How add-inflow and subsurface drainage affecting gully evolvement

in International Symposium (poster paper), 國際研討會(全文海報發表)
標題How add-inflow and subsurface drainage affecting gully evolvement
AuthorsWu, 吳嘉俊, 黃嵩, 吳亭育, & 魏平彥
出版日期Jul 13 2017 12:0

Processes of gully evolvement include downward scour in gully channel, headward retreat at gully headwall, and lateral expansion of the gully channel. These processes are often related to the threshold velocity and threshold shear stress of the flow. Less attentions have focused on runoff rates, soil moisture content of the soil matrix, and subsurface drainage conditions. Therefore, the objective of this study is to experimentally inspect the preceding parameters and to see how they affect the evolvement of a preexisting gully.
Experiments were conducted in a 2.2-m long, 0.6-m wide, and 0.16-m deep soil box that packed with loamy soil. A trapezoidal trench with cross section taking up 50% of that of the soil box was inscribed prior to each experiment to imitate a preexistent gully. Light sprinkle at 6.3 mm/h was applied continuously throughout the experiment to eliminate the formation of micro indentations and micro-rills yielded from raindrop impact. Three add-inflow rates, two slope gradients, and drainage valves in either close or open positions constitute all combinations of the experimental conditions. Add-inflow was added to the soil box only when the volumetric soil moisture content reached approximately 33%.
Experiment observations indicated that plung-flow scour did not occur at the toe of gully headwall. Seepage and slump from gully sidewalls as well as concentrated runoff due to add-inflow dominated most of the lateral expansion and rill headward retreat correspondingly. Slump of gully sidewalls was found to be highly correlated with volumetric soil moisture content when moisture content reached 82.3% ~ 98.7% and 98.9% ~ 99.3% of the maxima under drain disabled and enabled condition respectively. Slope gradient demonstrated its important role in controlling the lateral expansion and downward scour during gully evolvement.

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