Quantifying eroding head cut detachment through flume experiments and hydraulic thresholds analysis

Gully erosion is known as one of the most important environmental earth hazards in the world and especially in Iran where it is controlled by both environmental and human factors. This research has attempted to assess the effects of land use (LU) on gully hydraulic flow condition of head cut initiation under similar soil conditions through an experimental field base plot (15 x 0.4 m). Results reveal that critical shear stresses (tau(cr)) for head cut initiation in abandoned, dry farming and rangeland, land are 174, 43 and 192 dyne/cm(2), respectively, the remarkable differences being the consequence of soil surface condition (e.g., vegetation cover and micro-relief). Moreover, the flow turbulence and response of soil to an increase in flow condition (rate and depth) indicated a complicated behavior which can be addressed to the influences of surface micro-relief features and LU effects. Although a simple linear equation between tau(cr) and K-c can be used to estimate the parameter, significant differences of regression coefficients decrease the general equation for whole data sets. The main explanation for dramatic (3-4 times) variations of tau(cr) is the vegetation cover and soil surface conditions. In fact, the significant decrease in tau(cr) under dry farming can be linked to the effect of tillage operation on aggregate strength and soil susceptibility. Findings showed that a boundary shear stress of 35 dyne/cm(2) used in some physically based models for predicting erosion is subjected to high uncertainty and not appropriate for estimation of gully development. In addition, the duration of land abandonment has a crucial influence on soil erodibility, a factor given little consideration in erosion models. For the prediction of gully erosion rate, it is recommended that a single erodibility parameter is created from the functional relationship between tau(cr) and K-c. Due to the interaction of slope, flow regime, shear stress and type of ground cover, the relationship between head cut initiation and surface roughness varies. In addition, the Reynolds number is not a reliable parameter for estimation of surface roughness during head cut initiation. Therefore, for gully erosion modeling other hydraulic parameters should be used to produce a reliable model. For prediction of gully erosion rates a single erodibility parameter from the functional relationship between tau(cr) and K-c could be used.