nearby lakes. Thus, understanding erosion processes in concentrated flows and predicting sediment yields of soil materials depend heavily upon characterizing the nature, occurrence and dynamics of head-cuts.
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 0.1 6 m wide flume to a height of about 0.2 m. An additional height of 0.025 m of soil material was added and packed to form a head-cut A rainfall intensity of 30 mm/hr for a duration of 4 hours at a 5% slope produced a well-developed surface sea] with approximately 15 mm/hr of surface runoff. With the rainfall stopped and a bed slope of 1%, clear-water concentrated flows ranging in depth from 0.01 to 0.02 m and discharges from 25 to 82 I/min were then released onto a pre-formed head-cut thus allowing localized scour to occur. After a short period of time (1 to 3 min), the growth of the head-cut ceased and 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 1.3), and sediment transport rates remained constant (from 0.003 to 0.01 3 kg/S).
These and other on-going experiments are providing valuable insights and data on head-cut morphologies, sediment yields and migrations rates. Such information can be used to assess sediment yields in the presence of head-cuts, they can be integrated into soil erosion models, and they can be used to evaluate appropriate mitigation technology.